Comet 20 or 30?

older then dirt

older then dirt

Active Member
#1
#1
Hi Guys,
I have a few bikes I'm working on at the moment. One of them is a little custom Rupp low rider I'm making from years of spare parts buildup.
It will be powered by a late 70's H35 with a 1" crank snout. I will be using the stock Rupp driven unit with a 12 tooth sprocket on the jack shaft back to a 54 tooth sprocket on the 10 inch stock Rupp steel wheels.
I have two comet clutches for the H35. A 20 series and a 30 series. I'm not all that familiar with the comet drive units and I'm wondering which of the two would be better suited for this application. I have an assortment of different belts I can try to fit the application.
The drive units line up well with the driven pulley so I'm hoping it will work out with one of the two units.
What is the difference between the 20 series and the 30?
 
Last edited:
C

cfh

Well-Known Member
#7
#7
the stock driven unit on a rupp is symmetric and will demand a symmetric series20 drive unit. but also the rupp unit uses a 5/8" belt. so a standard series20 won't work (because they are setup for a 3/4" wide belt.) what you really need is what's known as a TC88 drive unit. OR you can convert the driven unit to use a 3/4" belt. that would be some work and some parts, which may not be available.

it's just simpler to replace the entire system, or use a stock Rupp TC1 drive unit. technically a complete series20 system would fit right on. But i have heard that a series30 system fits on a rupp also. You will need the correct jackshaft, the rupp shaft with the nuts on the ends (opposed to the pins that keeps the parts in place.) Black Widow MS sells both jackshaft styles.

The advantage to a series30 is overdrive. series20 does not do overdrive. BUT if your engine/gearing can not handle that, a series30 will kill power at full throttle. if you're using a stock H35 tecumseh engine with stock jackshaft cog teeth (11 tooth?), i would stick to a series20 system.
 
older then dirt

older then dirt

Active Member
#9
#9
sparkwizard said:
Potentially, there would be a problem finding these oddball parts for the 1 inch PTO...
Click to expand...
That's why I'm using the comet unit. I have stock rupp clutches but they won't go on the 1 inch shaft. With the information gained here, it appears I have two 20 series units. They are both symmetrical. One is an actual comet unit and the other is a used eBay knockoff I got a few years ago that was advertised as a 30.
 
J

Jim Donovan

Active Member
#10
#10
If you decide to take the crank out and turn it down to 3/4" you have to have at least 2.300" in length, 3/16" keyway and drill and tap it to a 5/16-24 at least 1.25" deep. The good news about American crankshafts is they are not heat treated on the 4-cycle.
 
J

Jim Donovan

Active Member
#13
#13
The 20 series was for go karts while the 30 series was for mini-bikes but many people used the series 30 on go karts because they didn't want to build there own jackshaft for the driven unit. You don't have enough room on a normal size minibike for the series 20 because the driven unit has the ramps facing inward towards the engine. The series 20 runs a lot cooler on the belt because you are not forcing the belt up the inside ramp which is almost flat with very little angle on the fixed sheave.
 
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