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I just purchased a 03 TRX 400ex from my girlfriend's brother's friend (I know, I know... that was my first mistake. ha ha).

I was told that the PO to him had an ATV performace shop in Mass buil the motor with a 416 Big Bore kit. Thing is, he doesn't know much about it and didn't bother asking questions, doesn't have the PO's number or email for me to ask him, and is not mechinically incline.

My question is how can I tell which Big Bore kit was installed as there is two out that I know if: a 10.8:1 and 12:1 compression kit? What should the compression be with each kit?
 

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I dont know the compression specs, but i can tell you this the mm and oversize is writen on top of the piston LOL as for knowing what compression piston u have, its kinda like a car take out the plug and use a screwdriver to feel for valve relief's
 

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I have a question myself...how hard is it to rebuild the top end of the trx400? Is Wiseco the best piston to put back in it?
I just did my first rebuild last week. I have an 02 400 ex. It is not too difficult if u have a little confidence. I don't have any mechanical schooling... Just had the manual. I put a new cam chain in and a new tensioner. I broke it down to the piston. Took me about 9 - 10 hours. Only recommendation is to STAY ORGANIZED. I used a tusk top end gasket kit for it. Good luck!
 
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