Handlapping is taking out the piston and use a polishing pad to polish the side wall of the piston. It is easy to do but you do need to be carefull.

It does help and with any other nomal engine just a few tanks on the track are needed to make it race ready but with the 2104 it seems that even then 2 litter is needed. I have no experience with it but many OS users doing their 1st 2104 are misjudging how much break in is needed compared what they normally have experienced.

Thanks Roelof.

Oil bath is really just the first stage, it needs heat cycles and patience. Do not even attempt to start engine until pre-heated to 90 degrees celsius and ideally keep it above that during heat cycles.

When the engine is fully broken in, you should be able to fully rotate the piston easily? Or there should still be difficult in the top end?

In cold condition it is normal that there is still some pinch to feel but it should be much easier to rotate the crankshaft through it. When you take the engine from the track stop it and feel it then you should feel almost no pinch.

Some pics of the inside. Is there anything weird? The rust in the glow plug is because of rich mixture?

Well, it seems it is not only to the R2104 anymore.....
I saw this this morning comming by, 2 offroad engines with a same issue.



This is what will happen if you go too quick, it is showed on the exhaust side. So your sight looks so far OK.

The brown dirt looks like burned castor oil but the pic is not sharp enoug to zoom in much closer. If it is already drying up (getting hard) I would advise to ckean the inside before the next start.

You mean clean to clean the piston? Am i doing it with wd40 and kitchen paper?
Regarding the glow plug does it need cleaning too?

Yes I agree with you what we do is heat up the oil @ 200 degrees F then run the motor at 900 rpm for 2 hours. Then we let the motor cool all the way down to room temp then again heat up the oil to 200 degrees F and run the motor at 1200 rpm for another 2 hours. Then take to to the track pre heat the motor to about 180 F and run 2 tanks on the starter box, let it cool down and then run 2 - 3 more tanks on the track very rich never allow to go into 2nd gear. After this then we start to learn it out little by little.

The 1st part of te break in is the most important one. On a piston it surface there is a very small screw thread like structure made by the lathe making the piston.




The top points of the piston structure are basically very fragile and need to get flat spots w/o burning and/or breaking them. When that is done the piston can stand more forces w/o getting damaged. The oilbath with 100% lubrication and no real expansion heat will give the piston a much better polished surface.

Good Info Roelof thank you for always giving out great detail.

4 hours don't do anything, and raising the RPMs doesn't do anything either. You have to think that when you heat it up it expands, making it easier on the rod and making the piston/sleeve fit slightly looser(the piston expands less than the sleeve) I do mine for 10-12 hours, 1st hour at 210, then I lower the temp to 190 and let it run overnight. Then I do one bottle rich(lots of smoke and very little power), let it cool. I start the next bottle where I left off with the previous one, but I lean it a bit each tank, by the the last tank the motor is almost on race tune, but 2 -3 hours rich, and letting it shift. I do one more bottle at that setting. Then its race ready, I haven't had any motors get scratched or heat spotted, as long as that procedure was followed.-

WOW 10 - 12 hours but good info thanks.

A nice read I came accross is the oilbath system by Blok engines. They clearly have looked to my idea to measure the pinch
https://blog.teknorc.com/2019/03/01/...eak-in-system/

But this pic does show an interesting thing:



You can see the pinch drops down very quick and rises again. This is probably a tiny grow by the 1st heatcicle at a low temp. As I did explain before, a heat treatment will give some permanent grow into the piston. Basically that happens again every time when the piston becomes hotter than before.
Heating up the oil for a 2nd run seems to give a kind of an heat cycle but it will be nothing compared with the combustion temps when it is fired up.
I myself does lower the temp at the end, it will raise the pinch a bit and so work a little bit harder again on the polishing.

I see that they only did it for 3 1/2 hours. My motors still have pinch when cold and a slight pinch when warm, for about a gallon of fuel. Perhaps Im over doing it, but I stand by my method.