After replacing bearings and ring in an old ST60, I took it out yesterday to break it in a bit. It fired right up but seized about 30 seconds in. It was plenty rich so I must not have gotten it clean enough. The crank pin is in excellent shape but the rod is tight on it. I thought I’d take it to work where I have a nice Sunnen hone. I figured I’d hone out a touch. Just need to know how much clearance there should be. If it doesn’t work, I’ll try to find a new rod. This is free to try at this point though!

Thanks, Matt

I would polish the crank pin anyway. A few seconds with a worn piece of 800 paper will do. Just hone the rod until it moves freely on the pin and it will be ok. I use hot soapy water and a tooth brush to clean parts and pre oil all the parts with 3in1 oil or similar. Have you set ball bearings before? If there's a bind it will stress the rod. Is there any castor in the fuel?


Motorman

I used a strip of diamond paper to lightly polish the crank pin. Did not measurably change the pin size. Fuel is Ritchs Brew 22% 11/11 mix. Hone was a bit more aggressive than I wanted. I ended up with just under .002” clearance or .05mm for the metric crowd. That’s the top limit of what Lauri Malila specified. It’s looser than what I wanted. I’ll try to get it on the test stand this weekend and see what I think.

Thanks, Matt