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Discussion Starter · #1 ·
Hi all!

Does anyone know if there is any difference in a pre-unit swing arm motor / trans and a pre-unit rigid (pre'54) motor / trans?

I know the primary cover is different... but what about the other stuff?

Thanks!

Sam.
 

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SamIyam said:
Hi all!

Does anyone know if there is any difference in a pre-unit swing arm motor / trans and a pre-unit rigid (pre'54) motor / trans?

I know the primary cover is different... but what about the other stuff?

Thanks!

Sam.
I was thinking there was a difference, but can't think what it is off the top of my head. There is some differences in the swing arm transmissions for sure. In the first few years of the swing arm pre-unit they had a standard type 4-speed, but then I think in 58 or 59 they came out with the "slick shift" 4-speed which had a little cam in it to actuate the clutch automatically in every gear except 1st. This was seen mostly on the Duplex bikes that came in 1960, but were on earliers as well, I think. A lot of people disable the mechanism (including myself) because it really didn't work too "slick" :rolleyes: (pardon my pun). The guts were mostly the same though. The biggest differences were in the mounting and the primary drive. Maybe someone can go into better detail about the differences.

One other thing I thought of... I think SOME of the slick-shift transmissions actually came with needle bearings and thrust washers instead of bushings, but it was also common to make the conversion as well.
 

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The differences are LEGION:

Gearbox. very different, incompatible swingarm to rigid without cutting and welding. Lugs top and bottom are different between the SA and rigid.

All preunit motors will fit all pre-unit frames, but when you have a later set of alternator cases, you need to think about how to run your lights on a rigid frame. I had a 1961 motor in a 1951 frame once and I had the very rare alternator primaries for the rigid bikes. so I could put in a modern voltage regulator and alternator.

If you have generator cases, then you need a generator or a set of alternator primaries and an alternator.

Up to 1954, the PU's had "small bearing" crankcases which mean they had just that a smaller timing side main bearing. '55 and up "big bearing" cases are preferrable and retro-fit to rigid bikes. Small bearing cases fit later swingarm frames just fine.

Primaries for rigids are longer and do not fit swingarm frames, swingarm primaries are shorter and do not fit rigid frames. Alternator primary covers for swingarms fit all swingarm frames, but alternator primary cases (very rare) for rigids won't fit swing arm frames.

Crank shafts are different early to late as well. Three piece cranks are a pain to clean and bolts that hold them together are expensive. Later one piece cranks are preferrable. All the compensator sproket stuff is not necessary if you use a later crank, but you need the different motor sprokets and you need a cush drive clutch center.

I could go on and on. . .
 
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