Compression Ring widths? WTH?

[Bryan]

Getting kind of antsy with little details that's not mentioned often.  First thing was compression height, and the fact nobody warns that "rebuilder" pistons are .02 lower on the top. If you don't deck your head and install them, you knock down compression.  Now it's the 3/32, 3/32, 5/32, 5/32 rings that pistons manufacturers list on their ring advertisements.  Took it for granted, but finally looked in the manual. The upper compression rings ARE NOT BOTH 3/32 according to the manual. One is 1/8".   Is there some little piece of obscure knowledge that nobody has mentioned?  "Oh, you have to add this other part to make it right".  

[Sniper]

Well, according to some the engineers knew what they were doing, lol.  Seriously though, those guys had a tough job compared to the analysis tools they have today.  When you consider that the flathead was originally designed in the 30's and likely as not the engineers didn't go back and redesign because it worked, after a fashion.  My guess, and that is all it is, was that the engineers of the day used the best materials and design knowledge they had to make something that would work.  Remember, in car ring obs were common back then, primitive oil chemistry and ring metallurgy, as compared to today, it what drove how they were designed.  Not to mention they mostly had to mentally visualize what was going on.  Probably was a balance between sealing and ring drag.

[Bryan]

24 minutes ago, Sniper said:

Well, according to some the engineers knew what they were doing, lol.  ... Probably was a balance between sealing and ring drag.

But which is correct?   3/32 or 1/8?   So far I've measured the pistons that come out of the motor, and the top ring land was wider.   The lower ring land was closer to 3/32. Not snug but close.  I've order a set of NOS pistons from VPW for $75 (plus unknown shipping).  I'll measure the ring lands on them and the compression height.  Matt on the phone said they were 2.0 CH.  Said I might have to scotch brite them.

[Sniper]

Best answer is what you just said, measure them.  Did you get some NOS rings to go with those pistons?

 

3/32" is 0.09375"

1/8 is 0.125"

 

A difference of 0.03125"

 

None of my service manuals gives any width info so I dug into the parts manual, no width info there either.  Not sure where you got that listing you posted.

Edited May 10, 2022 by Sniper

[Bryan]

1 hour ago, Sniper said:

Best answer is what you just said, measure them.  Did you get some NOS rings to go with those pistons?

 

3/32" is 0.09375"

1/8 is 0.125"

 

A difference of 0.03125"

 

None of my service manuals gives any width info so I dug into the parts manual, no width info there either.  Not sure where you got that listing you posted.

Not yet. Haven't arrived.  The ring info is from the Dodge shop manual in the engine section.

[Bryan]

I'm beginning to believe the 2nd compression ring is 3/32 and the top ring land is 1/8". Or the top land is getting the crap beat out of it.  Checked one of the 53 pistons, same thing. Top land way wider than 2nd.  Instead of .1000-05 it's .1130.

 

Anybody know if that's normal?