Vintage Flat Track Weapon: Bultaco Astro 360

Bultaco Astro 360 Tracker

The Bultaco Astro 360 was a unique machine, a purpose-built Spanish flat-tracker developed to win on dirt ovals across the United States, from small fairground tracks to the Grand National Championship series. Developed from the Pursang motocross bike, the Astro featured a higher output engine, no front brake, shifter and rear brake pedals on the right-hand side, a variable wheelbase with special rear axle adjusters, and weighed just 215 pounds. The 359cc/46-bhp two-stroke single-cylinder engine in the early models was notoriously high-strung:

“Faster than a speeding bullet, the 1971 through 1973 Astros had a lifespan about equal to a gnat trying to bench press a hippo,” –Rick “Super Hunky” Sieman, founder of Dirt Bike.

Bultaco Astro 360 Tracker

The “Astro” name came from the Houston Astrodome, which hosted the TT Nationals — a race where this two-stroke Spanish tracker particularly excelled.

Bultaco Astro 360 Tracker

Enter Gary Braun of Retrodyne, a former machinist mate on a nuclear submarine who began working at Minneapolis’s Honda Town after he got out of the Navy. There, he fell in love with vintage bikes and became obsessed with racing, taking a job as a machinist just so he could make race parts. Soon, he was working full-time out of his garage on customer bikes, doing the jobs nobody else wanted:

“I worked every day for four years from the day I quit my machinist job. 12+ hours a day, working, doing research, talking to mean old people just to glean some small piece of information to revive some lost art.”

Bultaco Astro 360 Tracker

All that hell and hard work paid off.  Today, Retrodyne is a well-known name in vintage racing circles, where Gary is known as a master in the dark art of building custom high-performance two-stroke expansion chambers. His pipes have graced several of our most popular two-stroke builds, and that isn’t all:

“Now I’ve worked on projects for the Barber Museum, I have pipes in seven countries on four continents. Wild four years I guess.”

Bultaco Astro Tracker

The 76-77 Bultaco Astro 360 you see here is owned by Steve Lambert, who sponsors riders Darren Carter or Kameran Miller in vintage flat track racing.

“It’s a weapon designed for flat track racing and you can tell when you ride it. Back in the day this was it. If you wanted to race you got yourself one of these. In district 14 this is the bike to beat. It can take most of the bikes in vintage heavyweight too. It’s just perfect.”

Bultaco Astro 360 Tracker

The bike is largely original, though it does have an upgraded rear brake, Weiss triple clamps, a box-section chromoly swingarm from flat track frame-master Led Szmek of Panther Frames, and of course one of Retrodyne’s custom expansion chambers — with 36+ separate pieces in the main pipe!

Bultaco Astro Tracker

Gary’s pipe-building process is mind-blowing, and he details it completely in our interview below. Suffice to say, it’s an intensive, highly laborious process that combines precision measurement, mathematics, full-size mockups, plenty of hand fabrication and ultrasonic welding, a world of patience, and probably a little black magic to create a pipe that performs well on the track…and doesn’t crack. It’s truly an art, and we highly encourage you to follow @retrodyne for more insight into the process.

“You don’t have to hammer any spots in the pipe to make it fit. Everything on this bike fits on it because it’s all made for each other. Perfectly balanced. A cross between a truly iconic vintage flat track bike and a modern art expansion chamber.”

Bultaco Astro Tracker

Amen. Below, we get the full details from Gary about his shop, build process, and the gorgeous Astro 360 you see here.

Builder Interview: Gary Braun (Retrodyne)

Retrodyne

• Please tell us a bit about yourself, your history with motorcycles, and your workshop.

Oh man, not sure either of us have time to read a novel. I was a Machinist Mate on a submarine in the Navy. Got out and worked at a place called Honda Town in Minneapolis. They’re one of the longest running Honda shops in the five-state area, so I got to work on some cool vintage bikes and sort of fell in love with them. At the time I only had one motorcycle that I rebuilt from a box of parts, a ’77 CB550K. I won a motorcycle in a raffle, sold it, bought a bunch of broken motorcycles. Quit my mechanic job to become a machinist so I could make parts for race bikes (I hadn’t even raced yet, I was just obsessed).

Retrodyne

After a year of that I started working on my customers’ projects full time. I was working out of my garage doing everything that people would let me. I took on jobs that nobody else would do because they were awful — I just wanted the opportunity to say that I did it. I worked every day for four years from the day I quit my machinist job. 12+ hours a day, working, doing research, talking to mean old people just to glean some small piece of information to revive some lost art. Went through six girlfriends just to say “I have a pretty nice shop with some decent equipment, please let me work on your bike.” Now I’ve worked on projects for the Barber Museum, I have pipes in seven countries on four continents. Wild four years I guess.

Bultaco Astro Tracker

• What’s the make, model, and year of the bike?

76-77 Bultaco Astro 360.

Bultaco Astro Tracker

• Why was this bike built/restored? For yourself or a client?

The bike is a factory flat track bike belonging to Steve Lambert. He has a few of these and other racing bikes. He’s a purveyor of fine things. Built expressly to compete in AMA flat track racing in the 70s. Bultaco actually almost directly copied the geometry of aftermarket Champion frames. It’s actually still very original with some upgrades for reliability.

Bultaco Astro Tracker

• What custom work was done to the bike?

The bike is surprisingly original. Upgrades to the brakes were huge, the original brake disk was aluminum and chrome plated as well as using a master cylinder and caliper from a forklift… Although some people don’t even use the brakes, this is a two-stroke and has a nice big 18mm compression release. Like throwing out an anchor that doesn’t make the rear wheel kick out. The swingarm is also handmade, done by Led Szmek of Panther Frames. Much superior box section chromoly and a touch longer, Led is the master of flat track frames. It has Weiss triple clamps and a list of other handmade stuff. It’s a well-sorted race bike.

Bultaco Astro Tracker

• How many pieces are in one of these pipes? Can you tell us a little about the process?

How long do you have? I don’t keep any secrets about this stuff. I hate when old guys gate-keep and try to take their secrets to the grave. There are about 36 pieces to the main pipe, not including springs and screws and stuff. It all starts with a design and a bunch of math. I take the head off, measure the ports to come up with the exhaust duration and all the engine details. Port size and shape, piston, rod length, exhaust port shape and duration. You have to talk to the customer too and say “what do YOU want this to do.” Most people have no idea and have never been asked. I think people are often given super generic parts and just told to “make it work.”

Bultaco Astro Tracker

Take all that information and go crunch the numbers. You end up with the lengths, diameters and angles of 5-6 cones. These will make a straight pipe which is useless if you don’t have a reverse cylinder bike. I use a welding rod with all my dimensions written on it with masking tape to mock up the pipe on the bike. This maps out the path of the pipe and makes sure I don’t run into anything. I take that and trace it onto a huge drawing pad to make a full-size sketch. I write down the angles, lengths, cone sizes, and divide them up into their individual parts.

Bultaco Astro Tracker

You print those out, mock them up into an actual 1:1 paper mock up all taped together. This process sucks, but if you want it to fit like a tailored suit it’s the only way to do it. I usually change one or two cones to make it fit really tight. Trace them onto metal, cut, roll, hammer them round and then weld the whole shooting match together.

Bultaco Astro Tracker

That middle part is the easy bit. 90% of the work is the last 10% of the job. The silencer fit, flange parts and rubber mount usually take me two full days to get them perfect. There can’t be ANY stress on the pipe when you mount it, it should float. You’ll see a lot of cracked hard-mounted pipes in the pits with some sad faces standing beside them. The silencer location and angle is aesthetically important too. I can’t stand when a pipe blocks the axle.

Bultaco Astro Tracker

• Can you tell us what it’s like to ride this bike?

From Steve: “You should ask Darren Carter or Kameran Miller. I sponsor them to ride it more often than not. It’s light and agile. It’s a weapon designed for flat track racing and you can tell when you ride it. Back in the day this was it. If you wanted to race you got yourself one of these. In district 14 this is the bike to beat. It can take most of the bikes in vintage heavyweight too. It’s just perfect.”

Bultaco Astro Tracker

• Was there anything done during this restoration that you are particularly proud of?

You don’t have to hammer any spots in the pipe to make it fit. Everything on this bike fits on it because it’s all made for each other. Perfectly balanced. A cross between a truly iconic vintage flat track bike and a modern art expansion chamber.

Bultaco Astro 360 Tracker

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2 Comments

  1. JOHN MUNNS

    Fantastic story, but why run Rick Sieman’s comment, he hates Bullies, constantly pans them. Astro’s are tough and fast, anything over revved will destroy a rod, even his Huskies! Bullies have so much torque you don’t have to over rev them, waste of time and money. These days Senor Bulto is regarded as up there with Walter Kaaden and Ernst Degner for his two stroke development. Great story, beautiful pipes.

  2. David Seitz

    Please provide short videos of your bike so we can hear the music!

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