I have a love for old two strokes. Spanish brands are cool, just because they are different. The short history is Francesc Bulto was the director at Montesa motorcycles. The economy tanked in the mid fifties and the other partners in Montesa wanted to pull out of racing. Paco Bulto had no intention of quiting racing so he boldly went off on his own starting Bultaco. Bultaco coming from Bulto + Paco. They went on to win many races and world championships on the road and also dirt, especially in trials. The Sherpa T was my first introduction to Bultacos in the seventies. Lightweight, nimble, and really beautifully crafted.
I have a really nice SherpaT but wanted to do a road race bike. I picked up an old Alpina 350 138 version for this project (pic 1).
I pulled it apart and went through the engine. I have to say that polishing a 40 year old, slightly abused dirt bike engine is a lot of work. I should start with museum pieces from now on. Oh wait…
I didn’t want to use gaskets so I machined an o-ring groove on one side of the case, and on the clutch cover. New bearings and seals and it went together like a watch. There is an old idea that Bultacos were questionably made, probably from Sammy Millers recounting of things, but he is a legendary racer and not a builder, so… I think they are beautifully built. Splitting the cases vertically is a little tedious putting back together, but everything fits and none of it is questionably designed or manufactured.
I started using Autodesks Fusion360 for design and programing the CNC machines. Coming from the old school of Solidworks design and then transfer into a programing system like Gibbs, Featurecam, Mastercam, etc, learning Fusion is a new and challenging experience. With Fusion you design the whole thing and then just turn off the features you are not focused on and then generate the machine code for that part. The swingarm/rear engine mount piece needed to be machined. The rest of it will be fabricated tubing. I decided to use aluminum tubing. Why aluminum?; Something different. The big single may just pull it apart, but I am doing it anyway.
That is where I will leave you. Be back when I have made more progress.
I have had welder problems. I bought a new TIG welder last year that welds steel fine, but I get horrible contamination in aluminum. I bought a new torch, changed bottles of argon, etc. Nothing fixed the problem. I finally called Everlast and told them I was done working on this problem, and that it very well could be an issue I created, but I needed to move forward and I wasn’t with this welder. Mike at Everlast, who is really terrific to work with, sent me a new control board. Fixed everything!!! I am so happy. Now the machine works perfect, I just wish my welding was as good as it was a few years ago. The skill always comes back about the time I am done with the job, and then I don’t pick up the torch again for six months and have to re-remember what I am doing.
I got the main loop done. Next is a swingarm, and to figure out a shock.
I machined a couple of swingarm stays. I tried bending a U to tie the swingarm together on Salt City Builds bender but it collapsed the inside of the tube, so i filled another section with buck shot and capped the ends. It worked excellent for the 1/8 wall but broke the 1/16 wall. I need to lay it all out and weld it together
I welded the swingarm together. It came out really square and the bore stayed aligned. I was concerned aluminum would move around more when I was welding it but it stays pretty stable. I had Dave Tagg at Wright’s motorcycles mount an old tire on the Bultaco rear for a mock up. I like the original hub and think I will use it. I need to get a shouldered rim like the front Moto Guzzi wheel. I machined shock mounts yesterday. I have this old Yamaha R6 shock I am using for setting up, but it is way too stiff to use. I hope I can pick up a Ohlins on ebay and respring it. Either way I will just tack the mount on for now in case I change my mind later.
I have been out showing off Alpha so slow going on this project. I designed and machined a new exhaust flange that i angled down 20 degrees. My header molds were made for Yamaha RD series that have inclined cylinders so they only need to go about 120 degrees to get them parallel with the bottom of the bike. Bultaco cylinders are straight up. I had to cheat as much as possible everywhere to utilize my existing header molds for the pipe. I had a fair expansion chamber that I made for a bike for my dad years ago. Since it was already made I just cut the header portion off and made a new one. The Bultaco exhaust port is huge. 1.8″ at the face of the flange. I am not sure why it has such huge ports. New designed use 70% of bore as the width of the exhaust port at the piston and very short, or oval. 70% is 2.295 wide exhaust port at the piston so maybe it is not that crazy. The area is about the same just not the best shape. Wobbly claims there is power to be made going from a oval to round exhaust port, which i do not doubt. This is round all the way, but I am getting side tracked… this is not a performance build. I have the RD400 for all out thinking.
The intake had to lengthened and angled out to clear the backbone of the frame. I have a 38mm Mikuni flat slide that may be too big. We will see. I just welded and extension on the intake v. making a complete new one.
Gears Racing made a new shock for me. It is a Yamaha R1 that has a lighter spring. I met the Gears Racing guys at the Handbuilt show. They are good to deal with and gave me a break on the cost of the shock, so I big thank you to them. I have a video review on Youtube about it. But for the readers, it is a really nicely made piece. Not sure how it performs, but the machine work is terrific.
I have started on an aluminum tank. It is something I have never done before. I will record the process and post it later. I bought .08 (2mm) 3003 sheet and have started hitting it.
I finished the tank. I will post one video here but I will have several more on my Youtube channel “mark atkinson”.