Circa 1974 or 5 I had a C15, actually one of 2, but this one was a bit special, engine built by Alf Hagon. Big valves, big Amal GP carb modified cam of some sort, and last but by no means least, a piston from a 650 Bonny. Was fast enough to keep me in trouble with the rozzers, good times :)

cool! looking forward to seeing how this project turns out!

Interesting project looking forward to the follow-up videos

Looks like a well planned modification. I've gone the same route with an aircooled VW Beetle and went from 1600cc to 1900cc. I also changed the crankshaft to have a longer stroke. That motor was a blast! I will suggest not machining (if possible) the sleeve to make it thinner. This can lead to heat distortion in short time when in use. There is also a pain to machine because as soon as you start cutting, the heat makes it expand making a tapered cut on the outside. This leads to a bad fitting with the fins and you will need a good one to maximize the heat transfer.

Looking forward to the followup. Thanks for the video.

Looking forward to another interesting series. Thanks for sharing. Stay out of the snow! Or would that be an interesting bike ride?

Sound good keep the videos vik

re lip on top of sleve .i think earlier b25 had no lip but ran into problems with the liner letting go with disasterous results .the lip was a cure . also on the rods .i may be mistaken but i think those rods are same as the 71 on triumph daytona and t100c . those rods were an upgrade from the earlier 500 rods which had a brass bush at the gudgeon pin .if they measure up it may be an option for a rod with larger diameter gudgeon pin hole .

If you Phil Irving a 441 top end and de bump inlet they can run pretty fast. I think head skimming also helps up the pep.

very,very interesting, looking forward to seeing how it goes,wisco used to do piston blanks,don't know if they are still available

Hi if the boring on the mill isn't satisfactory I've done barrels by large bolts through the barrel holes with nuts on then hold in a 4 jaw off the heads of these. Clocking for cylindricity to the centreline all the way along the bore. Saves making a fixture. Good luck

i had 5 b25 and found i could only keep one on the road at any one time.The biggest problems i had was with the camshaft bearings, the best thing i did was to put needle rollers in but that would do 20,000 before they died. this was all around 1970

Another great video. I am helping a friend with an engine rebuild on a 1968 Starfire/B25. I've owned and rebuilt many British twins over the last 45 years but not the B25 single. What is the best way to clean out the sludge trap in the crank, once the plug has been removed ? It had seized the big end on a recent run out and I suspect that sludge from the sludge trap may have caused the seizure. Also, once the engine has been rebuilt and all the old oil has been cleaned out, inc the oil tank and lines, is it advisable to refill with a mineral 20/50 or a more modern synthetic oil ? Hope you don't mind me asking but you seem to know these engines well. Thanks. Mac.

Michael have you checked the valve to piston clearance with the Yamaha pistons. Looking forward to this project. Cheers Eric

is Maryland in the UK ? :-) The only cylinder i bored was in the lathe - it worked out ok - stay safe

Mike: I think using your new boring head is the way to go. The only advantage of boring in a lathe (and it might be a big one) is rigidity. You can use a heavy boring bar. I have done it both ways. The problem with the boring head is getting some boring tools that work well with no chatter. The Chinese carbide bars don't work well on iron without a lot of grinding on the point, but might give a good result on the aluminum cylinder. I guess you won't even be boring the ID of the sleeve on this one. How about keeping the piston dome, just reshape it to fit the chamber? ....Dave

Two points, if you use a milling machine to bore the small end, there is an opportunity to change the length, should want to, by offsetting the hole. And, rather than modify the piston to reduce compression, make a thicker spacer gasket to go under the barrel. We did this on our Fleetstar gopher bike to make it more pleasant to ride through London traffic. Make that three, needle rollers are not really meant for short movement part rotation as on swinging arms, they are made for full rotations where the lube film works as it should. There are a number of scooters that use them and they are prone to failure, the rollers just dig into the hardened shafts with the back and forth motion. Taper roller bearings are better but I doubt you could retro fit them.

cool videos. when part three coming....

Given when this was filmed I expect it all worked out OK and you still have your knees. Just curious why you would turn the whole sleeve down and not just the lower section to fit the cases??

I don't know if someone already brought this up but the FJ1100 has a 74mm bore, the FJ1200 has a 77mm bore. i am guessing you actually used the liner from a FJ1100. Is that right?

OKO and Keihin float bowls are very easy to put on and take off. As long as side of the bowl closer to fuel intake is kept higher than the other side when taking off, its very simple and straightforward. Not that great an idea converting a B25 to larger capacity for 2 main reasons. First reason is that even as a 250 the flywheel weight is not really enough the trials purposes. The second is that the extra 50cc will generate a good deal more heat, which will cause all sorts of problems for trials use. The compression for an effective 250cc trials motor needs to be around 10.5:1, and ideally the port sizes in he the head reduced to increase gas speed. This 300cc conversion will mean a bike which is pretty nasty to ride, doesnt grip well and will run very hot. Way to go is to stay with 250cc, increase compression, modify intake port, inductive ignition, and a good exhaust and intake system. The BSA factory never did any of these big bore conversions, and I would guess the above reasons may well have been considered as to why no factory big bore motors...................

Have you seen Dave Ritchards new toy a hand cranked boring rig I think you two could have a joint venture. All the best for christmas Peter

Is it possible to stroke this motor an I am about to purchase a 1968BSA 250 I had a single cylinder 500 an didn't know wat I had I was 15 I'm 62

Someone is sure to remind me that the Chevrolet Vega had bore problems right from the get-go. Yes they did, but those bores were anodised.

I don't think that the smaller valve area for the displacement will be a problem. Big valves are for high rpm horsepower. Trials bikes, and even dual-sports want tractable power from 1000 rpm and up. I wonder why you are even bothering with an austentonic sleeve. Industrial hard chroming of aluminum bores has been going on for years. The trick with Hard-chromed aluminum bores is reversing the current flow right at the end of the plating process. This puts tiny pits in the bore, which prevent the bores going dry.

When is boring not boring? I can't see how going short stroke is going to improve a trials bike but as Confucius once said, 'What do I know'...

Circa 1965 I had a C15 - what a load of shite! BSA deserved to go to the wall if that was the best they could do!