Reply To: Mercury KE4 Piston Ring

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When the piston is inserted into the cylinder the rings are compressed to conform to
the diameter of that cylinder….rule of thumb is end gap of about 0.008″ for a 2 inch cylinder bore. This allows for expansion due to heat of engine operation.

After cylinder is prepared you fit the rings by inserting the ring into the cylinder it will
be used in and measuring end gap with a feeler gauge. New rings will be over size
and used cylinders will be larger due to wear. I like to use bottom of a junk piston
to set the ring square in the bore. Fee;er gage is an inexpensive tool consisting to
a range of thin metal leaves each marked to indicate thickness. When ring is in cylinder , the thickest leaf that fits between ends of ring is the gap.
If gap is less than required, remove ring from cylinder and file ends a little and
return ring to cylinder for recheck. Rings are made from cast iron and require
careful handling to avoid breakage. A simple holding device can be made from
two thin pieces of wood and a small vise….clamp ring between the two piece of wood with just the tips of ends exposed and gently file each tip a little. Check
ring in cylinder and repeat as needed to get the right gap
Cylinder wear is uneven with the most wear in middle and least wear at top and
bottom. A ring’s end gap will vary depending on where it is located in the cylinder
You can verify if cylinder by measuring end gap at several points in piston travel
in cylinder.
For present purposes set end gap for bottom of cylinder which will be least worn spot in cylinder.
Each ring has to be fitted to cylinder it will be used in.

2 cycle engines have ports in cylinder walls. If a ring should happen to rotate around piston in ring groove, the end of the ring will snag in a port and break off
SO two cycle pistons have some device to locate ends of rings. On the Mark 7 pistons it is a round piece of copper in piston that intercepts each ring groove so the end of rings are located on piston away from ports
Mercury Service Manual procedure calls for removing rings before removing piston
from rod AND installing rings after piston is installed on rods.
I found out the hard way why.
The rings will float freely in ring grooves…removing piston pin may involve some force (of U shaped Merury Service Tool) In either case if the rings may crush the soft copper pins. That will be a problem when you put engine back together. The mashed down copper pins may not stick up enough to reliably keep the rings in,e their proper place….ESPECIALLY if you need to rotate cylinder to line it up with crankcase. If a ring is riding on the anti rotation pins, ring is pressed tightly against
cylinder wall and crankshaft will turn over very stiffly.

Good used pistons are getting harder to find. Take very good care of them or you may have to go looking for them . I hope I do not sound like and old scold or a know it all…I am still learning.
Louis
The may be some way to undo damage to piston ring pins so don’t toss otherwise good pistons.