4X4 Frame rails

[Seanext]

I have a rolling 4x4 frame. I was told it was 71-72. I have a 71 4X4 but there seems to be some differences. Like rear spring length, front spring mounting plates and other minor things. I found two number on the frame rails
3933323 100 12 21 0
3933324 100 12 19 0
I am pretty sure these are the rail part numbers. Is there anyway to determine what year this frame is or were there just production run differences.
Any information would be greatly appreciated.

Melvin

[Mr Chevorlet]

Could post some pictures of the areas that are questionable. Or the frame itself. Right now I not sure.

[swamp rat]

Cant help with the numbers but there are differences in the frames, my 72 K20 has straight frame rails at the rear wheels and other frames are bowed outward, this also affects the spring spacing at the rear axle from (if i remember correctly) 40.5 and the other 42" at the center pins.

[Seanext]

Not sure how to post pictures. I have several and I will post if I can figure it out. The wheelbase is the same both are SWB ( I don't think they made a K20 in a SWB I could be wrong), front and rear axels are 6 lug and are the same, the cab mounts seem to be the same. The rear springs are 54 inches eye to eye with several more leafs than on my 71 they are 52 inches. Front springs are the same length but there are three leafs on the unknown frame. On the unknown frame the frame to axel distance is about 2.5 inches higher. I am assuming the extra spring may have been a lift kit at some time.
Any guidance on posting pic would be helpful.

[Seanext]

Seems I figured out the pictures without knowing it.

[jjzepplin]

Yeah that's wild stuff on the front there! Never seen that. Frame looks to be correct but with some mods. Looks like they moved the rear hangers back. Are they bolted on or riveted?

[WIDESIDE72]

Disc or drum brakes on front? That would at least tell you what year the front axle is. Drums=67-70, discs=71/72.
Posted via Mobile Device

[Seanext]

Disc on the front so I think it is 71-72.
The rear hangers are bolted on but they look like the correct
Ones. I will look when I get home and see if they have been moved.
Are GMC. FrAmes any different?
Posted via Mobile Device

[special-K]

Chevy & GMC are the same. That front spring mount in Post #4/last pic doesn't look original. Not sure what rear springs would be 54"<but I don't know of any GM lefs in these years. Did you get the VIN off the top of the left frame rail about where the front of the cab would go?

[jjzepplin]

If the rear hangars are bolted they have been moved. Measure your original and compare to this one and you may find that there is a perfect bolt pattern that you could bolt your old system back into. OR you could run these springs as long as you KNOW the wheel is centered in the wheel well. I had a really early Rancho lift on my 70 which had front blocks(NO-NO) and helpers (extra leaf) and the rear had blocks. Actually did not run bad on the road but couldn't trust it in a hard braking situation with my life so out it came.
What I'm getting at is do you want original or what ever the heck that is?

[Seanext]

I have tried to fine VIN on the rail with no luck. Next is a wire brush on a drill to see if it will showup. It looks like you are right the rear spring mount will move forward enough to put the 52" springs back in it. I am not sure about the front spring mounting. Seems to be something some one pulled from the junk drawer and put together. Going to tear it down and put all orginal back on it. If I don't want to run it then I wouldn't want anyone else to. I will assume it is a 72 and build it to that.

Thanks for all the info.

[special-K]

If it appears the VIN was ground off,they should appear if you heat with a torch since the numbers were stamped and the metal will be more dense where it was stamped

[Seanext]

After closer inspection the rear spring mounts are in the only place available the other set of holes does not lineup. The front rear spring mounts are riveted but the back rear spring mounts are bolted. After looking at the spring geometry I think someone just put a longer spring with more leafs. I think going back to the original springs will put everything back where it should be.
I did try heating up the rail all the way between the cab mounts and there does not seem to be a number anywhere. I also cleaned the rail upfront beside where the power steering pump is and there is nothing.

[CUSTOM/10]

It's possibly a GMC frame with some mods done to it. I've read that some of the GMC frames didn't have the VIN numbers stamped into them.

Gary