55.2-59 subframe question

[jmlloar]

Didnt want to hack my frame but seems like thats my only real option to solve my issues.

My question is, I have seen tons of car clips spliced in but I want a truck and keep it 8 lug 3/4 ton.
Has anyone spliced the front clip from a newer pickup to the older ones?
or should I just swap the whole frame?
If I splice just the front clip there would be no issues with cab mounts and bed location. Only radiator mount location.

[geezer#99]

I used the front clip from a 80 gmc on my 56 frame. Was just a 1/2 ton 5 lug though.
The frames were almost identical in size.

[jmlloar]

Originally Posted by geezer#99

I used the front clip from a 80 gmc on my 56 frame. Was just a 1/2 ton 5 lug though.
The frames were almost identical in size.

any pics?

1/2 and 3/4 from 73 to 87 are basically the same. Width behind front cab mounts looks the same. only question is 55 59 is straight and 73 to 87 drops a lot behind the front axle.
I think it would make more sense than trying to make the old system better, and just use the complete system that was designed to work together. Cant just use the front suspension since the newer frame is pinched in for independent suspension clearance.

[mr48chev]

Jmilaor pretty well nailed it, you would have to do the splice back under the cab between the cab mounts.

I'm not a fan of frame swaps unless you don't have a usable original frame to start with unless you are going to a custom frame but in this case I'd have to think you will be ahead of the game to just swap a complete 3/4 ton later model chassis of choice under the truck. That just means in this case Fabricating or reworking cab and nose mounts and mounts for the front bumper brackets. The key there is taking a bit of time to do a decent looking while plenty strong job of it.

Either way by splicing the frames or swapping frames you will have to redo cab mounts and fab the core support mounts and bumper mounts. I don't think that there is room in front of the firewall to cut and splice frame rails and have the strength you need when you are done by just using the frame stub. An overlapped splice that is plated and boxed is what you want and that has to be back where you have plenty of straight rail to work with the same as if you were taking a long bed truck frame and making it into a short bed frame. Thats a lot more work when the back of the two frames are pretty well the same from front cab mounts back. Plus you get the newer wider springs and the later style 3/4 ton rear axle as part of the package.

[jmlloar]

pretty sure I will stick with front cab mounts and back.
I have already finished all of that out including a newer rear swap that is completely finished except spring bushings.
Doing the eyeball method it looks like a slight overlap at front cab mounts and from there back the 73 to 87 front clip would set under the orginal 55 since the 73 to 87 drops down at its front cab mounts a lot.

guess this would really be called a front clip graft and not a subframe.

If I over lap the 55 from front cab mounts back and 73 to 87 from rear cab mounts forward I should have pleanty of meat for strength

Hoping someone had some pics of a similar splice project.

Hate to cut the original frame since I would put it at between an 8 or 9 on condition.
Also not looking for a restoration job. This thing will be more in the resto mod catagory. Only thing original on it will be body and most of the frame.
Daily driver pulling gooseneck and camper on a regular basis.

[jmlloar]

I did a 42 chevy mock up 30 years ago with a 73 to 80 frame and set the cab section of frame on top of the newer frame with decent success (put it in the drop section). I didnt have the width problem since truck I used was a 42 2 ton cab and feders.

Never finished it since my main project was a 42 chevy 2 ton with 2 speed rear. And I kinda lost interest. Still have some of the extra pieces around.

[geezer#99]

Sorry, no pics!
This was done 30 years ago.

And yes I spliced the frames togethor under the cab. Was easy to fab front cab mounts. I wanted the truck to look like a truck. Not slammed down. I could have z’ d the splice to raise the nose more but left it straight.
Must have done it right because it’s still running around in northern BC with a blown big block in it.
Any frame swap is a compromise. You just need to figure out what is best for you.

[mr48chev]

If you weren't wanting disk brakes a 67-68 and maybe 69 GMC 3/4 ton chassis would get the wheels inside the fenders better but you loose the disk brakes of the later trucks. That gets into the Hard to find one territory now though. 73/88 3/4 ton trucks are still pretty easy to find and you can probably find one within a reasonable drive that has an unmolested chassis but has been stripped of what the seller sees as valuable. meaning less to get rid of.

[dsraven]

check with the local authorities before doing a frame graft, it may not pass inspection. try a frame shop or a shop that does autobody inspections after a write off as they should know. it would suck to do that work only to have it fail due to not allowed to do that procedure for a road vehicle.
track width would be a little wider on the newer trucks would it not? maybe different wheel offset would make up for that but also change the front suspension geometry some so SAI (or kingpin inclination) and scrub radius would change which affects how it feels when driving. or maybe you like the tires sticking out past the fenders and the wide track look?

[jmlloar]

I've done both in a past life. I got that covered so it's not an issue for me.

[mr48chev]

The big problem with cutting and splicing those two frames is that the 56 frame pretty well runs straight from the core support back to what little hump it might have over the rear axle. The square body frame this being a 77 1 ton has a big dip where it drops under the firewall.


This after I caught my breath after hauling that frame home on my 77 dualie with the 10 ft bed. How I drove from East Wenatchee to Toppenish without getting stopped is beond me.


With that one I am able to use the front crossmember under the 51 1-1/2 ton and with a bit of finessing use the rear springs and hangers on the slightly wider 51 frame. I've got a Dana 70 dualie rear axle for the 51 as it isn't intended to be a load hauler when done.

[jmlloar]

you're thinking backwards, it bulges up in front of firewall to clear suspension. thats why I need the front piece of frame.
Its also pinched in making the frame on the newer truck much skinnier that older trucks in front of the firewall. thats why you can't use just the front suspension.
the frame width under the cab on both trucks is almost the exact same width.

[mr48chev]

What you may want to do is some fitting and trimming where you mate the later frame stub with the trimmed to fit frame end in front of the firewall pretty much as my buddy did a ton of Camaro stubs on TF frames.

Gray area being the TF frame.


For that you have to measure your current static ride height of the complete 56 truck, measure the complete donor or a very similar trucks ride height and bottom off front crossmember to shop floor height and with the TF frame on stands at the desired final ride height and level you start fitting the stub up to it with the crossmember at the measured ride height. That way when you have them mated up you end up with a final ride height that is real close to exactly what you want without any other mods.

This is a 56 3100 frame that my buddy did for someone a few years ago. This one ended up being super low with the bottom of the frame the same height as the bottom of the Firebird stub.
That part isn't important except for what I said above that you figure out the final ride height you want and using the method I suggested mate the two together. Not this low I would hope but at the height you want.

Leaving enough of the bottom of the frame rail from the donor to have a line to put parallel with the bottom of the 56 frame will help have the stub mounted right.

[mr48chev]

Well you can draw them pictures but that doesn't mean they can visualize what the pictures say.

First it isn't strong enough to butt weld the frames together.

Second unless you want the truck lower than it is. you have to figure out what the end product ride height is. That is by blocking the 56 frame up at the static height you want with the frame rails perfectly level end to end and side to side.
Third you slide in under the doner truck or a twin to the donor truck an measure from the bottom of the crossmember to the shop floor to get the height that the crossmember is going to sit when it is mated to the 56 frame. The overlapping splice is for strength plus it makes it simpler to get things square.

You will have to do a pie cut in the front of the 56 frame rail about where the later frame curves up to bring the 56 rails in so they match up with the frame stub. That is ahead of the firewall. All of that frame mating is trim, fit, check trim, fit some more until you get it spot on and then tack it and measure and check for square.

I'm not new to this subfame thing even though I fully believe that subframing with a Camaro, Chevelle or S-10 subframe to get a truck low is obsolete 80's tech and there are far better ways but this I don't believe is somethign that you plan to drop to the ground.

[jmlloar]

As stated in earlier reply
55 cut at fire wall
73 up cut at rear cab mount.
straight line down the center of both
center line parallel on both
height of center line diff to the ground between the two
and pleanty of length to splice them together.
angle at the front to match the newer frames bump up
I dont know where the assumption that I was planning to butt splace came from, not the case.although a butt splice would be pleanty strong with a double layer patch across the splice to strengthen it. I've done this before.
The 55 frame is much taller from the ground. Thats why I was assuming the newer frame would be pretty much running under the older one. I'll figure the exact specs as I go. Also taking the time to finish in the little details so it doesn't look half**sed.

I built a 64 F100 by using a 64 4x4 front and a 66 rear and slid the 66 frem inside the 64 for about a 2 foot overlap. Strongest point of the frame. used front bed mount hole as my alignment hole on both. Truck ran great for years and frame nevr was an issue.
Also seen many large trucks butt spliced and then overlapped with a long piece of steel that was bolted on. that hid the weld and strengthened it at the same time.

[jmlloar]

P.S.
I appreciate all the "dont forget this" stuff
It definitely helps me figure out what to do and what not to do.

[dsraven]

there are frame drawings for the 55-59 trucks in the assembly manual. it would be good to get a frame drawing of the donor frame and work it out on paper before you start. you obviously know the ins and outs if you have done it before but a "blueprint" doesn't hurt and when drawing it you may come across an anomoly (my big word for today, haha),
I suggest to start with a donor truck that is fully assembled and pin the suspension at ride height with the weight of the complete truck on the wheels, then you know how the suspension is made to sit. otherwise check on other similar trucks to see the angle of the lower control arm at ride height and then pin your donor's suspension to achieve the same angles before you start. an alignment shop or frame shop may be able to supply this info as well. that way your suspension will sit how it was engineered to sit when you're done. springs can be adjusted or purchased to correct the newly assembled finished truck to get the suspension back to where it should be when completed.
is the plan to use it as a 3/4 ton hauler or simply an 8 lug big fender truck that looks more like the original instead of slammed to the ground?

[jmlloar]

Originally Posted by dsraven

is the plan to use it as a 3/4 ton hauler or simply an 8 lug big fender truck that looks more like the original instead of slammed to the ground?

3/4 ton hauler. Close to original look as possible. Modern drivetrain. Already has '02 5.3l, '99 4l80e, '99 semi floater with 3:73 gears.

[dsraven]

is your truck a first or second generation 55? if second, the assembly manual is available online as a free download on the tri5 site, just google 55-59 chevrolet truck assembly manaul and pick that site. frame drawings are in there for all the models as well as many other good to know dimensional items.

[mr48chev]

The amount of hacking will depend on how in line with your stock frame you keep the top of the stub at the A arms.

These diagrams show a TF frame (I think 58) and an 84 3/4 ton frame.

All I am going to say from that is you are in for a lot of measuring and figuring to be able to set the ride height where you plan to.

[jmlloar]

That helps a lot.
thanks

[dsraven]

I have a 57 gmc but it uses the same rad support as a chevy truck, it's Canadian so maybe thats why? the GMC specific parts are like hen's teeth and the GMC sheet metal assembly stuff doesn't appear in the chevy manual I don't think. sounds like you have experience doing this sorta thing so I'm sure you have a plan and will get'er done. the frame drawings will be nice as they show how much drop the frame has from the cab mounts to the rad support mounts and also the dimension between those-fore to aft. the bumper mount dimension is another one. everything seems to be measured from the cab face as a starting point, which is the firewall. kinda like reading a blueprint, you gotta lot of math to figure out the dimension you need. I'm sure you have thought of this already but a new set of cab and rad support mounts would be good to have installed just so you can get stuff lined up correctly. there is a video on youtube showing how to get things lined up and gapped properly, it uses the drivers side front cab mount as a pivot point and shows how to rotate the cab from there to achieve proper alignment of the hood and fenders. it is "old man fixing stuff", google 55-59 gmc truck panel alignment, it's a few minutes of time that may pay off if you are new to these bodies.

[jmlloar]

US models 55 to 57 are different

[jmlloar]

the whole front cross member is different. it uses a thin piece of rubber and a u bolt down through it with springs and washers giving it a lot of flex.

[dsraven]

55-56 share fenders, 57 is different around the grille area, GMC also has holes to mount the mustache parts and some dimpled spots in the area around the grill. Just info in case you're looking for parts. 58-59 have longer frames from the spring mounts forward and their rad support brackets are further forward, not sure about the GMC compared to check differences there. Some models were fleet versions that used leftover parts from previous years, like instead of 4 headlights they had 2 and a cheaper version of a grille, bumper etc.
Post up some pics of what you have to work with, you have us interested now.