Compression test failure--advice needed

I assume they decked the timing cover along with the block?

Answered my own question.

I’m unsure about all of this. One problem is that I’ve never bothered to check compression on an engine build prior to running it, so most of what I’ve said is opinion backed by no facts at all.

What you have is lower compression in one cylinder, and oil leaking (maybe?) out from that cylinder (maybe?) suggesting that there wasn’t a seal on the gasket when torqued down.

You have one long stud longer than another which (maybe?) could have allowed the nut to bottom out and not properly torque.

I can run this one round and round in my head all day and not come up with one solidly convincing explanation for the lower compression.

It could be all or any of gasket not sealed, rings not seated or damaged, torque not properly applied or a machining error that has been missed. Or absolutely no problem at all.

I know from experience that machined valve seats may not seal perfectly until run due to minute irregularities due to cutter flex. So that too could be relevant.

I’ve stripped a fresh rebuild (someone else’s build and reputable machinist) with a knock to find 5 piston to bore clearances at 2.5 thou and the sixth at 7.5 thou.

Were I in your position at this point I’d want a sanity check. You have $10k worth of machining and parts, and the prospect of having to remove and strip the engine if there actually is a problem.

I’d run the whole thing past your machinist, and get him to double check his measurements. Even the best do make mistakes.

If at that point all is as it should be, then I agree with you; put it all back together with a good gasket.

Isn’t no 6 the front cylinder, the oil feed holes are at the back?


Because the equipment was there I made a leakdown test and the hand lapped valves were not perfect (the new rings seemed to seal well). It has improved after running the engine and is pretty good now.

If it’s parked downhill oil may run forward but… unlikely at best?

Glad to hear its running well, I’m sure it’ll improve with further running in.

Randall, yes, you are correct, I often forget that little fact! But I still believe the same could have happened from oil from the front chain area draining back. Just a thought. As wet as some of the gasket looked, I have trouble believing the head was that loose.

I see this when CCing newly rebuilt heads.

If you pour kerosene into the combustion chamber it just runs out the ports. You need to put some grease around the valve seat, and I have gone to using oil rather than kero.

If you look at the cut seat with magnification you can see a very fine wave pattern, fractions of a thou. This is why valves were hand lapped back in the day, although our machinist says this isn’t really necessary.

To check the valves I put petrol in, the valve springs are installed and they are either dry or just slightly wet, but if I see petrol dripping from the valve I lap it a little more; this works for me. With just some dampness I can live, and I’m sure that could show up on a test. Here valves aren’t lapped anymore either I think.

I‘d been very happy with that and checked again after a bit of driving.

Dick can verify if Coventry West laps valves anymore, but I suspect they do. I found vestiges of what felt to be grinding paste in the ports.


That’s not good? :thinking:

That sounds fine to me.

We hand lap all the valves. Not only for better and instant sealing but to also check for the seat width and matching. We cut a 3 angle cut on the seats. After that, the heads are steamed cleaned. I am surprised that there was any valve grinding paste left. We also wipe with a rag with lacquer thinner to get it clean before it is steamed.

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