Piston ring part # 39-21665 application…All 1941 thru
Mark 7 modes with 2 inch bore
(1940 models use 2 wide rings)
Some later Merc use same rings,
If seller says ther rings will work, ask if he will take them
back if they do not,
With new rings you need to check end gap with ring in the cylinder it is to be used in,
Rule of thumb is 0.004 per inch of cylinder bore… better too loose than too tight.
If you order rings from OTTO
Gas Engine you have to measure
piston for ring size.
Cast iron rings will flex a bit and break if flexed to far. Been there done that.
Good Luck
Clamp ring between 2 thin boards in vise with just ends
exposed when using file to limit flexing/breakage,
Check end gap of ring at several points in cylinder bore to check wear. You can
calculate amount of wear using
difference between unworn bottom of cylinder by comparing end gaps and
math formulae for circumfrance
of circle. My exjperience has
been cylinders wear is least
at top and bottom and most in
area in between,
Cylinders often have a glaze
A light hone may help new rings
If cylinder bore is pitted as often happens when piston rings rusted stuck, find a
better cylinder. Do the same
for badly worn cylinder.
It is best to remove rings before removing a piston and
installing rings after piston
is installed on rod to avoid
crushing anti rotating pins
or breaking a ring,
Been there and done that also
