73-80 Front Clip Swap on 81-87 Cab

[66gmcguy]

I just completed this swap on my 83 blazer, so I thought I would share my experience with it. I've heard from some people it's a direct bolt on swap with no frame mods needed (apart from the obvious issue with the bumper mounting), and other people say the core support frame brackets need to be moved. I discovered that the core support frame brackets are completely different, and are located in different spots.

The top of the core support frame brackets are flush with the top of the frame rail on the 81-87 trucks. On the 73-80 trucks, the top of the core support frame brackets are canted forward (relative to the top of the frame rail), and they are also about 3/4" further forward than the 81-87 frame brackets. Also, they sit just a hair lower than the 81-87 core support frame brackets.

I was completely unable to get the fenders to bolt to the core support when using the 81-87 core support frame brackets. I could get the top fender bolts in, but not the bottom bolts in (since the frame brackets were leaning back, thus causing the core support to lean back) The bottom bolt holes were about 1"-2" off. Luckily I had a spare 73-80 4wd chassis so I just took off the core support frame brackets and put them on my frame. I had to weld the old rivet holes closed, then re-drill new ones for the different frame mounts. Everything went smooth after that.

In all the forums I've read, I've never seen anyone talk about exactly how they are different, so I just thought I would post my findings and some pictures on here so anyone thinking about doing this swap knows what to expect.

1st pic is the 81-87 frame brackets. 2nd pic is the 73-80 frame brackets.

Lastly, this truck has a 6.5 diesel engine in it. I had to modify the 73-80 core support to fit the big diesel radiator in it. If anyone is interested in seeing some pictures of how I did it, feel free to message me. If enough people show interest, I will do a short write up about it and post it on here.

[66gmcguy]

Here's a better picture of the 73-80 core support frame mounts

[66gmcguy]

Originally Posted by bL.A.zer90045

How about the front bumper mounting? I know there is a way to cut the 81-87 frame to accept 73-80 bumper...any chance you have knowledge or insight on this? I have a complete diesel engine harness is interested I'd barter for?

I haven't really gotten that far yet, and I'm not 100% sure I will, since I am going to make a custom winch bumper

However I did mock up a 73-80 bumper (using the inner 73-80 bumper brackets and trimmed 81-87 outer brackets) and I didn't see it necessary to trim the frame. If you are installing a 81-87 bumper on a 73-80 frame then you would have to do some trimming.

If I was going to run a stock front bumper, I would just make some custom inner frame brackets that use the stock holes in the frame, but have tabs that pickup the holes in the 73-80 bumper. I guess another possibility is cutting off the frame horns from a 73-80 frame and welding them on your frame, but I would advise against that for a number of reasons.

[cadillac_al]

I wonder what GM accomplished when they cut the front frame horns at a diagonal? Was there some advantage in doing this? I will be dealing with this in the next couple months, putting a 73-80 nose on a 80's cab and frame......or rebuilding my 76 with an 80's cab and frame.

[66gmcguy]

Originally Posted by cadillac_al

I wonder what GM accomplished when they cut the front frame horns at a diagonal? Was there some advantage in doing this? I will be dealing with this in the next couple months, putting a 73-80 nose on a 80's cab and frame......or rebuilding my 76 with an 80's cab and frame.

My guess is it was just to accommodate the new front bumper design. I don't really see any other reason than that.

[Honest Bob]

Odd that you had to do that. I put a 75 body on an 85 frame and didnt have to do any bracket changes. I just had to use the correct body mounts for the year of the frame. Maybe its a blazer thing?

Originally Posted by bL.A.zer90045

How about the front bumper mounting? I know there is a way to cut the 81-87 frame to accept 73-80 bumper...any chance you have knowledge or insight on this?

I have a 73-80 bumper on my 81-87 frame.

The 73-80 side brackets bolted up for the outer bumper bolts.

I used the 73-80 brackets that bolt on top of the frame for the top center bumper bolts.

I used the 81-87 brackets (I dont believe I modified them?) that bolt inside the bottom c channel for the bottom center bolts.

No frame cutting required!

[66gmcguy]

Originally Posted by Honest Bob

Odd that you had to do that. I put a 75 body on an 85 frame and didnt have to do any bracket changes. I just had to use the correct body mounts for the year of the frame. Maybe its a blazer thing?

I doubt it's a blazer thing. If I had to guess, it's just a tolerance stack-up. The body could have been too far forward on the frame not allowing me get the fenders bolted to the core support. I even tried bolting the fenders to the core support without the core support bolted to the frame so I could see how far off the frame bolts were, and they were a whole 3/4 to 1" off, and that was with the fenders practically jammed into the doors, and the core support body mounts slid as far forward as they can go in the frame mounts.