The BSA unit single



My B50 project

I was first introduced to the BSA 500cc unit single in the summer of 1971. I had just graduated from high school and I was working during the summer at Motorsport, the Triumph/BSA/BMW/Suzuki dealership in Albuquerque NM. One of our customers (my friend Rick Heyman) had bought a B50T (the trail model) and he met me out at one of the local riding areas and let me try it out. I was riding a TS185R Suzuki at the time so the BSA felt pretty heavy, but I was very impressed by the power and the way the power was delivered.

Riding Rick's B50T got me hooked on singles, and over the next 15+ years I owned various Ducati, Matchless and Yamaha thumpers. But I didn't own a BSA until about 1987 when I purchased a used 1972 B50MX. This model originally came with the small fuel tank, the E.T. (energy transfer) magneto ignition, and a 20" front wheel. When I got the bike it had a 21" BSA front wheel on it and the larger B50T fuel tank. It also had a set of Dick Mann footpegs that moved them back about 2 inches. The stock footpeg position is considered by many to be too far forward for comfort when standing on the pegs, and I was pleased that the bike had the DM pegs on it. Otherwise, the bike was quite stock, including the stainless steel fenders.

I prepped the bike (the fork tubes were bent) and went racing vintage motocross with the California Vintage Racing Group (CVRG) in 1988. In the first race the previously pristine stainless steel rear fender split. I also discovered that if I fell down the front wheel would always twist in the forks. The B50 wasn't the best turning bike I'd owned (I had a 1972 Maico 125 that I bought new) and the front end wanted to "push" in the corners. The bike was also not terribly easy to start. On the other hand, it was fast, fun to ride, and sounded great!

Time then to begin modifying it! Plastic fenders replaced the stainless steel fenders. I switched to a set of 35mm Marzocchi center axle forks in Betor triple clamps (no more fork twisting or bending) and a conical hub front wheel that was common to many 1970s/80s Yamaha dirt bikes (lighter and better braking). I also installed a Lucas Rita electronic ignition which gave easier starting from the more retarded ignition setting at kickstarting speeds. The rest of the bike was still stock.

The Maico and Husqvarna 400s had a couple more horsepower than the BSA, but as you can see from this dyno chart the BSA has both a very high and flat torque curve as well as one that covers a wide RPM range. It is the area under the curve that is important, and I was usually in the first three to the first corner off the starting line. I ended up being the CVRG high-point Open Sportsman Novice for 1988.

Later I had the tank and seat off and I was considering modifying the frame to try and improve the steering. I noticed that the seat subframe was bent, and after fiddling with that a bit I decided that the stock frame was too heavy and too much modification was necessary to make it the way I thought it should be so I pulled the engine, cut the steering head off the frame, and sold off the rest of the BSA parts, keeping the front end, engine and ignition. I couldn't find anyone who'd take the stock (heavy) rear wheel so I was stuck with that.

And that's the way things stayed. I thought I'd be getting the bike built back up in a new frame soon so I bought an NEB 3 speed transmission (the B50MX was no longer staying in first gear), a long rod and piston kit from Stan Millard (the rod/stroke ratio on a stock B50 is very short which is bad for both power and longevity of the piston/sleeve), and a "core" B50MX motor along the way. A few years ago a TR5MX (still a B50MX, but badged as a Triumph) followed me home one day. :-) But I didn't do anything with it beyond sending the badly worn AMAL 932 off for renovation and fitting with a chrome-plated brass Mikuni throttle slide as I was busy taking an early retirement, thinning out the project bike herd, and nibbling away at other projects.

The TR5MX. The original owner of the bike wanted a B50SS but they'd stopped making them, so he bought the TR5MX and set it up for street use. He also installed the 580cc ForgedTrue piston kit that TriCor used to sell.

In July 2008 I went with my friend Jim Connelly to the AHRMA national races at Mid Ohio. There were a few B50s there and my enthusiasm was stoked enough for me to come home, sell my Suzuki TL1000R project bike to a friend to raise some capital/make some space and then drag the TR5MX and some miscellaneous parts home to start building another motocrosser.

My plan is to start by NOT taking the bike to pieces. This is very hard for me to avoid, but I really do want to go riding! I'll put the complete front end (as mentioned above) from my B50MX on, fit a Rita ignition, and **maybe** put a lighter rear wheel on. I've got CZ, Maico (sheet steel) and Montesa VR rear wheels on hand and I'll have to see what looks easiest to do. It is also possible to lighten a stock rear wheel but the CZ rear wheel with alloy rim is about 5 lbf lighter than the BSA hub with steel rim, and I'm not going to put money into a good rim for the BSA wheel. If the BSA wheel can be lightened enough to be a close match for the weight of the CZ wheel I might use that because, after all, it does bolt on. I need to make rearset footpegs and possibly a new kick start lever that doesn't foul my right leg when riding and isn't prone to bouncing around over bumps and trying to engage the kickstart gears.

Here are some photos of rearset footpegs I made for another rider's B50MX. They were taken before the final trimming/cleanup was done. The bike had sat outside overnight in the damp and that's the cause of the rust. I was supplied the footpeg and matching mount and fabbed the bits between them and the frame. The BSA frame tubes are, especially on the left, well under the engine and that makes it a stretch to get things attached. As you can see I was concerned with making things very sturdy, though it was largely not very thick sheet/tubing that was being used.

What then? I've still got the engine from my first B50MX and the "core" engine. I'm going to be sending a spare crankshaft and cylinder to Ed Valiket at E&V Engineering to have him install the long rod and fit the piston to the cylinder. Over the last 25+ years I've learned that you keep the running engine together so you can go racing and send the spare parts off to be modified.

The B50 frame is not a lightweight part. Here are some frame tube sizes to illustrate that. Since the tubes I measured have paint and burrs on them the dimensions are very close approximations::

I know one B50 in Northern California that has had the steering head deraked and a longer swing arm built. Mike Johnstone in Southern California has a number of B50s that he's done a lot of detail modifications to (and with excellent workmanship too) and he also modifies the steering angle and swing arm length, and also lowers the engine an inch. I've learned on past projects that there is a point where it is actually faster and easier to build a complete new frame the way you want it instead of fighting to modify a stock frame. And keep in mind that even if I did those kinds of modifications to the B50 frame it would still be significantly heavier than a proper race frame. BSA wasn't building these frames primarily as an all-out MX racer so I don't fault them (much) for the extra weight.. But since I like to design and build frames I may eventually do one after I build up another motor with the race parts.


John Burkhard was kind enough to pull his Dick Mann B50 out of storage so I could take some photos and measurements. John, raced his first MX on it at the Sand Hill AHRMA national in the spring of 2008. He said he was really impressed with how the bike turns. The rear wheel is BSA conical and the front wheel is Bultaco.

This is a medium rear travel bike, probably about 7". It would have been nice if it were a vintage 4" to see how Dick would have dealt with that kind of bike.

Measuring from the front axle (same length forks as stock) Dick moved the engine forward 1.25" and the foot peg back 1". Measuring the angles from the photos it looks like he was running about 28 degrees on the rake where my TR5MX is more like 30-31 (even though the factory frame drawings show 27 degrees).

Mike Bungay, of Sacramento CA, sent me some photos of his B50 racebike. It has a Cycle Factory frame, which was built in Northern California. Notice the detail work, including swapping the shift and brake levers. The engine is by E&V Engineering and is reported to be quite a runner.


Links to B50 sites

Rickard Nebrer's B50 Website

E&V Engineering Website


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© 1996-2008 Michael Moore, last update for this page 02 Mar 2009

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