Triumph unit twin rebuild
With nothing more involved to do than sourcing a later cam and purchasing an electronic ignition kit,there were high hopes for some progress…
It’s fairly safe to say anyone reading this will have been to an autojumble, those wonderful places where boxes of parts wait to be rummaged through – and for us enthusiasts it’s a bit like Santa’s Grotto. Agreed, autojumbles have been a bit thin on the ground recently, but no matter, there are on-line autojumbles and of course Old Bike Mart.
Had I been able to go to an autojumble it’s probable a cam of the correct design would have been found for a model as common as a unit ‘C’ range Triumph twin. In this happy event the electronic ignition kit, or the trigger mechanism at least, would have something to attach to in the old points housing. Sadly without an autojumble the only recourse to cam heaven was the internet which turned up lots of cams at reasonable cost… all for the distributor engine. These lack the extended nose containing a taper for the auto advance unit to sit in and the points cam to operate.
In the electronic kit there will be no need for the auto advance unit, which is just as well as the only one I have is for a BSA
B40, but the tapered bore in the cam is still needed to hold the trigger mechanism. There were NOS cams available from the USA but the cost of purchase and delivery to the UK was prohibitive.
So, the problem to solve is making the original exhaust cam drive the points cam or electronic trigger when it isn’t designed to do this task. For the unit 350/500cc engines Triumph decided to run the cam direct in the alloy of the crankcase and retain them in position with a plate screwed in the timing chest. The drive cog presses on the end of the shaft sticking in the timing chest and a nut secures the cog in place. Some people can work through these things, I’m not generally one of them.
Solve the problem
Eventually it dawned on me I may not need the later cam after all. Instead, an accurately made cam nut, with an extended nose to accept the trigger mechanism, might just solve the problem. There is a precedent for this thinking and it is in the timing chest already… the oil pump drive. Triumph’s plunger pump is operated by the inlet cam nut which has an off-set peg machined in it to locate in an alloy block and convert spinning action to vertical movement to operate the plungers in the pump. Would a similar idea, obviously without the off-set peg, work to drive the trigger unit I wondered?
Well I couldn’t see why not, but I’m not an engineer, so I called someone who is and he couldn’t see why it shouldn’t work either. He qualified his thoughts by adding “the final machining should be done with the cam in the lathe and the cog and nut in place too…”
To be honest I wouldn’t be surprised to find just such a modification has already been tried in the past and I’ve just not heard of it as it seems so simple, and not at all Heath Robinson in anyway. If it hasn’t been tried before maybe the reason is there has always been a supply of correct parts at reasonable cost. Given the exacting nature of any machining required I won’t be attempting the job as unlike the oil pump drive which could cope with a slight inaccuracy the same cannot be said for an ignition drive. Any inaccuracy here would cause problems for the running of the engine. In any case to create this part would require a left-hand thread to be cut in one bit and a tapered hole bored in another part, and these skills are beyond me at the moment.
What I am capable of though is determining if there is room in the timing chest for all this to work. The tolerances are quite close and the nut does come up to the back of the points housing, it clears but it’s feeler gauge territory. Mind you, boring out the hole in the timing case wouldn’t be an
issue as the case has been so hacked about I’m not destroying a pristine part and as long as the hole is bored to a size of an oil seal then all will be well.
Other issues for the inside of the case are where the later end feed crank would sit, this is redundant now, as are all of its drillings – some have been welded over by a previous owner – but I may have a little alloy milled off the housing just to give a bit more clearance to the end of the crank.
Because the electronic system has advance and retard built into its workings, the original auto advance unit isn’t needed and the pick-up is on a plate which is the same size as the old points plate but a lot less busy. Even Triumph realised this points plate thing wouldn’t be ideal for their high performance machines which were taking on the world and they fitted a Lucas racing unit which bolted into what I originally thought was the standard housing and gave the elements of the ignition system an easier time as there was more room for them to operate and a cooler environment to be in.
On reflection and looking through Claudio Sintich’s book on the racing 500cc unit bikes, it seems a special timing case was cast. Now there’s an option for me to complicate things even more, I do have pattern making experience… perhaps not. While better in a high performance setting, these units didn’t look quite as nice as the original and likely cost more and they would soon be superseded when the early electronic kits became reliable. These units were driven by a small slotted shaft which seats in the end of the exhaust, as would the points cam so the concept of making up a special drive nut isn’t too out of the way and because it won’t be me making it the special nut will likely look right.
Stalled
In doing this modification I’ll need to know the length and diameter of the extended portion of the later cam, this could be scaled up from a photo I have but it would be nice if someone says “oh, easy, it’s XX long and XYZ diameter” – in practice I doubt a few thou difference would be a great issue as the nose runs in an oil seal. So, the ignition is on the way to being sorted but stalled through lack of autojumbles.
While all this attention on trying to create a spark has been going on I’ve ignored other parts of the bike and one big area of concern is the clutch. Triumph used what seems a bewildering number of bits and pieces to create ‘this year’s great clutch idea’ and all the bits almost fit so the potential for getting it wrong is very high. I did get it wrong and found a while ago the clutch lifted too far and the plates went past the edge of the drum which isn’t good. Still, I have parts books and a steel rule plus a phone to ask those more knowledgeable than me.
It is also time to do something about the cycle parts such as the footrests. Being a trials rider by default, road bike footrests tend to feel a little too far forward for me and I’m not keen on Triumph’s reliance on an interference fit on a taper to mount them either. What will happen is a plate will be fabricated and some spring loaded folding footrests will be welded on and the plate mounted on the various brackets which held the pillion rest hangers on the original bike.
Forks too need to be considered now and at the last autojumble I was at – The Classic Dirt Bike Show 2020 at Telford – I rummaged on the stalls for 35mm stanchions which would fit in the BSA yokes this bike wears. Nothing was quite right and I thought “Stafford in a few weeks, I’ll get some there…” Luckily there are signs of a return to normality here in the UK and autojumbles are returning, so this bit could be solved soon too.