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The BSA-Bobber story


Conceived really as an ornament, this project became a brit-bike tech-fest.
 It made the front pages of both Brit Chopper and 100% Biker. Its build-up was detailed on the Chopperbuilder web forum. Here it all is again.
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There were a few reasons for embarking on this;

First, I  was just finishing off my '59 Chevy El-Camino, I thought a cute little matching bobber would nicely complement it.

Secondly, I fancied a change from Harleys, and as it was at least 10 years since I built a brit bike, I had (almost) forgotten what a pain they are to build & maintain! And I thought that an A10 might be reasonably cheap to do. This is no longer the case!

I got a lot of history with A10s and there was ‘unfinished business’. There are several features to A10s which I really cant be doing with; dynamo electrics, the timing side main bearing bush, unfiltered oil, primitive ignition to name a few.

I started accumulating bits in 2005. A bloke near Cambridge with 2 shedfuls of BSA parts supplied some of the engine hard parts; a pair of A10 cases with a hole in them, a pristine pair of thick-flange barrels that were only at +20 and many of the small internals.

It was about this stage I decided to try to use parts from as many different bikes as possible

Geoff Hoare supplied a Norton crank, a 5x16 rear wheel with a XS650 hub, and an A10 plunger frame that was really too good to cut up. So I did anyway.

BSA alternator primary cases are hard to find, so I grabbed a pair of short alternator Triumph ones off ebay. The inner one was so shotgunned that I would have been better off fabricating a whole new one! I had to widen it anyway to accept the belt-drive which was another ebay score.


Here is a shot of the oiltank. I used an old Taff Morgan trick here, which is to make the ends out of old hubcaps. Hillman Imp ones used here. Of course when Taff used to do it, steel hubcaps were a lot easier to find!

The springers are repro ones from Zodiac. Made in poland, exact replicas as far as I can tell. I was going to use a pair of teles off a softail custom, but they looked way too massive on this so I shelled out for these.

This was the point where all notions of 'low-buck project' went out the window.
I knew from the start that I wanted a Norton gearbox, but so it seems does everyone else, and prices are ‘healthy’ I bit the bullet and paid a bloke at the Newark autojumble 180 for one. The next month there was a better one for 120. Piss.

 It was time to start mating all this junk together. I made engine plates out of ” H30 to mate the A10 cases to the Norton box, the right distance apart to take the triumph primary. Having made them and on offering the assemblage to the frame, it was clear that I had set the gearbox too low in relation to the lump, so making virtue out of necessity, I tipped the assembly forward so that the engine is now canted somewhat like a Commando.

 Anyone who knows BSAs knows that the swingarm type motor wont fit the plunger frame, so I extended the top tube 2.5”, pulled the front downtubes forward another 2”, (bending the front of the top tube to rake it slightly). The headstock was altered to take harley taper-rollers.

I shortened the centre tube 1” and the plunger units were replaced with fabricated axleplate ‘castings’, extending the rear end about 2”. New bottom cradle tubes were spliced in adding the bottom gearbox mount from which all the other mounts would be referenced.

Much time & thought went into the engine side of things here, and much of it will not be apparent when looking at the finished bike,  Heres the gist of it:

Matters to be dealt with were these:

1) Timing side crank bearing
2) Oiling system
3) Charging system
4) Ignition
5) Combustion efficiency
6) Primary drive

1 & 2.

The crank bearing is a notorious trouble spot on A10s. Stock, it’s a bronze bush and oil is fed into the crank for the bigends via a groove in the timing side main journal. Trouble is that it wears quickly, and the widening clearance bleeds off oil pressure to the extent that the LH bigend is starved of oil & wears out quickly too.

 The solution is well known: to fit a special INA combination ball / roller bearing instead of the bush and feed the oil into the crank from a quill on the end as on Triumphs & Nortons etc. This means also that the oilpump only needs to feed 2 bearings instead of 3

A local bearing stockist got it in for about 28

The crankcase needs to be bored to accept the bearing, which was done in the Bridgeport.

The crank also needs  to be drilled & a quill shaft pressed in. Not needed on mine as I used a Norton crank, which is already so drilled. I did need to re-machine it to accept BSA timing gear parts.


Other reasons to use a Norton crank: Its already the right size to accept the INA bearing, Its easier to find than a big-journal A10 one, It has 5mm more stroke, giving 705cc. The Bigends are 1.75” so even a badly worn one will regrind to the A10 1.6875” std size. The drive side shaft is the same size as the A10 one. And it has a shaft on the driveside to take an alternator rotor. AND it readily accepts the Norton belt drive that I scored on ebay.

 Of course, the extra stroke means that either the rods or the pistons need to be shorter. I took 1mm off the rod length whilst re-circling the big end bores. The rest was taken care of by using 1275cc mini pistons, which I’ll get to later.

 Normally when doing this bearing conversion you have to rig up some means of getting the oil from the oil pump into the end of the crank. There are several ways to do it, but I chose a new one.

Since the space normally occupied by the dynamo would be vacant, I opted to fill it with a modern cartridge oil filter, similar to the way its done on HD Sportsters. And while I was at it, I made it so it filters the oil after it comes out of the pump, and before it goes into the crank.

To do this I made a new inner timing cover out of 1” ally plate, with drillings to & from the oilfilter housing where the dynamo drive parts would have been. Oil is then fed into the crank via a bolt-on seal housing. I threaded the end of one of the drillings to take a pressure gauge.

The rockers on an A10 are normally fed from the return pipe, but some times this can result in more oil going back into the engine than the return pump can handle, The crankcase fills with oil & it comes out the breather.

So I made another oil take-off from the filter housing so the rockers can be pressure fed from the feed side. I made new rocker feed banjo bolts with a 1mm feed hole to restrict the flow.

That, hopefully will take care of the oil & bearing issues. Hopefully!

 I mentioned previously that I elected to use 1275cc mini pistons. Several reasons; One, a pair of mini pistons & rings are 45, A10 ones are 145!
Two, I needed a different pin height due to the 5mm extra stroke,
Three, mini ones are a more modern slipper design and more robust looking.
Four, I needed flat-tops. I’ll explain why later. 

Of course they don’t fit straight in, the pin is 1/16th” bigger, so I bored the pin bushes out to suit. The pin bosses needed to be machined for circlips (minis have press fit pins), and I needed to machine off the dish in the tops and add valve cutaways. All in all, a bit of a lot of work.
I got the barrels bored to suit, and then dry assembled it to do the valve cutaways. The whole rotating assembly then went to Basset Down for balancing.
This done, I send the pistons off to Camcoat to have them coated. Thermal barrier on top, dry film lube on the skirts, and heat-shedding underneath. Less heat going into the pistons leaves more to make power, and reduces any likelyhood of seizure. Also allows tighter piston to bore clearances.(the bloke who bored the cylinders looked a bit askance when I told him the desired clearance!)

The Head.

A10 ally heads in good condition fetch big bucks these days.  I scored an A7 one on ebay for 35. It wasn’t a bargain; I reckon it had sat in a salty puddle for 10 years. The bottom fin was almost gone and the head gasket surface was deeply pitted.

So...a bit of welding needed.

I got it bead blasted & got started. I figured that whilst it was hot I’d do a few mods too.
Twin carbs is the obvious one. I cut the manifold off & made a double flange to be welded on.
I wanted bolt on exhaust flanges. Old Bert Hopwood who designed the A10 must have been having a bad day when he drew up the exhaust attachments. “bollocks” he said, “I’ll just have the pipes shoved in the port” so as a result, A10s tend have fall-off exhausts.

I milled slots in the fins above & below the exhaust ports & filled them with weld to form bosses, which are threaded to take studs. The exhausts fitting the same way as on most Jap bikes.




The most radical mod, and the reason I wanted flat top pistons was to add squish pads to the combustion chamber, very much in the same as on evo harley heads.
Squish chambers are much faster burning, allowing less ignition advance, less prone to detonation, and are unleaded-gas friendly.

All this welding took ages and was a complete bitch due to many contaminants in the metal. Once done,& still hot, I installed ironhead sportster valve guides (machined to take evo stem seals). 

Next day, it was milling time. This also took ages as it was virtually a raw casting and every face & hole needed to be done. After this, it was just a matter of a bit of port-smoothing and valve seat recutting and it was ready for assembly. Just like that.

The Rest:

I'm not a great lover of points ignition.
And whilst a magneto can be an excellent way of providing sparks, the traditional british deployment of same leaves a great deal to be desired.
To any of you out there who have not had the pleasure of trying to time the ignition on an A10 or similar, it goes like this:

Rotate the crank until the piston is a certain distance from TDC, usually something like 3/8". You do this with a suitably marked biro down the plug hole.
Then place a cigarette paper between the points and carefully rotate the magneto until the paper loosens, indicating that the points are just opening.

Then you take the auto-advance pinion and jam something in it to hold it in the full advance position. Locate the pinion on the magneto shaft, meshing it on the timing gears.

Then you put a socket or something over the end of the auto advance unit, cos you don't want to whack the end nut, and sharply clout it onto the taper with a hammer.

Of course, during all this clownery, something will have moved, so you do it all again. And a third time.
Finally, you'll get the timing close enough and you rotate the crank 360 degrees to check the timing on the other cylinder. It'll be way off, but there is no adjustment, so you spilt the difference between the 2 and start all over again.

When you've done, there is no way of adjusting the timing unless you pull the timing cover off and do it yet again.

Sod all that.

What I did was to use Harley parts. An evo pickup plate triggered by a specially turned-up rotor, all on the end of the crank.
I used a screaming eagle module and an ordinary dual-fire coil.

As it is running crank-speed instead of the harley cam-speed, the cutout in the rotor had to be twice as long, and obviously there is only one cutout instead of 2 on the HD.

I mounted the pickup plate on an ally housing I turned up which stands on spacers over the alternator stator.

The ignition rotor is located on a roll pin drilled & pressed into the steel inner part of the alternator rotor, A thin steel plate separates the ignition from alternator magnetism, and a small notch in this plate juxtaposes with centrepunch marks on the ally pickup plate housing to provide timing information. TDC, full advance etc.

This means the timing is adjustable whilst running, and can be done with a strobe.

There is no way that timing can differ between cylinders, as they both fire at once.

The screaming eagle setup gives a feckoff great spark, and at low rpm, and while starting, gives two feckoff great sparks.

I mounted the coil in a magneto-looking housing bolted to the old mag flange. Inquisitive types get told that it's a Huntley & Palmer magneto.

You'll have gathered from this that I also used a lucas type alternator for the charging system, regulated via a podtronix reg/rectifier.
All of this stuff hides away behind the Triumph short-alternator primary cover.

The primary drive itself, as mentioned before is a belt drive setup of unknown origin, but  a belt from Norvil fitted OK. The gearing is all wrong, it runs out of revs too soon. When time allows, I'll fit a bigger engine pulley.

I'm toying with the idea of converting the bike to electric start using a sportster starter motor. (my right foot isn't what it was) It'll involve making another inner primary case and will only be feasible if room can be found for the ring gear. Dont hold your breath!