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Valves hitting piston?

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Hey guys... interested in some feedback on my piston. Motor had a knocking sound... started pulling it down and noticed my piston had what i think are unusual marks - possibly from exhaust valves... Could my exhaust valves be kissing my piston? Doesnt look like its hitting hard but unusual mark on my piston... see pic..

7D96ACBF-560A-44B0-8ED7-EDD7CC0B9338.jpeg

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That’s some serious cause for concern. You don’t want that to smack once too many times and grenade any more parts.

Time for a rebuild. Recondition the head and I’d just go ahead and replace the piston due to the stress of collision. 

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Also a concern is what allowed it to come to that point and, how to prevent it from happening again.

Weak exhaust valve springs (valve float at high rpms), wrong cam timing / spun cam etc.

Just to be sure also check for play in the connecting rod and wristpin.

Edited by mlatour
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Thanks guys... appreciate the validation. Going through rebuild regardless. Cheers!!

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sounds like your timing chain and gears jump a tooth or two. You better fix that first. Keep us posted

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The head may have also been decked or thinner base/head gaskets may have been used causing the valves to kiss the piston.

Have you had the bike from new? First tear down?

If not, do you know its history/previous owner?

 

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Naah, bought the bike running with the knock. Its got a prox rod so assuming its been rebuilt before. Doesnt look like its had any performance work/machining. I was planning on full rebuild anyway.. at least i know what to double check and definitely replace. Thanks for the comments. Cheers!

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I would guess with a timing chain jump.  Had a Yamaha street bike that did that years ago, when I tore it down looked very similar. Just barely touching piston.

 

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What was the valve clearance before tear?  Down, in spec?  What bike?

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I was so sure it was bottom end i tore it down without checking.. all good. Was going to do bottom end anyway... ill reco the head and check cams etc as well. Its an RMZ250 2013

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6 hours ago, OFS said:

What was the valve clearance before tear?  Down, in spec?  What bike?

You cannot tighten the clearance to the point of touching the piston. It will stop running long before you get this tight. 

Piston to valve contact does not normally make much noise. The knock might actually be piston slap or a rod bearing which will also cause contact. 

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