tell me it's not true...interchangeability question

[lofly'a]

i just read a post about interchangeability on the paint and body board and i hope it's not true, but here goes...................my latest project, a 71 gmc swb fleet has leaf springs and i want to go to coils and trailing arms. i have the whole trailing arm setup with a nice 12 bolt 3:73 limited-slip to go in but on the paint board "ckhd" said that the frames are made different.....how so and what has to be done to make the swap.

[JimKshortstep4x4]

The frame for the trailing arm setup has a large crossmember under the cab(short frame) that reacts the force transmitted by the trailing arms and the leaf spring frame does not have this member. In addition the rear cross member is different and the shock hookup is different. I would not want to say that it can't be done, but it would be very difficult. I would look for another frame if it were me.

Jim

[Mike C]

Why not keep the leaves? Much easier to run a good exhaust, and with Early Classics springs easy to drop. Just add a set of Cal-Tracs or slapper bars if you are worried about axle wrap/wheel hop. If you do decide to go with the coils, I believe you can swap on all of the brackets, etc. to your truck but will have to remove most of the other brackets for the leaves. If you are dead set on it, the new frame is probably the way to go. I held out for quite some time to find a leaf spring truck myself.

[ckhd]

I'll say it can't be done. If you had two frames, a coil and a leaf, stripped clear down to just the frames, then looked at them from the top, you would notice that the leaf spring frame is straight where the coil spring bows out at the rear wheels. The coils go directly where the bow is. You would have to do some major modifications to the trailing arms or the crossmember to get them to be narrow enough for a leaf spring frame. Of course if you did that, then you will have jacked with the structural integrety of the pieces, and I wouldn't ride in that death trap for a million bucks.

See this crude drawing I whipped up about coil frame vs. leaf:

[crazy longhorn]

I think thats a doable swap, the 4x4 & longhorn frames are different than the other 2wd frames(straight rails). I havent tryed the swap, but from pics that have been posted, it looks to me that the 2wd leaf trucks have the same crossmember,&" bowed" out frames(where the coils would sit) that the coil trucks have. It should just be a matter of bolting the parts on. With that said, I do agree with MikeC on exhaust placement on the leaf spring trucks......my longhorn is a straight shot on ex, muffs can be placed pretty much anywhere! i have thought about getting rid of the leafs on the longhorn, but I have thoughts on a custom 2 link set up (long bars on the outside of the rails) ,with the front spring mounts slid ahead & AIR BAGS ! good luck.....crazy AL

[Yellow Cheyenne]

I'm not familiar with short frames, but on LWB 2 wheel drive 1/2 ton pickups, the frames themselves are the same shape. The swap can be done, I know cause I've done it (went from coil to leaf on a friends pickup). The frame rails are straight on 4x4's, and CKHD's illustration is correct to my knowledge (again, I'm not all that familiar with 4x4's), but from what I've seen, he is correct on the 4x4.

Back to the swap I am familiar with. You will want to get all of the hardware from the donor pickup. It will be best to have both frames next to each other as you do the swap so that you can see all of the necessary bracket changes. Even the E-brake cables that go into the drums are different. The main frame brace just below the rear of the cab is the same, just has more brackets bolted to it for the coil suspension. The carrier bearing and drive shafts will also be different, although you could use the leaf drive shaft and bearing with a coil setup, you would probably have to change the input on the rear end.

Please note, all of my experience was on a long wheel base, 2 wheel drive 1/2 ton. Don't know how much of it is correct for the short wheel base, but would guess a lot of it would be the same. I prefer the leaf setup for trailer towing stability. If ride and/or racing is your main concern, coil is probably the better setup.

Happy wrenching (and torching)!

[NSANE68!!!]

Whatckhd said is not accurate. I have owned both short wheel based trucks. Leaf and Coil, the frames both have the curve at the axle. The rest of the crossmembers, I don't recall. I am adding a pic of my 72 LEAF truck, you can see the curve.
The easy way out would be a weld on 4-LINK, and probably give you better all around traction.

[ckhd]

Leaf frames are curved too? I wonder what the heck I'm thinking of then. Used to have a '68 Chevy 1/2ton 2 wheel drive with optional leafs and I'd swear the frame was straight back there.

[COBALT]

Diff between 1/2 ton 4x4 and 1/2 ton 2wd maybe?

[Desert Rat]

Ive watched guys swap from coil to leaf,i dont know why,but what was needed was the coil crossmamber,the trailing arms,panhard bar,shock brackets etc.it isnt a big deal,but i like my leaf rear trucks for some reason,over coils.guess thats why everything gets a 67 GMC frame,well that and 67 GMC earlier ones have no VIN numbers.if you need a coil frame $250 in AZ,ill give the trailing arms for $25 nice rust free,can sell a rolling chassi if needed.

[Palf70Step]

This may sound strange, but why worry about adding coils to a leaf frame. If you are going to take things apart to do a sweap like that, why not just get a chevy frame with coils and put your body and all onto that. Then sell your leafe frame to someone who might want the leafs instead of coils.

[lofly'a]

i do have a chevy frame, 2 as a matter of fact but i wanted to keep the gmc frame with the rest of it. i don't like leaf sprung rears, coils ride better and handle better. and i know ya can't haul much of sh*t, but i don't intend to. i have had many leaf sprung vehicles and nobody can sell me on one versus the other and the results are proven over the years. besides, once i bag it, it will perform comparable or better at hauling (if i wanted to) than any leaf sprung rear. thanks for all the input, i'm still not sure that i can swap in a trailing arm rear on the leaf frame but i'll find out for sure when i bust everything apart.

[Stano]

I have both leaf and coil 1/2 and 3/4 ton and the rear of the frames are the same also the leaf trucks have the trailing arm xmember behind the cab cause thats what the carrier bearing mounts to. As far as the xmember over the rearend I don't remember for sure.

[lofly'a]

thanks stano, i don't have a problem with swapping out a crossmember or 2, it's gonna get powder coated anyway. i just wanted to know it the frames are diff in any way. i've never had a leaf sprung truck of this era before so i appreciate any info i can get.

[PICKMUP]

Here is a nice exhaust system that is prebent to clear trailing arms, coils, etc.

[PICKMUP]

Notice it is mandrel bent...no flat restrictions at the bends.

[PICKMUP]

...and it is made wide enough to clear rear tanks, subs & blzrs...plus 2.25" oversized. It will flow the same as a 2.5" system bend on any regular muffler shop bender. Nice, huh!

[lofly'a]

thanks for the pics PICKMUP, i never could get a muffler shop to make nice exhaust like that for my 68. i intend to punch holes in the trailing arm crossmember for exhaust pass-thrus. that is one nice setup. i want to gain some ground clearance by avoiding the dip under the trailing arm crossmember

[Fast68Chevy]

all 1 tons and 4x4's were leaf straight rail frames

leaf spring setup can be swapped onto any frame, coils can only be swapped onto 1/2 ton and 3/4 ton frames without fabrication.

leaf rear was optionally available on 1/2 and 3/4 ton 2wd chassies and the frames are all technically coil frames,
the frame width in front of and behind axle is same width on them all, roughly 34", so that leaves can be optionally installed if are ordered, the only real difference being that most coil holes are not drilled in leaf ordered frames and leaf ordered frames dont have locating ridges near the hanger brackets. its easy to spot and see once youve had a couple or more of each type of frame and also of 1 ton ones also.



38.5" roughly over axle on 1/2 and 3/4 ton 2wd frames


crossmember over axle does differ between leaf and coil, are swappable, no biggie. crossmember that trailing arm pivot brackets attach to lok same between leaf and coil, at least the ones i have out here do anyways, identical


good luck