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MG MGB Technical - lumpy idle on a 1972 1860cc BGT

I recently had to change the headgasket after the old girl overheated and blew through the gasket between cylinders 3 & 4. Obviously the head had to be skimmed, they took 11 thou off the head in order to get it perfectly flat, I also decoked all 4 pistons and metal polished the faces as described in the Haynes manual. after reassembling and torquing the head down to the specified 45-50 lbs per sq'in, I compression tested all four. the readings were lower than she used to read (10-10.2 bar) I assume because the head needs to "bed in" the readings are now around 7-8bar except cylinder 4 which is only getting to 5 bar.
She doesn't blow any smoke out the exhaust so I suspect the rings are okay. Is it likely to get better compression once bedded in or am I missing a greater problem such as refacing the block if thats possible?
S Munton 1

Did they use the same type of gasket or a thicker one?
Paul Hunt

PS. Even low compression is 9 bar/130 psi, high compression is 12bar/170 psi so yours are well down. Does a teaspoonful of oil in each plug hole make much difference?
Paul Hunt

A reduction in pressure after skimming 11 thou defies logic. are you testing it the same way? ie all plugs out, throttle wide, same air temp etc. also might be valve clearances too tight? Were the valves disturbed?
Was it torqued evenly ?
Art Pearse

Thanks for your replies,
The gasket was a similar one to the one that blew, it was comp tested the same way as before with plugs out,engine warm etc and yes it does defie logic!
I haven't tried the teaspoon of oil in each plug hole, interesting? I presume this helps seal it?
yes it was torqued perfectly evenly starting from the middle outwards at a quarter turn on each nut.
S Munton 1

You did readjust the valves, didn't you? Taking 11 thou off the head means the rocker is pushed up that same amount more than it used to be.
Matt Kulka

The Vaves were not removed from the head, it was a brand new head with unleaded valve seats about 4 years ago, yes all the valve stem clearances have been readjusted and are not too tight, initially after refitting the head the clearances were huge and they rattled along as they should. they all sit at 0.15 as specified in the workshop manual, any larger a gap becomes far too noisy for comfort!
S Munton 1

Huge gaps on refitting after a skim is wrong, they should have been tight. Yet another indication that something has lifted the head *further* away from the piston. Are you sure you got the right head back? And they skimmed it with all the valve gear in place?
Paul Hunt

Yes Paul it was most certainly my cylinder head that returned and the valve gear was not disturbed. Is it possible the block face could be damaged? I always thought these B series lumps were practically bullet proof! I did remove the rocker gear prior to removing the head but like I say reset the clearances on reassembly.
S Munton 1

Rocker clearances might be wide on reassembly because the gasket is not yet compressed to the final thickness. Surprising that it was more than enough to compensate for the -11 thou though.
I did once get a gasket set which had two new head gaskets stuck together, easy for an experienced person to catch, but for amateurs....
Retorque the head hot and reset the clearances.
Try again.

What does "brand new 4 years ago" mean in terms of miles?

I suspect the real answer for the low & uneven compression is in this: "after the old girl overheated and blew through the gasket between cylinders 3 & 4"
Why did it overheat and for how long? Oil out the back is not a good indicator of ring condition, especially compression rings. Those are the ones that are killed first by overheating.

"I always thought these B series lumps were practically bullet proof!"
They are, when put together correctly. A skilled operator can kill anything.

FRM
FR Millmore

This thread was discussed between 26/09/2009 and 27/09/2009

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