Lost compression - after head change

[Chris KC]

I can't see the logic of looking at bores and rings if a car which was running well won't start after a simple head swap. Either the head is not seating correctly or something else has been disturbed.

[Howard Wright]

Hi Bob

If you play the sleuth then as everything was OK before and the only thing you’ve changed is the head then it must be something to do with the head or gasket.

However if the old head was warped or bowed but fitted the block OK possibly the block is not completely flat.  Have you checked this with a straight edge?  Whilst you are at it I would meticulously clean around the head studs in case something is holding up the head here.

Last possibility is a porous head but if it’s a standard A7 cast head that’s not very likely.

Cheers

Howard

[John Cornforth]

It occurs to me that a knock-on effect of changing from LC to HC head is a change of spark plugs. I know that you have fitted brand new ones but there is a small possibility that they are faulty. Do you have another set that can be borrowed and tried ?

Another thought is that the raised compression will give the starter motor more work to do, so if the battery is a little tired it may just not be cranking over quickly enough.

Beyond that, I would take a close look at anything that was removed/replaced or otherwise disturbed during the head swap operation.

Cheers, John.

[Alan]

Have you put the plug leads back on in the right order. Don’t ask why I say that.

[Colin Swinbourne]

Probably a silly question, but have the plug leads gone back in the right order?
...having spent ages in the past looking for a serious problem it’s easy to miss the obvious ?

[John Mason]

Thank you Dave for correcting my post on wet and dry compression tests. I don’t know what was going through my mind when I made that stupid mistake put it down to age.

John Mason.

[bystander]

Just a thought that I don't think has been mentioned yet, that old chestnut - is the petrol fresh? How long since the car was last started up?

[Tony Betts]

i notice the numbers you give on your spark plugs are not a usual number supplied by the standard suppliers? how well do that number work on an austin 7 ?

that makes me wondering were you might have stocked the head gasket? is it really a high compression.

do remember a high comp gasket will work on a low comp head. but a LOW comp head gasket WILL NOT WORK on a high comp head.

again, check HT leads are in correct order. the engine usually backfires if they are not.

tony

[fandm]

Hi did you do a crack test on the head, before fitting, when torqued down it could open up the crack,

[Zetomagneto]

Is the replacement head correct thickness, should be 1.5” a bit less if skimmed.
If excessively skimmed are the nuts bottoming out before clamping down on the head.
I can’t understand why you have reduced compression just by changing the head.
From what you described ie head changed it should run .
Was any levering involved in getting the old head off?
If it started and ran well before then it’s not going to be rings or valves.