1969 Alternator Mounting on a 427 w/ PS
For a 427 w/ PS, the '69 AIM shows the upper alternator support brace mounting to the alternator and a support strap on one end and a water pump stud on the other. This is the same stud that I believe the AIR pump brace is anchored to. It looks like there might be just enough room for both. I have also seen the upper support brace mounted to the alternator bracket instead of the water pump stud. Which is correct? Was the mounting location (and upper support brace part number) changed during the 1969 model year? Thanks
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Re: 1969 Alternator Mounting on a 427 w/ PS
Craig-------
The upper support ("brace") mounts between the alternator and the water pump left side upper stud. The lower support ("brace") mounts to the alternator and to the lower bracket. The AIR pump brace also mounts to the same water pump stud.
The upper support did change part number during the 1969 model year. Early used GM #3878239 while later used GM #3955180. Just when the change occurred, I do not know.
The interesting thing is that the GM #3878239 remained available in SERVICE until December, 1981 when it was discontinued without supersession. The GM #3955180 was used only for late 1969 and was replaced for SERVICE by GM #3986109 in February, 1972. The GM #3986109 was used in PRODUCTION and SERVICE for 1970-74. This series of events would lead one to believe that the 3878239 is not interchangeable with the 3955180 or 3986109. However, I believe it actually is interchangeable, at least for 1968-69 Corvettes if not 1965-67.In Appreciation of John Hinckley -
Re: 1969 Alternator Mounting on a 427 w/ PS
Joe, thanks for the reply.
I bought a Paragon Reproduction upper support brace that was represented as equivalent to GM #3878239 (lower part in the photo below). Its listed as correct for 1965-1969 models w/ 396/427 & PS. This brace fits well between the alternator/lower support brace on the right and the alternator bracket on the left. It does not reach the water pump stud. The top part in the photo is a genuine GM part, GM #3986109. Even though it is bent to the same shape as the repro part below, its reach is about 3/8" longer. As you speculated above, the two parts are not interchangeable. GM #3986109 does fit between the alternator/lower support brace on the right and the water pump stud, and is too long to fit on the left side to the alternator bracket. If the proper configuration in 1969 is to use a support brace between the alternator and the water pump stud, then of my current two choices I have to use GM# 3986109 - which as you point out is not in the AIM and hence my confusion. A possibility is that when the part changed in 1969 from GM#3878239 to GM #3955180, the attachment location also changed. I suspect that these two parts are not the same, and that GM #3955180 is interchangeable with GM #3886109. I would like to know when the date of the part change occurred. My quick research yesterday on the topic indicate that people have had issues with bolts breaking when the upper alternator support bracket (GM #3878239) is attached to the alternator bracket on the left. Perhaps this is why the change was made during the model year.
I intend to attach the support to the water pump stud. Does anyone know how the support braces are stacked up? This obviously will depend on build sequence. Were the alternator brackets installed before the AIR pump brackets so we would have the water pump w/ stud, alternator support brace, AIR pump support brace then the lock washer and nut? or the other way?
Thanks again.
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Re: 1969 Alternator Mounting on a 427 w/ PS
Craig-------Joe, thanks for the reply.
I bought a Paragon Reproduction upper support brace that was represented as equivalent to GM #3878239 (lower part in the photo below). Its listed as correct for 1965-1969 models w/ 396/427 & PS. This brace fits well between the alternator/lower support brace on the right and the alternator bracket on the left. It does not reach the water pump stud. The top part in the photo is a genuine GM part, GM #3986109. Even though it is bent to the same shape as the repro part below, its reach is about 3/8" longer. As you speculated above, the two parts are not interchangeable. GM #3986109 does fit between the alternator/lower support brace on the right and the water pump stud, and is too long to fit on the left side to the alternator bracket. If the proper configuration in 1969 is to use a support brace between the alternator and the water pump stud, then of my current two choices I have to use GM# 3986109 - which as you point out is not in the AIM and hence my confusion. A possibility is that when the part changed in 1969 from GM#3878239 to GM #3955180, the attachment location also changed. I suspect that these two parts are not the same, and that GM #3955180 is interchangeable with GM #3886109. I would like to know when the date of the part change occurred. My quick research yesterday on the topic indicate that people have had issues with bolts breaking when the upper alternator support bracket (GM #3878239) is attached to the alternator bracket on the left. Perhaps this is why the change was made during the model year.
I intend to attach the support to the water pump stud. Does anyone know how the support braces are stacked up? This obviously will depend on build sequence. Were the alternator brackets installed before the AIR pump brackets so we would have the water pump w/ stud, alternator support brace, AIR pump support brace then the lock washer and nut? or the other way?
Thanks again.
Well, apparently I was incorrect about the 3878239 being interchangeable with the 3955180 or 3986109. And, that's entirely consistent with the 3878239 being discontinued without supersession in 1981. So, GM had it right and I had it wrong.
From what I can tell, it's like this: for the 1965-E69 configuration, the GM #3878239 support was used and mounted between the forward and rear holes of the alternator upper bracket, GM #3878236. The rear hole bolt passed through the 2 supports (upper and lower), the alternator, and the upper bracket. The problem for me is that I cannot understand how this support would have provided any real functionality when installed in this manner. But, I'm virtually certain now that's how it was installed.
For later 1969, the GM #3955180 support was used and replaced the 3878239 in the alternator mounting configuration. However, this support was mounted between the alternator and the water pump left side upper stud. The same configuration was used for 1970-74 big blocks with PS except that the 3955180 was replaced by the 3986109. There is no doubt, at all, that the 3955180 and 3986109 are interchangeable since, as I mentioned, the 3955180 was discontinued from SERVICE in February, 1972 and replaced by the 3986109.
In the L1969-74 configuration, the order of the support, bracket and alternator installation viewed from the front is :
3955180 upper support----3878240 lower support-----alternator-----3978236 upper bracket
As I mentioned above, the 3986109 can be used as a direct replacement for the 3955180 and I HIGHLY doubt that anyone will be able to discern the difference.
As to when the change in the upper support occurred during the 1969 model year, I do not know. However, based upon the AIM and based upon the 3955180 part number, I would opine that it occurred very early in the model year. There is absolutely no doubt that when the change from the 3878239 to the 3986109 occurred that a change in the mounting location also simultaneously occurred. Based upon what you have discovered regarding the difference in the effective lengths of the two supports, it could not have been any other way.
As you surmise, GM did not make this change in configuration because the first design worked just as well. So, for a 1969 and all things considered, I would assume that the second configuration is what my car had and that's what I'd install. As a matter of fact, if I had a 1965-68 I'd use the L69-74 configuration, too.
As far as how the AIR brace figures into this, the AIR pump support bracket was installed at Tonawanda. The brace was installed at St. Louis and the same brace was used for N-40 or not. Just what the order of installation was at the water pump stud, I do not know for sure. My guess would be that the AIR brace was installed on the water pump stud first, then the alternator upper support. However, I think that when installed, the correct installation order would become apparent based upon fitment.In Appreciation of John HinckleyComment
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