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Getting a tacho drive fixed on a classic racing bike
bogstandard:
A few days ago, a friend brought around some bits and pieces of a classic racing BSA Bantam. A 1960's+ 175cc two stroker. The road bikes were very quickly found to be easily tuneable, and a new class of racing was born. This bike had been sitting in a ditch for over 2 years, with half the engine under water, and was thought to be a gonner, until he got the engine apart and found it was clean as a whistle inside.
Now to the tacho drama.
The bike had suffered some damage, hence it's life in a ditch. These first few pics show what it was.
The drive piece that actually transferred the engine revs to the right angle tacho drive was rock hard, and as such, the end had snapped off, hence no drive.
A racing bike has no need of a speedo, but it is of paramount importance to know your engine speed for gear changing so that you can stay within the engines power band. This engine as a stock item doesn't have a tacho drive, so what is done, they get hold of a drive off another model of bike, and bang it on. In this case, it was welded (and by the looks of it, not using TIG, this one was done the old way with oxy/acet).
The gear sits in the big hole, with the shaft disappearing into the depths.
And popping out here, on the inside of the timing cover.
Where you see that well manicured machinists finger pointing is the drive off the end of the crank, the sprocket is held on with a 1/2" BSF nut, and onto that nut was a piece that is now missing, that used to engage with the drive gear spindle.
So I need to make a new nut with a drive on it. Unfortunately, I don't have my lathe ready enough yet for single point cutting. I just don't know how accurate imperial threads will come off it until I have tried it out. Another problem, I don't have any 1/2" BSF taps. But I know a man who does.
A quick call to Stew, and very soon I had in my grubby hands just what is needed.
So now to the fix.
I have decided that the hardened drive is just too brittle, so if I put something on the end that is tough and fairly hard, it should do the job.
I am going with a piece of silver steel that will be silver soldered onto the end of the shaft. If I do it right, the shaft will stay glass hard for use as a bearing surface in the bronze bush, and the silver steel won't harden too much so that it gets to a brittle stage.
There is no way I can turn the shaft, it is just too hard. So out came the toolpost grinder and I soon had a spigot ground onto the end of it.
After shortening the spigot on the offhand grinder, this is what it looks like.
The slug on the end will have a fairly close fitting hole drilled into it, then that will sit over the spigot, and both parts will then be soldered together.
Thats as far as I have got at the moment, the next instalment when I get a round tuit.
Bogs
John Hill:
My James used to leave Beeza Banties in the dust! :med:
I thought imperial threads would be perfect if cut with the 120/127 change gear? Maybe there is some other factor I dont know about... :scratch:
bogstandard:
John,
I used to run around on a James with a Villiers 2T engine, it would go on forever. But the best Villiers engined bike I ever saw was a Greeves scrambler owned by a friend of mine.
It just so happened that the Bantam was very prolific in the 60's because I think it was classed like a Honda 50, a get you to work and back bike, so that is why it was used, availability.
I don't want to get into thread bashing on this post, but just as a summary, imp threads on a metric machine, even with the conversion gears as we have, as far as I know are only super close, not exact. I will know more in a few days, as I have to make a dummy spindle, and that has to be turned by single point tool, a die will just not be straight enough for the job it has to do.
John
John Hill:
John, my James was in truth, a very miserable machine though the engine did run well for about 30 miles before I had to clean the whiskers from the plug points. Nowadays I would know how to select fuel and oil to prevent that but back then I went by the inscription on the fuel cap. Bantams were popular here too but in this country they would have had to compete in the same price range with various pre-war and war surplus bikes of much bigger capacity of both British and American origin.
Mine had a pressed steel frame which must be been twisted as it would never run straight but eventually someone bought it and I trust they lived to 'enjoy' it.
I will be interested to read of your imperial threading experiences in due course.
bogstandard:
I got to the stage last time where the softer end bit needed to be made and fitted. So a piece of silver steel (drill rod), just a little longer and larger diameter than what is required was drilled to give a loose 0.002" (0.05mm) fit on the spigot. This is to allow the silver solder to form and flow thru capilliary action all thru the joint. If you don't leave a gap around this size, your silver solder will most probably only be sitting on the surface and will result in a very weak joint.
The two bits ready for jointing together.
And this is how it will look when finished (well almost).
Tiny rings of solder were placed down into the hole, and around the ground spigot. You have to try to imagine how much solder will be required to fill all the voids in the joint after it is completed. That can only come with experience, and no calculations can be given. The more you do, the better at it you get.
Flux was then spread around the spigot and down into the hole. ALWAYS use a good quality flux, old wives recipes are just no use, and by doing it correctly, you will end up with a perfect joint and very little cleanup required.
A little hearth was made up out of broken firebricks, and because I was trying to protect the previously hardened shaft, that was protected with a bit as well.
The long bolt serves a purpose here. When the part is being warmed up, the flux expands, and like a piston in a bore, will try to push the two bits apart. The bolt holds everything together by pushing the bits together in the initial warming up stage. When the right temp is reached, you can see the solder flowing, then another push with the bolt displaces any excess solder out of the joint.
It was left to cool down naturally for a few seconds, until the solder had solidified, then the part was quenched in water, but with the silver steel bit not being quenched, being held slightly clear of the water.
A quickie rub over with a bit of worn emery shows just how little excess soldering material there is. A file test was done, and it showed exactly what I was after. An original very hard shaft, and a softer drive end to absorb any shocks during running.
The toolpost grinder was brought into play again, and once the original shaft was set up for zero runout, the end attachment was ground down to exactly the original shaft diameter.
Finished with the grinding now. If you look really hard, you can just about see the silver soldered joint. Later, the soft end can be machined to the required drive paddle profile.
It fits perfectly thru the bronze bush and is just slightly overlength. The inside of the timing cover will be machined and cleaned up, to make the area a bit more presentable, and give me a bit more space to work with.
A bit of prodding with a rod and marking it up at certain depths then allowed me to get a rough sketch made of the nut that is required. Get that made, and it will just be a matter of tweaking things to fit.
The next bit of the exercise will be to make the required nut. That will mean I will have to single point cut an imperial thread. A thing I have never done on this machine, so a bit of a learning curve on my part will be required.
Bogs
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