A larger gearbox sprocket?

[muskrat]

G'day GB
I'm pretty sure there is only one size engine sprocket for the plungers. Both A10 and A7 use 27 tooth part# 67-2050.
For the g/box sprocket the whole lump must come out and G/box removed from the motor. 18 is std for A7 and 19 for A10. The only change you can make is going to a 20 or 21.
The only other way to change the gearing is to turn off the rear sprocket from the drum and weld (or scotch key) another of a smaller size on. I have done this on my cafe and A65.
Cheers

[duTch]

 Like I said, that's my first choice partly because of what Kiwi said,

Speedo is of course unaffected by changing the engine sprocket.

,  but if you have an 18T, your speedo will be out of whack anyway, so sooner or later you'll have to change the G/box sprocket anyway, might be worth splitting the difference...go up oneT on each(one at a time)??

[Greybeard]

After reading through the previous comments I began to doubt my counting of 18 teeth on my GF gearbox sprocket; it's not easy to see, let alone count its teeth.
I put the long pipe nozzle head from a can of WD40 onto a can of gray primer paint and carefully spray painted one of the tooth valleys so I could see when it came around again. I definitely counted 18 teeth! I am planning to do some work on the 'ole girl this winter so I will get myself a 20 tooth gearbox sprocket.

...and a new chain!

[Klaus]

I have 21 T gearboxsprocket on my RR but with a tuned engine. Easy run with 4000 revs and scratched 75 miles. But at top gear 5500 is the end.
If you get one teeth more at the gearbox, is that the same as you count 3 more at the rear.
The racer is more tuned and lighter with fearing, so l get a 22 T sprocket.
All sprockets are old ones, welding up with blanks.

cheers Klaus

[bsa-bill]

I went along this route when building up the RGF and someone was good enough to post a chart of engine, gearbox rear chain sprocket sizes and radio's.
I have ungraciously lost it, but it should be in here and maybe the member who posted it might recall where it is or some one with better search skills might find it, it  might not apply in the same way to plungers though

[muskrat]

MG did this one     http://dl.dropbox.com/u/40922180/RPM_speed_A10_A7.xls
Adrian did this one    http://www.t20suzuki.com/cz/BSA_gearing.xls
Richard posted this one   http://victorylibrary.com/graphics/A10-ratio-chart.jpg
Cheers

[bsa-bill]

I found it - had it stored up in a cloud  (hows that for getting down with the kids)
nice little  sheet allows you to change the number of teeth on any of the four sprockets, wheel circumference and gives a final ratio, also speed at revs down the side

Right the problem is it's a Spreadsheet, needs Exel or open office would do it to, but how do I put it into a reply as a executable file ?
 

[Greybeard]

Just paste a link to it here.

This should work: https://www.dropbox.com/s/juonla0zuzjust9/Test.xls?dl=0

[bsa-bill]

I don't have a link as in website for the sheet, it's been a download at some time in the past but can't find it.
I've pasted the gearratiocalculato sheet into the test sheet but not sure how to proceed further can you get it from your dropbox.
If I try to save it it seems to do so but only to my PC.

[Greybeard]

I thought I'd dig out the rear sprocket that was on my GF rear wheel when I bought it; it has 49 teeth. So my machine used to have a final drive ratio of 18 to 49. I reckon someone must have hitched up a gigantic double adult sidecar; or used the bike for mountaineering!

I'll put the sidecar rear sprocket on the 'For Sale' posts

[bsa-bill]

Cheers guys, bear with me this saved from ods as xlx file but I find usually works

ah you need to click on the "enable editing" box - hope that's it

[Topdad]

Hi Bill ,just checked out mine and assuming  ( dangerous for me ) that clutch, gearbox and rear are 43,19and 42  ,to which I have added a 21 tooth engine sprocket I get a reading of approx. 2250 revs at 50mph which equates very much to what I reading through my standard sensory monitor ( ie my big backside .) Means cruising at 60 is very comfortable , thanks Bill.

[Greybeard]

Thanks BSA-Bill,
That is very interesting. Makes me think that if I change from an 18 to a 20 tooth gearbox sprocket my current 65mph should become 73mph; which would be nice!

[Tomcat]

I thought I'd dig out the rear sprocket that was on my GF rear wheel when I bought it; it has 49 teeth. So my machine used to have a final drive ratio of 18 to 49. I reckon someone must have hitched up a gigantic double adult sidecar; or used the bike for mountaineering!

I'll put the sidecar rear sprocket on the 'For Sale' posts

Wow Greybeard, I like your outfit! Ive never been a sidecar fan, but your's is one I'd love to get my hands on! 

[Rocket Racer]

I thought I'd dig out the rear sprocket that was on my GF rear wheel when I bought it; it has 49 teeth. So my machine used to have a final drive ratio of 18 to 49. I reckon someone must have hitched up a gigantic double adult sidecar; or used the bike for mountaineering!

I'll put the sidecar rear sprocket on the 'For Sale' posts

I suspect artists licence as the door should have been on the outside 
and more probably towed.

Unless running a chair I tend to stick with stock gearing, as BSA had a good idea what they were doing 
Over gearing typically bites you when you have headwinds and hills to deal with and effects your acceleration when passing.

If I was doing both sprockets I tend to fit B series sprockets which come in more modern 520 chain width good enough for most high performance moderns and giving more chain options.