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washers needed under spark plug for hc piston?

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Im looking at fitting a new piston next week in my 05 250f my bikes now done 50 hrs and im going to swap the stock piston out for a vertex high comp one ,a mate of mine just done this on his kxf and when he started it up it ran poorly ,they then pulled the plug out and the plug gap had closed up as the piston had just nipped the spark plug they then called up the store and were told they had to run 2x washers under the plug to lift it and give it clearance and said this was normal and standard on any hc piston ,is this correct ?

Anyone fitted one?

Are there any other tricks i need to know other than ring gap and retiming the cams ?

Any info would be much appreciated cheers

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cheers mate , what years your bike and did it give it a bit more bottom end get up and go ? did you also have to rejet ?

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My bike's an 02. I don't have anything to compare it to since I bought the bike with a seized crank and replaced the crank and piston also with a high compression one from the get go. I did rejet it though and it has no bog.

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I'll chime in for this.

I added a Wiseco 3 ring HC (13.5:1) piston to my 03, and never had this problem. Must be just a Kawi thing 👍

With the piston in there, I did not have to rejet at all. It was basically a drop in and run. Plenty more bottom end.

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I have an 06 YZ250F and I have the hi comp piston out of the standard 2008 model in mine. There are no clearance problems and the bike runs great. Fairly noticable off the bottom, but good general gains.

Just watch for all the usual problems when doing a rebuild, like barrel wear/damage, big end play, the condition of the little end of the rod, cam chain, check the valve clearances, even remove and inspect the valves/valve seats.

If you have an owners manual, it gives you a good idea on what to inspect during the rebuild.

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just finished the rebuild late last week also fitted up a full dep pipe and header

no washers were needed 👍 and everything was spot on ,proberly didnt need a new piston yet but would rather change it sooner than later (old piston still measured 76.93mm and new a piston was 76.94mm ,bore still had traces of old scuff marks from factory honing / surfacing , cams were mint everything still tight as , i run it in carefully then tested jetting ,bike ran perfectly through out all throttle positions but a plug check showed to lean with the stock 182 main jet tried a 185 and no difference so brought a 190 and drilled an old one to 200 , tried the 200 and bike still ran mint ,a plug check showed a little rich but is running the best it ever has loads more bottom end power and no flat spot low to mid (thanks to the pipe )so a great result ,thanks to all :worthy:

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