I still haven’t decided whether to re-machine the swingarm pivot to eliminate the swingarm offset, but considering it led me to re-order a few tasks. I did some measuring and determined that the swingarm would need to shift about 10.5 mm to the right. With the tools I have available in my garage–calipers, bubble level, plumb bob, steel rule, laser and chalk lines–I am confident that measurement is accurate to within about .75 mm, or 1/32″. That’s close enough to look visually correct, but regardless of how I position the swingarm, I will need to center the wheel properly at some point, without using the swingarm as a reliable point of reference.

So, what reference points can I confidently use to find the longitudinal centerline of the bike? The answer is not as many as you’d think. Motorcycles are often remarkably asymmetrical in detail. Few places on any frame are definitively on the longitudinal center of the bike (the Y-Z plane) because you can shift nearly any component to the right and left at least a little. Chassis bits are often shifted around to clear drive-line parts, the exhaust, single-sided swingarms, and the like. In actuality, not only is my YZ125C swingarm pivot asymmetrical (as I’ve discussed at length), but the Bultaco M192 frame I am using for this project offsets the whole engine slightly to one side. (This is not all that uncommon–BSA unit singles, Triumph TSX, etc., all did it. Heck, the classic Vespa hung the engine out completely to the right of the rear wheel!)

So what truly MUST be along the centerline of the bike? The answer is surprisingly straightforward: the steering stem pivot and the contact patches of the wheels. That’s it!
Look at this diagram:

Even with no other part of the bike described, we have already established a longitudinal line that know must continue back to the rear tire contact patch. Even though my drawing does not include necessary details such as fork offset, etc., none of them fundamentally alter this concept. All I have to do is place both the center of the front tire and a plumb bob dropped from the center of the steering stem on a chalkline, and I can know fairly precisely where my rear tire must go. By deduction, I can (if I wish) also position the swingarm so that the side plates that accept the axle are equally spaced from the centerline, truly centering it, rather than just visually making it look correct. Therefore, I need to first install the front wheel, then the rear.

Back in 1985, I swapped the front end on my Pursang roadracer for a GT250 front end, so this is one part of this project I have actual experience doing. Since all the interaction between the front end assembly and the frame is concentric around the steering pivot, combining them is really easy…if you have a lathe. I don’t, yet, so I had to hire this out to a machine shop near my home.

The Suzuki GS650L front end uses a 25x47x15mm upper bearing, and a 30x55x17mm lower bearing. The Bultaco frame is designed to accept two identical 30205 bearings top and bottom that measure 25 ID x 52 OD x 16.5. The Suzuki steering stem is therefore a wider diameter at the bottom than the top. It is designed to fit a steering head tube about 21mm longer than the one in my Bultaco frame.

Since the two bikes use the same upper bearing inside diameter, we already have a fit there. The minor difference in bearing thickness won’t matter; the bearing “floats” up and down on the shaft a bit to allow the bearing to be properly snugged up against the races after they’ve been pressed into the frame. All I have to do is have the shaft turned down to allow the bottom 30205 bearing to slide down and stop about 21mm above the bottom triple clamp. This is a scale drawing that shows the necessary modifications:

The blue lines illustrate the Suzuki GS steering stem. The orange lines show the steering stem as I need it to be. The purple shows the spacer I’ll add to seat the lower bearing at the proper distance from the top bearing to fit the Bultaco frame tube. This design leaves the lower bearing surface unaltered, for strength and ease of machining. As a bonus, the steering stem could still be reinstalled on a Suzuki GS at some point in the future, if desired.

I dropped the steering stem off at a machine shop down the street from me this morning before work. They are going to charge me $140 to machine the stem and make the spacer. That’s a lot of money in the scope of this project, but when you consider it’s all that is necessary to combine two major components from two vastly different motorcycles, it’s not too bad. And until such time as I have a lathe with a 12″ swing over the bed, I can’t do it myself!

Once I get the front end mounted, I will be readily able to configure and mount the rear wheel. I am making all sorts of assumptions about the parts I have, such as the fact that the fork tubes are straight, my cast wheels are true, and the swingarm pivot is perpendicular to the centerline. But given the parts I’ve started with, and the ways I have modified them to fit them together, I am confident that all these factors will end up within acceptable industry tolerances…at least for the mid-1970s!

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