Author Topic: converting 50/60 transmissions  (Read 1764 times)

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Offline JackA

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converting 50/60 transmissions
« on: March 13, 2018, 04:36:32 PM »
I have edited this post because I guess I was asking the wrong questions -- nothing new for me! -- to which 57 readers had no answer.

I am looking to build a couple of 50cc engines for vintage roadracing.  For openers, I am leery of rotary transmissions for that use.  Looking at the parts diagrams (what a helpful site this is, just for those and the other manuals!), it looks like the Tora has only about 4 more parts than the Chibi to make the shifter a return type instead of rotary, and my next step will be to ask Richard if he has those parts. 

The drums are also different, presumably for 3 vs. 4 speeds (?).  They both appear to differ from the Sport 50 drum, but query whether that could be used with the Tora stopper parts.

As an alternative, does anyone have any experience and/or photos of shop-made solutions for this?

« Last Edit: March 15, 2018, 08:07:24 PM by JackA »

Offline OldSwartout

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Re: converting 50/60 transmissions
« Reply #1 on: March 16, 2018, 09:35:40 AM »
I'm not familiar with the specifics of the small bike transmissions, but on the 175, the change from rotary to return change is just whether a moveable guide pin in the 5th gear shift fork hits a stop in the groove in the shift drum.  I would think something similar could be accomplished by simply drilling and installing a stop pin in the shift drum groove for 4th gear shift fork guide pin so that the groove didn't go all the way around.  I agree that a rotary shift would be a bad idea for a road racer.

What racing organization has a class for 50cc?
Karl Swartout
Mooresville, IN
BS175 Roadracer

Offline JackA

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Re: converting 50/60 transmissions
« Reply #2 on: March 16, 2018, 06:38:34 PM »
Thanks, Karl.  I will study the parts pictures for the 175 (again, it's great to have those douments available on this site!).

The USCRA -- United States Classic Racing Association ( to be confused with the Court Reporters or the Calf Ropers) -- which currently conducts most of its races at NHMS and the Canaan Motor Club track, both in New Hampshire, has 3 classes for 50cc machines:  Classic, Super Vintage and Modern.  Craig Hirko is a member and races his Bridgestone 175 there very competitively in 200GP (while the crank holds together), and Hiroshi Murata is at least a former member, but I don't think he has been around lately.  We also cater to multiple classes of sidecars (a B'Stone 350-engined rig would be eligible...).  It's a great club!  Many members also race with AHRMA and VRRA, but neither of those has 50cc racing as far as I know.


Offline al_pritchard

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Re: converting 50/60 transmissions
« Reply #3 on: March 21, 2018, 02:10:44 PM »
I recently went through this with my 175 prepping it for road racing. The pin Karl mentioned locks the shift fork into a groove on the shift drum. Allowing it to go from rotate to return. In the 175, in my case, if you locked the pin for a return style 5 speed. The shift pattern would be 1 N 2 3 4 N 5. Note the second neutral between 4th and 5th. This was a "feature" of early 175 transmissions, and was removed from later models.

Not sure of this helps with the 50. But it was a lessen for me to learn about the "not optimal" early 5 speed shift pattern.

You can see the pin and spring in the lower portion of the photo below.

Al Pritchard
Highlands, NJ

BS175 Racer

Offline JackA

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Re: converting 50/60 transmissions
« Reply #4 on: March 23, 2018, 12:43:35 AM »
Thanks, Al.  Your photo is much clearer than what the parts diagrams show!  However, the Tora shifter diagram seems to show that this rotary stopping function is done a little differently than in the 175, but I will definitely have to open up an engine or two and see how things actually fit together and move.  I have what is purported to be most of a BS50 Sport engine and what is purported to be the bottom end of a Chibi engine coming from eBay, but as we all know, people don't always know what they're selling, and in this case I don't know what I am buying, so I'll just have to dig in and see. 

Incidentally (although this comment probably belongs in the other half of this discussion, under Taka/Tora/Chibi, but anyone interested is probably looking in both places, as I did), the parts manual seems to indicate that the Chibi does not have a return shifter, contrary to what some of the advertising literature seems to say, but maybe this is just another feature that varied over time.

BTW, I have enjoyed going through your 175 racer thread.  I can really identify!  I hope to see you and that bike racing with the USCRA soon. 




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