This has probably been covered before, for which I apologise.
I'm contemplating splashing out on a new Triumph four spring clutch unit to replace the current six spring unit on my ZB32.
All the ads for the four spring units state specifically "for S/A bikes" while mine is obviously a plunger-framed bike.
The mainshaft on mine has both a key and a taper, while my memory (from a long time ago!) is the the Triumph mainshaft is taper fit only.
Judging by the numbers of bikes now running a four spring clutch there obviously is/was an adaptor of some sort available. Anyone know where from?
Any other issues to be aware of with this conversion?
I don't know too much about the ZB32, but I have a 1954 Plunger frame M33 and I bought and fitted a 4 spring clutch to it. I got mine from Kidderminster Motorcycles; it works a treat and the kit includes everything you need to fit the clutch, though you'll need a few gaskets too.
You'll have to phone them up to ask as they don't list what they have online (perhaps that's why you couldn't see them in a search), but you can find them here; there's a phone number on the site: Kidderminster Motorcycles (https://www.kidderminstermotorcycles.com/)
Adaptors here: Feked (https://www.feked.com/bsa-4-spring-clutch-hub-adaptor-standard-taper-a-and-b-group-models-circa-1954-63.html) & LP Williams (https://www.triumph-spares.co.uk/bsa-clutch-adaptor-hub-large-taper)
But I believe there were 2 different tapers so you will need to check.
The Triumph is taper and key too, as per the BSA, the taper is different on a Triumph and the roller hub will probably need to be specific for the conversion (the BSA hubs need to have the oil flinging scroll for the sliding plate, or a modded sliding plate). I had two hubs, each slightly different offsets and gave one to a local GS owner to try (a Suffolk wheeler dealer it would now appear) who has neither acknowledged whether it worked for him nor returned it. You would need to experiment with spacers for the chain alignment as I am not sure how spaced out along the mainshaft taper either the A10 version or the conversion version will fit so some experimentation is likely.
If I can share my (limited) experience with these:
I ran a standard 4 plate 6-spring clutch on my touring spec ZB34 without issues for a while. When I dropped the ABSAF600 engine in the same bike together with the Nova 5-speed conversion I had problems with slip under heavy load (even when adjusted correctly). I upgraded to 5 plate (deeper) inner basket from Dragnfly but the spot welds popped and my inner hub cracked on the keyway leading to loosening and the clutch coming off! Fortunately no damage done. I upgraded that bike to the NEB clutch, still with chain drive (vs dry belt) using the 'pink' drive plates. So far I've experimented with ATF and TQF and it drags slightly from cold so next I will try the same oil I use in my Competition 500. I ran 500 Daytonas for many years with the Triumph 4-spring clutch which always needed freeing off from cold and could occasionally drag when stuck in hot London traffic, so not a panacea for all clutch woes.
The Competition is running the original ZB34GS touring spec engine (7.5:1, 2450/2448 cams) with a rebuilt 5 plate, 6-spring clutch. This uses Surflex plates (although still 'cork' chainwheel) polished and flat plain plates set up as per service bulletins. I use Castrol 4T 10/30 semi-synthetic oil (same stuff modern bikes spec.for shared engine/gearbox/clutch) and somehow the clutch is working absolutely perfectly. No slip, no drag even when properly hot. Box is a STD T in good nick.
So it is possible to get one of these 6-spring clutches perfect (maybe a total fluke, never to be repeated) and I caution you that the alternatives are not necessarily perfect either.
I got my replacement 4 spring clutch for my 1949 M33 from Kidderminster and it fitted with no problems.
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Thanks for all the in-depth replies. Most helpful!