BSA Thompson 1100cc V-Twin

BSA Thompson V-Twin

BSA Thompson V-Twin

Remember those old BSA V-Twins? Me neither. Well, here's one to think about, it's a 1963 BSA A10 frame with an engine created by a P.Thompson in 1991-1992. It uses the BSA A10 gearbox, with cylinders and cylinder heads from a B50 along with a B50 carburetor. Total displacement is 1100cc.

The seller of this bike, over in Germany, says there were only 5 built, of which this is number 3. He also says the bike is complete and runs fine. Since it's made from BSA engine pieces, you wonder why BSA didn't do something like this themselves. Of course, we can ask all sorts of questions of why BSA didn't do one thing or another, but it's nice to know someone went to the trouble to create something like this later on.

A couple of years ago we showed you another V-Twin, the BSA B66 built by Doug Fraser. I guess there is a sub culture of builders that look at BSAs as fertile ground for this sort of thing, though you have to have some serious machining skills to carry it off.

The link for this came up in a comment by Barry a few days ago and it looked like an interesting build. Thanks, Barry!

Link: BSA Thompson V-Twin on eBay

BSA Thompson V-Twin

BSA Thompson V-Twin


  1. scritch says

    I’ve always had soft spot for BSA’s, and this thing looks very good to my eyes. Way better than that Enfield twin that was ginned up a couple of years ago, if only because the only good-looking 90-degree twins I’ve ever seen come from Ducati.

  2. Hooligan says

    Interesting, never heard of that BSA before. Put that engine in a Ducati frame what do you have? a Bscati.
    But looking at the listing at the bottom a advert for a MZ250 going for nearly 3000 Euros! I must admit it looks a very nice MZ250, but that money? These wonderful East German bikes are more of a collectors item than I thought.

  3. B50 Jim says

    Can I say how much I want one? Oh, yes, this IS the bike BSA should have made! However, they had enough trouble making the bikes already in their lineup, much less starting an entirely new line. They had already reduced their number of models to cut costs, and tooling up for a mostly-new engine was out of the question. Being the proud owner of a B50, I can only wonder at the problems that would ensue from having essentially two or those big pistons churning away down there, but when I think of the mountain of torque and respectable horsepower it must make, I’d forgive all the broken parts, oil leaks, crushed light-bulb filaments, blurred vision and rattling teeth the first time I twisted the throttle on that Amal

    One thing I wonder about: How in Heaven’s name do you start it? The B50 already is a potential leg-breaker and requires intimate knowledge of the compression release to fire it up. With two of those pistons 90 degrees apart, even with a compression release, one cylinder will hit compression during kicking. Having been nearly tossed over the bars on a few occasions, I can only imagine what would happen when one piston goes over TDC but the next one hits compression, and if it should backfire, OMG!

      • B50 Jim says

        I couldn’t not say something! There are only about 3,000 B50s left in the world, and we B50 riders have to keep the faith. Say…. I have enough spare parts in my basement to build a B50 twin…….

  4. Yeti2bikes says

    That looks like a 45 degree V-twin to me. Shouldn’t be any worse than kicking an old Harley. I say that sarcastically as I have been out of breath with a bruised foot many times kicking those old Harleys.

  5. Paulinator says

    I find it absolutely inspiring to read about successful engine projects like this. I’ve got a drawer full of close-tolerance parts that were supposed to assemble into an innovative new ICE concept… but I inadvertently developed a new type of Sprag clutch. It locks up solid :)

    ps. I had an old C-10 basket-case and spare engine. The jug weighed an imperial ton!!!

    • B50 Jim says


      I agree. The B50, for all its faults, is a great-running, sharp-handling machine when everything works right. Boyer ignition goes a long way toward this goal, and a new Amal every 4,000 miles makes up most of the rest. It puts down good power and revs willingly. Now, if only I could find a way to stop oil from leaking around the rocker box!

      • todd says

        Jim, many people swear by the “Bunn Breather” kit.
        I remember enlarging my breather line and I think I have at least one rocker cap with a hose fitting on it somewhere in a bucket. Both B50’s I have/had (MX and T) are/were pretty oil tight. My AJS 16CS on the other hand…

        I admit to having a massive Mikuni on the bike but I did keep the points and added a Mighty Max battery eliminator (with an B50SS stator). The bike always fires up and runs stronger than my GB500. I’m a little ashamed that I haven’t ridden it much in the last few years though, thanks to various Hondas, Yamahas, and BMWs, and Ducati in the garage that get that privilege.

        I’m always impressed to hear how much you use yours.


        • B50 Jim says


          Thank you! I am planning to add breathers to one or both rocker caps; that should help considerably. Since I replaced the rocker cap gaskets with O-rings the caps don’t work loose and provide “automatic” breathing. The Bunn breather kit appears to use a check valve, which makes sense; timing the pulses should maintain a slight vacuum in the crankcase (or at least less pressure). I’ve read of a particular Ford part that works well.

          Yes, I do use my B50 fairly often, as it’s my only running bike right now (and was my only bike for many years). I’ve had it since 1974 and commute on it to my office regularly when weather permits and I don’t need to carry a lot of stuff. With a 19-tooth countershaft sprocket and a windscreen it’s a good rider and stays with highway traffic (when running less than 70 mph; it’s happiest at about 55 but runs 65 easily).

          I found that replacing the points with Boyer was the best upgrade; the old advance mechanism had about 0.006″ “slop” so the points gap could vary by a great amount, which allowed the spark timing to jump all over the map and often resulted in the engine going “chuff” and quitting, usually in traffic. The engine wasn’t happy at low revs, either. With the Boyer, it will run down to 30 mph in top gear and walk right out with a twist of the wrist. Try that with points! Plus the spark plug stays much cleaner. Along with the Boyer ignition I used an automotive coil ($18 at a parts store and 8mm plug wire — I took and ohmmeter to the store to check for proper resistance. I also used a 16-amp alternator (double stock output, which makes life much easier and I don’t have to put a charger/tender on the battery at night.

          Oh, yes; this year I also rebuilt the clutch using all-new parts; when that thrust washer gets off-center, all kinds of mischief ensues. A B50 is a long-term, emotional love affair, and if an owner isn’t prepared to put a lot of effort into it, the best thing to do is sell the bike and get something less problematic. But when you twist the grip and feel those pulsations, the bike charges forward and keeps pulling, get into third gear and it’s like riding a freight train, it’s all worthwhile.

  6. 7R Pete says

    Back in the distant past I remember learning to pilot a sidecar with a BSA outfit, pre-war vintage, maybe 1934ish. The engine was a side valve V twin of, if I remember correctly, 880cc. and the sidecar was also made by BSA I think. It was big, it was heavy and it lacked hp.s and torque. Perfect to learn on (not left handers). So BSA did manufacture at least one V twin.
    Back in the early 1950s many pre-war bikes were to be had for a song and incidentally a 1947 B31, hard heel (a 350cc version of the B51) was one of my first and it introduced me to bruised feet and knees.

  7. Jiro says

    My fantasy british bike was a V-4 using a Norton Dominator/Commando head destroked to make a 1000cc unit construction using a cam in the valley of the V and OHV. I thought such an engine in 1967 would have changed Norton’s history.

  8. shane says

    This bike was owned by me about 13 years ago,I got it from Boothill motorcycles in Faringdon, it was in a sort of cafe racer trim with a long alloy tank, the tanks on it were painted by their custom painter over there. The engine has, if my memory serves me right, a Harley vl crank, homemade barrels with petter diesel liners, C15 cams with Triumph timing gears, the timing case is unit Triumph. When i got it the timing was wrong they had fitted a Harley ignition set up on it and its a 50 degree so it didnt run well. The frame was also re-done it was cobbled together. I also made the alloy engine plates, the gearbox is bolt up A10, it was more of a bobber when i sold it at Kempton Park about 2003 i think. It ran quite well once I got it running, I spoke to Peter Thompson after i got it he said there was only two bikes but I think there were more engines. Glad to see its still around, Shane.