I've nearly finished running im my C15F - and realised I have no idea what that actual top speed of a C15 should be.
Mine now has the 15137 piston giving 7.5:1 compression. Also my speedo seems uncannily accurate against GPS device.
I have run it in mainly at about 40mph where it seems very happy. More recently I have given it the odd blast up to 50mph - where it also sounded very happy. But I didn't get the feeling that there was much more to go. (And given the brakes etc 50mph seemed quite fast enough!) I have, however, begun to doubt my pal's stories of the C15 he had in his late teens doing 80mph!
I was interested to know what the genuine, real-world top speed of a standard basic C15 is reckoned to be? Mainly so I don't inadvertantly go there!
It's between "yippeee" and "oh f*ck".
Keep between those two and you'll be fine..
(Your mate's claim is well into the latter exclamation}
There was the C15 SS80 (https://www.motorbikespecs.net/index.php?flag=2&oldflag=1&make=BSA&model_id=5866981&model=C15+SS80+Super+Sport&year=60) but I doubt if they ever made 80mph.
Yes I think the 80 in SS80 was aspirational but it should reach 70 on a down hill lying on the tank. This was a right of passage for every learner. I got 70 out of my C11G for a few seconds before the valve float snapped a cam follower, it was quite a long hill ;D
I had my C15G's speedo needle wavering either side of 70 on the local by-pass. No need for a hill or for me to strip naked .
I remember thinking to myself that "now l know what she does flat out l don't need to do it again. "
I think the speedo was pretty accurate .
55 was very comfy .
60MPH seemed to be the default country road speed for my mates with bigger bikes or Jap 2strokes .
The bike buzzed along happily enough but at that speed l kept mentally muttering the " Two Second Rule" rhyme about braking distances. Then doubling it to 4 seconds to take into account the C15's stopping credentials. 😀
Quote from: Tigerfeet on 19 Sep, 2026, 10:16 I've nearly finished running im my C15F - and realised I have no idea what that actual top speed of a C15 should be.
Mine now has the 15137 piston giving 7.5:1 compression. Also my speedo seems uncannily accurate against GPS device.
I have run it in mainly at about 40mph where it seems very happy. More recently I have given it the odd blast up to 50mph - where it also sounded very happy. But I didn't get the feeling that there was much more to go. (And given the brakes etc 50mph seemed quite fast enough!) I have, however, begun to doubt my pal's stories of the C15 he had in his late teens doing 80mph!
I was interested to know what the genuine, real-world top speed of a standard basic C15 is reckoned to be? Mainly so I don't inadvertantly go there!
My machine is boxes of bits which hasnt run in 40 years but back in 1969 I had a standard c15 star, it was pretty gutless, upto 55 on a straight, maybe 60 downhill, lying on the tank with a following wind. I rebuilt it as a cafe racer (ok was a teenager at the time) alpha big end &timing side rollers, Joyner sport cam, 376 carb, 10:1 piston, ss80 inlet valve & gas flowed. It went well for a C15 between big end siezes but Im not sure it ever reached 80, certainly my mates bog standard Honda cb250 left it for dead. From memory I suspect its top speed will be limited by your nerves and the brakes ;D
Yes I had my C11G, one mate had a C15 then one bought an early Honda Benly 125. We were amazed, it had TLS a brake, OHC and revved to 10,000 rpm, it was also faster than all our 250s.