Jumping into my clean-slate revision of this project, the first thing I needed to find was an appropriate swingarm for my Bultaco frame. I needed a twin-shock design with a larger pivot shaft and robust construction (preferably square tubing) that would still fit within the frame and not be too long. After spending an inordinate amount of time surfing Ebay, poring over OE parts diagrams, and cross-referencing part numbers, the swingarm from a 1986 Yamaha SRX600 (or “SRX-6”) seemed to be an ideal choice. I found a nice one on Ebay for $100, shipped. It turned out to be unexpectedly, almost shockingly close to perfect. The stock width at the pivot fits the frame precisely. How precisely? In this photo, it is being held into place without the pivot bolt. It’s so perfectly snug, it will not fit in between the round frame tubes at the back of the side plates. I have to put it through at an angle and then pull it into position from the front. No cutting, no filing, no spacers needed.
Once I was confident it would work, I enlarged the swingarm mounting holes to 16mm using a combination of a metric step drill and a Rotabroach tool. The stock SRX600 pivot bolt slipped in perfectly. It is much longer than needed, but that’s a minor quibble. The width of the shock mounts also perfectly matches the top mounts with the bolts and spacers I have on hand.
When I mounted the rear shocks I have on hand, the swingarm angle is a bit short, so I will need some shock extensions. That’s just a simple $20 Amazon order.
Something else I was unsatisfied with was the GS450T 17″ diameter rear wheel I had been using, so I have purchased an 18″ GS400E wheel that has the same hub dimensions and uses the same rear brake and sprocket carrier. The SRX600 is set up for the same axle diameter, so there is no problem fitting the wheel to the swingarm. I am currently waiting for an SRX600 axle from Ebay in order to get the proper width.
The new 18″ rim is only 1.85″ (WM2) width, which requires a 3.50×18 tire. There are not a lot of options in this size, so I picked up a Shinko 712 tire. I didn’t want to break the bank, considering the last set of tires I bought for this bike aged out before ever going further than my driveway. Normally, a narrower tire is seen as a detriment, but I am totally okay with this. The result will be very appropriate to the size, era, and use of the bike, and similar Bultaco’s final Metralla GTS, a major source of inspiration.
More clearance above the wheel may be needed. I originally kept the frame flat because I wasn’t sure what style seat I would be using, but I might want to angle the rear frame loop upward, regardless of what bodywork I end up using.
The finish on the 18″ rim is really grody. I have a new rear tire on order for it, and the front tire is already 12-year-old rubber, so I might just have both wheels blasted before mounting new tires. I have always liked the look of this style wheel without accent paint, as on the original GS1000S Katana:
