Bottom End / Transmission Compatibility

[Oldmanriver]

Hello- I have a 2004 CRF70 with a bad Main shaft 11T and Countershaft 36T gear. There is someone near me with good bottoms from a 1972 ct70 and 1977/1978 z50. Would I be able to use any of these with my top end.
Thanks

 

[69ST]

I've not swapped individual pieces (gears, shafts), other than shift forks, between the early "6v" and later "12v" lower ends. That said, it is possible to swap an entire transmission assembly, of any year, into Z50/CT70 cases. The "12v" (GB4) cases have needle bearings, instead of integrally-cast bronze bushings on the non-thrust sides of the transmission shaft assemblies. They have the same dimensions as the bronze plain bearings they replaced. The later style, constant-mesh, kickstarter assembly can be used with the 3-speed tranny, it's a lot stronger (full-width tooth contact) and is sold as a "heavy duty" kickstarter assembly...it just has the wrong number of teeth for an H-model 4-speed trans.

 

[Oldmanriver]

Thanks for the response racerx! Do you know if I could use one of the bottoms mentioned above with the top from my 04 CRF
my engine serial number is de02e5316602.

This is my first time into one of there engines. I was going to buy a new main and caountershaft gear but they are on back order and would see them for a month. Checked my local craigslist and found a guy with the bottoms from 1(972 ct70 and 1977/1978 z50).

It would save me time and $$ if I could use one of those older bottoms

 

[69ST]

1972 CT70 & 1977 Z50 are, essentially, the same lower ends. You'd have to check a parts diagram + list with part numbers to verify that every single gear, c-clip & thrust washer is the same between them... or do a side-by-side comparison, on the bench. That's why I suggested doing the transplant as an assembly.

One thing I can all but guarantee you is that the shift forks will be worn and in need of replacement, even if they are still within spec for overall thickness.

 

[Oldmanriver]

I understand, but with the older bottoms i was going to leave them sealed up as they were in working condition and mate my top end from the 04 if it will receive it with out any issues.

I'd run it like that, as I am trying to complete this ASAP so the kids will have something to ride at camp. In the mean time I can take my time to order the main / countershaft gears and reassemble the bottom from the 04

 

[69ST]

With what you've just added to the "equation",I don't think you realize just how involved this might be. There's the ignition assemblies, alternator assemblies, crankshaft assemblies, piston compression heights (that depend upon connecting rod length), piston domes (that depend upon combustion chamber type), to consider.

For example, if you try installing a 12v era top end on an untouched 6v lower end, the 12v piston will sit far below deck height at TDC. And, you don't know the condition of the used 6v crank assembly. If you attempt to install a 12v crank, to mate with the 12v top end, compression height will be correct, as will piston type...but what will you use for ignition & an alternator? Can't use a 12v alternator with a 6v crank, nor a 6v alternator with a 12v crank. And the CDI pickup won't fit a 6v case half, without mods.

Going with a used, "sealed", lower end without opening it up for cleaning & inspection is huge leap of faith. How do you know the condition of the gears, shift forks & crank assembly...or how clean/dirty it is internally? If there's any sand inside, the engine will shred itself almost instantly.

 

[Oldmanriver]

You are correct I didn't realize the complexity! I will make one more attempt to find what I am after (used and available) before I hurry up and wait on back ordered parts. Thanks for the insight very much appreciated! Have a good day!

 

[dirtbkr188]

For what its worth, the XR/CRF50 lower end uses the same mainshaft and countershaft, to broaden your search and keep you within the 12V parameters.

 

[69ST]

Transplanting a transmission assembly is fairly easy and very straightforward between any two 49cc lower ends. Mixing & matching other 6v & 12v parts...not so much. FYI, that covers all 49cc & 72cc horizontal singles.