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MG Midget and Sprite Technical - Head gasket failure & cleanup

Having thrashed my 1360 Sprite round a Trackday last week, I ended up with big clouds of smoke from oil on the exhaust manifold. It was coming from the timing cover breather that had spat its pipe off.
Having checked the car over I had to add about a litre of water which then appeared in the sump.
There's mayo in the rocker box and water drained from the sump before the contaminated oil.
I'm guessing that the head gasket has gone. Could I have done this with a too early down change? If HGF is the problem, is there an easy way to clean the internals, or do I need to do a full strip and deep clean? Might the radiator and heater matrix be compromised?
Would performing a compression check do further damage if there's water in the bearings?

Mark
M Crossley

I don't immediately see how your added litre of water got into the sump unless you have a cracked block? (hope not).
A compression test should be OK but I think I'd just take the head off and have a look-see.
David Smith

I've now stopped sulking and taken the head of my motor. The gasket has gone adjacent to cylinder No. 4.

On initial inspection the block and head surfaces appear undamaged.
The motor was fairly recently built and has done approx 1000 miles. Its running 10.2:1 compression on a 73mm bore (1362cc). It was running well until it blew and had recently produced 100hp at the wheels on a RR setup.

I was quite careful when I put the motor together. I'm a fitter not an engineer, but I can follow instructions.
I used a BK450 gasket and Mini Spares uprated studs/washers/nuts, torqued to 45ftlbs dry. The block surface had been skimmed and the stud holes chamfered. The piston crowns were approximately .008" below the block face, so I don't think they have come into contact with the fire rings.

What I didn't do was re-torque the head beyond a go round after the first couple of hours running in, when I reset the tappets. Should I have paid more attention? What I'm really looking for is a reason for the failure.

Going forward, do I clean everything up and put it back together in the same way, or should I do something different? Different fancy gasket? ARP studs? I didn't think that the engine spec was particularly wild.
I drained 5 litres of oil/water from the sump (no anti-freeze). Should I strip the motor for a deep clean, or will lots of oil changes be sufficient?

Mark


M Crossley

not often seen a gasket that bad! What spec dizzy are you using, and what did you set the ignition timing to?
David Smith

I'd be inclined to clean everything carefully and put it back together. Run it for couple of hundred of miles, change the oil and retorque the head nuts and check the tappets. If yours have the dimple or indentation on the stud then they should be 50ftlbs. I am running 10.5 on the standard gasket and its been fine.
Bob Beaumont

David,
I'm not sure of the exact timing on the dizzy. I've lost the timing marks on the front pulley. It was adjusted for best results on the rolling road.
Fuel-wise I usually use the good stuff from Shell with Castrol octane booster, although I filled up with the ordinary 95 at a garage local to the track in Anglesey. The dizzy is on my 'to do' list. Its the standard one that came with the car all be it with the vac advance disconnected. The carb is an HIF44.

Bob,
Considering the time of year, I'd certainly prefer to be driving rather than stripping the engine.
I seem to find a lot of conflicting advice with regard to head stud torques, whether to oil, use moly assembly paste or leave dry. The bumf with the stud kit said that they would withstand 70ftlb, but warned that a figure that high would damage the threads in the block. Is your 50ftlb dry or lubricated?
M Crossley

Mark
The photo is good news. The gasket damage would explain the water in the sump, there's a clear track between cool and passage and pushrod hole so unlikely to be anything menacing like cracked casting. So long as there's no damage (scorching) of the head or bloack face, I'd do the same as Bob as I think there are avoidable reasons which if addressed would head off a repeat episode.

I think the cause could be a case of a perfect storm: Not enough clamping and pressures too high.

Clamping:
Personally, I would torque on lubricated nuts, then re-torque immediately after then engine has warmed up enough to open the thermostat. There are of course many other ways people like to re-torque, but this way has always worked for me, including each time I have built an engine for a week-long rally with 700+ miles of flat out closed roads. Key thing is that I do think a re-torque is essential and if I understand you correctly this didn't happen? And lubing the nuts gives a much more even result and doesn't lose some of the clamping torque to friction in the threads. The book also gives the torque for higher tensile studs (marked with 22 or a dimple) as 50 lb-ft. We do this torque lubricated. And the advice about possibly pulling the studs out of the block at higher torques is correct, been there done that.

Pressures:
Then comes the combination of CR and fuel octane. 95 is marginal on anything over 10:1 on a mild or fast road cam (the later closing inlet timing of a full race cam lowers the dynamic CR and so the engine is more tolerant of fuel octane). Yes, the engine can be set up on 95, I've done that with even 11.5:1 9though it costs power), but if it was set up on RR on higher octane that would make it definitely marginal on 95 with a tendency to detonate ... particularly under the prolonged full throttle conditions of a track day and the associated higher internal temperature gradients.

Finally perhaps check your CR calculation. With only 8 thou deck height and 1360cc, and original chamber capacity (which is unlikely but presents a lowest-case-CR scenario) you would need a 9.5cc dish in the piston, whereas approx 6.5cc is a common dish size including the purpose-made 73mm pistons we stock 9which may or may not be the same as your of course) A 6.5cc dish would nudge it up to 10.9. If this is the case, academic interest only other than it would exacerbate the problems above when things aren't quite right. At all other times it would be all good!

PS also picked up your post and replied on the K midget thread
Paul Walbran

Paul,

Thanks for your input.
I did re-torque the head, but just the once after the first hour or so of running. I'll pay better attention next time, and use your 50lb-ft figure and the oil. It sound like I just didn't do 'em up tight enough!
Which gasket do you use? I've noticed fancy (expensive) Cometic ones and Swiftune offer a 73mm specific one as well.

I've checked my compression figures. I juggled pistons and rods so the heights were pretty even, and 8 thou was as close as I could measure.
I measured the piston bowls at 8.4cc using a burrette, a perspex plate and alcohol. They were from Australia ( MiniSpares?) quite a while ago.
The chamber volume is 22.6cc and I allowed 4cc for the gasket and 1cc for the ring land.
Whilst the head is off, I'll re-measure the piston bowl.
I'm using an RE130T cam, 286-ish, which I believe also originates from Australia!

I'd really like to fit a K, and I'll probably start collecting bits in the background, but I don't like to be beaten, so I'll get the A series fixed. I've a programmable ignition in the wings which will hopefully improve things curve-wise and help the tick over etc.

Ta Mark
M Crossley

Yep sounds like you have CR nailed. Dynamic CR on that sort of cam will be about 0.5 lower than standard cam. We use BK450 on our big bore engines, and many others do too. They are marginal lengthways, just on 73mm so will overlap sligthly when crushed but it's never proved a problem for us. Across the engine, where your problem was, there's adequate clearance.
Paul Walbran

You could try a cometic multilayer headgasket.

http://www.cometic.com/
S

Nice post Paul, as you say it looks like insufficient clamping pressure exacerbated by excess heat/pressure from poor fuel quality.
The gasket looks like we see when the head has lifted under hard use conditions.
We use Cometics on our race Bs as we are experiencing head lift which we are associating with increased levels of ethanol. The heads can still lift even with 70lbs/ft after 40 minutes flat out, this can still let the water blow out but the gasket doesnt usually burn away saving the block and head gasket clamping surfaces. If we use posh Sunoco race fuel we get no problems and a large comfort level of safety!
Probably just personal preference but I do not retorque (recheck 20 mins after first fit before starting) Payen Paul only the copper head gaskets.
Mark, take fuel with you, it is a waste of time and money not having the brew you dynoed on, in fact even a waste of what you spent on the dyno.
Peter
Peter Burgess Tuning

Thanks for all the help everyone.

'Lessons Learned' as my wife the NHS nurse might say:-

Torque the head bolts enough. (50ft-lbs oiled?)
Re-torque after settling time.
I've ordered a Cometic gasket to suit the 73mm bore, from Swiftune.
Peter, I'm an idiot! Of course don't rely on pump fuel from the local village garage if I'm playing at a track.


Without wanting to reawaken a crankcase breather debate, and assuming
1) that I don't want to feed breather vents back into the carb anymore.
2) that I have a proper oilseal on the rear of the crank,so vacuum isn't quite so critical for oil tightness.

Can I fit a catch tank high up on the bulkhead near the bonnet hinge and feed it with a hose from the timing chain cover oil separator? Would a second one from the rocker cover be useful? Would it need shielding on the inside of the cover? The boss on the crankcase side isn't drilled for the mechanical fuel pump.

Am I barking up the wrong tree? Is there a better way?

That K-series looks better every day. I must learn to weld. ;))

Mark

M Crossley

breather plan sounds good to me, so 'yes,yes,no'.
David Smith

This thread was discussed between 19/07/2016 and 21/07/2016

MG Midget and Sprite Technical index

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