I have a '48 also and I was wondering what I was going to do about it. I didn't know there was one out there. It's not an exact repro but it sure beats a blank. Has anyone used this set? I was not sure if my my worn out bumpers were in constant contact or if they just stood ready should the cab tilt, anybody know? I guess the bigger question in my mind is why did they move away from this type of mount? Was it cost, safety, performance, repair access? With that said should I change mine if possible. Has anyone been there? GM advertised the 3 point system pretty good just to ditch it 3 years later, I wonder why? I've been searching in the past on this but have not found much information, does anybody know, thanks Jim D
"To prevent excessive freedom in the mountings, there are soft rubber stabilizer bumpers attached to the rear corners of the cab dirrectly over the frame side rails. Lightly compressed between the cab and the rail, they prevent any posibility of side sway or rocking motion in the cab."
Grigg, thanks for the direction. I read but did not retain that (per Sister Mary Grace 3rd grade). Knowing that has anyone used this kit and does it supply the needed "bulk". My old part looks a lot bigger, pie shaped with the point down and about an 1.5 inches thick all the way through to contact. Perhaps reinforce the floor and install 2 per side. Install one and shim it down any experience with this. The cone shaped ones just look a little weak at tip, contact might be 3/4 square inches, at best, whereas contact on original would have been 1.5X1= 1.5 sq inches. More compression might lead to more movement, different ride height, early failure of center mount, any opinions welcome, thanks Jim D
Here is the explanation on why they changed the mounts:
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 crossmember, 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 3100s tended to twist and shake loose after many miles on rough roads. This caused splits in the radiator core, and cracks in front-end sheetmetal. 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. https://www.flickr.com/photos/advance-design_parts_co/17278450221/in/album-72157649864028584/
Here is the explanation on why they changed the mounts: ...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 crossmember, 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...
I'm not following how changing the rear cab mount affected the front radiator support mount. When you say "two smaller shackle mounts were set on the forward chassis rails" what do you mean by forward rails? I also don't understand "lessened tension of the shackle bolts"
I'm familiar with both kinds of cab mounts as I have a 48 and a 53.
The original diagrams in the some of the literature and manuals shows lines connecting the cab based on these mounting points. The front crossmember/radiator support carries the entire front end from the firewall forward. The rear shackle worked the same way so the linkage looks like a diamond shape from above. Then they went to the paired shackles which took away the rear diamond. This is the GM explanation so it must have been what they were seeing in claims back to the dealer that prompted it. The forward chassis rail I would interpret as the outside of the frame rail. The shackle location there helped so the cab was not bouncing on rubber bumpers but the shackle movement itself. The movement of the cab against the frame must have transferred the stress back into the cab structure instead of the shackle. https://www.flickr.com/photos/advance-design_parts_co/8349389839/in/album-72157632440242503/
Brad, thanks. This place is great. That explains a lot, when I pulled my cab off the frame the front supports extending to the trans cover were up higher at the floor center by about 1.5". I think from the weight on the outside of the mount leveraging down caused the middle to go up. Doubling the rear support, moving them more to the outside and forward seems like it helped this issue. Is it difficult to swap cab mounting systems and has anyone done that? My cab is off right now so there's no better time and I won't have to mess with the old system. Do I need to do this as I won't be off-roading or anything. Just driving the St Louis area paved roads, any opinions, thanks Jim D
By moving the mount forward they put more weight behind them thereby teetering and/or holding most of the weight. Then the front mount only has to hold it in place, well not exactly but you get the idea. From my view with the floor bulging in the middle front I can see the advantage if one was to be on rough terrain. You can see my concern with the replacement 3 point mounting system, and which way to go, thanks Jim D
Been away for a while but thanks all for the input. Truckernix (nice album)if you have the service sheet please tell me where to look for cracks and how to reinforce, thanks. Huck if you think your rear axle bumper looks like it I'm going to look up the part number to see if it is available or maybe an image of it, thanks. Brad (also nice album) I agree I may not need to do the swap to the later mounts BUT I do have access now with the cab off and all. It looks so easy. I'm so tempted. I think I should keep checking into this, it's only four holes! and the 1 ton mounts are still available? thanks for the picture and your input. Jim D Note: before I checked Stovebolt, I was thinking of how I could stack old tire treads and hold them in place in lieu of the bumpers. Or how feasible it was to swap to newer mounts.
Now that I started checking I have found another set to rebuild mine ('48) at J Carter's and the bumpers look bigger at the base. http://www.oldchevytrucks.com/cart/product.asp?prodid=CB157&i=9648|||| Still have not found the later mount metal shackle parts 1/2 or 1ton anybody see them anywhere, thanks Jim D (hope I put the link in there right)
I am chimming in on this late but I may be able to add to the info. I have a 48 GMC with the center rear trunion mount with the corner rubbers and found a rebuild kit at Classic Parts. Everything fit properly but the rubber pads are 1/4" thicker than the originals. At first I thought the old ones had taken a 1/4" set but I don't think so. The way it was designed to work would require the cab to sit on the pads taking the weight and absorbing shock. I put a 1/4" shim under the center trunion mount so the system looks perfect except its not as designed. Shop Shark is correct in his info. GM redesigned the mount putting a trunion at each rear corner. The stress of the cab flexing 1/4" before it touched the corner rubber was tearing up the surrounding sheet metal at the center rear of the cab where the trunnion mounts. Mine is so destroyed by fatigue and a failed weld and further buggered by the woodpeckers that owned it over the years that I cannot make out the original sheet metal enclosure that covered the trunnion from the inside of the cab. If anyone has a picture os sound sheet metal in a 47-48, I would love to see it.
I installed the rubber bumper on my frame, but the provided holes are more on the top of the crossmember and getting the nuts on the bottom requires a few choice words. Letter f in the assembly manual. Part of the bumper is on the frame rail which sits higher than the crossmember. I am thinking about grinding the bumper to fit and sit flat. With these bumpers that are circle shaped and same size top to bottom, it puts the center mount up in the air almost 1/2". Is that normal
We have been using S10 body to frame mounts with good luck. Much more stable than either the center mount or the shackle type plus the S10 cushion has a lot of flexible rubber around the center tube. With our trucks being driven on paved roads now the flexing and cracking is probably a moot point. Picture of an S10 conversion showing white plastic spacers on the rear cab mount cushion which is necessary to fit the rear cab floor pan brace. A much sorter frame bracket is fabbed for mounting on an AD frame.
I notice there are cone shaped rubber bumpers available through the vendors my bumpers are the same size round top to bottom. Maybe I need the cone style. I'll look at it today again.
The bracket that bolts to the cab which is what holds the original rubber mount is still bolted to my cab. Since the rubber bumper is not available, I thought about fabricating a new bracket to accommodate the new rubber bumpers.
I believe it could be done. The upper shackle mount reinforcement is available from various vendors. I have not seen a 47-48 cab with the rear center mount. Maybe that reinforcement is already there. In which case you'd just need to drill for the bolts.
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.
The tricky part will be locating where to put the holes in the bottom of the cab AND the sides of the frame. I have a '49 cab on a '47 (or '48) frame. Since the '49 cab came with the later ('49-'55) rear mounts, it was easy enough to figure out where the holes in the frame belong. I attached the mounts to the bottom of the cab, placed 3/4" board between the mount support plate and the frame, let the mounts hang down and pushed them against the side of the frame and marked the frame through the holes in the mount.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
