Author Topic: Four spring modification  (Read 1257 times)

Online Brian

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Four spring modification
« on: 08.06. 2015 07:52 »
The subject of clutches has come up on the forum many times and everything from belt drive conversions to fitting clutches from other marques had been discussed.

I thought I would take a look at the standard BSA four spring and see if it was possible to improve it without buying many parts or any complex machining. I had a good look at modern clutches and the six spring as fitted to the plunger models, a clutch which I personally think is the best clutch BSA ever made.

Anyway after looking at them all I decided the two main areas that differed were the radial needle roller lift mechanism and the design of the outer "pressure" plate. The pressure plate is normally splined to slide on the inner hub but on the four spring it isnt.

I'm hoping most will be able to see what I have done by looking at the photos. I have welded the outer pressure plate to the last steel clutch plate so as the pressure plate lifts it is guided by the last steel plate. I've also fitted a radial needle roller lift mechanism from a Suzuki, this came with a Suzuki clutch I had but I'm sure you would be able to get one the same or similar from most bike wreckers or off e-bay.

The result I have to say is a definite improvement. The clutch is much lighter to operate even though I have slightly more pressure on the springs. It will select neutral quite easily at standstill and shifting through the gears on the move is improved. My next project is to replace the indent plunger in the gearbox with a roller one.

This is not some amazing transformation but it is a definite improvement which adds up to a smoother clutch and gearbox operation.

Online orabanda

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Re: Four spring modification
« Reply #1 on: 08.06. 2015 08:46 »
I dips me lid; you are a clever little vegemite!

Offline scotty

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Re: Four spring modification
« Reply #2 on: 08.06. 2015 14:02 »
Nice work Brian  *clap*

What model Suzuki was the roller lift assy from ?

Do you think the same mod could possibly work on the six spring clutch as fitted to swing arm A's & B's  *????*




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Online Brian

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Re: Four spring modification
« Reply #3 on: 09.06. 2015 03:29 »
Scotty that one was from a Suzuki GS500 clutch but if you went to a wreckers I'm sure you would be able to find something very similar from a variety of bikes.

I had to alter that one slightly as the shaft was too large to fit inside the BSA gearbox mainshaft, just needed turning down.

No reason at all that it wouldnt work on the six spring.

Offline trevinoz

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Re: Four spring modification
« Reply #4 on: 09.06. 2015 22:34 »
I've done both but also modified the 4 spring to 3 spring.

Offline BSA_54A10

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Re: Four spring modification
« Reply #5 on: 24.12. 2015 10:43 »
The problem stems from the fact that the pressure plate is a pressing .
This pressing is done to a blank cut or stamped out of a sheet of steel that was delivered as a roll so it will want to curl.
the blanks got tossed into a stillage then lifted out and put into the press so the spring holes have a random orientation with respect to the grin of the steel.

As such, some plates will always bend while othere will almost never flex in use.
This explains why some go mad and still never get a clean lift while others will get a pefect lift with no problems at all.

The next problem is the lift itself.
You have a flat end of the push rod pushing on a point with nothing to support it or hold it strait.
Thus the use of the top hat push rod where the pushing surface is a flange, held perpendicular by a length of rod inside the mainshaft.
Simply fitting the radial roller bearing and top hat lifter, available from SRM will make a world of difference.
Bike Beesa

Offline v8ivor

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Re: Four spring modification
« Reply #6 on: 27.12. 2015 15:11 »
I have just fitted ( From a basket case ) A thick plain plate then 5 friction plates + 5 plain plates and the spring pressure plate ( 6 springs ) to my 1952 plunger ( Duplex chain with domed clutch cover type) Like shown on dragonfly parts list, where as the workshop manual ( covers all models ) Says to remove the pressure plate followed by 4 plain and 4 inserted clutch plates. But the 5 +5 cluster is to thick so can anyone see a problem with omitting 1 plain plate or a plain plate and a friction plate ? Also I don't think the friction plates are cork, so can I omit the dome cover.

Offline terryk

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Re: Four spring modification
« Reply #7 on: 27.12. 2015 15:46 »
Im not sure what clutch plates you have or who made them but I know on other BSAs that some plates are made thicker these days by different companies than original so yes other BSA owners do leave out plates with no problem. In saying that I guess the test will be with if the clutch slips or not. The 6 spring clutch used on B33s 500cc single had 4 and 4 the B31 350cc had 3 and 3. These weren't a good clutch but thats how they are.
I wouldn't leave the dome cover off the A10 plunger clutch unless you know the plate are made to run in oil because plunger A10 clutches were dry. Im not sure if there are A10 plunger plates made these days to run in oil others may know, I was told that there was but I haven't had confirmation. I always just run them dry with the dome cover on. I purchased an A10 plunger once that had new plates but the dome cover was left off and the clutch slipped badly.
1950-53 A10 rigid/plungers, 1958-61 A10 super rockets, 1947-50 A7 longstrokes, 1949 Star twin,
1951-54 A7 plungers, 1940s M21, WDM20s,
1948-50s B33s rigid/plunger/swingarm, 1948-50s b31s rigid/plunger/swingarm