To big bore or not to big bore?

crf450 takes me for a ride instead if me riding it. My crf 250 is almost perfect, just needs a little more out of the corners and shooting 100ft tables. already have Dr d, I cat, power now and pwrnw plus, 1 more tooth on the rear sprocket. I just sent out for the 270 kit , hot cam, and i'm going to clean up the exhoust port. Someone tell me I'm doing the right thing and not adding to my problems.

i'll second that one in regards to the 450. sounds like you have a good thing going there. however i do believe from reading past posts that the connecting rod won't hold up well with the bigger piston. do a search. i think the magic word is "falicon rod."

other than that i think it will probably be the best of both worlds...

??? falcon rod ???

??? falcon rod ???


Shoot!!! thats one reason, besides the cost , that i did not do the 302 mod becouse of splitting the case. Is that common on the 270 mod to have the rod bearings go ?

That is the first I have heard of rod problems, keep in mind the compression is lower than stock on the TMR kit so it should be easier on the bottom end

I believe that the TR270 is totally safe with the stock crank-however if you go any larger than that, a Falicon rod or another built-up lower end would be necessary. I haven't heard of anyone having any probs with the TR kit. I can only say that it made a really noticeable difference in the power of the bike. :D


If you want more power, have the head ported, or sell it and buy bigger bike. Big bore kits dont do enough to make them worthwhile.

Where did I hear it was better on the crank to have a higher comp piston. It puts less stress on the rod. I think it was rap that said it?

>>Where did I hear it was better on the crank to have a higher comp piston. It puts less stress on the rod.


Higher compression Pistons put MORE compression stress on the rod/crank/bearings, not less. But most rods break on the exhaust stroke, as the piston is being deaccellerated before top dead center. A heavier piston will increase the load. If the piston weight is different than stock, you might want to consider having the crank re-balanced.

As the piston reaches (almost) TDC, the rod wants to stretch just a bit...the heavy piston wants to keep on going up, higher compression will provide more "push" back against the piston (and everything else) inertia...of course as soon as the fuel mixture ignites the higher compression puts more stress on the crank bearing itself. Like the falicon guy said, most rods break on the exhaust stroke and a higher compression piston gives a bit more "push" and may help a rod live longer under that narrow range of conditions.

But higher power means more wear on SOME part no matter's all a trade-off....

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