The fourth time’s the charm? I am now on my fourth swingarm for Bultakenstein, and I think we finally have a winner.
Looking for a path forward on the Bultaco, I considered:
- Doctoring the back of the engine cases to clear the swingarm. However, I would be messing up the one piece of this project that has significant, marketable value.
- Modifying the SRX-6 swingarm to clear the crankcase mounting bosses. This was doable, but it didn’t address the outstanding issue of actually mounting the rear of the engine somehow.
- Going with an entirely different engine, such as a CG250, or building the older 200cc Bultaco engine I have the modified crankcases and a bunch of other parts for.
At this point, I wondered what I would do with that nice, rebuilt 360 motor I have, if I didn’t use it in this chassis. Would it fit in the Montesa frame? I retrieved “El Fènix” from the storage unit and, sure enough, there’s plenty of room. Even better, it has a Yamaha SR250 swingarm, which is designed to mount the back of the engine to the swingarm pivot bolt—exactly the way Bultaco did it originally—thanks to an open gap in the middle of the axle, between the bearings. Super easy! But then there’s the whole issue of making up custom front and lower brackets. I looked at the two frames. What I really wanted was the original Bul frame, but with…the…SR250 SWINGARM! Of course, why hadn’t I thought of that? Both use a 16mm pivot bolt, and the width of the two frames where the swingarm mounts are within 1/8″. With surprisingly little effort, I was able to swap the SRX-6 and SR250 swingarms between the two chassis.
I’m disappointed to lose the strength and style of the SRX-6’s square swingarm, but I’m super happy to finally have the K.I.S.S. solution I should have set my sights on 15 years ago. The wheelbase and construction are similar to the DT175 swingarm, but I still retain the more robust 16mm pivot. The shock mounts are even further to the rear now, but switching to my longest shock extensions compensated perfectly. The only downside is that now I definitely need to modify the rear frame loop for clearance. Frankly, I should have committed to that anyway.
Besides the frame loop, there are couple of other tasks ahead. I need to make up rear mounting plates. I made up a couple of dummies out of 1/8″ aluminum, but they need some tweaking before I try to fabricate real ones from thicker stock. I will also need to make spacers to center the rear wheel…again. But that’s nothing I haven’t done multiple times already.
I am absolutely thrilled by this. Ever since I had abandoned the heavy, cumbersome rubber engine mounts I originally fabricated, there was this looming task of welding new crossbars and mounting tabs into the frame. The scope of that was in large part what had me dragging my feet on this project. Suddenly, that whole headache has gone away.
Yosoro! Onward!