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Old 03-10-2019, 01:33 PM   #1
71CHEVYSHORTBED402
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Rear Bumper Bolt Pattern Thread

Yet another rear bumper thread. New things come up. Hopefully this one serves as an education. The more 's and 's chime in the better.

For those with factory 1967-72 rear bumper bolt patterns, can you measure the bolt pattern on your frame rail? That and can you provide the:
  • Year
  • Model
  • 2wd or 4wd
  • LWB or SWB
  • Coil Spring or Leaf Spring
  • Factory bumper

The bolt pattern on my 71 C-10, 2WD, SWB, leaf spring is 5.3/10". It had a factory step bumper. A bumper isn't installed, so a paint stick made it easy to measure the frame.

The pattern on my 72 C-20, 2WD, LWB, leaf spring is 5". Factory step bumper. The rearmost hole isn't as far back on the frame rail as that on my C-10.
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck

Last edited by 71CHEVYSHORTBED402; 03-11-2019 at 05:35 PM. Reason: Corrected with new information.
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Old 03-10-2019, 08:18 PM   #2
71CHEVYSHORTBED402
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Re: Rear Bumper Bolt Pattern Thread

Someone gave me the clue to look at the part's book.


http://www.gmpartswiki.com/getpage?pageid=22606

3885614 - Chrome Rear Bumper Bar. 67-72 C-10-20 (Sub, Fleetside) Blazer

3890391-2 - Inner Brackets. 71-72 Ser. 10-30 (Stepside, Fleetside) and more.
Reads excludes leaf springs. Found an NOS on the Chevy site, it’s the bracket folks call “coil spring” inner brackets. NOTE: The same part was used for the front bumper on 67-70 "pick-ups" (excludes 4x4)


3952089-90 - Inner Brackets. 69-72 Ser. 10-20 (4WD) (Pick-up)
Reads Rr. Bumper, which is rear bumper. No web reference found. Possible these are what folks call “4x4” inner brackets.


3891985-86 - Inner Brackets. 68-72 SWB 2WD with leaf springs.
No reference found on the web for this bracket. Not surprising. My Chevy is an outcast


3891987-88 - Outer Brackets. 68-72 SWB 2WD with leaf springs.
Go figure my outer brackets are an oddity? Don’t know why outer brackets would differ, I’ve always heard they’re the same. I have two sets (repop & OEM) and they seem to clear just fine. Maybe there’s a variance for the rear hole in frames, I’ll research.

Ironically there are quite a few hits on the web for this outer bracket. The part number is referenced for replacements. May imply they’re wrong, may imply a lot of things. Been 50 years, stuff gets lost.


3892477-78 - Outer Brackets. 71-72 and more. Ser. 10-30 (Conv., 4WD), Blazer (Stepside, Fleetside) and more. Applies to many trucks. Reads “Aux. leaf springs excluded”. Can’t say what an “Aux.” leaf spring is, but OK.
Found an NOS on the Chevy site, looks like the typical outer bracket. Other than that nothing found on the web.
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck

Last edited by 71CHEVYSHORTBED402; 03-11-2019 at 02:52 PM.
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Old 03-11-2019, 03:21 PM   #3
71CHEVYSHORTBED402
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Re: Rear Bumper Bolt Pattern Thread

Chasing down why outer brackets may differ. If anyone has a SWB coil spring truck, a tape measure and picture would be great.
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck
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Old 03-11-2019, 05:31 PM   #4
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Re: Rear Bumper Bolt Pattern Thread

Here's where I'm at. They don't sell brackets for my Chevy. The rumor a coil spring bracket (with a cut-out to clear rivet) works may be a myth, because I'm 90-10 certain the bolt pattern on my truck 5.3/10" is right. The forward most hole is center the shackle bracket. A SWB 2WD GMC is in the same boat (leaf springs). Unless you're lucky enough to find the right OEM brackets (I'm looking), the coil spring brackets can be used with say a 5/8" extension and a new hole added.

Be great to receive some confirmation, but it's the outer bracket that has me stumped at the moment. Why would they be different for my application? That's why I'm asking for confirmation on the placement of the rear hole on the frame. For what it's worth, the bolt pattern appears to be 5" on most trucks.

Appreciate all input, pretty please thanks. And no, I'm not drilling through the frame
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck
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Old 03-11-2019, 08:16 PM   #5
Rick Bollinger
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Re: Rear Bumper Bolt Pattern Thread

I am following your thread but having trouble with your measurements. Are you giving them in 1/10's of an inch?
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Old 03-11-2019, 08:42 PM   #6
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Re: Rear Bumper Bolt Pattern Thread

Here is what I have going on. It does not even close to stock but have nothing to compare it to.
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Old 03-11-2019, 08:48 PM   #7
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Re: Rear Bumper Bolt Pattern Thread

Quote:
Originally Posted by Rick Bollinger View Post
I am following your thread but having trouble with your measurements. Are you giving them in 1/10's of an inch?
Yep, using my 10ths stick. They built the exterior in 10ths, and that seems to add up to the bumper holes on my truck. 8ths (i.e., 16ths) works too, close enough.
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck
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Old 03-11-2019, 08:49 PM   #8
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Re: Rear Bumper Bolt Pattern Thread

Ok, here's some pictures from my 1970 C10 Short Stepper, 2wd with coils & no bumper...yet. However, I have backup lights mounted in the holes you are interested in. Best pictures I could take with iPad while holding tape measure and balancing the flashlight. You'll have to eyeball the measurements.
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1970 C10 Short Stepper with 71/72 front clip.
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Old 03-11-2019, 09:13 PM   #9
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Re: Rear Bumper Bolt Pattern Thread

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Originally Posted by BugzC10 View Post
Ok, here's some pictures from my 1970 C10 Short Stepper, 2wd with coils & no bumper...yet. However, I have backup lights mounted in the holes you are interested in. Best pictures I could take with iPad while holding tape measure and balancing the flashlight. You'll have to eyeball the measurements.
My eyeballs say that hole is approx. 1.3/8" from the rear and 1.3/4" from the bottom. At least the vertical matches up. Interesting. So in other terms, if we used the same brackets my bumper would be 1/2" further back. This may be why the outer brackets differ between my application and others.

Looks like the horizontal position of that hole is the same as my C-20, 2WD leaf spring.


Keep em coming, I appreciate it a lot. At some point I'll dial this down. Had I known this would be an issue I'd have solved it before the frame was finished. Probably should have anyway. On a side, I know I'm not the only one with that rear bolt location. Here's another, post 6. Bet it's a SWB 2x4: http://67-72chevytrucks.com/vboard/s....php?p=6742315
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck

Last edited by 71CHEVYSHORTBED402; 03-11-2019 at 09:26 PM.
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Old 03-11-2019, 11:25 PM   #10
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Re: Rear Bumper Bolt Pattern Thread

I think it's finally clicked, thanks to input and research. I'm basically SOL.....................Just kidding, but if anyone has nice set of OEM rear sport bumper brackets off a 2WD, SWB with leaf springs, please PM....At any rate, here it goes.

A. If they remake a working set of rear bumper brackets for a 2WD, SWB with leaf springs then it's probably an accident. Makes me wonder if earlier years might work, when did they begin installing rear coil springs?

B. I'm 99% certain my bumper bolt pattern of 5.3/10” is factory intended. On most trucks the rear bolt hole is approx. 1.3/8” from the rear of the frame. That doesn’t work on a 2WD SWB with leaf spring frame, and you can see why by the picture under post #3. That’s why the hole is 8/10” from the rear on my frame. That and the forward-most hole is centered with the shackle bracket. The bolt pattern is 5" on most other trucks. Can't speak for 4x4.

C. They probably punched the holes in 10ths. That’s consistent with the rest of the exterior; it was all assembled in 10ths, or at least most. Not sure about the front bumper, but it is likely the same. Past experience 10ths can be confusing, it had to be explained to me. Inches are still inches, it’s just broken down into 10 parts of an inch instead of eight parts.

D. Next time someone says “all outer brackets are the same” you can edgeucate. If I were to use the common brackets the bumper would set approx ” further back than specs. Now I know why my bumper looked different when it was on using old 4x4 brackets. Irony is a lot of vendors use “my” part number to sell common OER brackets. They should be referencing 3892477-78 for the outer brackets.

E. There’s an option to use common “coil spring” brackets for my application, but the inner brackets have to be fabricated for a front hole to match the frame. Again, if these are used the bumper will sit approx. ” further back than specs. The “4x4” brackets can be fabricated to work as well, with the same result………….

There may be an option to use the front hole on a coil spring inner bracket, and fabricate a new hole on both the inner and outer brackets for the rear hole. Guess what, that puts the bumper where it’s supposed to be or close enough. But might that change the pitch? If that works the new holes would have to be nuts for flush bumper mounts. The more I think about this option the more I like it, but OEM correct would be awesome.

F. Wish I’d bought a coil spring truck……….HA!.Thanks for the help
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck

Last edited by 71CHEVYSHORTBED402; 03-12-2019 at 12:55 AM.
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Old 03-12-2019, 12:57 AM   #11
randy500
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Re: Rear Bumper Bolt Pattern Thread

Tony I will measure my trucks tomorrow and post up.
Where is your bumper rear hole in relation to the red mount hole.
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Old 03-12-2019, 01:18 AM   #12
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Re: Rear Bumper Bolt Pattern Thread

Quote:
Originally Posted by randy500 View Post
Tony I will measure my trucks tomorrow and post up.
Where is your bumper rear hole in relation to the red mount hole.
Check out this thread dude. I think zbm100's truck is the same, SWB, 2x4 with leafs. http://67-72chevytrucks.com/vboard/s....php?p=6742315

Do you mean if I use the existing front holes? It's going to be 3/10" off, guarantee it. You basically have to weld up the existing rear holes in both the inner and outer brackets, punch new ones. You could almost get away with a slot in the outer bracket, but not really because you have to cut the front off the bracket to clear what's shown post 3.
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck

Last edited by 71CHEVYSHORTBED402; 03-12-2019 at 01:40 AM.
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Old 03-12-2019, 03:56 PM   #13
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Re: Rear Bumper Bolt Pattern Thread

Pictures, note relation of rear bed bolt to rear bumper bracket bolt
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Old 03-12-2019, 04:05 PM   #14
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Re: Rear Bumper Bolt Pattern Thread

Other side
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Old 03-12-2019, 04:06 PM   #15
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Re: Rear Bumper Bolt Pattern Thread

My 72 4wd
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Old 03-12-2019, 04:09 PM   #16
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Re: Rear Bumper Bolt Pattern Thread

Rear bed mount hole
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Old 03-12-2019, 04:12 PM   #17
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Re: Rear Bumper Bolt Pattern Thread

Bumper rear hole, 4wd, to the closest 16th is 1 7/16
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Old 03-12-2019, 07:35 PM   #18
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Re: Rear Bumper Bolt Pattern Thread

Very cool, thanks Randy. That rear bolt hole in the wall of the frame rail is standard. I believe just the SWB, 2WD W leafs differs. Not sure on SWB 4x4. I believe the vertical location (i.e. height) of the hole is the same on all trucks.

Nice to see a clear measure from the back of the frame. It's probably 1.4/10", which is 6/10" further forward than mine. If I install "coil spring" brackets using the front holes in the bracket that takes up half the difference. Better than it was, seems it should work.
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck

Last edited by 71CHEVYSHORTBED402; 03-12-2019 at 08:00 PM.
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Old 03-12-2019, 08:05 PM   #19
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Re: Rear Bumper Bolt Pattern Thread

I think your on the right track using the 2wd front hole them making a new hole in the bracket at the rear. 2wd leaf spring bracket has to be all on its own or...

Maybe blazer is the same. 72 blazers inner bracket is part of the frame so that doesn't count.
Blazer behind the wheel is the same length as short bed trucks.
4wd blazers are leaf spring.
I wonder if they have the same length leaf spring as your 2wd truck, you could measure your spring length and compare to 4wd blazers, if it matches then a blazer bracket could be the same.

The folded relief at the top of the 2wd bracket is to clear the last bed bolt so you will have to cut the bracket like a 4wd one or use an open ended wrench to install the last bed bolt.
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Old 03-12-2019, 09:21 PM   #20
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Re: Rear Bumper Bolt Pattern Thread

Quote:
Originally Posted by randy500 View Post
I think your on the right track using the 2wd front hole them making a new hole in the bracket at the rear. 2wd leaf spring bracket has to be all on its own or...

Maybe blazer is the same. 72 blazers inner bracket is part of the frame so that doesn't count.
Blazer behind the wheel is the same length as short bed trucks.
4wd blazers are leaf spring.
I wonder if they have the same length leaf spring as your 2wd truck, you could measure your spring length and compare to 4wd blazers, if it matches then a blazer bracket could be the same.

The folded relief at the top of the 2wd bracket is to clear the last bed bolt so you will have to cut the bracket like a 4wd one or use an open ended wrench to install the last bed bolt.
I think the clearance for the rear bed bolt would actually improve. The link above demonstrates why, post 14. Push his bracket forward the 3/10" difference and the bolt is more better centered with the slot.

I'm on-board altering the coil spring brackets, but I have better than a year to score the right brackets.

Blazers huh........hmmmm, I had the impression they took the longer "4x4" inner brackets. Better than 25 years since I pulled the old brackets from a wrecking yard, but memory says I pulled them off a burnt orange blazer, and they're the longer brackets. I didn't install them, I hired some guys to pull the step bumper and install the brackets. It was sloppy, but what can you do with the wrong ones, and at least they didn't drill through the frame. Soon after I bent up the new bumper installing it
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71 C10 Custom Deluxe, SWB, fleet, 402/400TH, A/C...Resource intensive......And (*^(*^ leaf springs. Speak of, looking for OEM rear sport bumper brackets for this ride. They're unlike those more common, even the outer brackets are different.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Driver and tinker truck
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Old 03-12-2019, 09:37 PM   #21
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Re: Rear Bumper Bolt Pattern Thread

Post 12 shows the interference with the rear hole lined up, it will get worse using the front hole lined up.

I agree blazer I think uses the longer bracket not for sure though.
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