57-67 Non-SHP Harmonic Balancers

[Joe L.]

A recent discussion here related to the above-referenced subject spurred me to revise my previous thinking and do a little more research.

As some of you may know, there were a number of harmonic balancers used in PRODUCTION and once available in SERVICE for 1957-61 283 cid engines. All of these were eventually replaced for SERVICE by the GM #3896904. This is a 6-1/8" OD balancer with a ring thickness of 3/4".

1962-65 Corvettes with 250 or 300 hp 327 used balancer GM #3817684 (62-64) and 3861698 (65). Both of these were also replaced by the GM #3896904 for SERVICE.

1966 Corvettes with 300 hp used a new balancer, GM #3883204. This balancer was replaced for SERVICE in July, 1966 by the GM #3896903 which was used in PRODUCTION for 1967 Corvettes with 300 hp. As far as I know, the 3883204 and 3896903 are virtually identical in configuration. Both are 6-1/8" OD with a ring thickness of 3/4" which make them also identical in configuration to the 3896904.

For many years I was of the belief that the difference between the 3896904 balancer and the 3896903 balancer related to a difference in interference fit----the 3896904 having a heavier interference fit for cranks without balancer retaining bolt and the 3896903 having a lesser interference fit for crankshafts with balancer bolt. The difference, of course, would be only .001" or less.

Recently I came to the conclusion that my belief that interference fit was the difference between these two balancers was completely erroneous. In fact, I'm absolutely ashamed of myself for ever believing that was the difference between the 2 balancers. Why? Well, because NONE of the engines that either the 3896904 or 3896903 were ever cataloged for used a balancer retaining bolt. So, there would be absolutely no reason for any difference in interference fit between the two. And, as it turns out, THERE IS NO DIFFERENCE IN INTERFERENCE FIT BETWEEN THE TWO.

So, what is the difference between the 3896904 and the 3896903? The difference between the two balancers relates to a slight difference in "tuning". Given the fact that all of the component parts of these balancers (i.e. hub, ring, and elastomer) are exactly the same, I do not know how the difference in "tuning" is accomplished. But, that's the difference.

Now comes the question of WHY there needed to be a difference in "tuning" between the balancers. Both balancers were used for 327 cid engines of 250 and 300 hp (as well as other Chevrolet small blocks of the period). All of these engines used the same crankshaft forging. All of the engines were designed to operate in about the same RPM range and with about the same redline. So, why was it necessary to change the "tuning" of the balancers for 62-65 327's versus 66-67 327's.

My guess is that the reason for the change in balancer "tuning" was the change in the small block connecting rods which occurred "across the board" for the 1966 model year. I believe the improved, small journal 3864881 connecting rods used for the 1966-67 model years were somewhat heavier than the earlier design. So, I expect that was what also instigated the change in balancer "tuning" specs.

By the way, the 3896904 balancer remains available in SERVICE from GM. The 3896903 is discontinued without supercession.

[Bill M.]

Re: 57-67 Non-SHP Harmonic Balancers

So, what is the difference between the 3896904 and the 3896903? The difference between the two balancers relates to a slight difference in "tuning". Given the fact that all of the component parts of these balancers (i.e. hub, ring, and elastomer) are exactly the same, I do not know how the difference in "tuning" is accomplished. But, that's the difference.

I'm guessing that the stiffness of the elastomer was adjusted.

[Mike M.]

Re: 57-67 Non-SHP Harmonic Balancers

A recent discussion here related to the above-referenced subject spurred me to revise my previous thinking and do a little more research.

As some of you may know, there were a number of harmonic balancers used in PRODUCTION and once available in SERVICE for 1957-61 283 cid engines. All of these were eventually replaced for SERVICE by the GM #3896904. This is a 6-1/8" OD balancer with a ring thickness of 3/4".

1962-65 Corvettes with 250 or 300 hp 327 used balancer GM #3817684 (62-64) and 3861698 (65). Both of these were also replaced by the GM #3896904 for SERVICE.

1966 Corvettes with 300 hp used a new balancer, GM #3883204. This balancer was replaced for SERVICE in July, 1966 by the GM #3896903 which was used in PRODUCTION for 1967 Corvettes with 300 hp. As far as I know, the 3883204 and 3896903 are virtually identical in configuration. Both are 6-1/8" OD with a ring thickness of 3/4" which make them also identical in configuration to the 3896904.

For many years I was of the belief that the difference between the 3896904 balancer and the 3896903 balancer related to a difference in interference fit----the 3896904 having a heavier interference fit for cranks without balancer retaining bolt and the 3896903 having a lesser interference fit for crankshafts with balancer bolt. The difference, of course, would be only .001" or less.

Recently I came to the conclusion that my belief that interference fit was the difference between these two balancers was completely erroneous. In fact, I'm absolutely ashamed of myself for ever believing that was the difference between the 2 balancers. Why? Well, because NONE of the engines that either the 3896904 or 3896903 were ever cataloged for used a balancer retaining bolt. So, there would be absolutely no reason for any difference in interference fit between the two. And, as it turns out, THERE IS NO DIFFERENCE IN INTERFERENCE FIT BETWEEN THE TWO.

So, what is the difference between the 3896904 and the 3896903? The difference between the two balancers relates to a slight difference in "tuning". Given the fact that all of the component parts of these balancers (i.e. hub, ring, and elastomer) are exactly the same, I do not know how the difference in "tuning" is accomplished. But, that's the difference.

Now comes the question of WHY there needed to be a difference in "tuning" between the balancers. Both balancers were used for 327 cid engines of 250 and 300 hp (as well as other Chevrolet small blocks of the period). All of these engines used the same crankshaft forging. All of the engines were designed to operate in about the same RPM range and with about the same redline. So, why was it necessary to change the "tuning" of the balancers for 62-65 327's versus 66-67 327's.

My guess is that the reason for the change in balancer "tuning" was the change in the small block connecting rods which occurred "across the board" for the 1966 model year. I believe the improved, small journal 3864881 connecting rods used for the 1966-67 model years were somewhat heavier than the earlier design. So, I expect that was what also instigated the change in balancer "tuning" specs.

By the way, the 3896904 balancer remains available in SERVICE from GM. The 3896903 is discontinued without supercession.

Joe: if you have an example of both balancers, might be interesting to lay a degree wheel on each and comapre the degrees betweem the keyway slot and the slash mark for tdc. perhaps the general was messing with the timing by a few degrees so the low hp sbc's would pull better at lower RPMs. mike

[Jim T.]

Re: 57-67 Non-SHP Harmonic Balancers

Joe sometime ago you helped me in identifing a balancer that I had bought for a replacement to install on my 64 Corvette 327/300HP.
I installed this balancer when I rebuilt the 64's engine in 1968.
A short period after getting the 64 on the road I discovered the balancer was trying to come off and due to not having the time to repair it I took it to my local Chevrolet dealer in Coronado,CA.
The mechanic told me I installed the wrong balancer. The one he installed stayed on. Wish I had the paperwork to see which one he installed.
The mechanic gave me the one I had installed and I still have it. It has the number 6904H stamped on it.
The keyway and the timing line/mark line up.

[Joe L.]

Re: 57-67 Non-SHP Harmonic Balancers

Joe: if you have an example of both balancers, might be interesting to lay a degree wheel on each and comapre the degrees betweem the keyway slot and the slash mark for tdc. perhaps the general was messing with the timing by a few degrees so the low hp sbc's would pull better at lower RPMs. mike

Mike-----


The specs for both balancers call for the timing mark to be in the same location for both-----lined up with the crankshaft keyway slot.

[Joe L.]

Re: 57-67 Non-SHP Harmonic Balancers

Joe sometime ago you helped me in identifing a balancer that I had bought for a replacement to install on my 64 Corvette 327/300HP.
I installed this balancer when I rebuilt the 64's engine in 1968.
A short period after getting the 64 on the road I discovered the balancer was trying to come off and due to not having the time to repair it I took it to my local Chevrolet dealer in Coronado,CA.
The mechanic told me I installed the wrong balancer. The one he installed stayed on. Wish I had the paperwork to see which one he installed.
The mechanic gave me the one I had installed and I still have it. It has the number 6904H stamped on it.
The keyway and the timing line/mark line up.

Jim-----


The GM #3896904 balancer is identified by the "6904" stamped on the face of the hub section. So, there's no doubt that one you have is a 3896904. The 3896904 has been the SERVICE balancer for all 1964 250 and 300 hp applications since June, 1971. All I can figure is that the one you installed and which was replaced by the Chevrolet dealer was defective. However, the real mystery to me here is how you obtained a GM #3896904 balancer in 1968. In 1968, the SERVICE balancer for your application was GM #3861970. Although the 3896904 part number appears to be a late 1966 issued part number, I can find no indication that it was released for SERVICE until well after 1968.

I don't know what balancer that the Chevrolet dealer installed. However, if it was anything but the 3896904 then it was not the one that GM specified for your application (assuming this occurred after June, 1971).

[Joe L.]

Re: 57-67 Non-SHP Harmonic Balancers

I'm guessing that the stiffness of the elastomer was adjusted.

Bill-----


That's what I figured, too. I assumed the durometer of the elastomer differed. However, both the 3896903 and 3896904 used elastomer component GM #3882899 (of course, never available in SERVICE as a separate part).

[Timothy B.]

Re: 57-67 Non-SHP Harmonic Balancers

Joe, Thanks for the information, that clears things up a bit for me.
I wonder what type of tuning change is required for these engines because they are all internally balanced.

A while back someone posted on the factory balancing procedure and stated the flywheel and damper are used to balance the internal moving parts if I understood correctly. In the rebuild process flywheel and damper are zero balanced but internals are balanced to the lightest part.

If it's a tuning issue, balancing may not have anything to do with it, like you said, the extra weight from the new rods. I installed the 66-67 rods in my 63 engine when I rebuilt it using the original damper.

It's hard to believe this little bit would make any difference requiring a different tuned part.

[Michael H.]

Re: 57-67 Non-SHP Harmonic Balancers

Joe.... it sounds like you have the print for these balancers? If so, is there a different part number for the elastomer strip?

Do the lower HP 327 engines use one number and the higher HP 327 engines use another?

Is it possible that one of these was used on a pass car with a 283 engine that may require a slightly different rebound rate for the elastomer?

[Joe L.]

Re: 57-67 Non-SHP Harmonic Balancers

Joe, Thanks for the information, that clears things up a bit for me.
I wonder what type of tuning change is required for these engines because they are all internally balanced.

A while back someone posted on the factory balancing procedure and stated the flywheel and damper are used to balance the internal moving parts if I understood correctly. In the rebuild process flywheel and damper are zero balanced but internals are balanced to the lightest part.

If it's a tuning issue, balancing may not have anything to do with it, like you said, the extra weight from the new rods. I installed the 66-67 rods in my 63 engine when I rebuilt it using the original damper.

It's hard to believe this little bit would make any difference requiring a different tuned part.

Tim-----


The "tuning" difference between the 3896904 and 3896903 balancers is very small so that's consistent with the fact that the difference in the rod weight is small.

The "tuning" of the balancer is unrelated to engine balance.

[Joe L.]

Re: 57-67 Non-SHP Harmonic Balancers

Joe.... it sounds like you have the print for these balancers? If so, is there a different part number for the elastomer strip?

Do the lower HP 327 engines use one number and the higher HP 327 engines use another?

Is it possible that one of these was used on a pass car with a 283 engine that may require a slightly different rebound rate for the elastomer?

Michael-----


The part number for the elastomer component of both the 3896903 and 3896904 balancers is the same----GM #3882899. I don't have the information for the 8" SHP balancer. However, because of size alone it would have had to be different than the elastomer for the 6-1/8" balancers. So, even if we had the part number for it, we would not be able to tell if there was also a durometer difference.

[Jim T.]

Re: 57-67 Non-SHP Harmonic Balancers

Jim-----


The GM #3896904 balancer is identified by the "6904" stamped on the face of the hub section. So, there's no doubt that one you have is a 3896904. The 3896904 has been the SERVICE balancer for all 1964 250 and 300 hp applications since June, 1971. All I can figure is that the one you installed and which was replaced by the Chevrolet dealer was defective. However, the real mystery to me here is how you obtained a GM #3896904 balancer in 1968. In 1968, the SERVICE balancer for your application was GM #3861970. Although the 3896904 part number appears to be a late 1966 issued part number, I can find no indication that it was released for SERVICE until well after 1968.

I don't know what balancer that the Chevrolet dealer installed. However, if it was anything but the 3896904 then it was not the one that GM specified for your application (assuming this occurred after June, 1971).

Joe my memory was jogged after I made my previous post about rebuilding my 64's engine in 1968. I may have rebuilt it in the first months of 1969. It would of had to been before March 24, 1969 because I left Coronado, CA on the 24th for the east coast driving my 64 Corvette.

Your post on the information on the correct balancer #3861970 confirms something 18 years ago when I moved. The 6904H balancer I have kept all these years was stored in the balancer box the Chev dealer mechanic gave me that had contained the balancer he installed.

I remember the box having 1970 on it because of my 1970 Corvette. I threw just the box away because it was stored in my attic for 13 years before moving and silverfish feasted on it.

I have no explanation on getting a 6904 before it was a service part. Another number on my balancer I found is on the elastomer. AT-1433 ABS9 is clearly readable.

[Timothy B.]

Re: 57-67 Non-SHP Harmonic Balancers

If the elastomer, hub and ring is the same then how could it be tuned different?

I could be wrong but my memory tells me in the chevy by the #'s book 4577 crankshaft was installed in 66-67. Prior to that, the 2680 is used and I wonder why a different crank part #.

Same crankshaft I always thought but could new design rods have anything to do with crank part # change. Could this 4577 crank require a different damper.

[Bill M.]

Re: 57-67 Non-SHP Harmonic Balancers

Bill-----


That's what I figured, too. I assumed the durometer of the elastomer differed. However, both the 3896903 and 3896904 used elastomer component GM #3882899 (of course, never available in SERVICE as a separate part).

Joe:

Once again guessing:

The elastomer drawing specified geometry and basic material spec. (maybe even including an acceptable durometer range), but the assembly drawing performance requirement mandated a specific stiffness of the elastomer?

A case where the assembly drawing mandated a tighter control on the elastomer than was specified on the elastomer drawing?

Bill