low compression after valve job

[Jlmatt]

My engine is in my garage, I just did a compression test, cold, with the starter. The readings were at 60, 60, 55, 60. I then did a wet test: 70, 65,
55, 60. I do not kwow what the compression was prior to pulling the engine out.
I had a valve job done at a shop, one valve is new. they also resurfaced the head. I adjusted the tappet clearance myself.
My engine block is a late model, the head is original, 1961.
When I reinstalled the head I found that 2 stud holes were stripped in the block and I have not repaired them yet.
Could I expect the compression to improve significantly as the valves seat themselves better? Would 2 missing studs make that much difference?
Or is time to get a new head?
thank you.

Jean-Luc, Pearson Triton.

[Don Moyer]

Jean-Luc,

It's not uncommon to have to at least start an engine before the valves will begin to seat well. I have on several occasions had difficulty even finding TDC on the first cylinder due to very low compression, but as soon as the engine was started and later rechecked, the compression was normal in all cylinders. I recommend that you replace the defective studs with 7/16" repair studs and then move ahead and start the engine in your garage to recheck the compression. I see no point in replacing the head. I'll attach the instructions that come with the repair studs so you can see what's involved in that process.

Don

[Jlmatt]

I did install 2 repair studs over the week-end and was able to torque the nuts to 35 lbs. Earlier in the week I had tried to start the engine, I thought that with 15 studs that would be enough to compress the head gasket but, of course that was kind of stupid . I have a garden hose connected directly to the side plate, (my water pump is not functional right now) and as soon as I turned on the water, it started leaking through the cylinders and everywhere.
I cleaned everything up and squirted some marvel oil in the cylinders. Then I installed the 2 repair studs yesterday.
Today, I was able to start the engine. I let it run at idle in my driveway for about 10 minutes. Retorqued the head. And did another compression test. No change, #1 at 70 psi the others about 60 psi or less.
Why is the compression so low?
Is it possible that I messed up the head gasket by compressing it with 2 missing studs? Also when I did it I did not follow the nut tightening sequence as shown in the service manual, I did it progressively, 20lbs, 25, 30, 35, but randomly, starting in the center.
The block is a late model late 70's early 80's. I measured the cylinders and they were well within specs. I did not take the pistons out but I figure the rings should still be ok. Should I replace the head gasket again? I'm not afraid of it, after everything I did to get the engine out of the boat, that seems like an easy job now. And I don't want to put it back in the boat with the job half done.
Jean-Luc

[Don Moyer]

Jean-Luc,

I don't think changing the head gasket will help.

I recommend you squirt 5 or 6 squirts of Marvel Mystery Oil (MMO) into each cylinder (aiming the MMO away from the manifold side of the engine and toward the alternator side). Burn off the MMO and apply some more, at least 5 or 6 times, and check compression between each MMO application. After 5 or 6 of these MMO applications, you should notice a slight improvement if your MMO treatments are having a positive affect on your rings.

You might also recheck the valve clearance adjustment. Be sure that you set up each cylinder for top dead center of the power stroke when you set those two valves. You can follow the instructions in the attachment.

Regards,

Don Moyer

[Jlmatt]

Don,
last week-end I did just like you suggested with the MMO and I got the compression up to 80psi. Today I ran the engine for about 20 minutes and checked it again. It was down to 60 on all 4. I put some more MMO in there and got the compression up to 80 again. I guess that means it's time to replace the rings. Right? I was kind of hoping it was the valves not seating properly. It's just I did'nt think the block was that old as to require new rings already. There are 2 numbers on the port side of the block, I don't know which is the actual serial number. at the top is 112074 A and just below that, about level with the carburetor and in slighty bigger is: 295369. which is which? any idea how old this block is?
Thank you for this great site.
Jean-Luc

[Don Moyer]

Jean-Luc,

Sorry to hear that your compression is not responding in a more lasting fashion. Are you having performance issues or a lot of blow-by in the cabin area? If so, then it is probably time to consider serious rebuilding (which would certainly involve replacing the rings). However, I'm never quite comfortable recommending serious engine rebuilding based on compression values alone, as long as the engine is performing satisfactorily in every other way.

The serial number on late model engines is stamped on a flat surface on the front of the block just below the base of the oil fill tube over the flywheel housing. On early model engines, the serial number is stamped on the top corner of the block above the starter location.

Don

[Jlmatt]

Don,

I ran the engine in the driveway several times over the last week for a total of about 2 hours and with a lot of MMO treatments. the compression ended up at 80 psi or just under. The engine starts very well, but misfires and never sounds quite right to me. And I don't think it's electrical. Anyway, I know if I put it back in the boat like that I will always regret not having done more to it while it was out.
So I decided to work on it some more. I removed the head again today.
I have 2 questions: I noticed that when the pistons are at TDC, they are not quite parallel with the top of the block. On the side of the water jacket, they are all at 1/16'' above the block and at 1/32'' on the side of the manifold. Is that a concern?
I'm hoping I can remove the pistons myself and bring them to a shop to have new rings installed. I thought I would have access to them by removing the flywheel and flywheel housing and the oil pan. Do I need to remove the reversing gear and reversing gear housing also?
I have your book and you describe the piston removal at the end, after everything else is removed. I was hoping I might be able to bypass some of the steps, like the reversing gear, or the valves, since they were done recently.

Jean-Luc

[Don Moyer]

Jean-Luc,

Many of the steps in the MMI manual will apply to what you are planning to do. The following info may help:

You can lay the engine over on your driveway, then remove the flywheel, remove the lower three retaining bolts of the rear flange and loosen the upper three bolts, then remove the oil pan. You can then remove all rod bearing and main bearing caps and the crankshaft.

NOTE: Loosening the upper three bolts of the rear flange will allow the front of the crankshaft to drop down just enough to slip out of the reversing gear without removing the reversing gear itself.

Then remove the pistons.

Don

[Chip Hindes]

Sorry to ruin your day, but assuming that you'll be able to correct your compression problem by fitting new piston rings to old pistons in their original bores (only) could have you in exactly the same situation as now: you'll get everything back together and will still have low compression.

Once you've got the pistons out, IMO it would be foolish not to go the next mile: have the bores, pistons, and all the bearings measured to see if they're in spec.

Even if everything is in spec, you normally will require as a minimum a glaze breaker hone in the cylinders to bed the new rings.

When I did mine, I had generally mixed results. Pistons and bores looked pretty good, but it turned out they needed a .020 overbore and new pistons. Main bearing journals looked good, and they were, requiring only a polishing and new standard sized bearings. Rod crankspins looked bad and they were, needing to be ground .010" under and bearings to match.

Turned out as well even if everything else had been fine, the idler gear had over 1/8" of lateral slop and required replacment. This would have gone undiscovered until it failed if I haden't torn the engine down.