Hi All,
Here is my attempt to put together a guide to primary chain alignment issues that arise when fitting a 4 spring clutch to BSA A7/10 swing arm models and also issues arising when building up with parts of unknown origin
(Some information will refer to B series models)
There are three main considerations to address,
1) primary chain alignment
2) sliding plate to shaft adaptor fit and clearance
3) primary case / primary chain clearance problems.
It would seem that some replacement clutches/adaptors sit farther out than others!!!
The shaft adaptors can vary in the depth that the internal taper is machined to.
This affects where the clutch chain wheel will sit in relation to the engine sprocket. It will also affect the depth where the scrolled portion fits into the sliding plate
Sometimes a bought in clutch will fit and line up “just so” more often than not it will not!!!
Clutches for single cylinder models need to sit farther onto the gearbox mainshaft than twins
The single cylinder models inner primary case position allows for this.
I will leave it to owners to decide which way they decide to adjust the alignment
If alternative shaft adaptors can be obtained or change the engine sprocket position?
I have measured up some components that I have to hand to provide some figures for comparison to reader’s machines
Firstly I measured how far a couple of gearbox mainshaft protrudes through the adaptor.
The original adaptor fitted on varying amounts on three different original mainshaft’s which leads me to the conclusion that the shaft tapers differ as well!!! Unless the shafts vary somewhat in overall length??
“Original” A10 adaptor, showed 14.3 – 16mm, thread protruding depending on which main shaft it was fitted to, almost the same as a 6 spring adaptor
A pattern adaptor went on about 0.6mm less than original A10. (This is one of the adaptors where the scroll is cut in the wrong direction that were on the market )
Pearson Gold Star adaptor + 2.5mm more than original A10 (no photo of this)
Before checking the primary drive alignment make sure all the engine and gearbox to frame mounting bolts are fully tightened.
It is also worthwhile to check that the primary chaincase mounting face on the crankcase is parallel to the gearbox mounting bottom plate, a straight edge and steel ruler will be good enough for this , If out of line loosen the engine / frame bolts a little and lever the engine in the required direction ..
Trial fit the adaptor and chain wheel etc onto the gearbox shaft and use a straight edge to check the sprocket alignment
It’s a lot easier to use a straight edge on the rear face of the sprockets with the inner primary case removed
The other factors that can vary are the engine sprockets and crankshaft cushdrive sleeve shoulder width
For A10 models I believe the shoulder on the cushdrive sleeve should measure0.312in.
I have a range of engine sprockets, which vary from flat on the rear face to one with a shoulder of 0.100in.
If required shims can be added between the crank spacer and cushdrive sleeve if the engine sprocket needs moving outwards a little.
Large adjustments here will compromise fitting the cushdrive nut!!
I would advise against fitting any shims between the sprocket and cushdrive sleeve shoulder as these parts move relative to each other
The cushdrive sleeve must extend past the end of the crankshaft spline so as the nut can clamp the assembly solid through the main bearing to the crankshaft face.
The crankshaft cush drive nut should be tightened to 65ft-lbs, I use some Loctite threadloc on the threads rather than the lockwasher
The photo above shows the limit of the cush drive movement, (no spring fitted for clarity)
The primary chain needs to be a very good quality “standard” chain
“H” (heavy duty) chains are wider and can rub on the chaincases
It may be necessary to adjust the inner primary case position,
Check if spacers/washers have been fitted on the lower rear inner case to frame mount
Remove this mounting bolt and any spacers temporarily,
Fit some temporary 5/16whit bolts in place of the front primary to crankcase screws
This will keep the inner case square and in its proper position, the same as when the outer has been fitted
Trial fit the shaft adaptor of the new clutch on the mainshaft to see where it sits in relation to the sliding plate and the amount the mainshaft protrudes through it
If the adaptor sits well into the sliding plate, without being tight against it then all is well in that regards,
If the inner case position needs adjusting to clear the chain or shaft adaptor, gaskets can be added or removed between the crankcase and inner primary case,
Once the inner primary case position is finalised add spacers or washers as needed to the lower rear inner case mounting bolt, do not strain the inner case.
The pattern sliding plates available are to my mind poor quality; the hole for the scroll is usually non-circular and or oversize,
The originals were definitely better but still quite a loose fit on the scroll
I have resorted to a couple of different methods to improve the sealing,
including a machined close fit bush silver soldered to the plate,
or a machined up steel ring that holds an X ring seal
Recently I read that oil can leak between the plate and the riveted in centre piece,
Yes it can!! Some sealant pushed into the v gap may help
The “Pearson “ or similar sliding plate with a proper seal and bush are probably the best remedy???
I found when I first built my SR that with the original shaft adaptor fitted, that a flat inner faced crank sprocket lined up with the clutch, however in this instance it caused another problem with the sliding plate being tight against the adaptor and lack of chain clearance!!
I fitted a thinner spacer between the crankcase and inner primary to overcome this issue
Some years later I wanted to increase the overall gearing and also to try and improve the clutch,
I fitted an adaptor bought from SRM which has a thrust washer arrangement like the A65 models,
This adaptor does not have a scroll machined into it, a felt washer is fitted between the adaptor and sliding plate to act as a seal
At the same time I fitted a new crankshaft sprocket, supplied by SRM but Wassel brand. This sprocket has a shoulder on the rear face (I forgot to measure it??)
Either by chance or good measure the primary sprocket alignment turned out perfect!!
I then had to refit the original steel spacer between the crankcase and inner primary
To take advantage of the thrust washer set up on the SRM adaptor a clutch friction plate is fitted at the rear of the clutch basket, to get this to fit I had to file the slots in the basket deeper so as the plate would sit against the inner face
The inner clutch drum will need the lip at the inside removing or what I did was to machine out the centre hole in the rear friction plate to fit over the lip
An extra plain clutch plate was needed in my case to make up the plate pack thickness
A homemade clutch locking tool made from some old plates bolted together and to a length of steel can easily be made ,
I use some Loctite threadloc on the centre nut threads and tighten to 65ft-lbs
HTH
John
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