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Everything Dirt Bike
Make / Model Specific
Yamaha
1978 YZ400 Piston Seized
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<blockquote data-quote="cleonard" data-source="post: 133808" data-attributes="member: 21"><p>Replace the crank seals. They are likely from 1977 and leak some serious air.</p><p> </p><p>How deep are the scratches on the cylinder? Do they catch your fingernail? If they are not too deep you might get by with some aggressive honing. I'd break out some sandpaper on that piston if it's not too bad. You will lost the oil holding surface machining on that spot, but I bet it would run.</p><p> </p><p>One thing that is critical is knowing the piston to bore clearance. If it's too small it will just seize again.</p><p> </p><p>If the damage is too great on the bore then it's new piston time.</p><p> </p><p>No shop will do this though. I've assembled and ran engines that were a lot worse. Is it the "right" thing to do? Of course not. Remember it's a $500 bike tops. It's a bit of a different approach than I'd use with a $10k new bike.</p></blockquote><p></p>
[QUOTE="cleonard, post: 133808, member: 21"] Replace the crank seals. They are likely from 1977 and leak some serious air. How deep are the scratches on the cylinder? Do they catch your fingernail? If they are not too deep you might get by with some aggressive honing. I'd break out some sandpaper on that piston if it's not too bad. You will lost the oil holding surface machining on that spot, but I bet it would run. One thing that is critical is knowing the piston to bore clearance. If it's too small it will just seize again. If the damage is too great on the bore then it's new piston time. No shop will do this though. I've assembled and ran engines that were a lot worse. Is it the "right" thing to do? Of course not. Remember it's a $500 bike tops. It's a bit of a different approach than I'd use with a $10k new bike. [/QUOTE]
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Everything Dirt Bike
Make / Model Specific
Yamaha
1978 YZ400 Piston Seized
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