[Imhoff43]

I have an engine that has some wear. I have heard about pinching the cylinder which makes it smaller and the piston can be re-lapped to the smaller size. Does anyone know anything about this? How is it done?

[Ken W]

A memeber from our club used a small pipe cutter to make a grove around the top of the piston.This increased the size of the piston slightly and restored compression.This is only a temporary fix, but he did get the rest of he season from that engine.

[AJCoholic]

Never heard of it, or read about that (cylinder pinching) in any of my various texts on model engine design and construction...

The only way I know to make a cylinder smaller internally, is to plate it up and relap it, or in some cases, the old style steel cylinders/sleeves may be able to shrink a bit if heated to a dull red and allowed to cool and then relapped.

Sounds like a temporary fix anyhow, but I would like to hear more from someone who knows more about it...

[Robcrash]

knerralling pistons was done in the old days to fit loose pistons into worn holes and it stopped piston slap or knock , I have used this to fit valves in old cylinder heads ,only not to the valve ,to the bore , using a pipe cutter would be very effective in this manner. did he do this on a ringed engine and did it raise the compression much, or did it just feel tighter? :

[AJCoholic]

How would knurling a piston possibly increase compression, since the pattern of a knurl is so rough?

And you dont want to increase the diameter of a ringed piston to increase compression - you need the few thousandths clearance for proper operation, the ring and the ring only (unless in a ringed abc type) seals for compression.

[Robcrash]

Originally Posted by Ken W

A memeber from our club used a small pipe cutter to make a grove around the top of the piston.This increased the size of the piston slightly and restored compression.This is only a temporary fix, but he did get the rest of he season from that engine.

AJ did you read this, I was asking Ken if using a pipe cutter on the piston raised the compresssion ,not a regular knurl tool ,but quite effective i'm sure. And he states that it did raise the compression, I would like to know if this was a ringed engine or non, to surmize wether it was compression or stiffness . As i have a few old engines ,I might even try this just for the heck of it ,

[Ken W]

It was a ringed engine and the cut was made just below the existing ring.I don't really understand the theory behind it. I was skeptical myself but seeing is believing.Before this was done the engine would turn over as if there wasn't even a glow plug in.Afterwards the engine had compresson almost as if new.

[skypupnut]

I have read on RCU of a guy offering this service
he had special equipment to squeeze the sleeve and was using heat in the process
was also rolling groves in piston and lapping it to sleeve
did a search and could not find it
t`was over a year ago

[AJCoholic]

OK when yoiu said knurling I thought you meant knurling, not the pipe cutter trick (which I wouldnt call knurling..)

I have myself hammered the top (crown) of pistons (lapped ones) and rehoned to fit a sleeve but the effect is short lived.

On a ringed engine, I still cant see the effect of the pipe cutter, as the ring and sleeve fit arent affected by it, and thats what gives the seal? Weird stuff...

If you could somehow compress down the sleeve, and then relap the piston that sounds like a better way to go, but you need the equipment.

Lots of ideas here, thats for sure, but I would only experiment on an old engine if its your last resort - ie, nothing else works.. Some of these ideas sound like they can do a lot of damage too... and lord knows I have wrecked my fair share of engines trying to "fix" them too!

[wise guy]

Knurling pistons to fit the bore of a cylinder is an old automobile trick. The skirt of the piston was knurled to raise up the metal and over size the piston . It worked to prevent piston slap. Some of us old timers added more castor oil to our fuel to get enough compression to start a worn out engine .