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According to Bacon, all A65s had 28/58 engine/clutch sprockets. By '71 the gearbox sprocket was 20 teeth. The A70 had 21 teeth. 47 teeth on the wheel except for the FS which had 50 teeth.
The opening in the primary for the gearbox sprocket was cast for the 21 tooth but usually had to have the flashing filed to clear the sprocket.
The T140 clutch basket is not the same as the A65. Both have triplex chain but the A65 is dished, the T140 is not. If you use a T140 clutch basket you need to space out the engine sprocket and the basket will probably hit the primary cover.
Not advisable to use a master link primary chain as they are not as strong as a riveted.
 

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I have never noticed the dishing ,so assumed it was as a Triumph clutch as the centre and plates are. The linked chain was on the bike when i got it 10 years ago and was fitted 15 years prior to that. I did take the link out once when changing the sprocket and it is hard to re-align. I was surprised it had a removable link. Some good information, thanks.
 

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Hi Mick, If you have 28t crank sprocket, I'd recommend 21t for rear chain.

I've ridden very few BSA. That explains why the seem so buzzy. Reving higher than most Triumphs at 60.

I must have miss read the manual I was looking at. Sorry to have caused confusion!

Looking at shop manual Classic British Spares I see a gear ration formula chart. I'll run some numbers when I get time. What is rear wheel size, & rear tire type, size?
Don
 

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Discussion Starter #24
Yeh Don , I reckon the 21 would help heaps

Goin from the 18 to the 20 changed my revs at 60mph from 6000 to 5000

Reckon a 21 would drop it down to 4500

Much more relaxed

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According to D Madigan info above, the BSA primary ratio is 2.07. Triumph unit engines are 2.00...Very slight difference ,maybe 30 rpm at 60 mph with the same final gearing?. A BSA with a 20/47 final drive is turning about 3800 rpm at 60 mph with a typical 18 inch rear tire...No way is it 5000 rpm...
 

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Discussion Starter #26
Then something is very wrong

Tacho or Speedo out maybe , as has been suggested

Or maybe I'm wrong about the clutch slipping but it does power up hills no problem

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Hi Mick,
Could I try the tacho and Speedo off my t140 ?
Speedo. yes, tacho. no; BSA used a different gearing from Triumph (3:1 vs. 4:1?).

Off the top of my head, might that be the problem at the moment - Triumph-geared tacho. being driven by a Beesa drive?

Hth.

Regards,
 

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Don't know if this would work- at least to confirm your tachometer?
A phone/satnav can check speed as can a mate following

Know anybody with a strobe that reads HT pulses?
 

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Internet say A65 tach is 3-1 ratio....Early unit Triumph is 2-1 ,then changes to 4- 1
 

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I have a Triumph pair of clocks on my Spitfire. It is doing around 8000 rpm at 120 mph. I bought these by mistake a few years ago and both run a lot higher than the BSA instruments. 60mph indicated is around 40 mph real speed.
 

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Then something is very wrong

Tacho or Speedo out maybe , as has been suggested

Or maybe I'm wrong about the clutch slipping but it does power up hills no problem

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you can tell whether the clutch is slipping by riding along a flat road (or better, up a slight grade) so that th emotor is pulling.

without backing off the throttle pull in and release the clutch lever momentarily.

if the engine suddenly winds up and then immediately catches and is pulled back down to the prevuious rpm, then the clutch is okay.

if the motor winds and then only slowly returns to th eoriginal rpm then th eclutch is slipping.

another check is to suddenly crack the throttle wide open at a lower gear. if the engine winds up without the bike speeding up proportionately then th eclutch iss slipping.

if your clutch is slipping badly enough to show a 1000 or so difference on the tach i imagine yuo d be smelling it by now.
 

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Hi,
I have found the A65 (Hornet) a little different to set up when compared to a Triumph, the engine revs easily and the power comes in high in the rev range, but there seems to be a lot of intrusive vibration at high RPM making it unpleasant to ride fast, (at lower RPM it is great). The gearbox ratio’s are strangely spaced, with 3rd and 4th almost the same and a Grand Canyon size gap between 2nd and 3rd.
I think you will need to decide what you want to do with the bike before you decide on the gearing, if you are mostly riding trails then gear it low, if you are mostly on the highway gear it higher.
I sacrificed a little acceleration for keeping the engine out of the vibration zone at highway speeds.(21 tooth sprocket).
Regards
Peg.

I would happily marry a BSA, but I would have a wild and torrid affair with a Triumph😊
 
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Discussion Starter #34
Thanks for the advice , everyone

I'm gunna check the Speedo against my wife's car

Then do speed rattles clutch test

Then make some decisions from there

I also double checked the back wheel , it's an 18

I never bought it to be a speed machine , Peg but I would like it to keep up with modern traffic easily and not be a mobile road block

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Ross its easier to download a GPS speedo app and then tape your phone to the handlebars - use a bit of foam rubber for the vibrations. There are several good ones out there, I use one for my android called GPS Speedo 115. BTW they are quoted as accurate to 1 km/hr, so very good.

Years ago I installed a tacho that worked off my coil [it was on a car], that could work if you can get hold of one. Usually they had a switch on the back for 4-6-8 cylinders - somebody could check me here - if you set it to 4 cylinders then divided by 2 for points or by 4 for a Boyer it should work out correct? This sort of tacho was reasonably common in the day. I might even still have one in my shed somewhere?
 

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Hi Mick,
back wheel , it's an 18
It's the OD (strictly-speaking the "rolling circumference") of the tyre that's important, "18" is just the size of the 'ole in the middle.

Aside, tyre ID has only a tenuous relationship with OD/rolling circumference - your bike almost certainly had a 4.00x18 Dunlop K70 fitted originally, any modern 4.00x18 will be a very similar OD/rolling circumference, as will a 110/90x18, 4.10x19, 130/80x18, 130/90x17 ... :)

Regards,
 

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paint a mark on the floor and a correzponding mark on sidewall. air up the tire and sit on the bike. then roll the bike forward until the mark on the tire is against the floor again.meazure between them with a tape measure. that iz the circumference of whatever tire you have

my avons are 81.4 inches in circumference
 

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Rossmick62 said:
"I never bought it to be a speed machine , Peg but I would like it to keep up with modern traffic easily and not be a mobile road block "
I found A65 perfectly capable of dealing with everyday traffic in Thunderbolt form, double carbs later head makes it more cumbersome in a slow traffic, but early 2 carb small port head makes it a great bike in any circumstances. I also found out very heavy handlebar weights move vibrations to lower frequency and together with properly set up front brake let a rider to ride BSA quite fast.
This is me on a freeway doing 75 - 80 mph:
and some acceleration:
 
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