Vintage Dirt Bike Q & A

hi is there a way of making a big bore cr250m 73/74.i bought a set of cases on ebay and they have been machined out to take a bigger sleve.is there a way of making a 1973/4 cr300m
jim

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Jim Garvey
1x honda cr250m 1973 in bits
1x honda cr250 champion dirttracker
1x yamaha tt500 dirttracker 1976
1x yetman triumph dirttracker
1x tt500 longtack speedway bike
1 x 1940 350 ariel speedway bike
1 x bultaco astro grasstrack bike antig

you may need to find this out for yourself unless you can locate a aftermarket cylinder from the early 70's that allows the bigger piston to be installed.

they made such kits for say a Yamaha DT1 that allows the piston for a RT1 with it's 80 mm bore to be installed that took it to 322 cc's.

then there is the question of rod lengths and the Bore and Stroke ratio itself.

the rod length question would be a decisive factor on what piston to use.

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

I wonder if an FL350 piston etc. might work:

http://www.partzilla.com/parts/search/H ... parts.html

Wiseco might also have something, maybe from a piston-port snowmobile motor.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

ummm why? the poor old elsie is barely good enough at 250cc why would you want to up that?
Yes I raced one and I can attest to ALL of its flaws. I used to go through cranks on my 250 as much as my 125...
THEN I switched to a hoosk and all I ever did to it was put gas in the tank.

IF you are dead set on trying it contact wiseco they can give you all the data on pin size, timing edge and pinning.

good luck

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1994 AHRMA National Champion Sportsman 125 Intermediate.
1996 AHRMA National Champion Classic 125 Expert.