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Almost no compression

Posted By dennis22 Last Month
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dennis22
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Hi guys,

I have a bit of a curly one I need help with... or at least use the forum as a sound board.

I just got my heads back from the machinist, installed my new head studs, gaskets (corners toward the front) and torqued down the head (80ft-lbs 3 times as the ARP/Mummert guide suggests). I then installed my new adjusters and pushrods and went about setting the valve lash. I filled the engine up with new oil and installed a new filter and used a drill to prime her up.

Since I had the plugs out, I was blocking the spark plug holes with my thumb while turning the crank with a wrench to get the compression stroke. The passenger side (1-4) gave me plenty of pressure by feel of thumb when rotating the engine but when it came to no5 piston... not even a breath of air, but 6,7,8 all seem fine, Something is wrong, I have tried several times holding my thumb over the no5 plug hole but nothing. I tried a gauge on all other cylinders and got roughly 40 psi but 0 on no5 (keep in mind I’m hand cranking).

I did use a dial gauge on both intake and exhaust and compared it to no1 cylinder to make sure the valves are opening/closing all the way and it was ok.

I am leaning toward a ring problem, as I put some oil in no5 cylinder and got some pressure by feel of thumb over plug hole.

Bit of background-

The heads received
- new hardened exhaust seats
- full valve grind job
- machined head surface
- .530 valve guide cut and positive seals
The machinist has over 30years experience and was recommended by a reputable mechanic, so I don’t think it’s the heads or head gasket.

Block history-
The 272 engine was sleeved after a failed cylinder boring attempt and 20thou pistons were fitted (didn’t sonic check and went for 292 std Pistons size) (different machinist)

Thinking about it, after the rebuild the engine has always sounded like it had a bit of a miss but I thought that was because I didn’t have the head work done but I was sure I checked the compression before and after the rebuild but it’s been a couple of years and only just got the heads done. The engine hasn’t seen many hours- less than 5 and most on an engine stand.

I did snap a piston ring when installing the rings, so there is one different brand ring but I can’t remember which piston it’s on.

On the weekend I’ll tear it all back down and pop out no5 piston and see what the story is. It’s a big hit to the morale having to pull it all back down again and discard the newly installed gaskets and oil but it’s all experience and valuable lessons.











Thanks, Dennis.

56 F100 - 272 Y Block
Sydney, NSW, Australia.



Ted
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If the engine has set for an extended period of time, then stuck piston rings are a real possibility and especially if the vehicle was not garaged.  There are potential fixes for this other than tearing the engine back down.
 
Before doing any disassembly, I’ll suggest performing a leak down test.  A leak down test will point to exactly where the lack of compression is with the likely candidates being the intake valve, exhaust valve, head gasket, or piston ring seal.

Lorena, Texas (South of Waco)


dennis22
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Thanks Ted, I will get a leak down test kit this week and see what the result is.

The engine has sat for 18 months while I was away but was stored inside and was dry.

How do you ‘un-stick’ piston rings?


Thanks, Dennis.

56 F100 - 272 Y Block
Sydney, NSW, Australia.



Ted
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dennis22 (7/14/2020)
How do you ‘un-stick’ piston rings?

Four ounces of ATF in each cylinder and then pressurizing each cylinder with 100+ psi air.  Let it sit overnight and pressurize the cylinders once more.  Clear the oil from the cylinders, reinstall the spark plugs, start the engine and bring it up to operating temperature.  This doesn’t work all the time but sure beats tearing down an engine when it does work.


Lorena, Texas (South of Waco)


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Thanks Ted.
I already have the engine out of the car and I have ordered some ARP main studs and rod bolts, which I should have installed when I rebuilt the thing a couple of years ago🤦‍♂️- i’m sure many of us have made that same mistake only to “do it right” later on with the benifits of hindsight, so I think I’m just going to tear into it anyway, get everything right.

Just waiting on the leak down test kit to arrive before I get to far ahead of myself.

I ordered a dial bore gauge and outside micrometer so for the sake of learning and my piece of mind, I’m going to pull every piston and check Piston and cylinder diameter and record everything and also have a look at the rings while the pistons are out. Already getting ahead of myself....


Thanks, Dennis.

56 F100 - 272 Y Block
Sydney, NSW, Australia.



dennis22
Posted Yesterday
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Update-

So I couldn’t get my hands on a quality leak down test kit, seems like they are all on back order and have a 2-3month lead time. So I ordered an 18mm spark plug/compressed air adaptor and put some air into the cylinders. I tried a couple of cylinders but I couldn’t detect any change or notice anything different.

I pulled the heads off and popped out No5 & No1 cylinder and had a look. Everything looks ok to me although I did notice No5 piston compression ring gaps had lined up but not sure if that would cause the almost loss of all compression. The rings moved freely and look to be in good condition on both pistons. I did snap a compression ring when I rebuilt the engine and had to replace 1x Bottom comp ring with a different type And I suspected that could be the culprit but It Turns out that was on No1 cylinder.

Next step was to measure both pistons and both cylinder bore diameters. I got a difference of -0.003 between piston/cylinder 5. What should the difference between piston and cylinder be?

I will have to remove the rest of the pistons on the weekend and measure them compared to their cylinders but 5 & 1 look ok to me. (Bore measurements were taken from top/middle/bottom on both axis with 0.001 difference, piston measurements were taken from about 1/2 to 1 inch down from the skirt)- please correct me it that is incorrect.

I don’t think it’s anything to do with the heads because they just got machined and new ex seats fitted. Can you test heads on the bench?

Happy to hear if anyone has any ideas!














Thanks, Dennis.

56 F100 - 272 Y Block
Sydney, NSW, Australia.



RB
Posted 9 hours ago
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This may not be relevant in your case, but had a 390 that after rebuilding heads and reassembly had low/no compression.  It turned out the head bolts bottomed out before clamping the head properly.  Guy forgot to install the hardened washers under the bolt head plus the block had been decked in the past
dennis22
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Thanks for the suggestion RB, the block has been decked but but I installed head studs after the heads came back from the machine shop.


Thanks, Dennis.

56 F100 - 272 Y Block
Sydney, NSW, Australia.



charliemccraney
Posted 2 hours ago
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Re bench testing the heads, I've heard of setting them up to that the chambers are up and the deck surface is level and spraying carb or brake cleaner into the chamber to see if it leaks by a valve.

- pretty sure it was heard on the internet, so take that for what it is worth.


Lawrenceville, GA
dennis22
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I’ll give it a shot


Thanks, Dennis.

56 F100 - 272 Y Block
Sydney, NSW, Australia.





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