For normal wearing.Does anybody knows When to change piston rod?

When it breaks? Never changed one for wear.

Ain't broke don't fix it.

While I would normally tend to agree, I have had rods fail and take out the case. I would recommend checking the lower rod end bushing before the start of a flying season for wear. If there is obvious wear that can be seen with the eye alone, change the rod. To me, this would be cheap insurance as compared to having to buy a new case or replace the whole engine. Obviously, this wouldn't apply to an engine that has bearings around the crank pin

In my recent years of engine running I have changed a few on ducted fan and pylon race engines.

Never on a regular sport flying engine operating at moderate RPM.

Back in the days of cast iron pistons and liners I changed a few at the end of the running in period. That may have been because of poor material choice for the rod but it was necessary.

My very first power plane I had the upper rod bearing got elongated. Started loosing power. I thought the sleeve and piston were bad until I took it apart.

You can check it by removing the muffler and put a eraser on a pencil up against the piston then rock the prop back and forth to see if there is any slop.

This would be a good question for Bax, I have never had anyone ask before. Be nice to see what answer Bax would have. I have never had to change a rod or piston in a two stroke. I have completely worn out a couple of the old OS LA .40s and just retired them. I have only replaced one piston and rod but in a very well used YS four stroke and that was after what the NASCAR boys call having my engine blowed up. Broke the rod in half and the crown of the piston was almost completely separated. Just too many hard miles on it but the engine was worth the time and money to rebuild.
Be helpful to know what engine and brand the OP is asking about though. Some engines are worth the effort and some are cheaper to just replace.

Can you please give more detail about using pancel and eraser to do the check?

The eraser is used to hold the piston from moving then rock the prop back and forth to check the bearing clearance.

I have O.S. Max engines that are over 36 year's old. I've never had to change a rod. I have changed the bearings every couple of years. Of course I never leave fuel in my engines and always use Marvel Mystery Oil after a days flying.

Also, the lower rod end should a very small amount of play. I think it was Clarence Lee that stated that the little amount of play is what lubricates the bushing. And if anyone knows about engines, it's Clarence Lee. You can catch his articles in Model Airplane News.

Frank

Not having to do a lot of maintenance on an engine is one of the benefits of aircraft engines. Car/buggy and boat engines routinely need new bearings along with pistons and sleeves due to the much higher RPMs they operate at. In the boats I run, getting more than a year on a set of bearings or more that three or four from a piston and sleeve would be a borderline miracle. Then again, my marine .21s run at 35K while my .67s run at 28K, running on 50% nitro, something not needed in an aircraft engine