Project Bike SOTU 2018: Bultakenstein

Bultakenstein will be seven years old next month, and if you’ve been following these SOTU updates, you will not be surprised to learn that it is still nowhere near complete. For the last five months, my build-a-motorcycle-from-random-salvage-parts project (along with its little brother, my ’73 Honda CL125S project) has been crammed into one corner of my garage, as the demands of adult life have forced my attention elsewhere. I always feel slightly embarrassed to reveal how excruciatingly slow progress on this has been, but it has always been a back-burner, as-time-allows project. When I first considered this year’s Hooniverse SOTU extravaganza, I was convinced that I had no accomplishments to write about for this past year. However, when I went back and looked at my notes and photos, I realized that between my last SOTU in August 2017 and early May of this year, I did actually make a lot of progress on the frame and engine mounts, while also acquiring additional suspension, engine and body parts.


I had struggled with the ex-motocross frame’s convoluted rear frame section, originally designed to accommodate lengthened off-road shocks for more wheel travel. I eventually hacked it off and determined to build my own rear triangle from raw stock, which is where I left this project a year ago.  Such extensive frame modification required a frame jig; After years of researching what was available, I finally pulled the trigger and purchased an excellent bolt-together kit from Chop Source. I also spent $78 for a cheap tubing notcher for fish-mouthing the rear seat support tubes to match the top loop.
Around that time, I happened to find a photo of a converted Bultaco Pursang on the web, somewhat similar to what I am trying to do. Looking at it, all I could think was that if I had simply stuck with twin shocks for this project, I could have skipped literally years of delays and heartaches, all of which stem from the monoshock YZ swingarm I unwisely started with. Digging into the Yamaha parts interchange, I learned that several of the ’74-’76 DT and MX models used exactly the same swingarm pivot bolt, bushing and caps as the YZ125C and is nearly a bolt-in replacement. I purchased a ’76 DT175 swingarm off Ebay for less than $32, including shipping. The twin-shock version is not the most substantial swingarm, but it offers an easy option for conventional shocks using all the hardware I already had. I’d simply need to add a couple of shock mounts to the new rear frame section I was currently constructing.

So, I made up some simple upper shock mounts out of plate steel. A couple of the no-nonsense curmudgeons over at said that my gussets were crap, for a number of reasons. And they were 100% right. That forum has a reputation for harsh feedback, but I cannot tell you how many times that crowd has forced me to recognize the shortcomings of my work, and do (re-do) things the right way. When criticism is accurate and constructive, it’s worth more than gold.

Despite having a small MIG welder and doing the preliminary fit-up myself, I elected to have a local pro welder do the final alignment and welding. Even though the whole point this project is learning the skills to do things myself, I think it was wise to have a pro weld the frame tubes. I picked up the frame from the welder on April 3, shortly before my spare time for this project evaporated. It’s well put together and accurately aligned. There’s a lot of beautification to do, but it’s nice to have something that can be called a complete frame, not the chopped-up assortment of tubes and fittings I’ve had for the past two years.


Another major challenge of this project is that the Yamaha swingarms do not allow the swing arm pivot to double as the rear engine mount, which was the Bultaco way. I came up with an exceedingly complex arrangement to overcome this while simultaneously rubber-mounting the rear of the engine. Progress came in the form of my False Neutral co-host Garrett, who had the required engine mounting plates laser cut (yes, really laser cut) at a fabrication shop he’s friendly with. They’re absolutely perfect. The lower mounting stud will mount through the frame on both sides to take some of the load off my skinny little swingarm pivot bolt, which is a good thing for rigidity and durability.
Obviously, I still have plenty of fabrication work to do here. I bought plenty of 1.25″ x 0.120″ wall DOM tubing and confirmed that Suzuki GT rubber mounts are a properly snug push-fit. They will have small mounting tabs welded onto them, as shown in my original napkin sketch. The engine is slightly offset from the frame center line. Rather than attempting to work out the exact engine offset ahead of time and then attempting to weld the tabs on in the exact correct location, I will place them further outboard from the engine mounting bosses and use alloy spacers to locate the engine into place between them. As long as the tabs are anywhere inboard of the chain run, I’m good.


I originally used a GS650L cruiser front end, solely because I happened to locate one locally for little money. Unfortunately, thir leading axle offset was designed for a raked-out cruiser frame; once installed on Bultakenstein the steering geometry suggested frighteningly unstable handling. Thanks to in-depth parts diagram sleuthing, I determined that a set of ’82 GS750EZ forks use the same axle, would and slip right into my triple clamps (which had already been custom machined to mate with the Bultaco frame), but due to the different axle offset, would provide a great deal more trail, for much more appropriate dynamic stability on the road. Eventually, I was able to score  off Ebay for $100 shipped. They turned out to be the exact same length from the tube caps to the front axle as the 650L forks, and nothing but a one small off-the-shelf axle spacer was needed to complete the swap.


Chassis fabrication has dominated my efforts, but I also have an engine to build. This will also be accomplished with salvage parts. I’ve picked up a fairly random collection of Bultaco engine parts along the way, but I was excited to pick up a nearly complete Bultaco Model 143 Frontera 360 engine. It has the larger cylinder opening and longer stroke of the big bore (363cc) bikes, the later (’75-on) left-side shift option, and the same desirable, wide-ratio gearing as the Lobito transmission I already have. When the “bottom end” I bought arrived, it was much more complete that I expected. It’s basically a complete motor, minus the barrel, head and clutch plates. It even came with the intake and exhaust manifolds, and a clean 2nd over piston that at least looks serviceable. The rod bearing is borked, but it’s a solid basis for a rebuild.


I picked up a very cool-looking plastic tank from a ’77 Can-Am Qualifier that is better proportioned than the old-timey Rex KL35 tank I originally bought for this project. It has a greater fuel capacity and fits the angular, late ’70s look I’ve had in my head. I’m specifically trying to make this look like a roadster and NOT like a dirt bike, and using a poly tank kind of works against that, but I think if I clean it up, re-dye the plastic (yes, that’s a thing), and buff it out, it will look acceptable.


Switching to the twin-shock swing arm meant abandoning one of the fundamental design catalysts that first prompted this project. However, the monoshock idea isn’t dead yet; I am trying not to do anything that would eliminate either option. If at all possible, I’d like to end up with a bike that can accept either swingarm and be able to swap between the dual-shock DT setup and the YZ monoshock at will. In fact, if you look abaove and at the lede image at the top of this page, you’ll see the YZ swingarm in place along with the twin upper shock mounts on the frame. In the meantime, the simpler rear suspension removes some roadblocks and will help the project move forward.
I probably won’t have a chance to touch this project again until sometime in November, but I have continued to tuck spare funds set aside throughout the summer, so when I get the time I should have the resources to make progress — however slowly — once again.

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6 responses to “Project Bike SOTU 2018: Bultakenstein”

  1. theskitter Avatar

    What tangled webs we weave.
    It is okay to continue along the original path, with all the solutions you’ve already worked out.
    It is okay to change to a different idea, if it is cleaner and easier.
    It is okay to want both, though the window where both work is that might smaller, the relationships that much more difficult, the finish that much further away.

    1. Peter Tanshanomi Avatar

      Skitter, my man! How have you been? Good to hear from you, even if you are bustin’ my chops.

      1. theskitter Avatar

        You as well!
        This is still my favorite project.
        Remember the last time we saw each other?
        And my 300k Insight had just started exploding?
        Replaced with Volt, but still haven’t managed to sell it.
        Edit: First comment was meant as a pep talk.

        1. Peter Tanshanomi Avatar

          Yeah, I won’t let the dual-mode swingarm be a deal-breaker. Just something that might be a bonus if I can make it work. Thanks for the pep talk. I am finishing up a big project for my dad’s 90th birthday, so I hope to have some discretionary time to devote to this after around Holloween.

    1. Vairship Avatar

      If at first you don’t succeed,
      skydiving may not be your sport.

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