Custom BMW Fire Road Flyer

Bike builder Josh Withers turns a parts bike into a sharp and stylish custom ready for a dirt road.

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by Josh Withers

Photographer and custom motorcycle builder Josh Withers of California is no stranger to taking on special requests from interested clients.

He’s the first to admit he isn’t getting rich from the motorcycle part of his life, though. In fact, five years ago, when he had just finished one particular commissioned café racer, on social media he joked he’d made 10 cents an hour completing the build. “A guy made a comment that he’d pay me a raise of 15 cents an hour to build him a bike,” Josh recalls. “In jest, I wrote back and said, ‘Sure, I’d do that!'”

That guy is Bill Horntvedt of Michigan. Emails were sent back and forth, and several phone calls made. Josh says Bill wanted a BMW café racer based off of a R75/5 foundation but kept mentioning there were plenty of dirt and gravel roads near his hometown.

Josh says, “A café racer and dirt roads don’t add up, and we couldn’t really agree on what to build. But, I did start searching for a donor BMW to serve as the starting point for Bill’s project.”

He found a 1973 R75/5 in Palm Springs, California, that had been disassembled. It was being sold as a parts bike, and while driving back from a photography gig in New Mexico, Josh stopped and looked at the carcass. The wheels were off and there was a BMW /6 speedometer in a handmade dash, but the engine, frame and fork were there. Josh had to pay extra for the signature BMW “Toaster” tank but managed to work out a deal, and $900 changed hands. He loaded the kit of parts into his 2002 Volkswagen Eurovan and went home to Los Angeles.

Engine: R75/5 w/Siebenrock kit, 1,000cc air-cooled OHV opposed-twin, 94mm x 70.6mm bore/stroke, approx. 60hp @ 6,170rpm, 9.0:1 compression ratio
Carburetion: 2 Bing 32mm CV
Transmission: 5-speed, shaft final drive
Electrics: EME crank-mounted digital ignition system, 12v, 280-watt charging system
Frame/wheelbase: Double-loop tubular steel w/bolt on subframe, 57.7in (1,465mm)
Suspension: Telescopic forks, twin rear shocks w/ reinforced swingarm
Brakes: 7.9in (200mm) TLS drum front, 7.9in (200mm) drum rear
Tires: 100/90 x 19in front, 4 x 18in rear – Continental TKC70
Weight: unknown
Seat height: 34in (863.6mm)
Fuel capacity: 4.7gal US (18ltr)

“Bill works out of the country for long stints, and at one point, I just stopped hearing from him,” Josh explains. “I started wondering if something bad had happened, and in that time of silence I built two other motorcycles.”

Meanwhile, the R75 donor machine was sitting outside Josh’s garage as a rolling chassis. So much time passed that Josh was thinking about selling the project and was close to posting an ad when Bill resurfaced.

“He was energetic to start the build, and I was ready to go, too — but we still hadn’t landed on what we were going to build,” Josh laughs. Regardless, Josh took the engine apart and cleaned everything, discovering several problems with the donor powerplant in the process. While Josh does enjoy rebuilding engines, he felt the excessive crank play, rusted and seized rocker arms and clapped out barrels were beyond his skill set. He handed the pieces over to a good friend and trusted Southern California BMW guru who prefers to remain anonymous.

Engine upgrades

Given the top end issues with the BMW engine, Josh and his mechanic chose to purchase and install a big-bore Siebenrock kit. Established in 1984 in Stuttgart, Germany, Jochen Siebenrock started the company because, as a poorly financed architecture student riding a BMW, he couldn’t afford new parts and used pieces were at a premium. At first operating out of his garage as a used BMW parts business, he soon expanded and moved the company into a local warehouse. Siebenrock worked as an architect until 1991, when he made the decision to focus on the motorcycle business.

In 1997, Siebenrock engineered and began marketing a big bore kit to take BMW’s famous boxer twin-cylinder engine to 1,000cc with replacement barrels, pistons and rings, wristpins and circlips. According to the Siebenrock website, the kit is “plug and play” and increases both torque and horsepower. On the stock R75 engine, the bore size is 82mm. The Siebenrock cylinders have a 94mm bore, while stroke remains the same at 70.6mm. In stock form, the 745cc BMW engine is capable of producing 50 horsepower at 6,200rpm and has a compression ratio of 9.0:1.

The Siebenrock kit increases the compression ratio to 9.5:1, but Josh says they fit a .5mm shim gasket under each cylinder base to keep the stock 9.0:1 ratio. By taking this approach, Josh says, “It will help the bottom end and engine case withstand the additional power for longevity. This bike isn’t meant to be a race bike.”

Siebenrock claims their kit should take horsepower to 63, but with the lower compression ratio in the engine Josh had built for this project, that figure would likely be closer to 60.

In the meantime, Josh sourced a 5-speed kickstart transmission from a R90/6 to take power to the rear wheel via BMW’s famous drive system that sees the driveshaft running in a tunnel incorporated in the right side of the swingarm. Speaking of the swingarm, Josh retrofitted one from a 1980 R100 and had it suitably braced to ensure it could endure more punishing adventure-style riding, because with engine and transmission work nearing completion, Josh and Bill finally came together on what the bike would be.

Obviously it wasn’t going to be a café racer, but something fully custom that could handle rougher terrain, akin to BMW’s original dual-purpose adventure bike, the R80G/S. Launched late in 1980, the R80G/S was essentially a street motorcycle that could be taken ” … down a fire road for a pleasure cruise,” wrote the editors of Cycle magazine in a December 1980 test.

And, that’s exactly what Josh wanted Bill to be able to do with his custom BMW. To that end, the build was to be minimalistic, and Josh sourced an early production run BMW /5 headlight bucket that did not have a hole for the turn signal indicator light. Aftermarket R80G/S handlebars were also purchased.

Frame fixes

In 1975, BMW began stiffening their factory frames with double-walled steel tubes, and as Josh’s frame was produced in 1973, he wanted it to be as strong as possible. Working with another BMW enthusiast who is handy with a brazing torch, they added stiffening plates to the neck area between the dual front downtubes and the backbone. Josh bought an aftermarket rear subframe kit, but he wasn’t pleased with the construction and how the tubes mated to the front half of the original R75 frame. He worked with his welder to clean up the gaps and reinforced the muffler bracket area of the main frame at the same time. The subframe kit came with a saddle, but before Josh finalized the deal, he asked to have the seat modified to make it slightly thicker with more foam for Bill, who stands tall at 6-feet plus.

Suspension setup

Josh had the frame and various pieces mocked up and back apart several times before fitting the stock fork, modified with Progressive Suspension springs sourced from Bob’s BMW Motorcycles in Jessup, Maryland ( At the rear, twin MDI shocks came with the subframe kit and Josh re-sleeved the rubber bushings with metal collars, replacing the cheap plastic collars as originally fitted.

“The shocks feel fine but they’re about 3/4-inch too short for this particular set up due to the taller subframe, and I recommend that once Bill feels like upgrading or spending more money, that he puts on different shocks,” Josh says.

For rolling stock, Josh powder coated R75 drum brake hubs and laced them into black Excel rims with stainless steel spokes sourced from Germany. With the wheels built, Josh trued up the brake drums on a friend’s brake lathe. A set of Continental TKC70 dual-purpose tires were installed and with wheels and tires in place, Josh could set up and locate the fenders.

Up front, a fiberglass fender came from It’s lightweight and has no lip or ridges to provide that clean and minimal look Josh was after. To give proper clearance and provide the correct radius the lower fender stays had to be extended.

At the back, an aftermarket stainless steel blank was employed as a fender, and Josh had to locate, fabricate and weld in place appropriate mounting tabs on the subframe. This fender, Josh admits, probably looks cooler than it will officially function, as it doesn’t extend forward far enough to meet the swingarm and will likely allow debris to be flung around that open cavity. A metal box was fabricated to hold the battery, and it was welded to the subframe under the seat.

Other electrical bits include an LED headlight from that Josh was able to retrofit into the /5 reflector and shell, while the taillight is a Bates-style unit. For instrumentation, he ordered a MotoMeter cluster from to fit the /5 bucket and also brought in a crank-fired EnDuraLast Electronic Ignition system to upgrade from the cam-driven points system found on stock BMWs until 1978.

While working on the mock-up, Josh kept thinking about what he’d like to run for an exhaust system and was looking at vintage British motorcycle mufflers — this was because Bill frequently mentioned liking some of Steve McQueen’s vintage vehicles, including a British racing green Jaguar. Instead of English mufflers, however, a set of brushed stainless-steel trumpet silencers came from SPARK Exhaust Technology ( of Italy. These are mounted to a set of Keihan stainless steel header pipes.

“That system wasn’t cheap, but it’s gorgeous, in my opinion,” Josh says.

On the intake side of the equation, a pair of 32mm Bing CV carburetors from an R80G/S were jetted to perform with the 1,000cc Siebenrock kit and modified to use the /5 choke lever.

Working on some of the finer details, the rear of the gas tank was raised ever so slightly so it would match the line of the taller custom seat. Of all the parts that came with the original $900 kit, the gas tank, Josh says, was one of the best pieces he got. It wasn’t dented, and there was no rust inside or out. Remember the British racing green? That’s the color Josh chose to finish the fenders, gas tank and headlight bucket.

He’d originally selected a green from a Pantone color swatch, but when he sent it to his painter at Jon’s Body Shop in Long Beach, he got a call.

“He said he’d mixed up the color and done a test spray,” Josh explains. “And, he said to me, ‘This isn’t the green you want.’ I said, ‘what do you mean?'”

It turned out Josh’s green was too light, more like a John Deere color. That wouldn’t do, and thankfully, Josh says, his painter knew exactly the hue he was after and sprayed all of the panels in the deeper, more appropriate green. All black components, including the frame, swingarm, final drive, fork lowers and upper headlight mounting shrouds were powder coated. Replacement chromed steel toaster tank panels came from Bob’s BMW and these were treated to a black anodized set of Oshmo Motorworks roundels ( Also from Oshmo are the swingarm caps and top triple clamp on the fork.

Almost at the eleventh hour of the build, Bill mentioned he’d like even more illumination than the LED headlight conversion could provide. To accommodate the request, Josh sourced a set of Denali D7 offroad lights. These were mounted on a set of bolt-on engine guards, and this is one aspect of the build Josh would change if he had the opportunity. “If I had known sooner about the lights, I would have welded bosses into the frame to accept the engine guards and would have figured out a platform or floating tab on the engine guards so the lights would sit more upright,” Josh says.

To provide more power for the additional lighting, higher output alternator windings were installed. Josh says the system went from 180 watts to 280 watts and 20 amps. Finally, all handlebar controls are stock /5, including the switchgear, but the grips are custom leather from Mirrors and bar end signals are from Motogadget’s extensive catalog.

After he’d covered approximately 150 test miles, Josh says, “Compared to all my other BMW Airhead builds, this one feels tall due to the subframe and 34-inch seat height, but it handles really well.”

Josh concludes with a laugh, “If I did this for a living, my 15 cents per hour wouldn’t cut it. This bike was a parts bike for a reason. In any given moment, the smallest thing can go wrong and take the better part of a day, or two, to fix. Talk about the ultimate lesson in patience. However, I enjoy the challenges and lessons each restoration provides me, despite all the curse words that can end up echoing throughout the garage.” MC

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