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YZ/WR 250F Piston slap?

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I recently reassembled an engine using two basket case engines for parts for my Cousin. After getting the engine started, there was a lot of noise from piston slap while warming up the motor. I had reused the one good piston, on the insistance of the owner. It looked good and the cylinder liner was not worn at all from the lack of evidence of any ring ridge. Since the piston skirts are really short on this engine, is piston slap a common/normal condition with the 2003 YZ/WR 250F? I did not hone the cylinder, but used new piston rings.

TIA.

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You can't really rely on visual inspection when it comes to rebuilding motors from used parts, or indeed new parts.

At the very least you need to measure the skirt diameter with a micrometer (normally, with most bikes just below wrist pin level in a front/back direction), as well as the bore of the cylinder, then check the difference between the two to see if the clearance is within specs.

Assuming the issue with your bike is piston slap and that the used piston was actually ok, I'd say your cylinder lining is worn beyond specs.

Greg

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Also, Some of the knock noise could also be a loose wrist pin. The last time I rebuilt mine, I noticed that the wrist pin was loose Up and Down in the bore for piston where it mounts, AND the wrist pin was loose on the rod. Meaning the small end of the rod was worn as well and making a knock noise from 2 places. When using used parts, you must check all items.

Best bet would be to order a new crank and piston with rings for this rebuild, or at least a rod kit and piston. Remember that the small end cam sprocket can wear and jump time. Inspection is look at the teeth and see if they are sharp and pointed. If so, they are done. Also, run a hone thru the cylinder for 6-8 turns so the rings will break in properly and seal. Remember, a job worth doing, is worth doing right the first time, and will cost twice as much if you have to re-do it. If you don't have the funds, the wait and save.

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if that motor has a loose piston pin,the damage that will result is quite extensive,and usually renders the motor useless.you better do it right or you will be sorry.peace and wheelies.

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