2008 YZ139 kit

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8/6/2018 7:51 AM

Wanted to share some info. regarding a 139 big bore kit I had recently purchased. For me things didn't go well, right off the bore was very noisy, ok it's a forged piston, may need to get up to operating temp till it quiets down some. My first ride didn't last to long, the piston broke at the ring land in about 20 minutes, from my experience this usually to high compression specially with a cast piston, not the case here forged piston. 1mm squish clearance at setup, dome cc'd to builders spec.

This is not to bad mouth any shop, just be careful when buy custom kits and pistons. The kit I purchased this piston is made very light and is not giving much durability, most of my 144 kits run a whole season before off season teardowns, so I disassemble the top end and took some measurements, .003 PTC clearance, .001 size difference between the two pistons.

In the end, the builder didn't offer up much help so I'll get a new German made specific YZ144 piston and send it to power seal and tighten up the bore.

Bore measure real square .0001 top to bottom just to big, see the piston on exhaust, after one day of practice, smeared the aluminum. I had the bike jetted rich purposely plug is on the black side, 100 AV gas blende with 1 gallon 93, same used in all my bikes