In the early 1990s I'd completed the Mark 4 frame for my Laverda 750 and decided to do the Mark 5 version and go vintage racing instead of BOTT. That was going to make the Astralite wheels/brakes surplus. One of the local racers, Russ Ward, approached me about building a chassis for an air-cooled Husqvarna 510cc four stroke single engine. The wheels, brakes and seat off of the Laverda and a leading link fork that I had (built by Grizzly Engineering) were part of the deal. Russ provided a Lectron carb and Works Performance rear dampers and I adapted a Lucas Rita ignition to the engine.

The Husvarna engine was probably the lightest big single you could get at the time, and it also had a 4 valve cylinder head. The engine had a combined rear motor mount/swing arm pivot bolt that was not very large (12mm I think). I decided that I preferred to use that only as a motor mount and had the swing arm pivot behind the engine. This let me use a larger pivot shaft and allowed some variation in pivot location, though it did move the countershaft sprocket farther from the pivot.

The engine has a wet sump instead of a remote oil tank but holds very little oil so there was a lot of room under the engine. An aluminum fuel tank that fit under the engine was eventually built.

• Husqvarna 510 one of the sides of the swing arm is being profiled to fit the pivot tube.
• Husqvarna 510 the top tubes were brazed together and then the front was profiled to fit the steering head.
• Husqvarna 510 top and bracing tubes attached to the steering head.
• Husqvarna 510 as above - right rear.
• Husqvarna 510 frame largely done with the swing arm pivot sheet metal being fitted.
• Husqvarna 510 swing arm sheetmetal clamped in place
• Husqvarna 510 the frame is done, the swing arm lacks the external bracing.
• Husqvarna 510 as above - right front. The MK4 Laverda is on the bench in the background.
• Husqvarna 510 as above but now the swing arm is braced.
• Husqvarna 510 on wheels and fairly complete except for fuel tank - left rear.
• Husqvarna 510 as above - left front.
• Husqvarna 510 as above - right front. B50MX BSA in the background.
• Husqvarna 510 as above - right rear.
• Husqvarna 510 as above - right side
• Husqvarna 510 on wheels and the exhaust is fitted - right rear. I think the aluminum sheet next to the rear wheel was intended for the fuel tank.
• Husqvarna 510 detail of the front of the frame/engine/forks. The carburetor does just barely clear the frame tube.
• Husqvarna 510 engine area - right side
• Husqvarna 510 detail of the swing arm and pivot - right rear.
• Husqvarna 510 Rita ignition mounted on crankshaft.