Board index Flathead Power-Technical Questions, Answers, and Suggestions Morthodites-"Non Factory" transmission choices for K custom

transmission choices for K custom

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Post Fri Dec 07, 2007 5:13 pm

Posts: 217
Location: Palo Alto, CA
Hi guys,

I have been doing a bit of searching for transmission choices for this K, and would like your opinions if possible.

My initial thoughts were that the simplest choices would be:

1. Use the transmission section from another set of donor cases as Tim did (LHS clutch, RHS drive)
2. Use a Harley big twin transmission (LHS clutch, LHS drive, although there may be RHS drive options available?)
3. Use something like a BSA A10, or even ES2/16H style box (LHS clutch, RHS drive).

Without too much thought at that point, and having briefly seen them in the past, I wondered if a cassette-type box from a Japanese sport 250 might not be usable. Similar hp rating, cheap, dry clutch, 5 or 6 speeds I think?, and compact, lightweight quality with further race ratios available should the bug keep biting.

I hunted down some pictures of a late 80s box:



If those don't work, here's the link:

After an initial inspection and much mental gymnastics it appears to my untrained and inexperienced eye that this is not feasible. It has a RHS clutch and LHS drive arrangement that I can't see would work unless the whole things was flipped over.

One can see the Norton box used here in Stellan Egeland's impressive "Hustler" bike:

Can anyone suggest any other interesting options?

Post Sat Dec 08, 2007 12:22 am

Posts: 1654
I'm unclear what you are aiming for here, but of the question is 'what transmission can I put behind a cut-off K type motor' as I think it is, what about a late Norton gearbox with whatever clutch you can find to go with it? You can get 5-speed versions - there is even a 6-speed version from some specialist suppliers although these are rare and expensive )

a properly overhauled Norton or AMC box and clutch will handle anything a streetable K model will produce. If you are going for a separate box then you would be best advised to have left side clutch and final drive because the torque loadings from the input and output sprockets tend to cancel each other out. Otherwise the whole stress of keeping the transmission aligned falls on the mountings.
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Sat Dec 08, 2007 5:03 pm

Posts: 217
Location: Palo Alto, CA
Thanks for the guidance. Yes, that is the advice I'm after. Do you have any further info about applicable Norton models - especially the 5 speed? An upright-style box would be good to keep things compact. Application is street.
How do these Norton boxes shift - is it a fairly positive, short-travel shift? Just curious, as I've never ridden a Norton.

Post Sun Dec 09, 2007 4:51 am

Posts: 1654
the Norton gearboxes are all interchangeable up to the very last models, the early 70s 850cc models. They are mostly four-speeds, some late ones have five, and as long as you get a gearbox and clutch together, you will have no problems. Basically you want the latest unit you can find.

there are various aftermarket 5 and even 6-speed cassettes and diaphragm clutches available, depending on what you want to spend, they are mostly produced for the Classic Racing market and priced 'according'

beware of belt drive conversions. Most US-made kits use a different belt from the 'syncroflex' used on British kits, so you will end up fabbing a pulley and may need a special length belt. If you go for a chain you will need to fab a sprocket - if you are using a duplex chain make sure the chain is the right length before you finalise your engine and transmission mounts.

the Norton box is a good unit by the standards of the day. If you are used to Big twins it will seem very quick and light, it is about like a Sportster ironhead. By Japanese standards it is a right old bag of nails!
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Sun Dec 09, 2007 4:23 pm

Posts: 217
Location: Palo Alto, CA
45brit, thanks for the advice - I will definitely consider the Norton box, especially the 5 speed variant. This would seem one of the simplest options.

Playing around that much with custom belts/pulleys etc, it seems almost as much effort as to machine up a LHS transmission access cover/shift drum/bellhouse to utilise a sport bike's gear cluster. Does anyone have any experience with this?

If this bike has one theme, it's going to be that of experimentation with parts that are non-standard or have already been messed with in some way, or those considered leftovers from other projects. So I will wait and see what comes my way.



Post Mon Dec 10, 2007 12:12 am

Posts: 1654
I have just fitted a belt drive conversion to my 45 with British gearbox and it was simple. All ( all!! ha ha ) I had to do was buy 2 pulleys, one to fit the crankshaft and one to fit the belt; cut the centre out of the one that matched the belt with a rotabroach, mill down the one that fit the pulley to suit and weld them together. Any horizontal alignments issues are accomodated at this stage

check belt sizes, you don't need one of those huge belts like a potato-grader which some Big Twins have, for purely fashion-driven reasons. You will probably find that a 40mm belt is quite sufficient.A 30mm belt will handle 50bhp under racing conditions

you might even find that you can buy a clutch drum to suit in the US . Most belts over here are syncroflex type, US ones like harley aren't.
Shoot, a man could have a good weekend in Dallas with all that stuff...

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