Lost compression - after head change

[Bob Olive]

I have been forced to change the low-compression head on my 33 tourer, due to corroded water inlet bolt/stud. I have now fitted a high compression head (in very good order) and new gasket to suit. Torqued down to 20ft/lbs (cold), but compression seems to have dropped considerably, and it won’t start (always been a good starter). Any ideas anyone? Thanks.

[Alan]

Did you use gasket sealant? If there’s too much, the bead can stop the valves closing properly. Or, the valves are fouling the combustion chamber of the new head.

[Bob Olive]

Thanks Alan, no, just a very light grease on both sides. I thought the valves might be fouling the head, but I can’t see why that would happen. As far as I know the head is standard (unplaned) thickness. I’ll maybe have to remove it and check just that. Nothing else has changed - so maybe that’s it... I’ll report back in due course.

[Chris KC]

20lbft torque is good, and yes it should be done cold. Did you follow the recommended head nut tightening sequence Bob?
Did it run at first then stop, or hasn't run at all?
Is there water in the bores?
Is the head warped? (alloy heads especially are prone) - may be worth laying it on a surface plate to check.
As Alan said, anything stopping the valves closing properly?
Have you used a HC head gasket? (I can't immediately think what would happen; but HC and LC gaskets are different shapes).
Changing your compression ratio theoretically could put your ignition timing out a bit, but I'd still expect it to fire.
Have you altered anything else?

[Bob Olive]

Chris, thanks, yes, followed the head tightening sequence religiously, and yes a proper (new) HC gasket. The head appears to be true (checked on plate glass). It didn’t fire at all (fitted brand new plugs B5HS) and we have spark and fuel. Haven’t checked the bores for water, but will tomorrow, and look at the timing.
Will report back.

[John Mason]

Bob, Have you done a compression test to see if all four cylinders read roughly the same. Also a dry then wet test (Wet test is after a squirt of oil is put down each spark plug hole) If you get a much better reading after the wet test then you know it is the valves not seating correctly.  With high compression head it will find weak spots in poor gasket fitting or poor seating valves easier than with your previous low compression head.
If you have fuel and spark after cranking it over on the starter for a bit with ignition switched on if it doesn’t start take a couple of plugs out and see if they are wet with fuel. If wet and you have petrol put a small amount of new fresh petrol into the plug holes refit the plugs and try again. I think you should at least get a cough or splutter.


John Mason

[David Witton]

If compression improves with a wet test then it indicates a piston/ring/bore problem. If there is no change from a dry test, then the problem lies elsewhere.

[Derek Sheldon]

Bob,
I’m pretty sure that I have a LC  about here somewhere if you want to back travk

[Bob Olive]

Thanks everyone. I have removed the head this morning and can see nothing wrong. I'll try the wet test - and do a compression test as suggested.
The car was running very well prior to this minor incident which makes it all rather odd.
I'll report back.
Derek, thanks for the offer I'll bear it in mind, although I have always been keen on a HC head to go with the Phoenix crank, Renault rods etc set-up.

[Colin Wilks]

I have been following this thread with interest. You say you checked the head on a sheet of glass, which would tell you if it was warped or bowed, but have you checked ii is not concave in the centre of the face by using a straight edge with a light behind or a feeler gauge?