Dino,
I have lapped my piston before I broke the engine in and had no crankshaft problems so far.
But i think you should probably re-think your tolerance between the piston and sleeve, since two liters of fuel is a real pain in the behind.
Originally posted by dino.tw
Dear all STS racers,
I spent some time to research the crankshaft issue those days. There is only one batch as you knew we don't have that big sales. All of those cranks are the same. It confuse me why some racers may have problem with his crankshaft but not everyone. If the crank has fault really,it should be broken when you start the engine at the first time. And to my many experience with this crankshaft,I never have problem with it. But it's my job to find the real reason.
First,I used a big hammer to hit the crankpin. No matter how I beat it,the crankpin still there. After I changed to a biggest hammer(1000...t?),the crankshaft broken in the induction port. It's really hard to believe to me why you can broke the crankpin? It's made by Japanese bearing steel(SUJ2) which is a very strong material.
So I asked 4 A-main racers to test for me because I know if I test it personally,the result is still the same. I didn't tell them what should do or what should not do in advance. Just let them feel that it's a free gift. Those racers spent some time to break in their engine. I just watch beside them and keep crafty smile on my face. First day,there is nothing happened. In the second day,two of those racers started to lose their patience on breaking in. They start to lean their engines. To my observation it was about 10th~12th tank of fuel. One guy's crankpin broke soon and another's later. The other two racer's cranks are still fine. Now I knew why you can break the crankpin finally. Because I usual break-in one engine with 2 liter of fuel so I never meet this problem.
So I have to notify all STS users that;
1)Be patience to break in your engine. Don't lean it before 2L fuel through it.
2)After 2 liter fuel(about 27 tanks/75cc),replace with a new conrod.
If you can follow with my suggestion,your engine will not have any problem. You need about 2 liter fuel through for sure that you can start to lean it. After 2 liter fuel through,let the engine cold down. Take out the glow plug and turn the flywheel by hand, let the piston pass the top dead center in the sleeve. When you don't have any significant friction or just a little snug, you can start leaning the main needle step by step.
We are starting to make the crankshaft softer. If you were unlucky and have problem with your crankshaft already,contact with me by email. I will do my best to help.
Sincerely,
Dino