The Stovebolt Site Forums The Parking Lot Big Bolts 2 speed rear diaphragm

I posted yesterday asking anyone for the part no. for the diaphragm for the 2 speed on my 48 Loadmaster and possibly a source.Again does anyone have it.The one I removed does not have any numbers on it.

Thanks,
JMH

The GMC p/n is 2211884. They have not been available for many years. I saw a NOS one on eBay a while back that went for $125. I had no interest because the Diaphragm could have been rotten. I have one of these 2 speeds and I intend to modify a 20/20 air chamber to make it work.

Thanks for the part no. It is a start.I am a machinist and I have been toying around with the idea of some sort of mechanical shift mechanism or maybe an air cylinder setup.Before I try that if I cannot find one I am going to attempt to fab one out of sheet material.If you come up with something please post it.

Many thanks,

JMH

JMH,
See my discussion in your thread about oil in the vacuum lines, I think I may have found the Chevy part number.

Kent

JMH, is your shift mechanism completely gone? This is the chamber I was referring to. http://www.newlifeparts.com/catalog/section3.pdf It is the common chamber for brakes on the steering axle. It pushes with air pressure. I am thinking of cutting a hole in the diaphragm and gluing it to the push rod or bolting it with fender washers.
Scroll down to page 4 of 5 for an exploded view of the chamber to which I am referring.

Crenwelge,

I stopped at my local part store and per your idea purchased a diaphragm for a brake assy.It is the right diameter but is somewhat thicker than the original part and also stiffer . I am going to try and assemble it in place of the original.What I am afraid of is the truck may not have enough vacuum to make the unit function.By the way the part no. was a no-go at my local Chevy dealer.He said it should contain eight numbers? Oh well thanks anyway. I will let you know how it goes.Almost forgot to tell you my unit is complete minus the diaphragm.

Thanks,
JMH

I have made diaphragms out of heavy-duty rubberized gasket meterial. Is the diaphragm riveted to the operating rod, or attached in some other way? It should be possible to fabricate a shaft with a threaded stud to attach the diaphragm with a nut and washers if necessary.
Jerry

The GM part number has been obsolete a long time. The number I cited is out if a 1954 GMC parts book. Your engine should have enough vacuum to shift it if you don't have leaks elsewhere. You let off the gas when you upshift and that of course raises the vacuum. And vacuum is what shifts it into high range.

JMH,
Again, see my discussion in your previous thread about oil in vacuum lines. After thinking about it, I thought it would be a good idea to have an extra diaphragm in case my new one should wear out. I went to my local Chevy dealer (the place I detailed in the other thread), gave them part # 3847058 and they printed two pages of dealers across the U.S. who had the part on the shelf. They ordered me one, $49, and it will be here in two days. It is the correct part number.

Although obsolete, they believe there is some more modern application to this diaphragm or it wouldn't be listed.

If your dealer is not able to help you, call Chapman Chevy in Tempe, AZ and ask for the parts dept.

Kent

I didn't read all of the previous post to notice the p/n. It must have worked on something later. For $49 it is not worthwhile to jerry rig anything.

Sometimes a manufacturer unwittingly duplicates a long-obsolete part. When the Chrysler slant six engine came out in the early 1960's, they started to sell an unusual number of connecting rod bearings, more than would be justified by warranty work on the new engines. They found out they had duplicated an obsolete bearing from a 1930's engine that was no longer available.
Jerry

I looked through the obsolete list and found one at a dealer in Oklahoma farming company. He was an older guy and remembered seeing my trucks at the local grain elevator years ago and sold me one for $25 and was glad to get rid of it.