Piston Ring end gap?

[1939 President]

1939 Studebaker, 250 cu. in. flathead 8cylinder, 3 1/16" bore. Installing new rings, Studebaker manual calls for .013-.018 Piston Ring end gap. Does that mean .018  at the top of piston travel and .013  at the lower end of travel?  I'm new at this.

[Oldtech]

Not quite. The end gap is measured at the tightest part of the cylinder. On a used one that would be at the bottom.  Stay with the low number if it's not a rebore. The old rule of thumb was .003 for each inch of bore so that would put it only at .010 or .011.  Modern rings are very accurate. It's a while since I had to file a ring, but you do have to check them. 

[joe_padavano]

To add to this, the dimension is specified as a range because all manufacturing processes have tolerances. As noted, the small end of the tolerance band is preferred in this case, but the factory has determined that even the large end of the range will provide acceptable performance. If you have file-to-fit rings, you can set the number as you want. If you have pre-gapped rings, the tolerance band accommodates the variability inherent in the manufacturing process and the bore diameter.

[1939 President]

THANKS for the very helpful info so quickly! I was kinda close but you guys came through!

[Nailbitten]

While both replies are great and correct information,I have built many high horse powered circle track racing engines.I file fit every piston ring i ever installed and i always fit the largest gapped ring at the top. The reason for that is,that is where the most heat is generated when the fuel is ignited causing more expansion than the ring below.

 

Edited January 14, 2023 by Nailbitten (see edit history)

[joe_padavano]

21 minutes ago, Nailbitten said:

While both replies are great and correct information,I have built many high horse powered circle track racing engines.

A 1939 Packard flathead isn't a high horsepower circle track motor. There are a lot of things that go into a race motor that are unnecessary or even undesirable for a stock street motor.