GMC Motorhome Main Frames
Turn Key Installation or Do-It-Yourself Kits
PO Box 105 3249 Inwood Rd
Inwood, Ontario CANADA
How do you know if you need a new frame? Along the underside of the two long frame rails looking toward the front of the coach, if the lower flange has been bent upward, it’s no big deal-it means someone has tried to jack the coach up in the wrong place. (The only place you jack up the rear of a GMC is by placing the jack under the bogey bracket.) If however, the lower flanges are bent downward, that’s bad news; you have major rusting in the area where the bogey H-frame nestles inside the side rails, and in most cases that rust cannot be seen until the old frame is dismantled. When it is taken apart, there will be slabs of rust as big as a good-sized steak and a half inch thick, many of them. Some guys have said they only need new rails at that point. Not so. I have yet to see a frame in that condition that didn’t need the H-frame also. Now, as a matter of course, I always put brand new H-frames in as well as new forward cross members. Sometimes when my customers come to pick up their coach they wonder if they really needed to go to the full extent of a new frame. To answer their scepticism, I’ll rattle their old frame apart with the impact gun and show them. It tends to get their attention when they see the mess laid bare. Another place to look is where the rear subframe bolts on to the rear of the side rails on both sides. Gravel and dirt tends to collect in the vee and after poking the debris out, there will probably be perforation visible around the bolts. This area becomes even more important if the owner intends to tow a car because this part of the frame carries a major part of the structural work in towing.
The original frame components on our coaches were mostly 1/8” mild steel. There is evidence that GM did use some type of paint on them, but it’s doubtful that they ever expected the motorhomes to be on the road much more than 15 or 20 years. The first 30 or so frames that I built, after cutting, drilling and welding everything together, were sandblasted and coated with a two-part epoxy primer and then painted. The paint mainly to cover up the ugliness of the sickly green primer. That system worked OK and the frames will last a long time, but I wasn’t satisfied that it was the best way to preserve the steel. Since the largest galvanizer in North America is about 2 hours from my shop, I decided to talk to them. All my frames have been galvanized from that point on. There are several grades of galvanizing and my GMC frames have the same as used for culverts and guard rails. They won’t rust away soon. In addition to galvanizing, I have, from the first, been using 3/16” heat treated steel with close to twice the tensile strength of the original. Also, the flanges are almost 3” wide compared to the original 2 1/2”. Where the inner frame components nestle inside the side rails, I paint them with grease which helps them slide together and is more insurance against the rust worm.
Any coach that has spent much time in the salt air of the coastal regions, salted roads of the northern states and Canada, or sat on grass “behind the barn” will need frame attention. For those contemplating buying a GMC, the fact that a coach needs a new frame should not discourage you from making the deal. Just factor the cost of that new frame into the price you negotiate.
How long does it take to put a new frame under a GMC? It takes me about 120 hours spread out over 3-4 weeks if I have a new frame ready to go ahead of time. If not, factor in another week or two to order in the steel, do the fabricating and take it to the galvanizer. Buskirk used to claim a 40 hour time frame with two experienced guys working together, which seems about right. Some guys want to do the install themselves which is fine with me. They’ll save some money and I’m happy to coach them over the phone as I have done several times.