Sonic Weld was another of those companies that built custom frames for racing. Think along the lines of Trackmaster, Rickman, Cheney or Champion. It was common in the 70’s and 80’s to take an engine from a stock Bultaco, Ossa, Triumph, Yamaha, etc, and put it into a custom frame for racing. The factory frames were not up to the rigors of jumping and general abuse a motocross race bike endures. The custom frames were stronger, lighter and specifically designed for race use. Some of these companies are still in business. You can still get a replica Sonic Weld frame from Cheney Engineering.
This bike has a 1966 Triumph TR6R 650 engine in a Sonic Weld frame. This is an original frame, evidenced by the label, and looks like it should be able to be titled. Different states have different rules, but they are generally more lax for older bikes like this. The seller says the engine is “fresh” – that is a technical term often used on Craigslist. Pipes, tubes and tires are new.
It is very hard to put a value on a bike like this, as there are so few out there and no comparable sales to reference. The seller says he just spent $3,500 on the build at Century Motorcycles (the self-proclaimed oldest running motorcycle shop in the country).
I would love to take this bad boy out in the desert somewhere. If you are in or near Torrance, CA, and you think Steve McQueen was cool, you should at least go take a look at it. It is listed here on Craigslist for $8,000.