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MG MGB Technical - Pressurising Coolant

I changed the head gasket on my '77, using a Payen gasket.
I torqued up the nuts in the correct sequence, and retoqued after the initial run, when cold, to 55ftlb (oiled threads)

I now noticed after about 100 miles that there is pressure in the header tank when cold, and the water is below the thermostat (but the header is full). Blowing the water back into the engine, the header tank is empty.

I retorqued the head to 55, there was a slight movement in the nuts. I pressure tested the cooling system, it holds 10psi for 20 minutes. I did a compression test, and the centre 2 cylinders are slightly down on the other two.

Topping up carefully, when cold there are no bubbles in the header tank at idle, but following a run there were signs that coolant had come out of the overflow, and the system stays pressurised when cold.

When idling hot there is a faint hissing from the overflow, and the escaping gas does smell a little oily. (It definitely isn't steam.)

There were no visible cracks in the head when I cleaned it up, before grinding the valves, but I didn't have it skimmed (Wish I had now!)

I could try some of the "ceramic sealer" (waterglass based), is it likely to work?

The alternative is remove the head, and have it skimmed and crack tested. (Or exchange it for an unleaded reconditioned one)

Is there likely to be an internal crack, or will any cracking be visile?
Looking at pictures of a head, I can't reaaly see how any leakage between #2 can pressurise the coolant.

I fitted a Payen gasket, as these are supposed to be the best, but would a copper gasket give a better combustion chamber seal? The Payen seems to use steel rings.
Martin Layton

Martin. I fitted a resin type gasket, part of the Fel-Pro set, on my last three rebuilds. All of them have worked fine and have held up for several years now. Thus, I believe the Payen style gasket to be quite excellent and fully as good as the copper type for the majority of applications.

When you replaced the cylinder head gasket did you have the cylinder head inspected for warpage and cracks? I have inspected about ten rubber bumper cylinder heads over the years. About 80% of them are cracked and all have shown some degree of warping require a skim cut to make the bottom straight. If the cylinder head was not inspected, I would think that removing and having the cylinder head inspected would be the next logical step.

If the head was inspected, I would have the cooling system tested for the presence of exhaust gasses in the coolant, a simple and relatively inexpensive test. I would also have the cooling system pressure tested and run a compression check on the engine itself. Those three tests should provide sufficient information to make a good guess as to what the problem is and what is the cause of the problem.

Cracked cylinder head, warped cylinder head, bad gasket, cooling system problem and pourous/cracked block all come to mind as a possible cause of the situation you describe and a teardown and complete inspection would be required to find a cracked or pourous block/cylinder head. Hence, some testing, along with more information on what was done during your gasket replacement, might narrow down the possible problem causes.

Les
Les Bengtson

Thanks for the reply Les.

I'm convinced it is combustion gases geting into the coolant. It can't be anything else really, as the system stays pressurised days after a run. The only other possibility is some form of electroyltic corrosion, but it has new good quality antifreeze, with corrosion inhibitor, and it doesn't repressurise when cold once the pressure is released.

I guess my only option is to take the head off, inspect carefully, with a magnifying glass, and have it re-faced with a light skim.

If that doesn't fix it, I will try the ceramic sealer, as I won't have anything to lose.
Martin Layton

Only other possibles are a problem with the block or studs. I once had to resurface a block that had been "pouted" round the studs due to over tightening and that cured the problem.
The manual I have gives 45 to 50 for the torque, but maybe that was upped a bit for later cars.
Stan Best

My studs have the dimple in the ends, so they can go to 55ftlb according to Chris.

There was no sign of pulling around the stud holes, I didn't undercut them, as the Payen gasket has quite large holes around the studs.

I will check around the studs and maybe chamfer the top thread this time. If the block needs facing I've got trouble, engine out time!
Martin Layton

Martin, in your initial post, you say "oiled threads". Do you mean lubricated with oil? If yes, then they were overtorqued. Lubricating the threads significantly reduces the torque required to generate the clamping force that the bolts are designed to create. You'll see in the chart linked below, as an example, a 7/16-20 bolt should be torqued to 55lb-ft dry, but only 41lb-ft lubricated. The values for a Grade 8 fastener are 77 & 58lb-ft.

http://ourworld.cs.com/MJVanVoorhis/techdata/t205.htm

Here's another one you could print out and hang on the wall over the workbench.

http://www.jasperengines.com/pdf/Torque%20ValuesTB.pdf

For an abbreviated explanation of the why's, you can look here, even if there is some confusion over the K value for rusty fasteners. But we never have rusty fasteners on our LBCs, do we!

http://www.vfbolts.com/torque_value.htm

I'm not saying you've done irreparable damage by tightening to 55lb-ft lubricated, I'm just saying there's a difference. Especially with anti-seized.



Derek Nicholson


Martin,

I don't know a lot about MG's and I certainly don't know anything about torque values, oiled or otherwise. However, I just did a complete cooling system overhaul on my E-Type. When I finished, I had problems that were very similar to yours. Turns out that I had air trapped in the system. Did you "burp" yours? Try squeezing the bottom radiator hose and see if you get any bubbles. It can't hurt anything and it doesn't cost anything. Who knows, it might cure your problem.

Mack
A Sneed

Derek,

I'm not sure I agree about the oil/dry thread issue.

I was advised by a mechanic who worked here with BMC/Leyland from the fifties that they always oiled threads prior to applying the specified torque. He described the practice as standard workshop practice of that era.

When I reinstalled my head I too used the Payen gasket and followed this practice. I actually lubricated with an anti seize paste, wiping excess off when completed. My reasons for this were to maintain some degree of lubrication on the studs outside the rocker cover in the event I needed to re-torque. Given the four studs under the rocker cover are bathed in oil, my plan, if re-torquing was necessary, was to do it by flats measure of movement rather than the back-off/re-apply technique.

To date all is holding well so re-torquing has not been necessary.

Martin, pulling the head/engine et al is an extreme solution at this stage. What figures do you have for the compression test? It is common the see some variation between cylinders. What temperature are you running when warmed up? Are there any unusual (for your car) variations in the temperature readings when underway/slowed in traffic/climbing steep gradients?

Mack's point about trapped air is worth investigating. Check that your bottom hose is not collapsing when the system is hot and restricting flow.

One difference in my case was that I decided against adding glycol-based anti-freeze at the time (it was coming up to summer here) because I did not want to risk the slip factor of glycol forcing coolant through the settling gasket. Would it be worth while to change out any glycol in the system and do a re-torque, say 1/2 or max 1 flat stud nut turn in sequence, and try a test run?

Hope it turns out to be a solution at the simpler end.

Regards
Roger
Roger T

Derek,

Factory manual torque figures ARE for oiled threads.

Your last ref shows the K value for dry and lightly oiled is the same!

Only reduce torque if using a moly grease like ARP supply with their bolts/studs.

If using new studs & nuts you should torque and undo the nuts 5 times in the correct sequence to "bed in" the threads in order to acheive the correct clamping force.

The studs/nuts are at least 150,000 psi rated and are made of alloy steel which typically 1.5 / 2 times stronger than a grade 9 bolt!
Chris at Octarine Services

Sorry to be a geeky engineer type, but torque is applied to a head nut to generate a clamping force. In operation, the engine tries to blow the cylinder head off the block. The bolts are sized, and torque specified, to preload the joint so that there is still the necessary clamping force despite the pressure inside the engine.

When you turn the spanner, some of the torque you apply is used to overcome friction, some to increase the clamping force. If you oil the threads you reduce the friction, therefore you increase the amount of clamping load for a given torque. The condition of the threads (rust, surface treatment) etc will all modify the friction coefficient.

The clamping force is actually provided by tension in the bolt itself. There is a maximum tension that can be applied to a bolt before it yields and/or snaps which depends on the size, material and grade. Tables of maximum fastener torque tell you the maximum torque that can be applied to a bolt before it breaks under tension.

The torque you need to generate the required clamping force (eg for a gasketed joint) and the torque that can be applied before a bolt snaps are two entirely different things and should not be confused. Usually the nut is made from a lower grade material such that over-torquing strips the thread on the nut rather than breaking the stud/bolt.

Anyway, none of that helps with the overpressuriation problem. In that regard, sorry - I have no idea.

Neil
Neil Lock

Roger & Chris, I had a quick peek in my MGB Bentley Manual before posting and didn't notice any reference to lubricating the fasteners. I'm not saying there isn't one, just that I didn't see it. One way or another, as Neil points out, there is a difference between the two that people should be aware of. If it should be lubbed, then people are under tightening if they torque dry.

So Chris, what torque should be used for the ARP studs?

Hey Neil, I'm a geeky engineer type, too! HND (Mech Eng 1971, Kingston Polytechnic). However, I threw out the pocket protector full of pens long ago. Still have a slide rule around here somewhere.

TTFN



Derek Nicholson

Derek,

What's a slide rule? And no, I haven't got a calculator watch! I'm no longer an engineer either, but I do work in engineering.

Neil

PS I guess a degree in Mech Eng from Brunel in 1990 trumps...! Now and again I worry that some of the technical advice on these Boards is not founded in any real fact...
Neil Lock

Derek: ARP calls for 60 lbs. with 30 wt and 40 lbs. with their moly oil. Ray
RAY

Thanks for the comments.

I have decided to remove the head again, clean it up and check carefully for cracks. If I can't find any, I will get the head refaced (£25) and refit (New gasket set on order from Chris B).

Hopefully, that should fix it, fingers crossed!
Martin Layton

Derek,

I have no issue with the theory of differences in outcome of applying a given torque to a lubricated verses a dry thread. It was my concern about that difference that led me to do some research when refixing the head. Archives carried a range of views, and a considerable number of thread discussions where the issue simply did not come up.

The point I raised was to do with whether the recommended torque figure for an MGB stud should be applied to a lubricated or a dry thread. The factory manual makes no mention either way, like your Bentley. I was uncomfortable with the idea that 'no mention' meant one or the other approach. That was why I decided to ask around workshops I know here. All advice I received was that workshop practice of that era was to apply torque to lubricated threads.

The specific mechanic I mentioned is a chap who wound up owning the business that had been the MG/BMC/Leyland agent here in Canberra in the sixties. He told me that it was probably he who had affixed the agents sale plate to my B. It was sold through that agency in October 1969.

Regards
Roger
Roger T

Martin. You might find my tech article on crack inspection of use. It is on my website, www.custompistols.com/ under the MG section. This is something that can be done by the hobbyist, but it requires a fair amount of tooling to do the job properly. A good machine shop can do a magnetic inspection (Magna-Flux) in a few minutes. They will also have the proper tools to check the cylinder head for warpage. If you have a crack in the cylinder head, it will, from your description of the problem, be in the combustion chamber area. Most frequently in the number three chamber. The chambers will, most often, be very dirty from the years of carbon build up. Until all of that is removed, neither a dye-penetrant test (good), or a visual inspection (not so good) will be possible. I would have a machine shop strip the head, clean it throughly, and inspect it for cracks. Anything else is a waste of money, time, and effort.

As to the oiled threads. This was what I was taught, by a mechanic at the local BMC/BL dealer back in the 1960s. He made a point of mentioning it to me when I was replacing my first head gasket on a Sprite. It, like many other things, is not covered in the Factory Workshop Manual because the mechanics were supposed to have been taught such things as part of "their general knowledge". Thus, there was no need to cover it in a manual intended for use by a factory trained mechanic. It does, however, leave questions in the minds of those of us who are not factory trained mechanics. I have lightly oiled the threads on the upper end of the cylinder head bolts on a number of rebuilds and have never had a problem with that procedure.

Les
Les Bengtson

The workshop manual leaves out a lot of things because it was just that - a specific guide on the car for trained workshop staff. All the basic stuff like oiling threads before torquing up is taken for granted.

Chris at Octarine Services

Les, I have been to your site, it is interesting reading!

However, I am blocked from accessing it at work, probably the "pistol" in the URL now has me marked as a terrorist suspect :-)


The crack testing kit is available here for £20, and as I have managed 45 years without ever needing one, I'm not spending that much.

I will enquire about crack testing when I take it for machining, depending on the cost I might just risk it.
Martin Layton

"I have decided to remove the head again, clean it up and check carefully for cracks. If I can't find any, I will get the head refaced (£25) and refit (New gasket set on order from Chris B).

Hopefully, that should fix it, fingers crossed! "

"I will enquire about crack testing when I take it for machining, depending on the cost I might just risk it."

So what do you think is the source of the problem Martin?

Roger
Roger T

Martin. I have, in the past, given thought to dropping the current website because the inclusion of the firearms section might, to a few, give offense. Particularly so since firearms hold increasingly little interest for me over the years. An analysis of the site's statistics, however, demonstrate that there is still a very significant interest in both the firearms and MG sections. Since I cannot afford to operate, and ask my computer literate wife to administer, two websites, the basic site will have to remain as it is.

The main thrust of the tech article on crack inspection was to describe the various methods, in an understandable manner, for the hobbyist. It also demonstrates that doing the job yourself is both time consuming and, as you found, can be expensive. My dye-penetrant test kit only cost about $25, significantly less than L45 that you mention. However, the machine shop I use does not charge me a fee for inspecting a head they have been given to machine.

The only scientific crack inspection series I have done consisted of five late model cylinder heads. For of these, or 80%, showed cracks in the number three cylinder area. Subsequent inspections have given me the impression that this figure may be slightly low and that the actual percentage of cracked cylinder heads is between 85% and 90%.

What Roger is telling you, in a very gentle way, is that you are considering the cylinder head to be the problem. The information I have developed over the years tends to strongly support your conclusion. Thus, to fail to have the cylinder head fully cleaned and inspected would, most probably, result in the problem still being there with the cost of a new headgasket and your time wasted.

I understand money being tight. All of us have "been there, done that" in our lives. But, you must have the cylinder head inspected and brought into proper specification if you are to have any chance of correcting the problem.

Les
Les Bengtson

Les,

I don't object to guns, just at work we have a nanny filter, that stops any access to games, porn key words, and presumably also guns and weapons.


Anyway, I have taken the head off, and it is obviously warped, as there are brown blow-by stains between 3 and 2, also 4 and 3.

I now have 2 problems.

1) how to clean up the block.
2) it looks like .010" has to come off the head. (I did say it was warped!) With the .030" overbore and our low octane fuel it will likely end up pinking, so will need the ignition retarding.
Martin Layton

Martin

If, indeed, you do get 'pinking', bite the bullet, throw out the octane boosters and run it on Shell V-power. I know that it is a heck of a lot more expensive than 4 Star unleaded but the difference in performance is well worth it. (There's a lot of information about the correct Octanes for MGB and MGC on these BBS).

I had a similar problem with my MGC earlier in the year. I removed the head, had it skimmed and converted it to unleaded at the same time. Then running it on unleaded fuel resulted in 'pinking', whatever I set the timing to. So I tried the high octane Shell petrol and 'voila', no more problems!

Now this may sound like 'teaching you to suck eggs', but do you have an overflow pipe from the radiator filler to the expansion tank on your MGB? - I can't remember from my memory. Try reseating/replacing the overflow pipe and Jubilee clips. It may sound simple but if they are not sealed properly, air can be drawn in from outside the system rather than water from the expansion tank. I also did this and my MGC is running fine since.

Good luck

Robert
R Lynex

4-star unleaded? No such thing. There's 4-star leaded (which is a lot more expensive than V-Power), so-called 4-star LRP which funnily enough Sainsbury's have just started selling, 'unleaded' (which is probably what you mean) at 95 octane, Super unleaded which is 97 octane and more expensive than than 'unleaded'. There used to be Shell Optimax at 98 octane which has nor been replaced by V-Power at 99, which is probably only a bit more expensive than Super. Can't say whether Optimax was any better than Super, but V-Power doesn't seem any better than Optimax. There is also Tesco Greenergy at 99 but I don't know the price and haven't had the opportunity to try it. *All* of the supers at 97, Optimax and V-Power are significantly better at supressing pinking in my roadster compared to 'unleaded'. If you want 'a heck of a lot more expensive' then go for BP Ultimate 102 at about £4.50 per litre! Not to be confused with 'ordinary' BP Ultimate which is 97. All pinking should be able to be controlled by retarding the ignition somewhat, but that does sap performance, economy and increase running temps in my roadster. I suspect work on the combustion chamber to remove thin areas would be of benefit. Mine always tended to pink at the book timing figure even on 4-star leaded, whereas other BL engines I have had could go higher than the book figure, with consequent improvements in performance and economy.
Paul Hunt 2

Your absolutely right Paul, I still refer to the unleaded petrol we have today as 4 Star, as opposed to the old 2-stroke go go juice.

Anyway, when I'm in Solihull, I only fill up with Gas (that's LPG for our our friends across the pond) at Broad Oaks Garage where it is only 41.9p per litre. Sometimes I wish I could run my MGC on the stuff!

Robert
R Lynex

Paul,

My souped-up GT will barely run without pinking on ordinary unleaded, a bit better on super but by far the best on Optimax/V-Power. I've tried Valvemaster with octane boost, Valvoline pro-boost and Aldon octane boost. The Aldon stuff is the best, but Valvemaster with octane boost is much cheaper, pretty good and you can buy it in Halfords. All of the octane boosters wreck the fuel consumption though, and some turn the plugs a funny colour...

Neil
Neil Lock

Millers CVL worked well on my racer. I am now running it on the road midget, as I have some left, but it does turn the plugs a strange brown colour.

Why do they still have 98 octane 'super' in France...and it only costs 1 cent more than regular unleaded?
Dave O'Neill

Martin, I remember my MGB blowing into the waterways resulting in overheating and lack of water in the upper tank. Removed the head - couldn't see any cracks and using a straightedge it was flat within a thou or so in both planes. (It had been previously planed)It was only when I measured across the upper face of the block that the problem was obvious - it was warped across from one side to the other so there was much less clamping pressure between the cylinder centers. It was as if the studs had pulled the block up on each side - overtightened? - the only solution was to plane the block flat. Might be worth checking the block face is flat before re-fitting the head just in case the same thing has happened. It was fine after the repair. Good luck, Bob.
Bob Elwin

I tried the octane boosted Valvemaster but couldn't tell any difference so it wasn't worth the money, so I now just use the 'ordinary'.

"Why do they still have 98 octane 'super' in France...and it only costs 1 cent more than regular unleaded? " - why not? Unless you are asking why it is only 1 cent more rather than "why do they still have it?" The answer to that is "Brits will pay a higher premium, like we do for everything else". Except MGB parts, which by and large are much cheaper than in America.

BTW, watch out for (as in 'avoid') Sainsbury's '4-star' on the Stratford Road (at least) which is advertised with the red signage and four stars. When you get to the pump and read the small print it is nothing more than LRP, and as such contains no lead, is only 97 octane, the additives it *does* contain are at only half the required level to protect valves and seats like lead did, and are incompatible (as in causing sticking and burned valves) with other types of additive. Why they have just started stocking this when everyone else stopped some time ago is a mystery to me.
Paul Hunt 2

Martin-
After you have your head skimmed, you can reduce the liklihood of preignition by making sure that there are no sharp edges on the periphery of the combustion chambers to form "hot spots". You'd be surprised how many machine shops routinely do not bother to take the time to attend to this significant detail.
Steve S.

Latest News: I've refitted it.

Steve S,

You were right, there was a razor sharp flash around the chambers, (probably they just shove it across a snow grinder) I took it off with the trusty Dremmel.

There is a small crack from a large coolant hole, that hasn't reached a chamber, so I put it back on. If it only lasts 10,000 miles that is 3 years before I have to mess again!


Picture my disappointment when I ran it to hot, then let it cool and did a compression test to find I had 40psi in the only cylinder that had a good compression last time!

Turned out that the tappet gaps had all healed up from the initial adjustment, a re-adjust and a 20 mile run and all seems well.

At least the pipes don't stay pressurised. It has used some coolant, but that might be just dispelling trapped air.

If we get any dry weather, I will try another run, and see what happens to the water- I will fix up a catch tank to the overflow.
Martin Layton

This thread was discussed between 27/11/2006 and 03/12/2006

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