#35 double row chain is not all the same

[Brandis]

Turns out that the pitch of the rollers shrank a bit in the last 75 years.  I didn't notice it on the drive sprocket, but on the clutch side, it's pretty obvious.  I started with an old 80 link chain on Ebay and it's correct, but it is one offset link too short.  A normal link adds a roller looks like it will make it too long. 
I assume BSA shipped bikes to America with a higher gear ratio.  Long, straight, open highways and all that.
So here's the question. Should I look for a one tooth smaller drive sprocket or a correct offset link with the old style thinner pins?  The old and modern links are not interchangeable.

[trevinoz]

As far as I know, there is only one size sprocket and chainwheel but can be duplex or triplex.
I have bought chain in 10 foot lengths and cut to size and 80 links is correct.

[Brandis]

Well, bought a new length and connecting links, but the pitch of modern chain a wee bit shorter.  And I had to shave the sides of the sprockets because the new chain is narrower.  If all BSA sprocket were the same number of teeth, then I need an old offset or 3 roller link.

[Brandis]

And I just discovered that the pitch on the rear sprocket is slightly different as well.  Funny because the modern chain that won't fit was perfect on my '51 Matchless.

[Swarfcut]

  Semi Unit rigid and plunger bikes had the same duplex chain and sprockets, from start to finish the exception being the Super Flash with Triplex Chain chainwheel and engine sprocket to suit. Strangely these bits are not in my 1953  Catalogue which lists Super Flash parts.

 For standard engines

 Engine sprocket    27 teeth.

 Clutch drum         54 Teeth

    Chain,  Duplex  80 links.  Imperial size ....Pitch 0.375"   Rollers 0.250" diameter, Inner measurement between the plates 0.225". So says Roy Bacon's book.

 Sourcing the correct chain, new or used and it should all fit back together. Shame about the sprockets you've modified, Wear to sprocket teeth is also something to consider, it's always better to go by the book rather than trying to get a modern chain to fit. Err...You did slack off the chain slipper adjuster?

   Swarfy.

[Brandis]

As always, thanks to Swarfy.  It's Sunday and there's no Saturday beer talking.

I have messed with a lot of old Brit bikes and always took the chain for granted.  So, imagine my dismay . . . .
I'm going into this situation with the obvious knowledge that I'm an idiot. 

First, the primary chain.  When I lay the chain on the bench side by side it appears to be the same length.   My machine shop had shaved the sides of the sprockets to make new chain fit.  When I fitted it with the offset link, and tensioned it, I couldn't keep the tension constant.  it would be too loose or tight as I spun it.  I have now come to the conclusion that the machinist did not shave it equally over the entire diameter and that it hangs upon the the thicker teeth.  So I need an offset link for old chain or back to the machine shop for new chain.

Rear chain is similar.  Old Brit and new Japanese chain side by side appear to be the same, pitch and width.  It looks like neither chain slides down far enough over the sprocket and so the links appear to be too short.  I know there were a lot of weird issues with the first 500 bikes made and that production was stopped to fix them, but I can't imagine that they made the rear sprocket to fat and found chain to fit.

[Swarfcut]

 New chain on old teeth can never be perfect, down to the differing wear pattern. Sounds like a case of the original parts having a fair amount of wear on the teeth. Tight spots are to be expected, wisdom was always to tension the chains at their tightest point.

 Final drive chain is again imperial,  0.625" x 0.400" x 0.380"  Length varies according to frame type and the gearbox and rear sprockets fitted. 100 links on Plunger A10, 19/42, rigid and plunger A7 18/46 it's 102 links. Gearbox sprockets are subject to wear, in extreme cases the teeth hook. This sprocket is the same fitting on all A series gearboxes, Rigid, Plunger S/A. However early versions have a scroll on the back of the sprocket, in place of a conventional seal. If replacing the sprocket with a later type, fit with the later type oil seal, it's the same across all models and years. If sourcing a replacement rear sprocket, take note of the width of the teeth. Lower powered models in the B range use the same basic part, but the teeth are narrower in width, not mentioned by some shady sellers.

 Swarfy.

[Brandis]

I fixed the rear chain problem.  Remember that this is a bike with 12K miles on it so wear seemed unlikely.

The more I messed with it, the more I was convinced that the rear sprocket was too thick.  I can't explain it either.  I lifted the wheel so it spun freely.  Measured the width.   Then I got a small angle grinder, spinning the wheel I touched the outer edge.  Soon the friction of the grinder kept the wheel spinning. I stopped when I had taken off .010. 
The new chain wrapped around perfectly.  Did the usual tension adjust.  I never saw a smoother spin.

As for the front, I've put out requests for an offset link or 3 piece link for the older chain.  I nothing comes of it, I'll go back to the machine shop, do another shave and then fit the new chain.

I have no idea why 3 of the BSA sprockets were fat. I guess it's the curse of the first 500.  The Burman on the transmission was perfect.