Vintage Dirt Bike Q & A

hi
i have a complete low hour mr250 motor,also i have a 1974 cr250m crankcases,barrell and head.can i build a motor for dirttrack using all the intermals and side covers of the mr250 in the cr250m1973/4 crankcases.the motor will go in a longtrack speedway frame,i have a 1973 cr250m and no most of the diferences but not the internals
cheers
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

The trans should work along with the clutch and other right side parts. The crank has a different length rod, but may work with a spacer under the cyl (not sure though). It will also be a wide ratio trans compared to the CR.

You gotta remember the cr had a tiny little flywheel and the mr had a huge one and I doubt it would fit in the cr case.

the CR,MR and MT ignitions can be interchanged,the cases where the ignition is are identical.

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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!!

As MX317 pointed out, the MR and the 75-76 CRs had connecting rod that is 4mm longer than the 73-74 CRs. If you look hard enough, you probably can find a spacer because people with the early CRs often have to make this modification when rebuilding their crankshaft (easier to find the longer rod than the shorter rod). The other thing you need to look out for is that the ring pins are in different places on the MR and 75-76 CR than on the 73-74 CR. I think, however, that you are safe to run the MR/75-76 CR piston in the 73-74 CR cylinder. It is running the 73-74 piston in the MR/75-76 CR cylinder where you would probably run into problems with the ring pins hitting a port.

I think this is worth pointing out twice. While the MR250 doesn't have the ridiculous "500cc desert racer" gearbox of the '75-'76 MR175, it's in the same ballpark. From the speedway and dirt track races I've watched, it seems to be all about max acceleration within a fairly narrow range of speeds, and the MR gearbox will give you less gear choices and drop you below the meat of the CR powerband after every shift, even if geared down. The MT250 gearbox would be better, but the CR box would be by far the best choice, in my opinion.

I've got the ratios for all three motors if you need 'em.

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