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MG MGB Technical - engine compression

Recently returned from a good long run with my 1974 MGB GT. I noticed that it was getting harder and harder to keep it idling but at speed it ran great with lots of power. I did notice that I was using a little more fuel than normal but not alarmingly so. Upon returning home I checked all of the valves first, then ran a compression test with all plugs out and throttle wide open the results were
#1 - 135
#2 - 135
#3 - 90
#4 - 90

I have never had a "B" engine lose a head gasket between 3 & 4 but it almost appears to be the problem. By the way this is a low compression engine so the 135 is/has been normal for this engine.

Has anyone got any ideas before I start pulling the head off?

Thanks
Brian

Brian Smith (1950 TD3376)

Pretty conclusive. Lift the head and make certain that both faces are flat, ensure the stud holes are all countersunk by half a thread, It is possible to carefully use a file on the block to remove distortion, use a second cut file , dont try to get too fine a finish as the marks left will bite into gasket better. Same goes for the head, if you have it machined, again, dont have a polished finish. Use a Payen composite gasket in preference to the copper/steel types.


Jim
jim soutar

As Jim said. I had the exact same problem severn or eight years ago and the gasket was blown between 3+4.
After removing the studs I found the area around each had pulled between .002" and 003". I used large drill bit sharpened to a lower angle in a slow turning cordless drill and it worked well. You only need to remove to about 050" out from each stud, and you can check with a straight edge. The Payen gaskets are great although I did use a new set of ARP head studs as well. Check the head for flatness both length and width while its off and as its off already may as well have a look at the valves. Denis
Denis4

Second Dennis above, except back in the 70s I borrowed a surface plate stuffed grease in all the holes in the deck and went over it with coarse then fine emery paper. I had pouted the deck by not using a torque wrench and you could clearly see for a few mm around the studs metal was coming off. That solved the problem and I learned a lot. I now have 2 torque wrenches, one for alloy and one for steel, and use them.
Stan Best

Thanks for the comments. I had previously countersunk the stud holes so I can easily check them again. I had just never had a head gasket blow between 3 & 4, between 2 & 3, yes. These are really great engines as it ran approx 670 kilometers without missing a beat on the highway, didn't idle very well but it got us home!

Will pull it apart in the next few days, still have the TD to go on runs etc.

Brian
Brian Smith (1950 TD3376)

My bet is that your original head studs have stretched over the last few decades. Nothing lasts forever. This probably allowed your cylinder head to lift just enough to blow out the head gasket. The same thing happened to my supercharged '67. After some spirited driving, I discovered some oil in the cooling system. I pulled the head, checked the head and block for warpage, and then countersunk the stud holes in the block. I followed up with a Payen head gasket and a set of ARP studs. That was five years ago and there have been no problems since. RAY
rjm RAY

Replaced the head gasket today and the car is SWEET! As suspected it had blown between 3 & 4 and thankfully no damage. Straight edge shows no warping of head or block and it once again idles nicely and runs smoothly. Thanks for the confirmations, it appeared to be a head gasket but as I have never had one go between 3 & 4 in the 40+ years I have been driving MG's I thought it best to check with those who know.

Thanks all.
Brian
Brian Smith (1950 TD3376)

Brian--if you haven't cured the reason it blew, then you'll be doing it again. Better keep a close eye on the oil and coolant.
JM Morris

Retorque now, hot, and again in a few months, else you will repeat the process. This would have prevented the first failure as well. I made a lot of money replacing B head gaskets around 3-5000 miles because the local dealers did not do this.

FRM
FR Millmore

Thanks for the additional input. I have always retorqued the heads on my cars once I have taken it for a short run. It usually does need an extra little tweek. I don't appear to have a problem other than it has been a long time since the head was off and everything checks out in good order. Brian
Brian Smith (1950 TD3376)

I made an extensive study on this, over a period of years. Result is I retorque all hneads several times in year one, and about every two years after. This is true for "no retorque" gasket as well, although they take longer to fail.
If you want to read, email, ask for "Boltbabble"

FRM
FR Millmore

This thread was discussed between 17/07/2012 and 24/07/2012

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