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Inportant tip when rebuilding 450 sxf engine

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I figured I'd pass this experience on to you guys to avoid a potential issue when rebuilding your 450 sxf engine.


I bought my 08 450 sxf and just completed my 3rd race/ride season with it. I typically replace rings every year and the piston every other. This time around, I have it apart for new rings. Last year I installed a Pro X 13.5:1 piston and ran it for a full race season (about 50 hrs) without incident. Upon disassembly, I was alarmed to see valve imprints in the carbon on the piston. Further disassembly showed no actual damage (lucky!) to either the valves or piston. BTW, Pro X makes a good part, my ring endgap still measures @ .017" (good) and the piston skirt has minimal scuffing. Anyways, so I contacted my local small engine machine shop and asked if they could flycut the reliefs on this piston. Like Prox told me when I installed it last year, the machinist felt the higher compression piston wasn't the issue. He asked if I had ever disassembled the cams and timed it with the alignment tool. I had never done this but, given the hours on my bike (170), I had planned to do thisso and made the tool. So, last night, I timed the cams using the tool and clayed the cylinder. Inspection of the crushed clay shows that I have over .085" of valve to piston clearance (good). Bottom line, although this is contrary to what the service manual says, I think I'd do this during every rebuild. Keep in mind that the bike started, ran, and performed well all this summer despite having valve timing which was dangerously close to having a valve piston collision. Just thought I'd share.

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