Managed to get a 81 YZ465H with matching numbers engine / frame.... except for the barrel.. the stampings show its a 82 YZ490 Barrel.
I've searched the web but haven't managed to find a definitive answer to the piston clearance... would 6 thou be the answer ? A quick measure with a feeler gauge whilst the barrel is on shows 15 thou + way too much...only did this as I could physically see the gap between the piston and bore is way too big... walked away feeling depressed. I'll take the barrel off and get a correct measurement...
would it be worth getting a new piston / rebore or try and source a 465 barrel ? from what I've read the 490 wasn't as good / vibrates / etc.
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