45 Piston Ring Side Clearance

[Rex]

Specified as .004, how much is too much?

[chuckthebeatertruck]

That’s .004 per inch of bore. IE a 3” piston equals a minimum gap of .012.

Too tight is worse than too loose.

[Rex]

That sounds like end gap, I'm looking for the maximum allowable side clearance in the groove.

image_33438.jpg

[chuckthebeatertruck]

Ah, sorry about that.

anything much over .005.006 will have you potentially breaking rings or achieving a very poor seal. The motor will run, but will eventually gum the rings in groove and lead to other problems.

generally a piston is plumb worn out if the grooves are showing excess wear.

[Rex]

Thanks, just found max permissible clearance to be .008 on page 25, and .006 on page 41 of TM 9-1879 which I am now reading cover to cover.

[Rubone]

It varies based on ring width. On the old style wide rings, lower compression engine, iron pistons, etc., it was not as crucial as on the later narrow rings which will twist and flex more. When a ring flexes excessively it will quickly work harden and be subject to breakage. And certain types of ring materials are more brittle than others. Evey time an engine goes through a cycle the rings are subjected to friction on one side more than the other causing flex. If the clearance is high the flex is more. Upper compression rings are often beveled on the inside face to allow flex without binding, under compression the rings are forced against the piston land and to a certain extent due to expanding gases out towards the cylinder wall creating a better seal but also loading the rings with pressure. Higher compression heads on a 45 increase ring load exponentially.

[Rex]

Sorry for the response delay, I'm no longer getting email notifications for subscribed thread responses. Sometimes it works, sometimes not.

These are +.040 aluminum pistons and heads with narrow rings which will be replaced.