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Hello Forum,
I was wondering what is the difference is if any between these two GM#'s?
They are both big block upper alternator support straps.
GM#3878239
GM#3986109
My reason for asking is I replaced my alternator over the weekend.
I realized from looking over the great photos on this forum that my upper strap was mounted incorrectly.
It belonged on the stud on the water pump.
The current strap fit well on the upper outer alternator mounting bracket bolt where it had been forever.
It was a very tight fit to get it onto the water pump stud.
I had to mount it to the alternator mounting bolt first and tilt the unit to get it started onto the threads of the lower stud.
The strap seem like in its proper location about a 1/2 width bolt to short.
I took the strap off and made sure the bends were straight as well as the entire length of the strap
I can't stretch it no more.
I was thinking I had maybe the wrong one?
It is not marked in any way with a part # or manufacturers strike.
Hoping some one knows what I'm talking about
Thank you,
Marshal
I've got the one off my 72 which will be the 3986109, I can measure the offset and center-center distance between the holes if you want, at least you'll be able to compare with what you have.
I've got the one off my 72 which will be the 3986109, I can measure the offset and center-center distance between the holes if you want, at least you'll be able to compare with what you have.
M
Mooser,
That would be great.
Much appreciated if you have the brace off.
I just looked at Corvette Centrals page and they list both.
I need to stretch mine a bit it's very snug between the mounting holes.
Marshal
I just moved mine down to the water pump bolt,I had to straighten out the bracket to make it
fit. Now I think thebelt is loose,never a dull moment!
I thought about opening up the lower bolt hole a little, but thought it should really fit it was there originally.
PO must have reinstalled incorrectly?
Marshal
Hello Forum,
I was wondering what is the difference is if any between these two GM#'s?
They are both big block upper alternator support straps.
GM#3878239
GM#3986109
My reason for asking is I replaced my alternator over the weekend.
I realized from looking over the great photos on this forum that my upper strap was mounted incorrectly.
It belonged on the stud on the water pump.
The current strap fit well on the upper outer alternator mounting bracket bolt where it had been forever.
It was a very tight fit to get it onto the water pump stud.
I had to mount it to the alternator mounting bolt first and tilt the unit to get it started onto the threads of the lower stud.
The strap seem like in its proper location about a 1/2 width bolt to short.
I took the strap off and made sure the bends were straight as well as the entire length of the strap
I can't stretch it no more.
I was thinking I had maybe the wrong one?
It is not marked in any way with a part # or manufacturers strike.
Hoping some one knows what I'm talking about
Thank you,
Marshal
Hi Marshal and Mooser,
I am kind of late to the party.
But JUSTTODAY I found my "stash" of brackets to put Power Strg on a BB 67
I have two GM NOS brackets 3986109 and one Paragon PR-1023 bracket which Paragon states is a replacement for 3878239.
The Paragon bracket is exactly a 1/2"shorter on the mtg hole C/L's then the GM 3986109 bckts.
I also I have what I believe is an original GM 3986109 bracket which is very gungy and rusty - someone years ago slotted the one hole on this rod to make it agree exactly with the Paragon copy of the GM 3878239 bracket - the witness marks bear that out.
I believe at some point GM changed / modified the BB Corvette power steering assembly - studs instead of bolts being used or vice versa ? requiring a longer bracket - 3986109.
I believe the 3986109 bracket does not fit early (C2 ?) BB Corvettes properly
Now the mystery is WHEN did they supersede the 3878239 bracket ??
Chevrolet / GM is prone to replacing / superseding similar parts without checking or worrying ?? about dimension changes
I have observed this with choke rods in Spec Hi Perf applications - specifying a rod that is 1/4" too long for replacement purposes.
Marshal sounds like you have the early bracket but need the later replacement 3986109 ?
Regards gentlemen
Jerry from Mooser country LOL
It shows both the 3878239 (66-69 427 w/PS) and then the 3986109 (70-74 w/PS) and if you look they show a slightly different size 0.90 vs 1.01 so a little over .100" even those these are pretty sketchy numbers
That would make it look like not so much of a supersede but a change required when the 427 switched to the 454. Maybe a slightly different location of on of the holes between the engines. I believe they used the same SIZE alternators (69 BB being the unicorn from what I understand but only from a numbers standpoint)
Oddly enough there is also a 3955180 that also used the 3986109 as a replacement
That shows up as a 69 427 w/PS in a 68 parts book and looks even shorter somehow
M
Last edited by Mooser; Jan 24, 2024 at 11:05 PM.
Reason: that's the cost not length D'oh...
Mooser are you into the Moosehead LOL
I believe those are the prices 90 cents and $1.01 LOL
The two brackets are quite different in length as are the bolt locations at least 1/2" wider on the bolt spread.
Something changed between 1969 and 1970 - alternator mounting width ?
Weird !
I didn't look to see if the other bracket changed as well. This is with a CS144 but the mounting flange was the same as the original alt
I do know that on some of the 427 pics I've seen the strap bracket went to the same bolt on the head as the other bracket instead of the waterpump so that might be the difference you're seeing. It would be a little shorter
M
Just looked at your original photo's - maybe the '69 and earlier setup didn't have the strap going to the water pump.
I just looked at a 1967 AIM and the 3878239 bracket DOESN'T bolt to the water pump like in your photo - it bolts directly to the big bracket using the same bolt into the head !
I didn't look to see if the other bracket changed as well. This is with a CS144 but the mounting flange was the same as the original alt
I do know that on some of the 427 pics I've seen the strap bracket went to the same bolt on the head as the other bracket instead of the waterpump so that might be the difference you're seeing. It would be a little shorter
M
That's exactly why the early bracket is shorter - Eureka LOL !
on the double-ended stud at the water pump, the stud for the alternator bracket is 5/16", so should be a smaller hole on that end, unless it's been hogged out.
on the double-ended stud at the water pump, the stud for the alternator bracket is 5/16", so should be a smaller hole on that end, unless it's been hogged out.
Yes you are correct : the two holes ARE a different size - BUT a 3/8's bolt still goes through easily (both NOS brackets)
I see that this started several years ago, but there have been recent postings.
The two brackets are confusing and the parts suppliers also seem to be confused as to which bracket is which. I was told that I was missing the bracket from a picture that I posted on the forum.
Tried to get the one that goes from the upper alternator to the large alternator support bracket. The supplier told me I wanted the 3986109 (Zip M-3067) for 70-74 so that is what I ordered and when I got it, it only fit between the alternator and the water pump bolt.
Re- ordered the 3878239 (Zip M-278) for 65-69 and that fits to the large alternator mounting bracket.
Yep those are the correct brackets for the applications you stated
Either one will work as long as they are installed in their intended location.
I would think the '70-'74 bracket makes for a more rigid set-up
Altenator strap difference?
