Thought I toss out some details on the Dual Twin Bridgestone 175 that I am about to begin the restoration.
Bridgestone was a Japanese tire maker and desired to move in to manufacturing first bicycles and then motorcycles after World War II to compensate for the sagging tire market. So in 1946 Soichiro Ishibashi began production of bicycles and then looked to motorized them. So they partnered up with Fuji Seimitsu Kogyo (is now Fuji Precision Engineering Co., a subsidiary of Nissan) to produce clip on motors for bicycles. By the mid 1950’s they were developing and producing motorcycles and eventually separated from Fuji Seimitsu Kogyo and moving to their own plant. Bridgestone even had a racing department which, in part led to their development of the Dual Twin.
The 175 Dual Twin did well in racing and on the street, as did the other Bridgestone models. This lead to the exports to the U.K., other Asian countries and eventually to the U.S. by Rockford Motors in Rockford Illinois.
So what is the 175 Dual Twin?
It was the first motorcycle equipped with a dual rotary disc valves for fuel induction. It had (has) aluminum alloy cylinders, kick-start; dual transmission 4-speed rotary shift gear allowing the bike to be shifted to to a 5-speed gear while running or when stationary with the “sport shift lever’, large brakes hubs and totally enclosed carburetor for protection against water and dirt. It has a max speed of 80 miles per hours and can get to a ¼ mile from a standing start in 18 seconds (this is without the racing tune…oh yes…they were race-able!!).
So let me stop right here for a minute. I had no idea what a rotary valve was, ok let me be very honest I’ve never worked on a motorcycle before, lawn mowers – yes, motorcycles – nope. I have worked on bicycles and car engines, so how hard can it be? (Famous last words, usually spoken very early in the planning stages of such projects.)
So what is a rotary valve? It is where the intake opening is controlled by the spinning of a disc that has cutouts that allow air/fuel mixture in and then closes the opening. The discs attach to the crankshaft. The crankshaft disc is a close-clearance fit in the crankcase and there is a cutout which lines up with an inlet passage in the crankcase wall at the proper time. Here are a couple of images:
Intake Valve Disc
This a dual disc valve set up (a newer Suzuki Rg500 engine)
I have more coming up on the Bridgestone Project so stay tuned.
Thanks for reading.
Tim
