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MG MGB Technical - COmpression Figures

Good morning,

I have just done the valve clearances on my 1965 B Roadster, gold seal engine done 39,000.

THe compresison figures are a bit wierd!

Cylinder 1 - 170PSI
Cylinder 2 - 140PSI
Cylinder 3 - 130PSI
Cylinder 4 - 155psi

THey have always been within about 15-20psi of each other but I have to admit I havent checked them for about 5,000 miles. 40psi swing seems a little but concerning really. The head gasket was replaced (and head skimmed) about 5,000 miles ago and I wonder if it is leaking between 2 and 3 (it was replaced due to water leaking to the outside between 3 & 4 and dripping on the distributor mount).

Engine runs fine, has as much power as normal and all the plugs are a nice light brown with no signs of burning oil. All the valve clearances are fine.

My plan is to get another guage and try putting some oil in each bore. WHen I normally check the compression, I take one plug out, check it by turning the engine over for 5-10 seconds then replace plug and move onto the next. Is this correct or shall I remove all the plugs and then check the compression. Also, I have heard people say the throttle should be fully open when doing compression checks. Can anyone confirm what the correct proceedure is!

Thanks for any comments

Chris
Chris

Chris,

Typically, all the plugs would be out and the throttle would be open. I do the same number of "hits" on each cylinder rather than thinking of cranking time. But, if you were consistent, the PSI swing probably does exist. Nevertheless, I would do it again in the normal manner both dry and wet and see what you get.

Charley
C R Huff

Was the block checked for warpage when the head gasket was replaced? If not, the leak may be between the block and gasket. RAY
RAY

As Charley said.
If you have a screw in gauge, or get someone else to turn the key, you can watch the gauge jump until it goes no higher.
3-5000mi is typically when a reasonably well installed gasket fails due to not being retorqued.
It was our most common new customer (from dealers) job in the 60's, done hundreds. Retorque now with engine hot and you might save it.
FRM
FR Millmore

The block was checked and found to be ok. I am nervous about retorquing as the block and head were flat and it was torqued up correctly at the time but maybe a hot retorque is the way to go, any advice on torque figures? ie max to go to or is it more a case of try get the bolts to turn a degree or two!

I will try the wet test first, I turned the engine over from the engine bay so was looking at the gauge at the time and so did turn it over enough for the guage to stop increasing.

Thanks for all your help!
CHris

Chris,

Opinions differ on the need to retorque, with some saying it depends upon the type of gasket used. I believe in retroqueing.

I do it one bolt at a time in the same order as recommended for initial torqueing. Loosen each bolt about a quarter turn, and then bring it back to the specified torque in one smooth swing.

Many say it should be done hot, including FRM. I have always done it cold with the rational being that the temp remains constant throughout the process. Given that yours may already be leaking, doing it hot might be better since a composite gasket might be a little more pliable in this condition.

Charley
C R Huff

Chris-
Send me an email and I will send you my complete dissertation on this. Ask for Boltbabble.

ALL gaskets settle with time and heat/cool cycles, miles has little to do with it.

Mark the positions of the nuts relative to head.
Then do as Charley says, being sure that you back off each nut a bit before you retorque. You will find that the nuts move in varying and some times surprising amounts relative to your marks. 45lbft is plenty.

FRM
FR Millmore

Just a quick update, head retorqued, did find that any really moved more than 5 degrees but still, they moved. Removed all plugs, put some oil down each bore and checked compression with throttle fully open, got much higher figures:

190PSI on 1
185PSI on 2
180PSI on 3
185PSI on 4

So, happy as larry now! Just got loads of smoke out of the exhaust now, need to take her for a good thrashing to clear out the oil. Have also opened the plugs to 35thou and changed my 123 dizzy to setting 6 rather than 1 which gives much more advance at lower revs so hopefully all will be much better when I get my drive out tomorrow!

Thanks for the help

Chris
Chris

Chris,

After you have burned the oil out, I suggest another dry compression test. When oil brings the compression up a lot, it usually means poor ring to wall seal, so the torquing may not be responsible for the change.

Did you back each nut off a bit as you retorqued them?

Charley
C R Huff

Charley, yes I did back each nut off about 1/4 of a turn before torquing back up.

I will try a dry test again as previously when I did the dry test, I only had one plug out at a time and the throttle was closed. If I remove all plugs and open the throttle, I am sure my dry figures will increase - it is probable that the low dry figures are due to the way I was doing the test.

Will let you know how I get on, the car is definately running much better with the timing setting on my dizzy at number 6 rather than number one (it gives max advance about 2000 rpm earlier now). It makes the car much more drivable and I dont feel I need to rev its nuts off all the time - however the noise it makes when I do rev it hard probably means I still will!!

Thanks

Chris
Chris

Chris, What do you mean when you say you set your dizzy to six rather than one?
Peter Murray

Peter, I have a 123 dizzy, it is a solid state distributor and has several settings for pre-defined advance curves. For example (not 100% accurate but from memory) curve setting 1 gave max advance of 27degrees at 4200 rpm, curve 6 gives 28degrees at 2200 rpm.

If anyone needs a replacement, I can recommend this unit, it has made the car run much better and always starts easier!

Regards

CHris
Chris

This thread was discussed between 21/06/2009 and 29/06/2009

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