Bigger 250 BSA/Triumph engine.

Is it possible to enlarge a BSA Starfire/Triumph 250 engine to 350 or larger using B40/B44 parts, or are the two engines totally different?

Martyn.

Going to the bigger bore of the B40/B44 requires surgery, the studs need moving out with welding of old holes etc and the crankcase opening increased in diameter plus new barrel and head. For smaller increases in capacity there are other options using Triumph pistons but retaining the head and barrel.

Thanks Kommando. I think it would be better to start with a complete bigger motor rather than mess about welding & machining then still having to find a top end.

Good luck, B44 stuff is expensive in UK as scramblers use and abuse them and there were few to begin with, lots more parts in the US but ebay's shipping rates have killed that as a source. I bought all the B44 stuff I needed in time. B25 parts are a lot more common in UK.

On you tube there is a B 25 conversion 300 cc by Michael Waller. I did something similar from 250 cc to 320 cc by using an old Bonneville 650 big bore piston which is 67 mm. (60 grams heavier). And I used an 850 Norton sleeve. I drilled crank 4 x 5/16 holes to make the same balance factor as a 250 piston. It runs very smooth, nice low end and lots of power. I'm using also a 32 mm mikuni carb. It still really wants to rev and it hasn't blown yet

That's interesting to know. When I first asked the question I was looking at buying a BSA 250 green lane bike but have since aquired a 1959 Royal Enfield 350 Bullet/Clipper. These can be taken out to 612 & even bigger, seemingly without problems.

What is the point of buying a slow bike and making it slightly less slow?

What is the point of any old bike as a new one is all around better.
Because it is fun.
...odd sense of fun I suppose. I plead guilty yer honner.

swooshdave said:

What is the point of buying a slow bike and making it slightly less slow?

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Some people buy a Norton commando and make it much slower by fitting a single carb to it
It seems senseless to me
But it's each to their own I guess

Matchless said:

That's interesting to know. When I first asked the question I was looking at buying a BSA 250 green lane bike but have since aquired a 1959 Royal Enfield 350 Bullet/Clipper. These can be taken out to 612 & even bigger, seemingly without problems.

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The b25 motor really is fun and fast (between rebuilds)
If you have a lathe or access to a lathe you can do a big bore yourself
You can push the liner out , bore the barrel bigger
Then fit an xs1100 Yamaha liner and piston
You need a 4 thou shrink fit on the liner
I went a stage further with mine by welding the head up to form a squish
This gave me from memory a 10.5 -1 compression and 283 cc
It was a very fast high revving motor I found it a lot more fun than my b44 and my b40
It was certainly a lot smoother , I put this down to a more rigid crankshaft but the con rod has to be perfect I had my one shot peened
If I went that route again I'd use a billet con rod or similar, there's more choice these days
Cheers

hasselt said:

On you tube there is a B 25 conversion 300 cc by Michael Waller. I did something similar from 250 cc to 320 cc by using an old Bonneville 650 big bore piston which is 67 mm. (60 grams heavier). And I used an 850 Norton sleeve. I drilled crank 4 x 5/16 holes to make the same balance factor as a 250 piston. It runs very smooth, nice low end and lots of power. I'm using also a 32 mm mikuni carb. It still really wants to rev and it hasn't blown yet

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I think you mean a 71mm 650 Bonneville piston?
The standard bore for a 250 is 67mm
Cheers