Weldsol,
By lap joint I mean the following:
To make it you make the ring purposely oversize and mill two slots offset from center. These slots interset at mid ring. Cut the ring where the corners meet and now you have a lap joint when the ring goes into the bore.
To size the ring before the cut lap, I use the following formula.
Dmachine = (Width of lap + clearance / Pi ) + D piston
So with a 1.00 inch diameter piston with .09" lap+ .010 clearance, the calculated machined diameter (Dmachine) would be
Dmachine = ( .100 / Pi ) + 1.00
Dmachine = .0318 + 1.000 = 1.0318
So machine a ring 1.031" diameter and cut the .100 wide slots in the compressed state
The ring will fit rather well with about .01 clearance on the lap for expansion.
For running on air, .01 should do, but for steam, you will need more.
Here's the material properties of teflon
http://www.matweb.com/search/DataSheet.aspx?MatGUID=4d14eac958e5401a8fd152e1261b6843&ckck=1
CTE is something like 100 microinches / inch / F
That's alot!
Adjust accordingly!
Dave