Chapter 7. Drive Time

I can’t begin without saying that I’ve had the bike running, albeit briefly! I’ll come to the detail later in the story, but it represents a milestone in the build. I genuinely gave it about 10% chance, particularly as it meant having to do a bump start in my back garden and stopping before I hit the garden furniture. I also had to fit the second hand Amal 276 carb as the Kehin look-a-like, referred to last week, was about 1-2 mm too big to fit in, plus a myriad of other last minute snags, such as the gear selector jamming.

That’s the good news story - you know I was being clever, going on about using plugs in the various ports and holes to stop bolts dropping in? Guess what, just about to fill the gearbox with oil and a small cover screw falls in. First thoughts (the devil on your shoulder) it’ll sit there at the bottom of the box and no-one’s the wiser. Second thoughts, what if it catches in the gear teeth and wrecks the box – remember no spare parts available for this dear old bike. Common sense prevailed, and luckily the gearbox end cover can be taken off in situ. There it was, lurking under the kickstart quadrant.

Back to the story: By now I had the main components assembled, frame, forks, wheels, engine and gearbox and needed to sort out the engine plates to literally hold it all together. The period parts manual had some thumb nail sketches of what they should look like, and Perry’s general arrangement drawing allowed me to work out the approximate dimensions between the mounting studs. I also found one ad on eBay for a similar set so I knew roughly what was required. The first patterns were made from plywood sheets, followed by some thin gauge alloy plate, and I homed in on the exact dimensions, each time setting the frame up and re-measuring. The final iteration was a set of plates cut from 8.8 mm alloy plate. To get the square edge look, as you get with CNC or laser machining, I used a cylindrical cutter mounted in my pillar drill and moved the plates by hand on the table.

 

I’ve made up some spacers out of solid 1” alloy bar and some eccentric bushes for the rear mounting stud (you can see in the photo above) that allow all the components to fit snugly without having to stress the frame. The primary chain adjuster is fabricated from steel bar and heavy-duty angle and jacks the rear gearbox mounting stud backwards and forwards.

 

Researching the BSA clutch got me nowhere. No original parts are available, although I have the gearbox clutch sleeve. Also, every article I found said that BSA clutches were not up to much anyway and most – certainly post-war – could be usefully converted to a Triumph type. I’ve always been an admirer of Perry’s clutch (if that doesn’t sound to risqué) so I decided to source one of those. 

Again I went from expecting there would be hundreds of used ones to choose from I couldn’t find too many complete units on-line and ended up buying a pre-unit, 4-spring, 5-plate unit. This would probably be adequate for a Triton so its well over the top for a 1932 250 BSA. Although I believe you can buy a clutch hub adaptor to suit a BSA main-shaft taper I’ve been unable to find any dimensions, so I’ve machined the Triumph hub with my Dremel set-up on the tool-post of my lathe and using a ½” diameter grinding wheel. This is not the perfect solution and it’s ended up setting the clutch a little too far in towards the gearbox. If anyone out there has any information on BSA / Triumph gearbox mainshaft tapers – or a good idea how to “adjust” a taper fit to make it tighter – please let me know.

For the engine cush drive I had a very productive day at Kempton. I had the cush drive bearing – the part that fits to the engine mainshaft and the nut. Rummaging though Martin’s tray of sprockets I found exactly the right sprocket, 17T and a perfect fit on the bearing. I was then sitting in the grandstand with my brother, enjoying a coffee and the vegetarian option from the burger van (just in case Jane reads this) and took a look in another vendors box of bits at the bottom of the steps. Incredibly I found a cush drive sleeve and a spring that matched the bearing and Martin’s sprocket. Finally, I made a couple of keys and cush drive nut on my lathe. You can see the whole lot whizzing around in the video.

I’m learning that you really have to take the rough with the smooth on a job like this; some things go really well, others are a real b***tard. I knew the primary chain was a ½” pitch 425 size, and the gearbox sprocket measured up as ½” pitch and what I though was a 420 chain size. I bought a 44T Talon alloy rear sprocket partly because it would be easier to machine to fit the rear hub and also because they are very inexpensive – I have no real idea how the gearing will work out and may need to change the gearing and sprocket. At present I’ve geared it for 60 mph at 5,000 rpm as a starting point.

Having fitted a couple of new chains, I wheeled the bike around about and got an unpleasant “click” every quarter rotation. It turned out that the existing gearbox sprocket is a Reynolds chain size, which has a larger roller diameter that metric equivalents. Given that the existing gearbox sprocket was very hooked anyway, and not wanting to source and machine another rear sprocket, with advice from Mike and Perry plus one of Perry’s contacts in USA - a very helpful guy, I set about fitting a new 420 sprocket onto the BSA gearbox sprocket hub.  In summary the only way to get the old teeth off is to use a handheld grinder to get it roughly concentric. 

I covered my lathe in sheets as you would a patient undergoing a surgical operation and spent a gritty couple of hours grinding away. Machining a spigot location for the new sprocket was even harder, both time and lathe tool consuming. Finally the parts were finished and quite a relief to complete the job with a TIG weld on the back side of the sprocket. I even managed to keep the little BSA – three crossed rifles – stamp on the original part.

Next week, the engine. I’ve skipped over the engine rebuild, from my opening comments you’ll know that it works, but this BSA has some interesting features being at the juncture in motorcycle development between total loss and more modern lubrication systems.

Neither my lawn nor the neighbours will stand for me riding the bike round the garden any more so I’ll now set to work on what will hopefully be the final stages of this project - dream on!

Keep well and stay safe

Gareth