Author Topic: '25 CW clutch rebuild  (Read 398 times)

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Offline franky

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'25 CW clutch rebuild
« on: 01 Jan 2024 at 06:46 »
 The flywheel clutch on my 25 CW has always been the problem child in my relationship with this bike. A friend accidentally broke the original die cast radial thrust bearing body neatly into two exact pieces. I tried a Torrington needle roller bearing, had to do some machining to get it to fit, but then there were often intermittent actuation issues. Added to that, the clutch would slip. In my home district there are so few actual level grades so this was always problematic. I tried different springs and after a new reline of the friction disc it was better but still there was the persistent slip on light grades. The presser plate was dented and bowed and the back plate was also very sad. Much time was soaked up trying to recover these two original components.
I had some 3mm and 2mm mild steel laser plates cut as a trial to see which would suit best. I believe the 2 mil plates, being lighter, would have done the job very well as they are closest in thickness to the original pieces. I went with the 3s knowing that there may be an increase of flywheel effect but considered that the three mil plates would be stronger.
The back plate protrudes about half a millimetre out from the flywheel. The only hand finishing that was needed was countersink work to bed the screws correctly and grind a mild curve on the outward side of the inner circle to clear the primary chain. This was done incase contact was made. If this does occur I can revert to the 2 mil plate.
Both of my plates are flat with no clever factory strengthening as evident on the original back plate which has an inward curve perhaps made using a die. The original presser plate was a piece of industrial art with the lovely waves stamped in to add strength without adding weight.
Fitting my presser plate was an exercise in patience and careful finishing. With not much spare room available in this area of the clutch and not being able to secure steel rivets it was decided to use nuts and bolts. There was a lot of assemble, breakdown, minor changes then refit. Time will tell regarding the integrity and service life of these components.
Moving on to my broken thrust bearing. Following a chat regarding my clutch my friend Graham took me out to his shed gave me a chunk of brass which was next to his old lathe saying see what you can find in there. Another old friend Brian arrived and then a plan was hatched to build a new one. Using the die cast original bearing as the template we were able to replicate the bearing case precisely including the lip which seats the clutch actuation arm. This project was complex to design. Each hole had to be drilled eighteen degrees apart using new numbered drill bits.
Following this the body was cut and the top lip was created. The correct drill depth was calculated so that each ball would drop to the correct level at the bottom of the case and then once pinned in on the top face this, theoretically, would be the same clearance at the top.
Due to the availability of the excellent piece of brass and the possibility that things could go pear shaped we made three cases.
The seating of the ball bearings by hand was tricky (read stressful) which required the making of a punch from a jewellers screwdriver set with just the right pin angle. Each dimple was hand pinned using a light hammer, and a huge industrial magnifier with a circular light.
I am still in the process of fine tuning the clutch, really is a work in progress. One thing is absolutely certain however, the clutch now has bags of grip and it is a delight to be able to go up my gently uphill sloping driveway. Heading up the street it is able to climb the slope on my street with no clutch slip.
The flywheel clutch was designed and created a century ago as a refinement by the Douglas engineers and it was a really interesting journey for me and my friends.

Offline graeme

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Re: '25 CW clutch rebuild
« Reply #1 on: 17 Jan 2024 at 06:54 »
Nice work Frank

These thrust races are well known to break, so this is great info for anyone looking to replace theirs

Cheers, Graeme