JB Atoms

Well I've lost the pics of boring the crankshaft for the transfer passage. (of all the things I've lost I miss my mind the most!) While on centre, I centre drilled, drilled 3.5mm to depth, then used a 4mm end mill to depth to make sure the 3.5mm hadn't wandered off. (Small drill sizes tend not to drill straight) Then drilled 4.5mm knowing the passage was now straight, then reamed to final size (3/16")
Nothing fits in this hole but the reamed finish will improve performance. I also noticed that the original crankshaft was drilled right through, I have stopped at the carburetor port, this will improve crank case compression by reducing the volume.

Phew! Then I have to cut the big end pin. To do this I have to off set the crankshaft by 4.8mm and then turn the pin. It's easy with the 4 jaw chuck, with the dial clock in place I zeroed the clock to 1.00mm, then with the clamp block exactly vertical I used the jaws to push the block across until the clock read 5.8mm. Now when I turn the pin I will have a stroke of 3/8" Where I do have to be careful is not to have too much spindle speed, the out of balance caused by the off centre block can give me an oval pin.


I've taken the pin to 4.2mm that leave 0.1mm of material to come off.
At this point I left the crankshaft to make a gauge for the final pin size, this gauge will also be the basis of the new connecting rod I have to make.
The centre distance is 18.5mm and the reamed holes are 5/32"


Next up final size for the crank pin.

At these sizes it's very difficult to get a decent surface finish so I decided to try a gadget I got from Banggood. It's supposed to be a drill, but!

Whatever works!


And size.

Next up, sort out the balance lobe by cutting off some material. First the marking out, in the factory we always had a 5ltr bottle of Meths with Engineers blue mixed in, since I don't need that kind of quantity I just use a marker pen and scribe my cut lines on it.




Never hold anything in a vice without soft jaws! I have 2 types, Aluminium and Brass.


I thought of using a milling cutter to do this but decided the set up time didn't warrant it. Modern thinking doesn't cut the lobe this shape anyway.

If any one's interested I used the Neanderthal approach (Alex Whittaker would be proud of me) a small angle grinder with a thin (1mm) cutting blade in it.
The final operation, and the most critical, drilling the hole in the inlet passage. To do this I went back to my rectangular clamp block and drilled the correct size hole (4.8mm or 3/16") down the split line into the crankshaft hole.


Having done that I then placed the original shaft in the block and pushed the drill down until it locked the shaft by going into the hole. I was then able to accurately measure the offset of the pin to one face and the protrusion of the web.


Put the new shaft in, replicate the measurements and drill.


Job done.

one tenor wrote: Very nice Klippy but the web could have stayed round I think !??? Never mind it's done now . Thanks very much indeed. can't tell them apart bar the colour.

The creation of the web is to make a counter weight for the piston, this will reduce vibration.
I find most types of Water pump pliers are useless, they always slip, usually pinching a finger in the process! The Fatmax were given to me and certainly they seem to be good.
We now need to complete the connecting rod.


The proto rod was cut out of 6mm Duralumin flat bar, I've drilled and reamed the small and big ends, I used them as a gauge for the crank pin, the first job is to create a round on the scrap end so we can turn it.


Now we turn it around so we can turn the main body in a 3 jaw chuck.

Having finished the conrod I revisited a self created problem.


When I turned the crankshaft I made it 1mm too long for refinishing the driver taper, of course Murphy stepped in and made sure I hit it first time! I don't want the shaft to float that much so I had to make a 0.9mm spacer.


Now I am ready to assemble with the new conrod.