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Discussion Starter #1
Hello, I've been a member for years but this is my first post here.

Ive read several posts of people putting the trx90 trans gears in a horizontal Chinese motor, but there aren't many detail listed. I know it's not a plug and play like some other trans, but the gear ratios are desirable.

I have purchased the trans and started measuring/fitup. Looks like the TRX gearset is slightly wider on both shafts (~.100-.140").
However there is a lot of space between gears/dogs so it looks like cutting a new circlip groove on each of the shafts should work. The shift drum grooves don't appear to move the shift forks the full gap, meaning that as it sits the dogs don't fully engage the gears, so the gears are able to be moved over. Just not sure how tight of tolerance could be run between a dog that is floating between gears changes. I have a lot more detail that I can't easily explain through text.

Looking for anyone who has done similar conversions, and willing to help.

Thanks,
-Sean D.
 

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Easy answer is do not do it. It doesn't fit.

TRX90 is sort of its own platform in the USA. Yes several Chinese companies used the TRX90/c90 as a starting point to copy from. So some Chinese engines and TRX90 parts interchange. The CRF50/c50 is a different platform, parts are all similar just a little bit different.

I think the shaft ends where it goes into the case bearings is different/different size. The cases have needle bearing in one engine vs ball bearing in other engine.

You may be able to make it fit into the case width with a modification. Only a guess but, it probably will not shift right after that.

The clutch mounting portion of the shafts are different. You may be able to hack up and machine the trx90 shaft to fit a crf50 type clutch. Or you want the trx90 clutch too. If I remember right the gear between the clutch and crank is different on tr90 than a 50 so it requires a special crank drive gear if using a crf50 type crank(takegawa makes a gear and CHP used to make them too)

Remember that the kickstarter gear engages the trans. So a change to the trans gear means you need a kickstarter gear that matches it to engage properly.

Depending on what you want to do that best options are:
+Get complete TRX90 bottom end and start from there.
+Pick a trans kit that fits hassle free into a CRF50 with ratios that satisfy you. direct fit and best ratios are Takegawa and Kitaco.
+There are alternate Honda parts from TRX70/ATC70/CT70hk1 and others that will fit in the crf50 cases with much less work and have different gear ratios.

example:
ATC70
1st... 3.273
2nd... 1.938
3rd... 1.350
4th....1.043

Some combos that fit in the cases will still have to have the kickstarter gear changed to match up.
 

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Discussion Starter #3
Thanks for the feedback 50race. I am really liking the ratios of the TRX90 trans. The big issue being that most trans swaps still have a large gap between 1st and 2nd, the TRX does not.

I've spent hours researching, almost went with a ct70 trans but found out that they are weak.

The clutch mounting portion I am not concerned about. The TRX trans input shaft has the same splines so an existing xr gear can slide right on, retaining the crank mounted clutch from the xr.

From what I've read a 22t kicker gear must be used for the TRX trans. I have a 22 tooth from an older z50 but the mesh seems to be on the loose side, wasn't sure if there were different part numbers maybe with different OD.

-TRX 90 bottom end.... Pull start only :(
 

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Dratv sells a trx90 Kickstarter shaft and gear kit.
 
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