small knock in YZ250F?

[72Javeloon]

Hi guys. I just picked up an engine from a guy that he claims has a small knock in it. What are the service limits on a yz250F engine?

i.e. how much play in a connecting rod side to side is acceptable?

How does one check bottom end bearings without tearing cases apart? Just visually and make sure they feel smooth? Is there a way to check?

I'll change the wristpin bearing that's simple and easy.

There's a guy who races that wants the motor in a hurry but I want to make sure I don't sell him a hand grenade.

Thanks in advance for all advice.

Have an awesome day boys.

Phil

[ronbuell]

A small amount of play will sound like a knock. I bought an unknown like that and ran it. At rebuild, the small end of the rod had play, the wrist pin was scored, and the pin was loose in the piston (BTW, there is no small end bearing on the rod). The Big End bearing didn't have any up and down but it did have side to side play. The crank main bearings were OK, no up and down. I replaced the piston, pin, clips, and crank, along with cam chain, valves, seals, gaskets, etc. and had the valve seats ground. That was over a year ago and that motor is still strong and cranks up first or second kick when hot. My advice is to tear it down and see what it needs (most likely a new crank and piston). We just honed my cylinder 5-6 revolutions to clean it up and put a cross hatch pattern in it and it measured right in the middle of the spec when I slid a new piston in there.