Ok, I found an clue to why No5 had almost no compression, turns out My memory had failed me. Last night I pulled No1 & 5 pistons and everything looked ok apart from the odd ring that I snapped and replaced with another new ring when I rebuilt the engine. But the ring I thought I snapped and replaced on No1 cylinder was actually the ring on No5. I just popped out No2 & 6 and they both have serrated bottom compression rings, same as No1 so No5 is the odd one out- see pics below.

Now I don’t have the knowledge or experience to tell me this is the cause of the issue but it sure seems like it could be. When I snapped the ring I called the supplier and ordered 2 more rings (one as extra). When I received the ring I realized it wasn’t the same and phoned the supplier back, they said the original set was about 30yrs old and it was the last pack so they sent me modern replacements. I’m hoping they didn’t send me 2x top rings and now I can’t find the instructions for the new rings. Anyway what do you guys think? Could it be the cause? I’m not sure it’s even the same size as the rest.

No1 serrated bottom face of bottom (same as 2,3,4,6,7,8) compression ring


No1 different angle


No5 non serrated bottom ring


No5 different angle



Thanks, Dennis.

56 F100 - 272 Y Block
NSW, Australia.

just a thought but i hope no 5 is not undersize. good luck.i thought these strange problems only happened to me. cheers

Ring gaps aligned will not result in no compression.  When you consider the exposed area between the gap and crown of the piston compared to the volume of the cylinder, any compression loss will not be detectable by conventional means.  It's something like .00004" square, likely smaller than that.


Lawrenceville, GA

Hi Blocky, yeah mate thats just how it was when I took the photo not at TDC.

Checking all the rings are installed correctly is one of my checks once I have all the cylinders out. I’ll measure each bore and piston diameter and compare differences, although the issue is with No5 I still want to check the rest while everything is apart.

Checklist
- check rings installed correctly
- check piston to bore tolerance
- check valves for tightness (carb cleaner or vacuum test somehow)
- any other suggestions?



Thanks, Dennis.

56 F100 - 272 Y Block
NSW, Australia.

going to make a fool of myself (yet again) but on pic no. 2 is the piston all the way up the bore or is that just the position it was in when you took the pic. also have you got the rings facing up  / down the correct way??

Thanks Joe,

I just checked with the machine shop an they don’t test the heads before they go out.

Would having the compression ring gaps aligned cause a loss in compression (almost none by feel of thumb over plug hole) when hand cranking but be less noticeable when the engine is running?


Thanks, Dennis.

56 F100 - 272 Y Block
NSW, Australia.

A simple adapter with rubber gasket to cover the intake port, and a vacuum port fitting.  Pull vacuum on the closed valve and see if it holds.  Local shop tests all valve jobs by pulling 80" vacuum before letting heads go out door.  Joe-JDC

JDC

I’ll give it a shot


Thanks, Dennis.

56 F100 - 272 Y Block
NSW, Australia.

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

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
NSW, Australia.