bed mount question - bent frame from towing?

[Chuck78]

After owning my truck for a short while, I noticed when the previous owner replaced the bed wood with some oak plywood on my 62 GMC panel, they left a out the bed mount bolts on all but the rear mounts. The ones over the gas tank were completely missing, as evidenced by the empty space left behind. Then, moving forward to directly over the axle, I alarmingly noticed that the side to side cross brace under the bed wood was sitting directly on the frame. I didn't think this was supposed to be like that, and also shortly after, when hauling tools and having a 6 foot level laying in the back, I noticed that the bed wood seemed to hump up in this location a slight bit. I then started wondering if the previous owner's had bent the rear of the frame down from towing, as it does have a heavy duty rec. hitch. Can anyone confirm this frame/bed support contact for me, or tell me that it's gonna be alright?
I need to go pull out the rubber bed mat and re-examine the problem wth the 6 foot level directly on the wood...

When I first got the truck, I looked at the frame rails and thought to myself, wow, those aren't very tall for using a receiver hitch to tow a car trailer! I had thought at that point that I should reinforce the frame with some heavy duty C-channel. Now I am wondering if I need to unbolt the bed mounts and jack the whole truck up directly in front of the back bumper mounts to the frame, and try and use something to bend the rear frame rails back upwards if this is the case. It is the stronger X-frame, but the frame channel height on this whole generation of trucks left me wondering about their towing capabilities due to the minimal frame rear of the back axle. If it had leafs rather than coils, I would think it would have been less of a problem, but the coils don't support the frame back that far.

Might be time to get out the welder! I can get pictures soon if needed.

Thanks guys.

[Chuck78]

this other guy's '65 build thread picture shows the later frame, and it does appear that the frame normally kicks up a slight bit over the axle, but is this normal for the bed's cross support to sit directly on the frame rails, where as all the others have the rubber body mount blocks under them?

attached is another 60-82 "X" frame picture I dug up. It at least shows the side to side bed support brace still attached to the frame. I guess this somewhat answers my question, but I wonder still if maybe a rubber mount was suppose to go on top of this, or if it was rigidly connecting the bed and frame, where as all the other connections had rubber bed mounts?

I still suppose that maybe all my mounts have rotted away, and have gotten crushed, and that could be why my bed wood is sitting lower everywhere other than the rigidly mounted cross brace. I need some input here.

[LILRED66]

Rear of the axle, the cross sills sit directly on the top flange of the frame rail. Forward of the axle, the cross sills sit on the stand-off mounts, in order to make up the distance to the top flange of the frame rail, due to the offset profile of the frame rail.

It would take a massive amount of weight on the rear portion of the body, in order to bend the frame tails downward. Additionally, this much weight would damage the floor structure. If it was tongue weight on a receiver hitch, the weight would be transferred into the frame rails and the suspension would bottom out. I don't think a conventional step bumper could withstand weight adequate to make the rear frame rails bend, without failing itself.

[Chuck78]

it's a rec hitch welded to the frame

[joe231]

I agree with Tony....it would take some massive weight and a hard bottoming out to bend the rear rails down...probably would have hitch failure also with that kind of stress

[Chuck78]

must be that my rubber body mounts that are there are just rotted and compressed, and then the one set that's missing behind the axle (second to last set to the rear) have just caused the bed wood to hump up a little where the bed is directly bolted to the channel that sits on the frame. I got a bit worried for a minute about that. I'll have to check my manual and see if there are any measurements for reference.

I have a new set of rubbers coming from Brother's (only about $10), but I wish I would have waited and ordered a full set with all the bolts and everything from LMC ($50-60), as I am missing three sets of the nuts and bolts, and completely missing two sets of the rubber mounts behind the axle. Is there anything special about the bolts that hold the bed mounts? Just a Grade 8 hex head? Or are they a specialty fastener? I only have the rears, and I can't see the heads unless I take off the bed wood. My bed wood has all the factory skid plate rails or whatever you call them, but is just a nice finished sheet of oak plywood with 1/4" screws and nuts holding the top metal pieces on. Gotta remove that all some weekend and do what the previous owner should have done, and bolted the bed body mounts down! Some people just lack common sense I guess.

[Chuck78]

Can anyone tell me what the bed mounting hardware looks like? Like I said, all I have are the cab mounting bolts and the rear most bed mounting bolts, which look to be a carriage bolt. All of the bed mounts in my truck that go under the wood bed are missing the bolts. I have all but one of the rubbers, but two of the rubbers are not in place (one I found on top of the gas tank). I just wondered if they were a standard grade 5 or grade 8 hex head bolt or something, or if there is anything special about them.
I got a set of 60-66 bed mount rubbers in the mail today from Brother's, but when I compared them to the mounts in my panel truck, they are maybe half the thickness of the panel truck's, and also square (originals are rectangular), smaller hole, and no fllange that locates te rubber through the large hole in the body mount bracket off the frame.
What am I missing here?

I again put my 6 foot level across the bed, and it seems that the center of the bed over the axle (where the bed support crossmember sits directly on the frame with no rubber mounts), the bed there definitely is a good half inch higher, than in the front or rear, or else the rear of my frame rails is bent down nearly an inch by the bumper!
Makes me wonder if the truck got rear ended, or if in fact the rails are bent. That'd be heck to try and straighten then! My buddie's garage has a basement under part, I can envison jacking the rear of the truck up slightly by the very back of the frame rails, and then at the axle, drilling through the concrete floor, and using some 1/2" threaded rod attach to the frame at the axle to pull it downwards...

If it were less of a difference, I would be inclined to think maybe that all my rubbers had just deteriorated, but they seem to be 1/2" thick, which seems reasonable.

My bed wood is not original, it is two pieces of oak plywood. I doubt the factory bed wood would have a recessed notch in it for the bed crossmember that sits directly on the frame with no rubbers, but who knows...
Between the bed wood and the bed side inside sheetmetal, I can see an 1/8" gap beteen the bed wood floor and hte sides of the bed, rear of the wheelwell, but the body is sitting pretty straight otherwise. That crossmember that sits directly on the frame goes between the wheelwells, and the wheelwells (inner wheelhouse I think it's called) could be flexing quite a bit to take up the slack from the much stiffer body, but this mystery is still confusing me.

Any and all advice much appreciated!

Thanks,

Chuck

[atomic60]

Chuck I just took my bed floor out of my 60' GMC longbed. over the weekend and have some pictures of all the cross sills for the floor to attach to. I am not sure about any mounting rubber as my floor had been replaced at some point with half inch plywood. If I remember I will post them for you to check out.

Eric

[Chuck78]

Looking at these two photos, in the lower one with the body completely off, the bed cross support is mounted behind the raised portion of the frame (the part over the axle), where looking at mine, I see that it is mounted directly on top of this part of the frame, on top of the holes visible in this photo that are halfway between the cross support and the coil spring. This could explain a lot! When I looked under the truck yesterday, to me it seemed like on my panel with the gas tank mounted where it's at, that there was no room to move this support rear anymore, as the gas tank was fairly far forward. I think mine looks more like the upper picture of the green truck, where the supports front and rear of the gas tank are more forward. Any suggestions? The cross support brace on mine definitely is sitting on top of the frame at the frame's highest point over the axle, with no rubber pads between the frame, support/bed wood.

Help me figure this one out!

Thanks,

Chuck

[joe231]

My first response is that I believe the Panel frames are different than the Truck frames, so you might be comparing apples and oranges....do you have an assembly manual?

[Chuck78]

I thought the 7-1/2' panel was the same as a short bed pickup? I know the Suburbans are a full 8' bed. To buy a new panel truck, on the factory order form, you ordered the pickup truck, and for bed options, you checked the box for panel bed.

It stopped raining, so I went out to look at it. the cross sill couldn't mount too much further rear because of the gas tank area cross bracing like in the upper picture (green cab) of my two examples. I did notice that the rear of the frame is sort of an extension that is riveted (with some massive rivets!) into the rear of the frame channel. This rear extension sits lower than the rest. I wonder if the previous owner got the rubber mounts mixed up, and put shorter mounts in the rear position? I wouldn't think the rubber mounts would differ, however.

Is it me, or are the big rubber frame bushings/bed mounts just not available anywhere?

I have a 62 GMC repair and overhaul manual, but no assembly manual. This would help me out a lot if I had one of those!

[joe231]

It is my understanding that the rear of the frames are different between the trucks and panels....if I'm wrong, I'm sure someone will correct me.

Where is CaptainFab, or Jonz? they usually has lots of info about the panel trucks.....

[Chuck78]

The rear of my frame on my 62 GMC 1/2 ton panel looks exactly like the green cab 60 or 61 chevy in my example picture, except that since it is built on the short bed pickup chassis, to get the 7-1/2' bed, it has frame extensions slid into the frame rail channels and riveted in place with some serious heavy duty rivets. I think it might be two pieces actually, and the rear piece steps down just a bit. I can maybe take pictures another day. I think this is the only difference that I can see at least from comparing that picture.

The picture with the body off is a slightly different frame than mine, due to the different bracing where the frame mounted gas tank would go. I think the main parts of the frame are the same, just different rear bracing.

the fact that the frames are extended to the rear on mine makes me want to try and add some vertical stiffness by doubling the height of the frame with some C-channnel up to where the frame is boxed, and maybe make some type of triangulated gusset out of plate steel on one side of the rail. I was checking out a late model GMC frame, and the rails were more than double the height of the 60-66! I think they were just a C and not boxed, but still, that's a huge frame! That and the fact that I have coils in the rear makes me wonder how much tongue weight this thing could really take. With leafs, you have half of the support from the springs at the rear of the frame. With coils, the frame is just hanging out there where the trailer load is pushing. I am happy to have coils though, to give it a more sporty ride.

[Chuck78]

A guy on the GMC V6 email list told me that the panels used different bed mounting pads from the pickups. The mailorder parts place in Canada looks to have these in their stock for $50-something. The ones they show a photo of are the larger rectangular ones that I have it seems.

Another gentleman on the GMC list sent me a link to a photo of the underside of a 64-66? GMC (the later years that had leaf springs), and although it is not the X-frame, this picture pretty clearly shows no cross sills directly over the axle where the cross sill in question on mine is located. See attached picture, and note location of perimeter wheelwell bolts in bed wood vs the next rearmost cross sill.

I would be very satisfied to see this same view on an X-frame panel truck on a 60-62 (or was the X frame until 63?).

[joe231]

62 was the last year of the X-frame

[protrash64]

I believe the bolts are going to be carrige style similar to the pick-ups. You should have cam-style washers for adjusting the body/mounts/etc when assembled. Your frame will kick down in the rear, the later burbs have this also.

Burbs and panels are the same length, unless its a 10'(?) 1ton truck. My burb has the exact wheelbase as the pick-up parked next to it.

[Chuck78]

Originally Posted by protrash64

I believe the bolts are going to be carrige style similar to the pick-ups. You should have cam-style washers for adjusting the body/mounts/etc when assembled. Your frame will kick down in the rear, the later burbs have this also.

Burbs and panels are the same length, unless its a 10'(?) 1ton truck. My burb has the exact wheelbase as the pick-up parked next to it.

burbs and panels I believe are the same wheelbase, but I know for a fact (GMC sales brochures) that burbs have an 8 foot bed, and panels have a 7-1/2 foot bed.

I don't know where the carriage style washers are on mine. Do they sit down in the hole under the bed wood???? Maybe I need to order a set of these for when I pull the bed wood up to install new mounts. As I said, I am missing at least two sets of bolts for the bed to frame mounts, and lost one of the sets of rubbers in the process. Actually, I think I lost half of hte ones that are there, as there are upper and lower rubbers, right? Do they have a metal sleeve that goes in between them? I am missing some of these also. Might need to find someone parting out a panel for this stuff, eh?

My frame does kick down in the rear, I realize that is the way it is supposed to be, but I am worried that the rear kicked down part might be bent downward slightly. My truck does have a really beefy receiver hitch welded to the rear of the frame, showing that it may have been used a lot for towing.
This concern is why I am trying to determine if my cross sill over the axle is improperly placed.

Thanks,

Chuck

[protrash64]

Ok, this is a 1964.....but should be similar......I believe. First pic is the toggle style bolt in the cammed washer type thing. Second pic is the shock x-member just in front of the rear axle. Third pic is the floor support just behind the axle and directly above the panhard mount. Its not the tall support, its the short one (roughly 3/4'' tall). Its just peaking out above the two lines running on the frame.

[protrash64]

I would disagree with other statements about the frame bending. I think its entirely possible. My neighbors had an RV/motorhome that they used to vacation and towed a car behind. After a few years the frame cracked over the rear axle. My father, being a mechanical engineer, said it had an 'extreme bending moment' over the axle with the weight suspended so far behind the axle. Basically the weight had a bunch of leverage over the frame due to its length. A coil spring/trailing arm suspension would be similar. The bending moment (point) would be where the coil meets the frame. Think of a teeter-totter, or fulcrum. I think its entirely possible if a trailer had too much tongue weight.

[padresag]

115" w/b frames are all the same except for 4 x4's which are 2" deeper in the mid frame area. p/u mounting is different from the rear cab mount back than suburbans and panels. you cannot buy the rear panel or burb insulators for the 60-66 models. I believe that the 67-69 insulators will work and I see that they are disappearing also as i saw them in the cataloques about a year ago but not now. I do have the measurements though for the 60-66 ones.
you cannot bend the frame by overloading a trailer. thosr 4 ply tires would blow with the amount of weight it would take to ever come close to bending that frame
ron

[joe231]

Jeff I hear you on the RV.....but from my experience with RV's they usually have a much longer frame extension from the rear axle to the rear of the RV(much longer fulcrum-with much more leverage)....I have seen some with 8-10+ feet of frame behind the axle......I'm not saying it couldn't happen....just that there would probably be other signs of a major occurrence

[protrash64]

Its a long shot, I agree. You just never know what happened to a truck over 40 years of use.....I would think that if it happened on a panel you would see some bowing across the quarters or the interior panels. The frames have NO problem bending the other way when you take the bed off....my frame had to be redone cuz the bed tipped forward.

[Chuck78]

Wow, this guy just gained a customer here from his email response to me. This answers a lot! I asked if he had the thicker bed mounts that are about a half an inch, like what I need for my panel vs the 1/4" pads that I got apparently for a pickup application. This is what he said:

_________________

Hi, Chuck
The 1960-72 Suburban / Panel truck body mounts were last available through GM up until about a year ago when the last available were finally sold. These are the mounts you need, and when new they are actually more like 3/4"-1" in thickness not a 1/2" and have a steel sleeve molded into the bottom cushion that is slightly taller than the cushion and the top cushion has a small depresion in it that the round steel sleeve fills when they are mounted and tightened. We have heard that there is a company having a manufacturer reproduce these and we expect them out sometime this year but it looks as though it will probably be late summer or sometime in fall at earliest before they become available.
Have a nice weekend!!!
My Very Best
Paul @ GMCPauls

Online Catalog- http://www.gmcpauls.com
GMCPauls Truck Parts
505 Adams Drive
Rockville, IN 47872
Fax: 773-442-0103
___________________

That answers a whole lot, as my mounts must be really smashed down and rotted! I think this amount of lift would fix my problem, or at least allow the bed to sit level. I still think that the one cross sill in question (that is currently sitting at the high point of the frame over the axle) would be touching the frame with no room for any rubber mount, however. I still think that it had to have been mounted in a different location, like one set of "skid rail" bolts back.

I did see a few sources that had mounts that looked like the ones I needed, but they did say 67-72 Suburban I believe. That was only one place out of all the truck restoration parts suppliers that I had looked into.

Due to it not being a leaf spring rear, and the possibility of towing, I still think I will try and take a piece of 3" structural steel channel and weld it to the bottoms of the rear frame extensions, and then where it comes up towards the axle, and the frame kicks up, I think I will take some 1/8" or maybe 3/16" plate steel, and add a side gusset to the frame, from the bottom of the channel and following the contour of the frame kick-up, with as much hanging down below that as I can without risking having the axle tube bottom out on it. Might do the 77-81 Z28 trick and throw an aluminum spacer block under the bottom-out bumpstop rubber snubber.

I feel pretty safe that if I had all new body mounts in place and all bolted in, that the body itself would lend a huge hand in stiffening/strengthening the tongue weight capacity of the truck, but I still may do that reinforcement. I rarely tow, but once or twice a year is enough to justify the extra insurance. My buddies ask me now and then to tow home a new lead sled project for them, and my 71 Camaro project might need a new storage place to call home soon when I move...

[padresag]

[QUOTE=Chuck78;3288291]Wow, this guy just gained a customer here from his email response to me. This answers a lot! I asked if he had the thicker bed mounts that are about a half an inch, like what I need for my panel vs the 1/4" pads that I got apparently for a pickup application. This is what he said:

_________________

Hi, Chuck
The 1960-72 Suburban / Panel truck body mounts were last available through GM up until about a year ago when the last available were finally sold. These are the mounts you need, and when new they are actually more like 3/4"-1" in thickness not a 1/2" and have a steel sleeve molded into the bottom cushion that is slightly taller than the cushion and the top cushion has a small depresion in it that the round steel sleeve fills when they are mounted and tightened. We have heard that there is a company having a manufacturer reproduce these and we expect them out sometime this year but it looks as though it will probably be late summer or sometime in fall at earliest before they become available.
Have a nice weekend!!!
My Very Best
Paul @ GMCPauls

Online Catalog- http://www.gmcpauls.com
GMCPauls Truck Parts
505 Adams Drive
Rockville, IN 47872
Fax: 773-442-0103
___________________

That answers a whole lot, as my mounts must be really smashed down and rotted! I think this amount of lift would fix my problem, or at least allow the bed to sit level. I still think that the one cross sill in question (that is currently sitting at the high point of the frame over the axle) would be touching the frame with no room for any rubber mount, however. I still think that it had to have been mounted in a different location, like one set of "skid rail" bolts back.

I did see a few sources that had mounts that looked like the ones I needed, but they did say 67-72 Suburban I believe. That was only one place out of all the truck restoration parts suppliers that I had looked into.

that is basically what I told you on the 60-66 site. I am glad that they are going to be reproducing them. following excert from an old GM man

Ron, this is Lewis. Please forgive me for not getting back sooner, but I have been busy in my work for the church I serve. I checked both my 65-66 parts manual and my 67-72 parts manual. This is what I discovered: 1964-6 body mounts are p/n 3794779, w/dimentions of 1.76" w; 3.00" l; 0.92" thick, needing 8 (up & lwr same). The 67-8 p/n is 3909351 up & lwr, 69-72 uses 3909351 as the lwr, but p/n 3946246 is the upr. No dimentions are given for these cushions, and it seems that the frame changed in '69. Qty's in 67-8 are 16, with 69-72 requiring 10 of each. If you have dimentional axcess to the later cushions, these 64-66 dimentions should help you. Like you, I suspect that the 67-8 cushions will work, but there is no superseded p/n's between the body styles. I can tell you that years ago I bought cab mounts for my 65 and the p/n's had superseded to the mounts for the 67-72 cabs. I hope that this info is of some assistance in answerring this question.

the good ones that I have measure about .720" thick but if one measures in the round raised protrusion( around the bolt hole) that was mentioned earlier
Lewis's measure of 0.92" thick would be correct
ron

[joe231]

Originally Posted by Chuck78

I still think that the one cross sill in question (that is currently sitting at the high point of the frame over the axle) would be touching the frame with no room for any rubber mount, however.

could this sill you are referring to be a reinforcement strip? or maybe there is supposed to be a reinforcement strip there instead of a cross sill, the reinforcement strips are not as tall as the cross sills

here is a link to the reinforcement strips explained
http://mar-k.com/catalog/catalog.asp