The long swingarm will need an extended drive shaft to match.
As a start I measured the stock shaft to get a kind of baseline. I used this height gage and a ground pin to measure the length. I zeroed the gage with the pin under it, then put the pin in the yoke bores and took the reading at the top of the pin as the length of the shaft to the bores. It measured 284.27mm.
The stock shaft is center drilled on both ends, so I put the shaft on a lathe between two centers and setup a dial indicator to check the runout at the center of the shaft. I measured .064mm max.
My plan for the extension was to cut the shaft at its center, drill a 5mm hole in the end of each of the resulting sections, then make an insert that had a section on each end turned down to 5mm. The turned down sections would press into the holes to hold the insert in alignment for welding.
Here's the machining of one end of the insert. I turned the end down to 5.1mm, and on this end I put a very small center hole to use when aligning the first weld joint.
After I got the shaft and insert ready I pressed the insert into the shaft with this arbor press
. The shaft plus insert was too long to fit into the press so I used these C-clamps and a piece of steel plate to make a base to press against.
After I got the insert and one shaft section pressed together I mounted them in the lathe and checked the runout. To adjust the alignment I supported the shaft with blocks on each end and tapped on it with a plastic mallet. After a few iterations of checking and tapping then checking I could get the runout to about .015mm.
With the insert aligned I tack welded the joint then put a bead all the way around the shaft.
I let the shaft cool slowly and then ground away the excess bead.
The stock shaft was heat treated, but I decided it would be better to leave the shaft soft at the weld points than to try a heat treatment and have it too brittle. If I have trouble with it when I put it into use I'll consider having it done by a heat treating service. After welding I found the runout was close to 1mm.
I used this setup to get the shaft aligned. It took a few trips between the lathe and press to get the shaft back to about .060mm runout.
After the first joint was finished I repeated the process on the second joint. Here's the shaft just after finishing the second bead.
After the insert was fully welded in I did a final alignment of both weld joints to get the final shaft runout to about .060mm.
With the shaft aligned I could then turn down the weld beads to the diameter of the shaft. I mounted the shaft in this four-jaw chuck
and used a dial indicator to get it precisely centered for machining.
I saw a monolever arm and shaft up for a reasonable price, so figured it wouldn't be a bad idea to have a spare on hand. Here's how the extended parts compare.
After finishing the extension work, I can say that I don't think my method of a press fit alignment was a good one. I think a better way would be to make a fixture that holds the shaft ends and the insert in position for welding.
Here's a shaft fixture drawing
of what I'm thinking about. It can handle +100mm and +125mm shafts. I have some 3"x2"x3/16" rectangular tube left over from the swingarm fixture I can use for the base and I'm thinking I'll pickup some rectangular bar stock for the V-blocks next time I'm at the scrap yard.