I'm changing the rear cab mount from the stock center mount on my 48 3/4 ton to shackles that Chevy switched to as the Advanced design developed. I bought a pair plus the support brackets and have a photo I got off the web of what they look like installed but have a few questions about how they function. 1. Is the purpose to keep the body from contacting the frame rails? 2. Do they somehow reduce the twist in the body as one side raises and lowers? 3. The 48 came with rubber bumpers. Are the bumpers not needed with the shackle design?
Last edited by Joel1948; 08/01/20195:39 AM. Reason: Make it more understandable.
Here are some details on the running engineering changes for these trucks.
"The cab itself, which had previously been attached to the frame at three points via shackle bolts and rubber mounts, was now mounted at four points. The idea of the three-point system was to let the frame flex without affecting the cab. This worked well, but the single forward mount put too much stress on the front frame cross-member, so two smaller shackle mounts were set on the forward chassis rails instead. The cab was still unaffected by frame flexing thanks to the lessened tension of the shackle bolts. Frame flexing had caused other problems as well. There'd been complaints, for example, that the 1947-1948 radiator support in the 3100's tended to twist and shake loose after many miles on rough roads. This caused splits in the radiator core, and cracks in front-end sheet metal. For 1949, Chevrolet added X-bracing to the radiator support frame, reinforced the bottom channel, and mounted the entire assembly more rigidly.Hood shake had been another complaint, so a set of cross braces was installed. These ran diagonally across the inner front corners of the hood."
FYI, there's a doubler plate on the floor above the cab brace where the shackle mounts bolt thru the floor. I'd be sure to install something there as well. Rather than nuts and bolts, the bolts thru the floor screw into a nut plate. That makes it possible to install mostly from below, once the bolts are initially threaded into the nut plate. This photo shows the shackles as removed from my truck for cleanup and repainting. I thought I had a photo of the finished product, but you can see the nut plates sticking out from underneath.
There are no auxiliary bumpers between frame and cab at the rear.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
