1972 BSA Thunderbolt Custom Hill Climber

BSA Thunderbolt Custom Cafe Racer Hillclimber

Today, we’re thrilled to present this BSA Thunderbolt hillclimb build by Darren Carter of the UK. We see lots of Triumph customs, but relatively few BSA cafe racers and customs. Darren’s build immediately stood out for it concept and originality. It may not be the most high-dollar, spit-polished build, but that was not the intent:  this bike gets ridden!

BSA Thunderbolt vs Lightning

The BSA Thunderbolt was produced from 1964 to 1972, fitted with a 654 cc parallel twin that made 46 horsepower.  It differs from the BSA Lightning mainly in having a single Amal Monobloc carburetor instead of twin carbs.  The Lightning put out marginally more power (52 hp), but most experts and enthusiasts agree that the Thunderbolt is the better all-around bike.

BSA Thunderbolt Custom

In the Builder’s Words:  Getting the Bike

The bsa a65 thunderbolt came to me by an unexpected set of circumstances.

The story begins back in the olden days when I was in my late 20’s and became the proud (?!) owner of a pink ’78 kawasaki Z1R.  (It was unfortunate that it was pink but I really wanted a big bike and was prepared to put up with the colour!) This was about 6 weeks before the bike insurance industry had a wobble and went a bit mad!  At the time I had a few z650’s and all of a sudden I couldn’t get what I considere–for a chap with a small family and budget–affordable insurance for the big new z1r. You can imagine how popular that made me with her indoors!

So, the kwack was now an expensive luxury and so had to go!  Now the worry was that if it was sold the money would get absorbed back into the family running costs and would never be seen again, so I offered the z1r up as a swap or px. I contacted the “used bike guide” and “old bike mart” and “bsh” in the hope that someone would find my pink (had I mentioned pink?! Not my colour really) Z1R interesting and offer me a deal…and then I waited.

The second call was the Thunderbolt I still have. Faced with little choice I organised a meeting with the Thunderbolt owner, which fortunately meant the bike came to me on a trailer from about 100 miles away.  Boy was he keen to get rid of that BSA, I thought…better be careful!  The day came and I rode pillion for couple of miles and thought, well, why not?  It rattles a bit and is slow-ish and not first choice or obvious soughtafter classic from the 60’s that I was hoping for, but hey! Beggars can’t be choosers! The obligatory haggling followed…tea was drunk and a deal was done.  My z1r went on the trailer and the thunderbolt and £100 folding pounds went into my shed! More importantly though I could still insure a classic and so, on 10th January 1993, life with a 1972 Thunderbolt began.

1972 BSA Thunderbolt
Before shot of Darren’s 1972 BSA Thunderbolt

Now lots of derision has been written about this range of bikes:  bad styling, high seat, blah blah, but I have to say it has served me well as a perfectly good motorcycle. I am not a big distance user and have other bikes to spread the mileage across, so some years the bike has done 500 miles, one year when I was” between” cars it was used daily in all weathers for the work commute and did about 5000miles.  I am the only classic user in one of my groups of Japanese bike friends, so being the slowest (allegedly!), I normally lead when we go out but my speed is rarely an issue on normal roads and never a problem on back roads, the lowdown torque makes it excellent for Wiltshire’s B roads. However, when I go out with my local Wiltshire BSA branch my speed and road knowledge can sometimes be a problem (but I always wait for them!).

So far through 21 years with the thunderbolt, it has seen more use and neglect than pampering:  basic oil change maintenance has been sufficient so far and occasional replacement of parts when needed, kick start spring, fork seals and two batteries has been about it in all that time–that must say something about this old bike!  I have never been impressed with the mpg, about 40-42 riding to work and no more that 50mpg on a steady run, but then again, I do only get about 2-2500 miles out of a back tyre!  Brakes I have normally found to be adequate, and with a bit of anticipation of the road ahead, brisk, smooth progress can be made.

So that brings us up to date and the fact that for all the good points I enjoy about the Thunderbolt, the speed, handling, reliability and social side with the BSA branch…I have always thought the bike to be UGLY! So having just completed a rather special z650 I thought what next?  The choice was obvious!

In the Builder’s Words:  The Build

I have been a fan of speed hillclimbing for some years now and have done a couple of Prescott bike festivals hillclimbs on a couple of my GSX-Rs and even tried a proper event on a GSX-R 750, which was great scary fun, but just went to prove that I am not quick enough for those bikes (yet)! 

The plan was to modify the thunderbolt to improve its looks and then use it for competing in hillclimbs at a pace more appropriate to my ability.

So…to the modifications.  I started off with the idea of just softening the look and trying to visually “lighten” the bike.  A rounded tank was the obvious place to start, something that still had a practical size for road use but didn’t look too bike.  The tank chosen was a Yamaha XS250 which was damaged and therefore cheap!  The tunnel and bottom of the tank was removed, the damage repaired, and I made a new wider tunnel to go over the massive top tube/oil reservoir of the frame.  I also gave it a central tank fixing so it looks a bit more “of the period” and cut a clearance section at the seat end to clear the oil filler cap.  The original bsa twin petrol taps are retained and a period Monza style flip up petrol cap was welded on–I think that is 70’s Suzuki GT range. 

The tail piece was always going to be from a Z650, quite simply the best back of a bike ever invented!  Although I did cut it down a little to suit the scale of the Thunderbolt and then made a seat to fill the gap between the tank and tail that was comfortable but not imposing on the design. 

The side panels were always going to be changed, but into what I didn’t know.  I cut up a pair of Z1000 panels which looked “ok,” but I didn’t want people thinking I was just building a Kawasaki around a BSA motor.  So, various cornflake boxes were used as templates, and slowly the finished shape evolved into what you see now (a bit 80’s Yamaha inspired perhaps?).  The side panels are hand-formed from 2mm alloy using a small fly press and a 50mm tow ball, cut and welded where needed to give a more interesting and flowing shape. 

I relocated the ballast resistor at this time, as I felt the panels would be too thin to mount it to nicely, that explains the chunky bash plate at the front, that is now the heatsink for the resistor!

At this point, the new air filter manifold had to be designed and made, as I didn’t want to go the formula route of off the shelf pancake style filter.  So, following more cornflake cardboard mockups, a series of cones–again in 2mm alloy–were formed, and I think turned out ok! Avoiding the big frame tube while reaching the single central carb was the biggest problem.

BSA Thunderbolt Custom Airbox
Custom Airbox

The swingarm was always going to be changed, when the old one came out and I started to measure it for jig making, I was amazed to see just how weak and out of shape it was!  The replacement was going to be straight and strong, and has become a bit of a talking point.

The swingarm started life on a big Kawasaki sports tourer from the 90’s, ZX-10 I think, and was a monoshock, and much too big!  So I cut it up into its basic component parts:  two arms, bearing tube, etc. And just set to work, reducing and shaping it until it fitted into the jig made from the original, so all dimensions are the same as original for chain fitment , handling, etc.  As I was welding it all back together and fitting the twin shock mounts, I thought an under brace would make it stand out even more and finish it off in a true 70’s style.  So a brace was calculated based on a couple of my other 70’s Japanese bikes and voila!  After several hours of polishing, it looks like a “proper” one!!

BSA Thunderbolt Custom Swingarm

The exhaust choice was simple–stainless and unique!  I bought a couple of 180 degree bends and set to work; the final angle of the one off silencer was dictated by a small section of a bend I had left over from a turbo conversion I had helped a mate with!  Did I say this was a tight budget?! The exhaust note is “outgoing” but acceptable in classic circles I think.

Frame-wise, bits have been cut off when not needed, braced and reinforced when they have been needed, especially around the headstock and swingarm mounting plates, which are truly laughable as a design!  The rearsets are designed and made from stainless steel, and the obligatory renthal 790mm mx bars used up front

Oh, and then I cut the front off!

After most of the body work was done, there was still something that wasn’t quite right…everything looked tight and compact, but the front wheel was too far away.  Only one thing for it, then–bring it in!  This was done with smoke and mirrors and bits of string (and a jig) and was then very securely welded back together, and I did extra bracing in a factory style to make it look like it had always been like that.  One or two people have already thought it was a rare competition frame but I have always let them know its just a normal one that’s been butchered!


I then did all the painting, with cheap ebay paint and a clear lacquer (the frame is only a brushed on gloss at the moment because of time running out) and cut the decals and stripes by hand using cardboard, then alloy templates, tidied and reduced the wiring loom, fitted the oil warning light in the old steering lock hole, fitted a custom mini speedo, and then took it for an MOT with the “get me home lights” taped over.  And bob’s your uncle, a finished and I think quite stylish bike that will no doubt in these modern times be described as a “flatracker,” which I don’t like!  (Much the same way people describe my special z650 as a streetfighter.).  To me it is a custom bike, an evolution of a bike that was fundamentally all right, but now just looks better.

First time out on the finished Thunderbolt was the Prescott Bike Festival Hill Climb back in April.  I was unsure about how it would feel after all the work and just over two years in the workshop.  (I only get to do my bike work during dinner times and after work, you see–my Z650 took over 10 years!) And I did wonder whether a hill climb was a sensible first trip out, but it felt good and natural straightaway; the steering is really immediate–some may say twitchy–but the edges of the tyres show just how well it now handles!  I think it has uprated fork springs, as it has always seemed taller than other OIF models in the local club, and Zephyr 550 gas shocks on the back, which on the softest setting work really well…so far!

BSA Thunderbolt Custom

To sum up, I am well chuffed with how it all turned out!  It’s by no means a showstopper, either in design or finish quality, but that was never the target.  The idea was to do something that would work as an occasional amateur racer and be cheap and easy to repair in that context, but also look credible in a paddock and be noticed in a pub car park–not in a “wow look at that, how much has he spent!” way, but in an understated “mmm…what’s been going on with that then” sort of way.  And I would like to think that is how it has turned out!

(Editor’s note:  Absolutely, we were attracted in just this fashion.)

There may be more changes in the future–I will see how the next few months go–but I do have a spare 17” supermoto back wheel, which will give me a disc rear brake and better tyre and gearing choice for hill climbing.  We will see…

The bike is now used daily for going to work, and will be going out to shows and club runs, and as it has free old bike tax status will be used all year round, and always with a smile!

Figures and Thanks

BSA Thunderbolt Custom

I will end with thanks and a few facts and figures.

Wheelbase now reduced to approximately 1400mm; fork angle brought in to about 25 deg from the original 28 deg; trail is about 90-100mm; seat height is now 850mm, as I am using gas shocks from a 550 zephyr on the back.  MOT weight is 182kg, about 400lbs wet, which is only about 5kg lighter than standard.

First and biggest thanks must go to Roy and Steve , my employers at R and P Engineering (01793 710421), who have allowed me to keep the bike in our workshops while the mods have been carried out and have always been supportive and interested in its progress and not minded me using stock and equipment to get the job done!  And also my mate Neil Pearson, who is my “go to” machinist if I think something is beyond my basic machining ability.

Local rock and roll band, Josie and the Outlaw–find them on Youtube for genuine good time 50’s rock’n’roll–gave me the Kawasaki swingarm to cut up and also the rare Honda 750 swingout kickstart to modify so I could design the rearsets.  Couldn’t have done it without them…

Peter Hammonds in Cirencester have supplied a few spares over the years and C T transfers in Stroud supplied the tank decals at short notice when the original ones were lost in the post!

BSA Thunderbolt on the Track


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  3. If anything, it gives the bike an almost bullish feel as if leaping those Great Escape fences will work up no more sweat than passing over shadows on the asphalt. Fuel consumption is in the mid-50s, which is less than you might expect from a 500cc BSA, and top end performance pegs out at around 90mph.

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