Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817173

[Joe R.]

I'm trying to help a friend obtain a correct-looking, high quality damper/balancer for his late '67 L79.

As I understand it, the correct GM part number is 3817173, which is 8 inches in diameter and 1-11/16 inches thick. Original balancers with this part number had "fins" on the back side until sometime in the 1967 production year, but the fins disappeared during 1967.

Fortunately, my friend's late '67 L79 does not require "fins," but even no-fin originals are very expensive.

My question relates to whether certain 1969-up balancers used for SHP applications are dimensionally and functionally identical to the 3871713, with the primary difference being that the timing mark is in a different position. The location of the timing mark is not a deal-killer for my friend's application.

For example, it appears that the 1970 LT1 damper 3947712 has the same dimensions and functionality as the 3817173, but with the timing mark in a different location. I have also read that 3947712 was replaced in service by part number 6272224.

To use one of these '69-up dampers, my friend is willing to consider filling the existing timing mark and creating a new one in the correct location, or, if the damper needs to be rebuilt anyway, sending it out to be rebuilt with the timing mark in the correct location.

My questions are as follows:

1) Are there additional GM part numbers from '69-up that have the same external dimensions as the '62-'68 3817173, with the only functional difference being the location of the timing mark?

2) I have heard that GM's SHP balancers were made from nodular iron, presumably to help them avoid fragmentation at high rpm. Do you know which GM balancers were made using nodular iron?

3) Do you (or anyone else reading this) know of a high quality, nodular iron aftermarket balancer that could be considered for this application?


Thanks,

Joe Randolph

[Joe L.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

I'm trying to help a friend obtain a correct-looking, high quality damper/balancer for his late '67 L79.

As I understand it, the correct GM part number is 3817173, which is 8 inches in diameter and 1-11/16 inches thick. Original balancers with this part number had "fins" on the back side until sometime in the 1967 production year, but the fins disappeared during 1967.

Fortunately, my friend's late '67 L79 does not require "fins," but even no-fin originals are very expensive.

My question relates to whether certain 1969-up balancers used for SHP applications are dimensionally and functionally identical to the 3871713, with the primary difference being that the timing mark is in a different position. The location of the timing mark is not a deal-killer for my friend's application.

For example, it appears that the 1970 LT1 damper 3947712 has the same dimensions and functionality as the 3817173, but with the timing mark in a different location. I have also read that 3947712 was replaced in service by part number 6272224.

To use one of these '69-up dampers, my friend is willing to consider filling the existing timing mark and creating a new one in the correct location, or, if the damper needs to be rebuilt anyway, sending it out to be rebuilt with the timing mark in the correct location.

My questions are as follows:

1) Are there additional GM part numbers from '69-up that have the same external dimensions as the '62-'68 3817173, with the only functional difference being the location of the timing mark?

2) I have heard that GM's SHP balancers were made from nodular iron, presumably to help them avoid fragmentation at high rpm. Do you know which GM balancers were made using nodular iron?

3) Do you (or anyone else reading this) know of a high quality, nodular iron aftermarket balancer that could be considered for this application?


Thanks,

Joe Randolph

Joe------


The 1969-80 8" SHP small block harmonic balancers were as follows:

GM #3947712

GM #12551947

GM #12555879 (this balancer has degree marks around the perimeter of the outer ring)

Each of the above was supersessive to the former. Each is, nominally, 8" OD, and, nominally, 1-11/16" thick. As far as I know, all were of nodular iron construction. The timing mark is different from the earlier 8" balancer as you mention.

I do not know of any aftermarket balancers that are of similar configuration. There may be some but, if so, I am unaware of them. There are suitable aftermarket balancers that from a functionality perspective would work fine but they would in no way mimic the configuration of the original balancer. Fluidamper is one and Summit offers several of their own brand.

Now, for the big question: can the 1969-80 SHP balancers be used in place of the earlier 327 SHP balancer save for the timing mark difference? That I am not sure of. Here's why: factory type harmonic balancers are usually "tuned" for a particular engine size and crankshaft type. Just how this "tuning" is done I do not know. Whether there would be enough difference in "tuning" between a balancer intended for 327 CID engines versus 350 cid engines I do not know.

[Duke W.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

As you stated these types of dual mass dampers are tuned to a resonant frequency that generates the greatest torsional vibration, and back in the day this is found experimentally, but I don't know the details of the test procedure. Nowadays structural analysis programs will usually accurately predict resonant frequencies.

Silicone fluid dampers are not frequency specific.Crankshaft torsional resonant frequency is a function of crankshaft geometry, mass distribution, and material, so the different stroke and journal diameters of a 350 crank results in a somewhat different resonant frequency than a small journal 327 cranks, and I believe cast iron has more internal damping than steel, so cast and forged cranks of the same geometry and mass distribution might have somewhat different resonant.

The above is why '68 large journal 327 cranks, both 300 HP cast and SHP forged have different dampers than earlier small journal 327s.

Duke

[Jim L.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

Silicone fluid dampers are not frequency specific
Duke

Other than not being "correct" is there any technical reason to avoid fluid dampers?

[Duke W.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

I've heard mixed reports about them. You might get more answers on the CF.

Duke

[Joe R.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

Hi Joe:

Thanks for your help on this. You identified two additional part numbers that I had not seen before.

You don't mention one part number that I had listed, 6272224. A post I found in a discussion forum said that the 6272224 superseded the 3947712 at some point in the mid 1970s. The 6272224 part number seems odd for GM.

What changed after 1980 that made the '69-'80 SHP balancers not suitable? Was it just the location of the timing mark, or something more substantial?

On ebay, I have seen used balancers that appear to have the same external dimensions as the '62-'80 SHP balancers, but the timing mark is in a very different position from the '62-'68 SHP or the '69-'80 SHP balancers. Another difference is that the back side of the inertia ring has a deep groove in it that would make it lighter. I'm guessing that these balancers were used in some applications after 1980.

[Joe R.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

It's my impression that the "tuning" of the damper is partly accomplished by changing the durometer of the rubber that isolates the inertia ring from the hub.

I'm not sure how critical this tuning is. In the GM service parts books, certain service dampers are listed for both small journal 327s and large journal 350s. So, if there was a difference in the production versions, GM apparently felt that a compromise was acceptable for service.

I've been looking at companies that rebuild dampers, such as Damper Doctor and Damper Dudes. It's my impression that they use the same durometer material for all rebuilds, but I could be mistaken. My guess is that the experienced people at these companies probably have a wealth of knowledge about damper technology, but so far I have not had a chance to speak directly with such people.

[Mark L.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

Joe, I think the knowledge pool that you are delving into is probably of interest to many members. Perhaps if you can "close the circle" with respect to suitable replacement dampers a Tech Article in the Restorer would be worthwhile. Thanks

[Patrick H.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

Joe, I think the knowledge pool that you are delving into is probably of interest to many members. Perhaps if you can "close the circle" with respect to suitable replacement dampers a Tech Article in the Restorer would be worthwhile. Thanks

I know that we have a 1965 that needs an 8" damper, and it doesn't have to be 100% correct, just 100% functional.
So, you are very correct that some of us would love to know what current dampers are correct with regard to function and timing marks.

[Joe R.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

I know that we have a 1965 that needs an 8" damper, and it doesn't have to be 100% correct, just 100% functional.
So, you are very correct that some of us would love to know what current dampers are correct with regard to function and timing marks.

It's my impression that there are no currently available GM dampers that have the timing mark in the correct location. So far, it appears that there are also no currently available GM dampers that have the correct 1-11/16" thickness, regardless of whether the timing mark is in the right place.

Even among the aftermarket vendors that I have looked at, it appears that most of the available replacements are less than the 1-11/16" thickness of the original SHP dampers. And, it's unclear whether any of these have the timing mark in the pre-1969 location.

I suspect that a thorough search may reveal a suitable replacement part available from commercial auto parts stores such as NAPA, Autozone, and Advance Auto. It seems likely that Dorman may make a suitable part, but it's hard to tell from just looking online.

If you don't mind having a damper that is less thick than the original, and also has the timing mark in the wrong location, there appear to be some high quality aftermarket dampers available from suppliers such as Summit Racing and Jegs. It's not a big deal to "move" the timing mark by filling the existing one and creating a new one in the right place. Some Chevy Orange paint will help cover your work to hide the old timing mark.

Alternatively, if you have an original SHP damper that needs new rubber, companies such as Damper Doctor and Damper Dudes will rebuild a damper for about $180. My guess is that a rebuilder can reassemble a damper with the timing mark in any location that you request. So, a 1969-up SHP damper could probably be rebuilt to have the timing mark in the 1962-68 location.

[Joe L.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

Hi Joe:

Thanks for your help on this. You identified two additional part numbers that I had not seen before.

You don't mention one part number that I had listed, 6272224. A post I found in a discussion forum said that the 6272224 superseded the 3947712 at some point in the mid 1970s. The 6272224 part number seems odd for GM.

What changed after 1980 that made the '69-'80 SHP balancers not suitable? Was it just the location of the timing mark, or something more substantial?

On ebay, I have seen used balancers that appear to have the same external dimensions as the '62-'80 SHP balancers, but the timing mark is in a very different position from the '62-'68 SHP or the '69-'80 SHP balancers. Another difference is that the back side of the inertia ring has a deep groove in it that would make it lighter. I'm guessing that these balancers were used in some applications after 1980.

Joe------


I should have included the 6272224 in the listing of part numbers for the 350 SHP balancers. It fits between the 3947712 and the 12551947.

This part number was from a series of part numbers that were previously unused and "salvaged" in the period between the 3999999 and the adoption of 8 digit part numbers. They were assigned in the 1972-73 period. I believe this series of part numbers was originally assigned to the AC Division of GM but transferred to Chevrolet when the need arose.

Another balancer of this series still survives in the GM system. It is GM #6272222. This is an 8" balancer of 1-5/16" thickness. It was used on 1969-70 350/300HP engines as well as many 350 engines in trucks. I think the truck applications are why it's still available.

[Joe R.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

Joe------


I should have included the 6272224 in the listing of part numbers for the 350 SHP balancers. It fits between the 3947712 and the 12551947.

This part number was from a series of part numbers that were previously unused and "salvaged" in the period between the 3999999 and the adoption of 8 digit part numbers. They were assigned in the 1972-73 period. I believe this series of part numbers was originally assigned to the AC Division of GM but transferred to Chevrolet when the need arose.

Another balancer of this series still survives in the GM system. It is GM #6272222. This is an 8" balancer of 1-5/16" thickness. It was used on 1969-70 350/300HP engines as well as many 350 engines in trucks. I think the truck applications are why it's still available.

Thanks Joe. I now have a growing list GM balancers that are 8" diameter and approximately 1-11/16" thick. If the only functional difference is the placement of the timing mark, I think that can be dealt with.

Do you know why the '69-up balancer compatibility ends at 1980? What changed after 1980 that made those engines different? I'm wondering whether the timing mark got moved again starting in 1981.

Below are two photos of an unidentified balancer that is 8" diameter and about 1-11/16" thick, but the timing mark is in a a position (relative to the keyway) that differs from the 1962-68 position and also differs from the 1969-1980 position. I'm wondering whether this balancer is from 1981 or later.

Another distinguishing feature of this balancer, shown in the second photo, is that the back edge of the inertia ring is deeply grooved. This differs from the other SHP balancers I have seen.

[Joe L.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

Thanks Joe. I now have a growing list GM balancers that are 8" diameter and approximately 1-11/16" thick. If the only functional difference is the placement of the timing mark, I think that can be dealt with.

Do you know why the '69-up balancer compatibility ends at 1980? What changed after 1980 that made those engines different? I'm wondering whether the timing mark got moved again starting in 1981.

Below are two photos of an unidentified balancer that is 8" diameter and about 1-11/16" thick, but the timing mark is in a a position (relative to the keyway) that differs from the 1962-68 position and also differs from the 1969-1980 position. I'm wondering whether this balancer is from 1981 or later.

Another distinguishing feature of this balancer, shown in the second photo, is that the back edge of the inertia ring is deeply grooved. This differs from the other SHP balancers I have seen.

Joe------


1980 was the last year for what GM regarded as a special high performance engine and the 1980 L-82 was, thus, the last year to receive an 8" balancer. Beginning for 1981 and through 1991 all Corvette engines used a 6-3/4" balancer. The same, basic balancer was also used for 1972-80 base engines. So, after 1980 the 8" balancers "went away" as far as PRODUCTION passenger car applications were concerned.

I cannot identify the balancer you pictured.

[Joe R.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

Joe------


1980 was the last year for what GM regarded as a special high performance engine and the 1980 L-82 was, thus, the last year to receive an 8" balancer. Beginning for 1981 and through 1991 all Corvette engines used a 6-3/4" balancer. The same, basic balancer was also used for 1972-80 base engines. So, after 1980 the 8" balancers "went away" as far as PRODUCTION passenger car applications were concerned.

I cannot identify the balancer you pictured.

Hi Joe:

Thanks for the clarification about what changed after 1980. If 8" balancers disappeared after 1980, the "mystery balancer" in my photos must have been used before 1981.

Today I purchased that ebay balancer so that I could get a better look at it. I'm wondering whether this is the elusive 6272224 balancer. My hope is that like most GM balancers, this one will have the last four digits of the part number, and a date code, stamped somewhere on it. I'll report my findings after it arrives in a week or so.

[Joe L.]

Re: Question for Joe Lucia: Alternatives to '62-'68 GM 8" balancer, part number 3817

Hi Joe:

Thanks for the clarification about what changed after 1980. If 8" balancers disappeared after 1980, the "mystery balancer" in my photos must have been used before 1981.

Today I purchased that ebay balancer so that I could get a better look at it. I'm wondering whether this is the elusive 6272224 balancer. My hope is that like most GM balancers, this one will have the last four digits of the part number, and a date code, stamped somewhere on it. I'll report my findings after it arrives in a week or so.

Joe------


It's possible that an 8" balancer was used for some truck applications after 1980. I don't think so, but it's possible.

I have an NOS GM #6272224 [not for sale]. I can't bear the thought of trying to dig it out but my recollection of it is that it is identical to the predecessor GM #3947712. I can tell you for certain that the 8 x 1-5/16" GM #3947704 is identical to the successor GM #6272222----absolutely identical. I compared these very carefully once-upon-a-time.