BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: MICKREEVES on 15 November, 2018, 07:47:35

Title: C15 compression increase
Post by: MICKREEVES on 15 November, 2018, 07:47:35
My C15 ss has , (I think) 8 1/2: 1 compression ratio.
If I skimmed the barrel by 1 mm, would that raise the C/R to 9:1?
Bore is 67 mm. stroke 70 mm. My maths is no longer good enough.

Would this be a good idea, for more power, but still nice to ride and start?
Would Higher C/R be worth trying? I could always fit thick gasket to take the ratio down again.
Mick
Title: Re: C15 compression increase
Post by: JulianS on 15 November, 2018, 09:42:11
Not a good idea to skim a 4 stroke head to increase compression. You need to consider the geometry - pushrod length/tappet adjustment/valve to piston clearance. Better to source a hc piston.

Then you may not notice any improvement anyway. More power when you already have a high comp piston is about the whole engine. You will put a lot more stress on the bottom end of the bike.

Personal view is that it is just not worth it.

If you want a faster bike then you really need to get a faster bike.
Title: Re: C15 compression increase
Post by: MICKREEVES on 15 November, 2018, 13:09:36
I thought I could avoid the cost of a new piston, and upsetting the balance of the engine.
I sold my B44 because of high vibration level.
I just did a quick run round our March bypass. (my usual blast)
Only just manages to pas large lorries doing about 70.
Two up would be worse, but unfortunately my Tart ( Sweetheart (wife)) will not consider it!!
AAAgh!
Mick
Title: Re: C15 compression increase
Post by: AdrianS on 15 November, 2018, 17:22:18
I agree totally with Julian.

Skimming 1 mm off the head wont make much difference to the power and may introduce further problems.
I certainly would not want to be travelling at 70 with a pillion on todays roads on an old C15!

Buy a cheap Jap pocket rocket if you need the speed! Brakes should be better and you will have plenty more performance.
Appreciate the C15 for what it is - its an old lady, treat it with a bit of respect and enjoy riding the quiet country lanes on it. They weren't built for the busy roads of today.
Title: Re: C15 compression increase
Post by: MICKREEVES on 15 November, 2018, 17:39:52
To me, the whole point about riding, is recapture my young days, 55 years ago, when there were no Jap bikes.
My C15 loves being driven hard!
Mick
Title: Re: C15 compression increase
Post by: DerekAnderson8 on 15 November, 2018, 18:47:02
 ;D

https://www.ebay.co.uk/itm/BSA-C-15-250-Star-piston-kit-040-NEW-15305-and-16114-high-comp/142981696619?hash=item214a5f8c6b:g:AyYAAOSwwAFbHN48:rk:1:pf:0

go for it mick!

 ;)
Title: Re: C15 compression increase
Post by: DerekAnderson8 on 15 November, 2018, 18:48:57
or even better

https://www.ebay.co.uk/itm/BSA-C15-BIG-BORE-KIT-69MM-BORE-CLASSIC-RACER-PRE65-SCRAMBLER-VINTAGE-CAFE-RACER/283254913748?hash=item41f34efad4:g:nrYAAOSwkNZbavTj:rk:1:pf:0

what the hell, your only young once!

 ;)
Title: Re: C15 compression increase
Post by: MICKREEVES on 16 November, 2018, 08:07:19
Thanks Derick for the link, but i dont know my bore oversize, and I think my piston is already high comp.
I will find out when i take the top end off.
Title: Re: C15 compression increase
Post by: MICKREEVES on 16 November, 2018, 08:09:55
I dont think the very small increase would help much, and it would need running in. I dont have time for that at 80.
(Your only young twice actually!!)
Mick
Title: Re: C15 compression increase
Post by: MICKREEVES on 07 January, 2019, 14:11:19
With further experience, I now have only the C15 bike, and it will reach 80  mph.
The compression ratio is 8/1, and it is over geared by one tooth. RPM at 80 mph is 6600.
0 to 60 mph is 15 seconds, and in 10 seconds it reaches 50 MPH.
I tend to ride it flat out most of the time.
Mick
Title: Re: C15 compression increase
Post by: AdrianS on 07 January, 2019, 16:21:52
For how long before something gives.....!!!