Big Bore kit for 2001 CR 80 Expert (103cc)


Aug 5, 2001
Sure it makes it LESS reliable, because you are taking what is meant to be an 80 and punching it out to 103. Beware, the really good shops that do this will ask you to buy the piston and send both the cylinder AND the piston to them.
If done right the bike will "rip"!


Super Power AssClown
Aug 24, 2000
Hi guys,
The reliability issue of increasing the bore size is related to these issues;
1) The compression ratio
2) The width of the exhaust port as a percentage of the bore
3) The weight difference of the piston
3) The ratio between the bore size and the pipe's ID in the header

The trend that I'm seeing is for companies just to bore the cylinder without adjusting the ports, which makes the bike strong on low end but flat up top. Another problem is sloppy port chamfer or the top radius of the cylinder. That can cause ring scuff and head gasket leaks.
There are several variations of pistons on the market for the CR80, I prefer the 766PS from Wiseco. Its close to the stock piston weight and they make a corresponding gasket kit so you don't have to bare the expense of o-ringing.

So to wrap it up, increasing the displacement of an engine won't make it unreliable as long as the work is performed carefully and the powerband matched to the riders' needs.:)
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