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hi i have a 400ex im doing the top end on. right now ive been searching trying to find out whats the best compression ratio to go with i have found 10.1 wiseco kits that come with gaskets but dont know there gasket thinkness to see what compression would end up being if i run honda xr400 gaskets instead. i also run 93octane and want to stay with it and dont want to change head studs.

quad was buring oil its stock bore now was gonna take bore to like 406 to stay close to stock bore as possible so i can get more rebuilds out of it.(still need to take jug to machine shop to see how out of round)

ive already got brand new hot cam stage 1,cam chain tensioner,oil pump,crf450 cam chain ,new valve seals.

i mostly ride trails, also quad already has wb e series, uni air fiter with pre filter,no box lid,spal fan, stock header with welds ground down.

i also am going to need to know what would be a good jetting also for this bulid after depending on compression choice im at 950ft elevation but some times ride where its 500ft elevation

thanks for the help
 

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If you compare the parts (I compared a 2004 XR400 to the 2005 TRX400EX) you will find the TRX and the XR use the same cylinder, head and piston. Only the head gasket is different, but they both spec the same compression ratio.

The Wiseco kits are the same whether it's an XR or TRX - whether you select the 10:1 or 11:1 compression, 93 octane fuel will be fine. As to bore size, go as big as you want. Cylinders are easy to find in stock and oversize bores. You can go as big as 426cc without having to sleeve and install HD studs.

Jetting will be determined after the job is done, the engine is broken-in and the performance is tested. It needs to be rich enough to keep the piston from melting, but not so rich as to build carbon deposits. If it is jetted correctly now, it will be OK after the rebuild also. Jetting does not change with changes to bore size - changes to intake and exhaust affect jetting and it's changes to the intake that affect jetting the most.
 
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