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MG MGF Technical - Discussion point: high oil temperature as a cause of HGF

From the thread 'Oil temperature', we've suggested that HGF maybe more likely in cars with high oil temperatures.


Because of the following reasons:

1. HGF is caused, at least in part, by the movement of the cylinder head over the block, which causes the head gasket's rubber beeding to fail through shear stress, and thus lead to escape of coolant.

2. This movement is caused by engine block twisting under high torque loads, and also - the import bit for this discussion - due to differential expansion due to different temperatures in the head and the block.

3. The cylinder head is primarily cooled by water. The block is primarily cooled by oil.

4. Under high loads, the engine temperature increases (more work, more heat). This heat is disappated by the oil and water circulation. Oil is cooled by heat radiation into the airflow under the car (marginal in a mid-engined vehicle) whilst water is cooled by the radiator; water cooling here is much more efficient due to that large air/water heat exchanger known as the radiator.

5. If the engine is working hard, the oil temperature will continue to rise, whilst, due to the opening of the thermostat, the water temperature is reduced.

6. Water temperature is lower than the oil temperature. This means that the cylinder head is cooler than the cylinder block: the block and the head will expand at different rates = shear stress on head gasket.

7. Temperature gradient problem is exacerbated when the thermostat opens and allows cold water (that has been sitting, being cooled by the airflow through the radiator) to enter the cylinder head. Suddenly, the head is even colder than the cylinder block... head contracts, whilst block expands = significant shear stress on head gasket.

So, given this premise, if your engine's oil temperature is regularly above 120C, this problem with differential head/block expansion is going to be significantly worse than when the oil temperature remains in the 90s (closer to the 'regulated' water temperature).

Based on these premises, a water/oil heat exchanger would be hugely beneficial in matching cylinder head and block temperatures, and limit the heat expansion mediated shear stress on the head gasket.

Comments anyone?
Rob Bell

Nice explaination Rob. You got me convinced.

tim woolcott

Fair enough for a theory.
In my case, the oil temp was always on 90, only climbing higher with "prolonged speed" (and that was rarely). Car owned from yer 3 to yer 5 yers old.
HGF occurred with engine at temp and oil temp beginning to climb (not being thrashed).
Paul Lane

The second mark on the oil temperature gauge (between 60 and 120) is, oddly enough, 100 (and not 90). I saw that once at my dealer when he had the testbook connected.


>>3. The cylinder head is primarily cooled by water. The block is primarily cooled by oil.<<
Rob, I've not seen this b4, where's it from ?
Considering the amount of heat in the block, I would expect to see a very rapid rise in temperature.

Is the head gasket a thermal barrier ?

I would still expect a significant amount of heat is lost from the oil to the coolant, even if it's indirect heat transfer.

Would a higher flow-rate help under some circumstances (eg idle) ?


I guess I am a little out of date but when I did my training the block is where the greatest body of water was circulated BUT it was also where the greatest anount of oil moved around.

But again perhaps I am out of touch BUT water was for cooling and oil was for lubricating.

Simplistic I know!

Ted Newman

Takes you back to school physics.

If we could find out the coefficient of linear expansion for aluminium and the length of the head, then we could work out the movement that 30 degree temperature difference would deliver. Since I suspect we could argue that we are only talking about half the length of the head then I suspect we are talking about very small movements as I seem to remember that ali has a low coefficient.

I was at school a very long time ago so could be talking complete bollocks!




Ali was not yet invented when I was at school:-)

My first bike had a 'bronze' head - a Raleigh 350 cc single cylinder twin exhausts.

Ted Newman

I am with Ted on this one. Oil is for lubrication and water is for cooling. Their functions shouldn't be confused.

Oil gets hot so to keep its lubricating properties it has to be kept within a temperature range. The oil cooler (air or water) is used to cool the oil and help keep it within that range. It is not used to cool the oil so that the oil can cool the engine.

Spyros Papageorghiou

Hi all,
looking down the waterways left around the liners when the head is off clearly shows how little water there is circulating down there! So IMO the oil does a much tougher job in both lubricating and cooling on the K-series engine than for ex. on an old iron lump like the BMC 850 /1100/1300 -engine. Probably this is the price to pay for lightweight and modern construction.
Last year I had the chance to poke around with a infrared temp. instrument borrowed from work. Dont have the figures at hand ,(was thou in an thread I dont find in the archive at the moment ).
The funny thing then was that at repeated measurements at different loads the highest temp. on the head aluminium was always found to be at the head just behind the temperature shield for the alternator !
My initial thoughts was that the form of that shield and its rather shiny structure concentrated heat onto that specific part of the head just as a parabolic dish.... And this is exactly the place where the gasket goes when we have an HGF !
The shield does a good job of protecting the alternator - I think there is NO way it can be removed without cooking the alternator. So I just painted the shield black. Unfortunatly the IR-tempsensor is part of a servicebag that nearly always is on travel so no measures after the painting yet.

As said before - any pict. and drawings on the TF160 oilcooler would indeeed be very interesting !

BR, Carl.

Your worrying me now.
I have a 99 VVC and my oil temp is always on the 120 mark - just above or just below. Last night, on my way to the Essex meet I had my mate in the car who weighs about 14 stone and me at about 12 stone and the cars oil temp went as high as the middle mark (cant tell you what that is off hand). So should I worry?

I have heard that the VVC runs at a higher temp then an MPI anyway so is my car normal?
If not what can I do?


Guys, it's a fascinating thing to talk about but don't forget that some people might be looking more on their oil temperature gauge then on the road while driving!! DANGER!!!

Don't become another statistic.

Drive Safe, Jorg
Jorg Mendgen

Wez: has your's got one of the "faulty" oil temperature sensors introduced in the early MY2000 cars? That could be an explanation if it has always read hot..


Unfortunatly, to the person that suggest that oil's function is ONLY for lubrication , your wrong. It's primary function is to lubricate, but also plays a part in cooling the engine too.
Gareth Kidman

Gareth I'm with you on this one. OIL is mostly for lubricating but has clearly a cooling job. Remember the Porsche 911? It wasn't water cooled, so it had huge amounts of oil.

Done some research and found this:

Linear thermal expansion coefficient (in/in.F)x0.000001

Ali: around 13
Steel: around 8. Therefor Aluminium has a greater expansion than steel.

But my research continued and I calculated the thermal capacity of some fluids, that is the capacity of "Storing" energy inside them (sorry the English is not helping...). It's the product of their mass density by their specific heat. Units are: kJ/(m3xK). Translating: KJoules/(cubic meter x Kelvin).

Water at 87c (360K)= 4203
Air at 27C (300K)= 1.169
Oil at 127C (400k)= 1928

That is: water is 2.18 times more efficient than oil when cooling.

HTH, Valter.

P.S. What I'm I doing? I should be on the road top down...


Gareth - I prefer your sister - perhaps she also noticed that NO ONE said that oil is ONLY for lubricating.

What I said was:-

>But again perhaps I am out of touch BUT water was for cooling and oil was for lubricating. Simplistic I know!<

Any one with a modicum of intelligence (which I hope includes me) knows that the oil will help in cooling as does the air around the engine - so come on mate stop 'nit picking' and perhaps make a useful contribution.

Ted - BTW before you get all uptight - I have my tongue in my cheek!

I have a Steptronic and my oil temp never goes above 90/100 degress (even on the motorway). Under normal use the engine sits about 2000rpm and the gearbox does all the work. In another thread I think that Rob suggested that there was a cooler fitted to this gearbox. This being the case the engine on this car seems not to work as hard as on other models. Can anyone tell me if Steptronics have fewer HGF and if so Rob's theory of high oil temp due to the engine working hard might have some validity.

David Clelland

>>>>>>>>Spyros Papageorghiou, Nicosia, Cyprus,
I am with Ted on this one. Oil is for lubrication and water is for cooling. Their functions shouldn't be confused<<<<<<<<<<

Ted, THIS is the reply that grabbed my attention, not yours.

My contribution was useful, i suggust you remove the tongue from your cheek as i'm simply pointing out the correct facts here, i would not comment if i thought it to be incorrect information.

Please read back and you will notice what Spyros said, which quite clearly stated that in his opinion, oil was for lubricating only.
Gareth Kidman

Rob, very interesting just like the email you sent me. :) Sorry haven't had time to reply yet, I want to give you a long reply as well, but exam is on tomorrow, and haven't had a chance to. But it is a very convincing theory, and if the TF part of oil/water cooling system can be retro-fitted, I guess I would have to install that one. (Oh yeah.. I told you I was going to stop at exhaust upgrade, but... but....)

Wez, all MGFs should stay around 90 degrees for normal usage. Above 130km/h it should rise to 95 or so. You must have the faulty sensor, just like Neil mentioned. Get it looked at!

Hanah Kim

Been busy over the weekend, so sorry for not comming back sooner! ;o)

>> >>3. The cylinder head is primarily cooled by water. The block is primarily cooled by oil.<<
Rob, I've not seen this b4, where's it from ?<<

This is an observation made by Simon Scutham (link: Simon, in case you hadn't heard of him before, races a modified Elise - tuned to excess of 200 bhp. He had horrendous problems with reliability of the head gasket - and this comment is derived from his efforts to sucessfully overcome the problem of HGF.

Valter, thanks for those figures! :o)) They are tremedously useful, because before reading your post I didn't know the relative heat capacitance of oil versus water or indeed the relative expansion of alloy versus iron. In the case of a K-series, I am not a metallurgist, so I am not sure what the specific expansion capability of the K-series block and head is, but we can start to make a guess based on the figure you've quoted :o)

Hanah, as Paul says, this is just a hypothesis - but at least one that could be tested. (Good luck with the exams!!!)

In terms of asking the question: "if you have had a HGF, did you notice that your oil temperature tended to run high" - well, unfortunately, this is a question is fraught with problems - recall bias, individuals tend to concentrate on the road rather than the gauge (agree with you Jorg!) - and then there is the potential problem with dial calibration that Will's pointed out previously, and am pretty sure is the problem in your case Wez (in common with a number of other early MY2000 models).

What might be worth doing, is comparing a car that has never suffered with a HGF and see what is different to a car that has suffered from repeated HGF (Dot's old MGF has apparently suffered 3 maybe 4 HGFs in its 5 year life span).

David, I think you're right that the Steptronic has a transmission oil cooler. However, the transmission and block have separate oil supplies (unlike the old A-series transaxle) - so this may not help - and the steptronic was introduced at around the same time as the upgraded HG and cylinder head dowels - so this is a further potential confounder. But your thoughts are along the same lines as mine :o)
Rob Bell

(All from memory, so may be a little wrong??)

The VVC engine runs hotter than the MPi.
To this end it has different values for various thermostats and the like. In fact, the VVC mechanism doesn't kick in until the oil has reached a certain temperature.

If you are correct Rob, then the VVC should see significantly more HGFs ?
Do they ?

Incidentally, have oft had my oil temp touching the red line (150degC I think on the VVCs) on track days.
On the road it's always "high" as I don't pootle much. I never simply switch off when the oil temp is high. I always let it cool with the engine on and the interior fan heater on full. In fact, where possible, I run with the heater on full on the slim possibility that it helps. :o)
I do occasionally (after a hard run and then unexpectedly pulling up to lights etc) get the water temperature rising when I come to a standstill. Putting the heater on full cools the engine enough to reverse this trend.

I've never had HGF - yet.

Paul Nothard

I am going to get a garage to have a look at a few things when I get back from my holiday, and the Oil Temp will be one of them.
I think I may well have a faulty reading, although I am not too bothered by this as I know now when the car is warmed up or not, but just so its perfect I will get it done.


There's no real reason why a VVC would run significantly warmer than a 1.8i Paul, unless the full engine potential was being used - because, as you can see from the superimposed 1.8 and VVC power graphs ( you need to be above 5000 rpm before the VVC starts to produce more power than a 1.8i (more power = more heat).

Most cars are unlikely to be driven to their full potential on the road - so in fact, I would not expect a significant difference in the numbers of HGF reported between the two engine types. On the HGF reporting page (, there is a slight numerical difference in the numbers of VVC related HGF - but I would not get too excited by that (107 VVC versus 99 MPi).

Paul, even you can't drive the whole time on country roads (ROFL! - shame, 'cause it's good fun following ya!), so the majority of your driving is going to be within cooler oil temperatures than the 150+ seen on track days.

At first glance, your car should be a prime candidate for HGF - and in fact I wonder if you shouldn't actually think of performing the thermostat mod? Having said that, occasional running up of very high temperatures for short periods (minutes to hours) of time may not be as damaging as long term (many hours over a period of years) raised 'normal operating' oil temperature.

There are clearly many facets to HGF - but I think if we can get temperature telemetry equipment, we may be able to determine whether some cars are truely different from others with respect to oil temperatures.
Rob Bell

Thanks Rob. I agree with your theory. And everytime we talk about HGF's and causes we learn something new.

You stated that there are slightly more VVC's with HGF than MPI's. But what's the manufacture ratio between VVC's and MPI's? If there were made 30% more MPI's than VVC's thats an allarming situation for VVC's owners (includes myself!).

Cheers, Valter.

But at the other end there are countries where ONLY the VVCs are imported by the dealers ! Sweden is one.


Oiltemp on my 1999 Mpi is always between 115-125 Deg.C.
There was no faulty sensors-issue in 1999 was there?
I have seen many people on this board reporting the same values. Whats the temperature on other cars then?

I don't really see where you're going with this expansion rate thing. Firstly, Its bound to be different with different metals and masses, but also what are you going to do about it? Have a cast iron cylinder head made?
All you can do is hope you don't get HGF, and if you do, make sure your mechanic is very competent to limit the chance of it recurring.
Water is ONLY there for the cooling, whilst oil serves a dual function. So what? In my opinion HGF is caused by flexing in the engine caused by its change to wet liners which weakened the block. CAreful assembly will help minimize the effects of this, and that's all you can do.
T Green


I think the point is to try and avoid regular *huge* changes from hot to cold inside the engine.

If there are regular cold slugs of water coming into the engine, the differential expansion - no matter how small - will slowly weaken the gasket.

If the temperature is regulated then the stress levels on the engine and gasket will be reduced. Thus fewer HGFs.

Thus, all mods (except the TF160 oil mod???) are trying to gain more temperature stability.

My point wasn't related to the higher power output of the VVC. I believe that the engine may actually run at higher temperatures given that I remember engine fans and other thermostats not kicking in until higher temperatures.
Agreed that the stats don't really back this up.

If I am right and the VVCs do run at higher temperatures - due to fans not kicking in until the higher temperatures are reached - and the stats show no significant difference in HGF rate... then can we broadly infer that there is no link between mean oil temperature and incidence of HGF ?

I'd love to know the design decisions within Longbridge that have produced the oil cooler mods to the TF160.

Paul Nothard

I think it is fair to say that oil temperature, in isolation, is not the primary cause of HGF. That onerous privilage goes to the thermostatic admission of cold water from the rad.

As you very rightly point out Paul, the oil temperature's contribution is to exagerate the temperature difference between the cold water in the head, and the hot oil in the block.

A sticking point at present with the oil temperature hypothesis is that there must a difference in the dissipation of heat from the oil and engine block between cars that may contribute to the differences in HGF probability in different cars (assuming that other factors are equal).

Air circulation around the engine bay maybe one aspect by which there maybe a plausible difference between vehicles. One of the bizarre differences between the MPi and VVC is the trip temperature for the engine bay cooling fan: 75C on the MPi and 85C on the VVC. Based on this, one might postulate that VVCs would have a preponderance of HGFs. However, how much of oil cooling is thanks to the engine bay cooling fan? I doubt that it is much - probably of use only when the car is sitting in traffic after a vigorous run. Most of the time, oil cooling will be through radiation of heat into the airflow under the car and around the sump area - and perhaps heat loss through the boot vents.

I think that an oil/water heat exchanger would be of benefit - to return to Tim's point - heat expansion mediated HGF would be ameliorated if both head and block temperature could be maintained in close proximity to one another - something that clearly does not occur in the standard system.

Is the TF heat exchanger up to this task? Based on the comments made so far regarding temperature readings - it would appear not, although one can't be completely confident of the water temperature readings from the water temperature guage.
Rob Bell

>Most of the time, oil cooling will be through radiation of heat into the airflow under the car and
>around the sump area - and perhaps heat loss through the boot vents.

I think that most of the heat loos of the oil will be thru the head where the oil and water gallys are close together, the oil serves as a carrier for heat from the engine block, I would think that _very_ little is lost thru convection. The engine bay fan is to stop the other stuff (wires, petol tank, MEMS, etc) from getting too hot - remember all the oil and dirt hanging around- if that gets too hot it will catch alight- it is that which the engine bay fan is there to stop.
Will Munns

Will, I am sure that you are right about the role of the engine bay fan - but why the difference in trigger temperature between the 1.8i and VVC??? I've never understood the logic of this.

You could well be right regarding the heat exchange from oil passageway to water circulation - but it would also be fair to say that there isn't much in the way of water circulation in the block - which would certainly mean that the block remains hotter than the head even if some of that heat contained in the oil is dissipated in the head?
Rob Bell

>but why the difference in trigger temperature between the 1.8i and VVC??? I've never understood the logic of this.

Rob, I think you should not worry about this confusion, MGR may not know that themself ;)

So many coincedences lead to solutions which can't be changed later when production got introduced.
Though I think the main reason is a follow up to the 'temperature red light' at the VVC. May be it didn't work when the Oil got to hot.
Err, I know it's a mad theory ;)

Dieter <--- missing the pictures of the mgtf160 Oil-Water cooler Mike talked about :(


Yes, it is odd and yes the block will remain hotter than the head. The VVC engine bay temp I think is a red herring- I don't see why it would make any real difference to HGF
Will Munns

I have to agree with you guys - engine bay temps are likely to be unrelated to HGF. The best evidence for this comes from Valter's observation's regarding the conductivity of air versus either water or oil. Which is to say - there's not very much capacity there in the ambient airflow to cool the oil.

The majority of oil cooling is already performed by the water circulation: the problem is that this results in a temperature gradient across the head (where the oil to water heat exchange occurs) and the block.

This lends credence to the assumption that the cooling system is more than adequate to take the loads of a water/oil cooler - afterall the current cooling system takes the majority of the heat from the oil as it is.

Advantage of an oil cooler (as I currently see it) is that instead of the heat exchange occuring in the head, leaving the block hot, is that the oil cooler would tend to lead to a more homogenous temperature over the whole engine (oil temp matching water temp).

Dieter - I haven't got those pictures of the TF160 oil cooler either. I promise to forward them to you if Mike sends them to me :o)

>>I'd love to know the design decisions within Longbridge that have produced the oil cooler mods to the TF160.<<

Me too Paul! Was it anxiety over oil longevity in certain warmer climate markets or was it anything to do with HGF as we're speculating here?
Rob Bell

>Advantage of an oil cooler (as I currently see it) is that instead of the heat exchange occuring
>in the head, leaving the block hot, is that the oil cooler would tend to lead to a
>more homogenous temperature over the whole engine (oil temp matching water temp).

Yes, I concur with that, but the real question is where is the tempture gradient- after all the water does go into the block, and the oil does go into the head, so I would expect a temp gradient thru the block and the head, not (just/mainly) at the point the head and block meet.
Will Munns

Maybe the new TF160 oil/water cooler is actually an oil warmer used to get the oil up to temperature quicker, which is better for the engine and for emission regulations.

I remember some comments some time ago by Nick Adams an Elise Development Engineer on the Lotus Life BBS. He mentioned that Lotus do recommend an oil cooler for any Elise that is used on track even occasionally. He also said that most oil/water coolers available are not really up to the job and that an oil/water cooler (unlike a oil/air cooler) does not increase the total cooling capacity of the car. If I remember correctly he also said that the standard elise cooling system is adequate to up to about 200 bhp.

Spyros Papageorghiou

>>but the real question is where is the tempture gradient- after all the water does go into the block, and the oil does go into the head, so I would expect a temp gradient thru the block and the head, not (just/mainly) at the point the head and block meet. <<

I'd agree with that assessment Will. We need some empirical data to determine what kind of temperature gradients that can occur. However, the info that I've been given would suggest that water cooling of the block is significantly less than that in the head - and given that the join between the head and block is a 'weak spot' in respect to potential movement - be it due to a lack of block torsional rigidity or through heat related expansion/contraction - this is the point that any temperature mediated movement would be seen. How much of a temperature gradient is required to trigger a HGF I have no idea - but it needn't be much if it is manifest in cycles over a long period of time... And we mustn't loose sight of the fact that when the thermostat opens, the temperature of the head will not be homogenous - and that this heterogenicity of the head temperature may be an important contributory factor in the genesis of HGF...

>>He also said that most oil/water coolers available are not really up to the job<<

Interesting bit of information there Spyros. Do we have any information on water/oil coolers that are 'up to the job'? I see that Elise parts dot com sells water/oil coolers. They are not to the same specification as Simon Scuthams' - perhaps we should find this bit of info out? From the info provided by TF160 owners, the MGR OE fit oil cooler is inadequate for the purpose of oil cooling. (Rapid oil warming? Interesting idea! Does it reduce emissions? Could be the reason Spyros :o)
Rob Bell

Spyros, I think youre right.

The new TF 160 fitment will work as an oil warmer untill the car is perfectly warm. It can also be a cooller later on! Either way is good for longevity, as friction is greater when engines are cold.


To a direct question on these matters that I asked the other day, Nick Adams on the Lotus Life BBS answered (as Valter suggests) that the water/oil cooler on the MG TF 160/Lotus Elise 111S serves the dual purpose of warming the oil after a cold start and of cooling it when the engine is very hot.

He also said that both the head and the block are primarily cooled by the water (sorry Rob), whereas the oil only cools parts like the pistons and crankshaft, not the block. He pointed out that the coolant system should always run cooler than the oil and that oil temperatures above 100 C are actually beneficial to the engine since they allow water and contaminants to evaporate off into the breather system (now, remember that the second mark on the oil temperature gauge of your MG indicates 100 C and not 90 C!).

As an aside, he also indicated that cheap mineral oils are still perfectly adequate up to around 140 C but that above that temperature, synthetic oil becomes necessary. Finally, he said that there is no significant difference in thermal conductivity and heat dissipation between a cheap mineral oil and an expensice synthetic one. The difference lies only in the fact that the synthetic oil won't break down at very high temperatures, whereas the mineral oil will.


P.S: Even though most people on this board drive MGs, I really recommend joining the official Lotus Club which gives access to Nick Adams' forum. He shows the kind of openness and helpfulness that MG Rover would do well to emulate!

Thanks for that info Per - Nick's comments would appear to pour cold water (!) ;o) on the concept that Simon (S) has proposed regarding the cooling of the block.

The truth is out there - and I'd like to try and discover it.

Anyone have access to temperature telemetry equipment and fancies a blast around Bruntingthorpe?
Rob Bell


the purchase comes through ;)

I remembered we had some points of this discussion already years ago.
Found it in the Archive Y2000 by use of keyword 'Mocal' or 'Laminova' may work as well.

Thread: Oil coolers
Bob Potter was the guy I had contact in that case

Now surprise, surprise :) Found IMO the source at:

I think I'll speak to someone in Sweden and I bet the MGTF160 cooler is from them.

Dieter <--- Still after pictures.:)
Dieter Koennecke

>>I think I'll speak to someone in Sweden and I bet the MGTF160 cooler is from them.<<

That would be interesting to confirm Dieter - but are we confident that the oil cooler fitted to the 160 is man enough for the job?
Rob Bell

Oh, this thread seems to have cooled off...
now what do I have in the bag, this 'ill do.

From the Lotus Elise Tech board

>Posted 25 June 2002 at 11:29:38 UK time
> Dave Andrews, MK,

> In have it on good authority that Rover have produced a new type of thermostat which is both
> temperature and pressure sensitive, this cuts down the heat gradient problems
> and almost eradicates failures due to temperature cycling. At the moment is listed for the
> freelander but will infioltrate it's way to everything in time.

> I'll see what further details I can glean..
> Dave

Ah, that should warm things up a bit ;-)

Will Munns

!!!!!!!!!!!!!! 8oO

Oh yeah, that's got my attention!

Will, I told you last night about my thoughts about how certain cars must have some pre-disposition to HGF, and mentioned my thoughts about thermostatic opening - a train of thought that I think you were on with respect to rate of opening.

Dave's discovery of a new, modified thermostat would certainly fall into this category.

Oil temperature is but a small component of a problem compared to the way that cold water is allowed into the head...

This needs more investigation!!!
Rob Bell

Can we conclude from the deliberations in here that minimising the thermal gradient when warming up the car is better (from an HGF point of view), than having a sharper gradient?

If that's the case I wonder if it's better to let the 'F' warm up before driving?

I live in the middle of Edinburgh, so have to drive for 15-20 minutes to get to a motorway. By then the car is warmed up quite a bit. I find driving along the motorway at 70 mph warms the engine up quickest, and wonder if people who do a lot of motorway driving when their car is cold, might be more liable to HGF?

Bit of a remote link, but thought I'd throw it into the pot anyhoo!



I'm a little late in this thread, and it's a long way back to the first post....

I don't think that the block will be anywhere near the temperature of the head. The only figures I can find are from Redline's website (a reputable source, one hopes), a few years old but fairly indicative. They are, all in deg C:

Combustion chamber 1650 - 2750
Exhaust valve 650 - 825
Piston crown 375 - 450
Upper cylinder wall 150 - 275
Crankcase, crankshaft, main bearings 100 - 200

All these are presumably from a low-stressed V8, but they're all I have to hand. The point is that there is a hell of a lot of heat pumping through the combustion chamber. From there on downwards it gets cooler, to about 90 - 100 at the oil pickup point. (I know others have reported very high oil temperatures, but I rarely see much above the 90/100 mark, which is the accepted running temperature for 'normal' running - the SAE viscosity ratings are measured at 100 deg C).

There are some aspects of the K-series engine which are, as far as I can tell - unique to the 'through-bolt' construction and directly affect the head to block joint, and how well or poorly it is sealed. I'm afraid there's a lot of it, but I'll try to be brief! I should not think that this is unknown either to MGR or to the many specialist engineers we know and love.

The K-series engine differs from a typical modern four-cylinder engine in that the lower halves of the main bearings are cast in one large flat alloy housing, the main bearing ladder. Instead of main bearing bolts being screwed upwards to hold each bearing together, long bolts are passed down through the cylinder head and crankcase to the main bearing ladder and pull the engine together forming a very tightly torqued-up sandwich. The bolts are effectively main bearing cap bolts turned upside down. As the K-series engine has five main bearings there are five pairs of bolts holding down the cylinder head (they actually screw into a steel oil rail under the main bearing ladder, as alloy threads would not take the huge stresses involved.) Fortunately there are additional unstressed bolts holding the oil rail to the bearing ladder and the ladder to the block, so that the engine won't fall apart when the head bolts are removed. There are no additional bolts on the cylinder head.

There are, as far as I can see, three ways in which this construction method can affect the cylinder head to block joint.

1) As the long Main Bearing/Cylinder Head bolts are trying to serve dual purposes there's a conflict. To maintain the integrity of the main bearings and to ensure that they do not distort out of round the bolts should be as close to the bearing area - the centre line of the engine - as practicable. This leaves the bolts holding down the cylinder head in a line very close to the cylinders, in fact just about within the diameter of the bores. This is no doubt very good for clamping the fire ring around the cylinder liners, but less good for clamping any sort of seal the further away from the bolts the seal is positioned. The outer areas of the cylinder head joint are the least supported, just where the thin red line of the elastomeric gasket seal lives.
The furthermost point away from the head bolts, and therefore having the weakest clamping pressure, is where the head/crankcase widens at the front left and right corners of the engine. The head casting has thin walls directly in these corners as it accommodates two coolant galleries. These thin walls are more likely to distort than the more solid and more evenly clamped cylinder head. Add to this the very high temperatures on the exhaust side, then the opportunity for the head to lift at the front left-hand corner is high - possibly irresistible.

2) The Main Bearing/Cylinder Head bolts are very much longer than the studs used on conventional engines to hold down the cylinder head: around 380 mm from the top of the oil rail to the underside of the bolt head. The bolts are high-tensile steel, and the crankcase and cylinder head are alloy. Both expand at quite different rates. High tensile steel has a co-efficient of thermal expansion of around 0.000012 per deg C, and alloy around 0.000024 (the exact materials used and thus their thermal coefficients are not known). The temperature rise when the engine is run will differ along the length of the bolts, but an average 100 deg rise from 10 deg C to 110 deg C will not be far out, and is probably quite modest.

So the bolts will expand by 0.000012 x 100 x 380, which is 0.456 mm. The equivalent crankcase/head assembly will expand by 0.000024 x 100 x 380, which is 0.912 mm, almost half a millimetre more. With the threads of an ISO M12 coarse pitch bolt at 1.75 mm pitch, that's equivalent to tightening the head bolts by over a quarter of a turn each time the engine is run up to operating temperature, and then slackening them by the same amount each time it's cooled. Drive the car twice a day for 250 days a year, and that's 1500 times the bolts have been tightened and released by the time the car reaches its third birthday. Do the sums for a temperature rise of 150 deg C and the difference is even greater. This constant and repeated stretching and releasing of the head bolts must eventually result in a deterioration of clamping pressure.

3) In a 'normal' engine the cylinder head is held down on the block by relatively short studs fixed in the top face of the crankcase, about 100 mm long (yes, I know, the head is actually held down by nuts screwed onto the studs). These studs are held firmly in their blind holes by being screwed down until they run out of thread and bind on the shank, or until they bind on the bottom of the tapped hole. They may also, depending on who's doing the assembly, be held tight by a thread sealant. The studs are effectively a permanent part of the crankcase. Being short, and fixed tightly, they do not flex easily. When the head is clamped down the relatively close clearance of the tunnels and the inherent stiffness of the studs holds the head firmly in place.

The K-series bolts operate entirely differently. They are not a permanent fixture, being unbolted for head removal, so the thread fixing imparts no great lateral integrity to the bolts. In any event, the thread is a long way away from the cylinder head joint. As the bolts are 380 mm long, around three times as long as fixed studs, the bolts are more easily deflected. A 380 mm long M12 bolt, no matter how tightly held by the screw thread, or what the tensile strength, can be deflected by hand.

The bolts are also not a close fit in the tunnels in the head or crankcase. Maybe this is because the tunnels are used as drain galleries for the oil; maybe it's because tight(er) tunnels would give problems locating the threads in the oil rail.

For these reasons the cylinder head isn't as positively located as it would be following the normal practice of using short studs in the crankcase. That job appears to be left to the locating dowels

All of this applies to the K-series engine wherever it's installed. I have yet to complete part two of the theory of what (might) push the MGF engine a little further towards the brink. I don't think it will be as easy as the above.

Regards, Kes.

This thread was discussed between 21/06/2002 and 01/07/2002

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