Clutch for racing

Restoring my Model 50 for racing is just about finished, just an exhaust to make to comply with current regs in the UK. My question is about the clutch.
In the early 90's I remember taking the clutch off and trimming the chain wheel, putting a band round the outside and also drilling the clutch centre in between the contact point of the plates. On fitting the clutch it would only take three friction plates (not counting the inserts in the chainwheel) not the 4 that were used when racing. Measuring the centre the width is approx 30mm where as an old very worn centre I have is 35mm and would take 4 friction plates. There is no way I used the worn out centre and I can't find the original used for racing.
1) would 3 friction plates be enough for the power of a model 50 that late 80's was turning out similar power to 7R's and 350 Manx's?
2) I can buy a wider centre for a reasonable price from RGM (https://www.rgmnorton.co.uk/buy/dominator-clutch-hub-centre_888.htm) but they say its for the later clutch that had a plain chainwheel with the friction material in the back plate. Looking at the picture I can't see why it wouldn't work? Any suggestions please?
I will be building a 500 at some point and will go to a belt drive but at the moment the money is not there to buy one so I just need the cheapest option that will work. The worn out one is way past filing the grooves out.
Thanks for any suggestions.

On my -59 40M clutch had approx clutch centre height 28mm, hard to tell without taking apart. Plain chain wheel. 3+ one single sided friction plates, dogs inside. 3 plain plates dogs outside. It had the usual omitted roller cage, filled with lots of rollers modification. As it was in poor condition a new pricey Manx clutch from AN is now fitted. Uses the weakest springs I have. Have never experienced any slip for the last 5 seasons.
The old clutch was fitted with 6 Allen screws holding centre cover plate, not countersunk. So there might be space for a wider centre.
As the 350 and 500 Manx has the same clutch, I can't see any need for 4 plates. Hopes this helps.

Mike40M said:

On my -59 40M clutch had approx clutch centre height 28mm, hard to tell without taking apart. Plain chain wheel. 3+ one single sided friction plates, dogs inside. 3 plain plates dogs outside. It had the usual omitted roller cage, filled with lots of rollers modification. As it was in poor condition a new pricey Manx clutch from AN is now fitted. Uses the weakest springs I have. Have never experienced any slip for the last 5 seasons.
The old clutch was fitted with 6 Allen screws holding centre cover plate, not countersunk. So there might be space for a wider centre.
As the 350 and 500 Manx has the same clutch, I can't see any need for 4 plates. Hopes this helps.

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Thats really helpful thanks. Proof will be trying it but don't want to get to a track and it's slipping. Have to take it up the road when no ones looking (steep hill).
What is the mod with the omitted cage? Is anything else needed except for fill with rollers?

Checked my old messy spreadsheet on clutch springs. Long time ago, but I think that the 65 lbs springs from RGM will work. They also have 85 lbs springs. I think I used the springs from the old clutch which was the weakest I had.
Reason for my spring problems was that the AN springs was way to strong. Because I have a TTI box. Lever pulling force which also means clutch rod travel differs with different gearboxes. Could be described as gearing between lever and rod. The laydown box has easiest pull, shortest travel. AMC boxes are harder. longer travel. Commando AMC boxes even harder, more travel. TTI boxes worst of them all. As usual lever pivot to cable nipple centres distance matters. So a lever with a shorter distance will give easier pull and less clutch movement, A 3 plate clutch don't need much opening to disengage.
So for me weak springs works ok. Any springs which gives a decent lever pull will probably work for you.
Hope this is understandable as English is not my native language.

Charkmandler said:

What is the mod with the omitted cage? Is anything else needed except for fill with rollers?

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As far as I know nothing but lots of rollers. Guess it makes clutch basket less wobbly. Also less contact force between individual roller and race. But rubbing between rollers. As a dry clutch has no lubrication on that roller bearing, you don't want much grease there, slipping risk. Think more rollers will minimize wear. More trouble fitting. Just guessing.

I use an early Norton clutch with my 850. It looks the same as a manx clutch. It has four friction plates. It never slips. Those clutches have been used on 1000cc Vincent drag bikes. My brother has a 600cc Jawa speedway sidecar - he uses a Barnett clutch. They are made out of aluminium and are much lighter.

My primary drive is single row primary chain. Floating Jawa speedway engine sprocket on carrier, and Norton clutch. If you hit the chain with chain lube, it will last forever. Wire the clip ! The rollers in the basket bearing never wear. The mainshaft in the TTI box is the reason for the floating engine sprocket - Had the carrier made. The taper on the Norton crank is 5 degrees - a Jawa sprocket carrier probably has that taper. The sprocket only needs to float 5mm.

acotrel said:

My primary drive is single row primary chain. Floating Jawa speedway engine sprocket on carrier, and Norton clutch. If you hit the chain with chain lube, it will last forever. Wire the clip ! The rollers in the basket bearing never wear. The mainshaft in the TTI box is the reason for the floating engine sprocket - Had the carrier made. The taper on the Norton crank is 5 degrees - a Jawa sprocket carrier probably has that taper. The sprocket only needs to float 5mm.

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I made a crank in the late 80's and also the mainsheets. The drive side taper was made for the Jawa carrier. Do you restrict the float of the sprocket?

If relying on Andover Norton (usually not always reliable) they seems to list the same clutch centre for all slimline including 50, except a special one for Manx. They say that the 50 has 3 plates. So if you rely on AN it should work with a wider centre and 3 plates. If it don't work, an extra inner tabbed plate closest to the alu pressure plate will shim it.
But RGM do offer a 29 mm aluminium centre for Manx. I'm not certain the longevity though. Lots of wear usually seen on clutch centres.

A taper engine sprocket works ok on a road bike. But on race bikes you need to quickly change engine sprocket to adjust gearing for different tracks, The Manx has a splined sprocket, not floating. Had to shim it for chain line up.
Changing gearing especially needed on speedway where changing weather (rain) makes it necessary to change in less than 2 minutes.

Charkmandler said:

I made a crank in the late 80's and also the mainsheets. The drive side taper was made for the Jawa carrier. Do you restrict the float of the sprocket?

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No I don't restrict the float , but I haven't really used the set up in anger. I cannot see how it can go wrong. The sprocket can only move about 5mm sideways. And I have been in a club road race where there was a Jawa speedway bike. Jawa sprockets are cheap, but in recent years, the two circuits I raced on required the same gearing. I did not have the quick-change facility back then. I think I had it, when I almost stuffed the bike into a fence at Winton, so it must have copped a very extreme load, and did not fail. I haven't hardened the carrier.