Two-speed Rear Axle Speedometer Repair

[Guest philedmonds]

I've gotten the main part of the speedometer for my '49 B1FA cleaned up and working. However, there is a vacuum activated unit on the back of the speedometer that apparently "shifts" the speedometer to match the rear axle shifting so the speedo reads correctly for whatever range the rear end is in. I think this is correct! Anyway, the vacuum unit has a small rubber disphram that has a hole in it and in testing would not move the plunger and shift the gears. Does anyone know where I can get a replacement diaphram? Or does anyone know of a vendor who could repair/replace it for me? I assume these parts are fairly rare and this may be a challenge! Thanks.

[anthonyb]

Phil,

I had the same problem on my 52 B3FA. I just cleaned the rubber really well and top coated it with a thin layer of RTV sealant. Works so far.

I seriously doubt you will find a replacement diaphragm for this. I searched for some time and couldn't locate one. I did talk to a guy who repairs speedometers (sorry, can't remember his name) and he suggested the above fix.

Even if it doesn't work for long, you're not out any big bucks.

If you do find replacements, please post the source. I know there's several folks with bigger trucks that probably need one of these.

Tony

[greg g]

How big is the diaphragm. Many motorcycles have a vacuum operated fuel tap. the prime, run, reserve thing. Usually, the bike will start on the fuel in the carb bowls, then the carb vacuum will pull the diaphragm to open the fuel flow through the run circuit. Shutting off the bike, kills the vacuum, and shuts off the flow of gas. Had mine apart for cleaning, the diapraghm is pretty light material, on my bike it is about an inch and a half square. and the operating part is about the size of a quarter, a little bit bigger maybe. So long story short, check you local MC recycler for a donor diaphraghm.

[Young Ed]

Lot of small engine carbs have diapraghms in them too. Might be another source.

[Guest philedmonds]

The guys at Collector's Auto Supply in BC had one. The item includes the puch rod that the diaphragm moves. It was a bit pricy ($45) but I was happy to find it. Thanks for the help.

[grey beard]

In the fifties, Eaton - largest 2-speed maker - went to electric shift instead of the older vacuum type. With this change came electric speedo gear boxes with one wire. These are quite common and available at most any truck parts counter of any account. Older applications would of course be six volt. JMHO:)