Piston ring installation update and questions as usual!

Hi all,

OK well the pistons are back in my f-head. You may recall that my pistons were .010 over and we did some measuring that indicated we should use .020 rings and file them down. My friend who restores very very high end stuff suggested a cold gap on each ring of 7/1000ths. Another friend, the guy who basically comes to my house and does all the work in exchange for beer, who builds race car motors, said we should be more like 10 to 15/1000ths so he filed to like 12/1000ths cold gap. Each ring was measured in the cyl where it would be going and measured about 2/3 down inside each cyl. Oil scraper rings required no filing.

So it is all back in and each piston has a bit of play in all directions inside the cyls, suggesting that the rings are not jammed against the piston heads....we made sure we clocked rings correctly, numbers on journal bearings lined up with numbers on piston arms and all numbers were in correct order and on left side of motor.

SO we have one outstanding concern that unfortunately we do not have the info to judge....according to the box of rings we got they said "these rings are designed to fit into pistons with original factory groove depths. Some replacement piston heads have shallower ring channel depths and these rings will be too large to fit those piston heads." or something to that effect. Problem is the pistons came with the Jeep and I do not know anything about them other than that they are .010 over.
Are the ring grooves shallower than originals?
Since I do not have any originals to compare to...no clue.
Joe my beer drinking mechanic is 'confident but not positive" that we are OK, he said under compression he was quite satisfied with the amount of ring that was flush to the piston head. We turned the motor over a few times and everything moved cleanly but it is all COLD, we are concerned about what happens when it gets hot!
We are happy to pull at least one of these things back out to measure the depth of the ring grooves, but:

1 - don't know the factory depth,

and 2 - can't decide if it is a problem or not to begin with!

Thoughts???? Experience here? Any tell tale signs to look for? Other than this issue it was a really easy job...

 

WHo made the rings? Maybe you could ask the supplier what ring groove depth they expect.

Another possibility is to measure the depth of the groove and the size of the ring, and calculate whether it's possible for the ring to expand more than the size of the groove. One source lists the expansion rate of gray iron is 5.8 millionths of an inch per inch in thickness per degree Fahrenheit. Guess at a top temperature (500 F? What's the metling point of iron?) and see if the ring can get too big for the groove.

My hunch is that the rings would sit proud of the groove if you had the shallow grooves. But maybe not.

 

I'm not a machinist but my opinion is, if they fit OK visually and you end up with proper compression then you are fine. But you won't know for sure until you get everything back together.

 

I did a BOTEC and thought I'd share -

Let's say that the ring is 2/10 inch wide (it's less than that, but this makes the BOTEC easier). Guess that the ring increases something like 500 degrees. So...

5.8e-6 * 500 * 0.2 = 5.8e-4 or 0.00058" change in width. So a ring groove one half of one thousandth inch deeper than the ring is wide should be plenty of room for expansion.

 

What is the piston to cylinder wall clearance?

I don't suppose you measured the land depth or the ring width, did you?

And why .020 over in a .010 bore?

 

020 filed down because the cyl walls were worn a bit wider by the disaster of rings etc that we removed so 010 would have too wide of a gap (or so we think anyway)... and size was not even across cyls... so we got 020 to make up the diff and we measured and files new rings specifically for gap in each cyl wall.

We did an eyeball comparison of the flat width of the old rings and new rings, new ones were slightly wider across plane...the old ones were so wide at the gap points that they almost seemed too small....(the ones that were not broken!)...
what the heck is a BOTEC??? LOL..bunny with a pancake

 

http://www.netlingo.com/word/botec.php

 

And here I thought it was some nuclear physicist-y math calculation...

 

My $.02. The cylinder walls are worn enough that you needed longer rings, so it is very unlikely they will ever fully seat in the groove anyway. If I'm following your post correctly you noticed the ring thickness was different, did you check the ring to groove clearance? Ring end gap is only catastrophic if too little and I guess your friend is in the habit of setting it on the large side to allow for the greater piston heat seen by competition motors. The worst thing you will see from .012 (assuming the spec is actually .007) is a bit more oil burn and a slight loss of compression, probably not even enough to notice.

Personally I would pull one piston and check the ring/groove clearance and the groove depth just so I could sleep at night.

 

OK so just got off phone with Walck's. Guy said the narrow groove warning on the wrappers for the rings is a generic warning and that he's never in his career heard of replacement pistons for an F-Head being narrow grooved. He said as long as we've gapped well we should be fine. Also confirmed with my mechanic friend that before installing he pushed on the rings from the side opposite each gap and that they were flush with the side of the piston so I think I stop worrying and start rebuilding. I will try to remember to take pix...now how do I know the top from the bottom of my head gasket???