Compression test failure--advice needed

This is a follow up to the Cometic Gasket Thread here:

Despite the poor cut quality of the gasket, I installed it anyway and did a compression check to verify the rest of my build to this point (no manifolds, no cam covers and no breather). My dry numbers are (#1to #6) with 6 compression pulses:
158 153 153 153 148 138 so #6 seems soft. I squirted oil in each and repeated the test:
169 168 163 161 160 148 so #6 still seems soft but not from rings.

My next thought was maybe leaking valves or head gasket. Since Coventry West did the head, I doubt it’s valves. But to be sure I brought #6 to compression and listened through a hose to the intake and exhaust ports. No sound. I tried the same through the breather hole and it sounds like the ocean! To me, this sounds like the head gasket and I can’t say I’m surprised given it’s quality. But before I tear it all apart again, is there another test I should do?

Rick OBrien
65 FHC in FL

If it’s through the breather, very likely it’s from the rings. Both your wet and dry numbers tend to suggest that piston is not sealing as well.

The next test would be to fire it up, with water in it, and then do a dye check on the coolant when the engine is hot.

That’s a good thought. I’m going to try moving the steth-o-hose around to get a better feel for it and maybe narrow it down. I’m a long way from running. Carbs still need a full resto.

Rick OBrien
65 FHC in FL (aka Christine)

Oh, if this is a new engine, I just simply wouldn’t worry about it: on a new engine, it’s not at all unusual to have uneven compression figures.


Yep rings need to be bedded into the bores.

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I agree with Paul, the rings need to bed in. Just get it running and you’ll discover the first 100 miles will make a world of difference in how well the rings seat. I learned a long time ago not to idle a fresh rebuilt engine a long time, get it on the road and run it up and down in speeds, never exceeding 3000 RPM. This is probably A controversial subject much like what motor oil to use.

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Sure, NOW you tell me :grinning: :grinning: :grinning:

In all honesty I was uncomfortable with that craptastic gasket anyway (has a nice ring to it). Took only 30 minutes to disembowel again having all the weird tools still handy. So here it is. There is a lot of oil around this cylinder compared to the others so I do suspect a leak. A lot is on the head and not much on the block.

The bore itself looks good. No sign of perhaps a broken ring.

So, I’m ordering a Payen from SNG. I’m just more comfortable with the OEM passage sizes. It was out of stock at the time, which led me down this whole Cometic/Victor Reinz rabbit hole.


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All that oil seepage around # 6 looks Verrrrry odd: dumb question… was the deck machined, and if so, was the front cover on the block?

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You may wish to check that the dome nuts did not bottom out on one or more of the head studs at cylinder 6.
To check this you put head back on, and put the dome nuts on without the washers and tighten them. When the nuts bottom out on the stud, there should be way less then the thickness of a washer between the dome nut and the cylinder head

69 OTS

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It has been a long time but the two studs seem to have shoulders on it and looks like a locating stud.

Are the holes in the head wide enough to take these shoulders?


Good question and yes the timing cover and block were match machined.

I did check this and 13 of the 14 had plenty of room to tighten. The exception was the dowel stud which did not sink nearly so far into the block. But even with that, it would tighten to the lift bracket without the washer. But now that it is accessible again I plan to turn another 1/8" of the dowel stud.


I’ve never put oil into a fresh motor, ran it for compression then removed the head gasket and looked at it.

So I can’t be sure but isn’t it just leak of oil out from that bore because the engine hasn’t come up to temperature and the gasket isn’t sealed yet?

Since you have the engine on a stand, as a sanity check, my approach would be to remove the #6 piston and confirm that none of the rings are broken.

If not I would reassemble it with a proper head gasket and not worry about it. You could mic the bore and make sure it’s the same diameter as the others.

My gut feeling is that there’s nothing wrong and once the rings bed in you’ll be fine.

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I may not hurt to put a straight edge lengthwise across the head and then along the block to check


Since the oil doesn’t seem to be around the other bores raises a huge red flag.

Could the oil have run out the back oil feed holes, down to the gasket and then across the gasket?

Shouldn’t have, in a clamped condition.

I am wondering if the seal was perfectly fine until OP went to remove the head. He would have removed the two banjo bolts to the oil lines, then removed the head bolts, and then when he was lifting the head, the oil runs out the oil holes, down the head and onto the rear of the gasket.

Anything is possible: I’ll refrain from any further speculation, till we get more data.

I got the impression it was the only bore he put oil in… I could be wrong.

If it wasn’t then I agree that head face and deck flatness need checking.

I can answer quite a few of these questions.

This is a new build with fresh machine work so the deck and head were both re-surfaced: .005" on the deck and .010" on the head.

The clearance on the piston/bore in question is .0023" which is pretty much where all of them are. The range is .0020" to .0026".

The first comp test was done dry then oil was added to each cylinder and repeated to get the wet numbers.

The number 1 bore (at the rear of the engine) has some oil around it too but I saw this happen as the heard parted the block and the rear drain holes cleared. Too my knowledge there are no drain holes in the front aside from the small one in the block to feed the idler sprocket bearing, but that one gets drain oil from the chain gallery.

The engine has never been run so the concern over lack of a heat cycle for curing gasket is valid.

My plan going forward is to install a Payen gasket and shorten the dowel stud top to match the height of the other three long studs.

Great questions and observations everyone. Please keep them coming.