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

[Joe R.]

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

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.

Hi Joe:

From my web research I have run across a Z28 balancer, part number 3947708. The physical dimensions look like they may be the same as the SHP balancers listed above. Aside from the fact that this balancer is probably "tuned" for a 302, are its external dimensions the same as the 3947712 LT1 balancer?

[James G.]

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

7708 looks basically the same the 3 inner spokes do not have the raised ring. The balancer off my LT1 slipped the ring and I sent it to damper doctors - the number 7712 and the date were both stamped on the face, like this 7708 off one of my 69Z's is.

FWIW I bought from GM what is marketed as a replacement for the LT1 damper. When I received it dia was 8" dia with a narrow inertia ring - 1.5" cost was $80
Damper Doctors rebuilds them for $150.
100_4083.jpg

[Joe R.]

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

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.

Hello All:

My research into the question of suitable replacements for the 1962-1968 SHP #3817173 "big" balancer has revealed a lot of interesting information, and the research is still ongoing. However, I thought I would provide an interim report for those who are interested. Following is a short summary of available options for getting a GM balancer:


NEW GM SERVICE REPLACEMENT

GM discontinued the original 3817173 part long ago. For 1969-up, the #3947712 balancer used on the LT1 was almost identical to the #3817173, *except* that the timing mark was moved approximately 10 degrees. GM made this change to the balancers (and the corresponding timing covers) to increase visibility for timing the engine. So, if you want to purchase a brand new GM service replacement balancer, you will have to add a new timing mark in the correct location and use that for timing your 1962-1968 Corvette.

With help from Joe Lucia and some web research, it appears that the original LT1 balancer #3947712 has gone through a long succession of GM replacements. Following is a list, in approximate chronological order, that has not yet been checked by Joe Lucia:

3947712
6272224
12551947
12555879
88960604
19260269

The last one, #19260269, does not show up at gmpartsdirect.com, but it is still available at some web sites. For people who want a brand-new GM balancer, this might be worth considering. Below is an add clipped from the web.

IMPORTANT NOTE: The ad below says this balancer is for "externally balanced" engines, but I think it was supposed to say "internally balanced engines." The 400 small block was the only production small block where a counterweight in the balancer helped to complete the engine balance. All other small blocks were "internally balanced" and used a "neutral balance" balancer. This aspect should be verified before purchasing a 19260269 balancer for use on a 327 or 350.


GM 19260269.jpg



HAVING AN ORIGINAL BALANCER REBUILT

There are at least three companies that will rebuild a balancer with a new rubber isolator between the hub and the inertia ring:

Damper Doctor (https://damperdoctor.com/)
Damper Dudes (https://www.damperdudes.net/)
Dale Manufacturing (www.hbrepair.com)

The cost for this service averages about $170. My guess is that when re-assembling the balancer, these vendors would be happy to put the timing mark in the 1962-1968 position.


FINDING A REBUILDABLE CORE

Provided that a balancer has not been damaged by incorrect installation/removal, the only two things that need to be renewed during a rebuild are the rubber isolator between the hub and the inertia ring, and the wear surface on the nose for the seal in the timing cover.

Over time, the timing cover seal tends to wear a groove in the nose. A well-accepted solution is to install a very thin sleeve on the nose to provide a new surface for the seal. An alternate solution is to use a different seal whose lip is slightly offset from the stock location where the groove has been made.

On ebay, a search on the older GM part numbers 3817173 and 3947712 typically yields several used and NOS balancers at prices around $400 and up. Most used balancers should definitely be rebuilt. While a NOS balancer might seem like a safe choice, keep in mind that if it is a very old NOS unit, the rubber isolator may be more than 50 years old.

HERE IS A USEFUL TIP FOR EBAY SHOPPING: Many ebay sellers think that the part number for the balancer is the one that appears on the hub casting, but that that is just the casting number for the hub. The most common hub casting used for the big balancers is casting number 3942538. If you search on this number, you will likely turn up many candidate balancers at about half the cost of the balancers listed under the part numbers 3817173 and 3947712. Yet, most of the "3942538" balancers are actually the 3947712 or its replacements. Typically, the last four digits of the part number, such as "7712" or "2224" are stamped somewhere on the face of the balancer, but it's often not visible in the ebay photos. The key thing to keep in mind is that if the hub casting number is 3942538, the balancer is one of the "big" balancers in the 3947712 balancer family.


OTHER CANDIDATE CORES

There is a very common 8" GM balancer that looks like the "big" 3947712 balancer, but is only 1-5/16" thick rather than the 1-11/16" thickness of the 1962-1968 "big" balancers. In the ebay ads, it is very difficult to detect this difference if the seller has not stated the thickness. However, the hub casting number is usually 3896966, and many ebay sellers list these as GM part number 3896966. So, a search on 3896966 will usually turn up lots of used balancers for about $50 that are 1-5/16" thick. The 1-5/16" balancer is a good budget compromise for replacing the stock 1962-1968 "big" balancer because it is the same 8" diameter. When installed, the primary visual difference is that there will be a gap of about 1/4" between the back edge of the balancer and the timing cover. The original balancer was almost touching the face of the timing cover.

It's worth noting that the GM part number for this thinner, 1-5/16" thick balancer was originally 3947704, and later superseded by 6272222. The AC Delco 6272222 is still available new for about $130, so there is little incentive to purchase a used core and have it rebuilt.

However, there is a "mystery balancer" that shows up on ebay when searching on the 3896966 hub casting number for the 1-5/16" thick balancer. This balancer uses the 1-5/16" 3896966 hub, but the inertia ring looks wider than 1-5/16". On close inspection of the photos, it appears to me that the inertia ring has an outer flange that extends the back edge of the inertia ring. When installed on the engine, this would give the appearance of being one of the "big" balancers, although it could be detected from looking at the balancer from under the car. I've purchased one of these "mystery balancers" on ebay for $34 just so I could study it. I will report what I find when it arrives.


REPRODUCTION BALANCERS

The above summary is limited to GM balancers. It should be noted that one or more reproduction balancers are available for the "finned" 3817173 used from 1962 to early 1967, and for the "un-finned" version that was used after mid-1967. Typical prices are $400 to $500. I have not researched these to find out where they are made.

[Rocco S.]

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

This very interesting subject got me thinking. A few years ago I purchased an 8" Chevy balancer from a garage sale for like $5. I knew small blocks used both small and large balancers back in the day. The large balancers were typically used on higher performance engines. I was hoping it would be a spare for my March 1967 L79 coupe. My L79 balancer is a late unit and does not have the fins.

After reading the eariler posts today I pulled this spare unit from storage to look for any identifing numbers. First I had no idea there were thick and thin 8" units and that later 69-up units had different timing mark position. It turns out my 8" unit is the thin one measuring 1-5/16" thick. I also noticed that the timing mark was exactly in-line with the keyway. So I assume it is was '67 or '68 unit since '69-up the timing marked was moved.

The inner hub was marked with a casting number "3896966", "GB", and "2". On the outside, one arm is stamped "F K 6". There are no other numbers. I thought the casting number was the part number. But I could not find 3896966 in my Corvette parts book but did find it listed in my 1977 Chevrolet Dealer Parts book. In Group 0.659, 3896966 is listed as Damper Assembly for 1967-1968 350 engine only. 1969-up had a different number.

What I don't understand is the meaning of "GB" and "2" near the casting number and the stamped "F K 6". Can anybody help out here? Is this unit for '67-'68 350 non-corvette engine only? I know the 350 was introduced in 1967 for the Camaro SS only and Corvette did not get the 350 until 1969.

What I would like is to confirm the application since it is not correct for my 1967 L79. Below are photos of the balancer.
PXL_20220107_202642522.jpg
PXL_20220107_202905017.jpg
PXL_20220107_202711404.jpg
PXL_20220107_202655309.jpg
PXL_20220107_202848108.jpg

[Joe R.]

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

Hi Rocco:

I think the "GB" is the supplier's logo. I wasn't aware that 3896966 had ever been assigned as a part number for an assembled balancer, but you have clarified that this actually did occur for 1967-68.

My guess is that this balancer was used on the Camaro 350 in 1967, and on many other 350s in 1968. Since the keyway-aligned timing mark was discontinued after 1968, this part became an orphan starting in 1969. The corresponding 1.25" thick balancer with the timing mark shifted about 10 degrees was probably the 3947704. That balancer still used the 3896966 hub from the 1967-68 version.

[Joe L.]

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

This very interesting subject got me thinking. A few years ago I purchased an 8" Chevy balancer from a garage sale for like $5. I knew small blocks used both small and large balancers back in the day. The large balancers were typically used on higher performance engines. I was hoping it would be a spare for my March 1967 L79 coupe. My L79 balancer is a late unit and does not have the fins.

After reading the eariler posts today I pulled this spare unit from storage to look for any identifing numbers. First I had no idea there were thick and thin 8" units and that later 69-up units had different timing mark position. It turns out my 8" unit is the thin one measuring 1-5/16" thick. I also noticed that the timing mark was exactly in-line with the keyway. So I assume it is was '67 or '68 unit since '69-up the timing marked was moved.

The inner hub was marked with a casting number "3896966", "GB", and "2". On the outside, one arm is stamped "F K 6". There are no other numbers. I thought the casting number was the part number. But I could not find 3896966 in my Corvette parts book but did find it listed in my 1977 Chevrolet Dealer Parts book. In Group 0.659, 3896966 is listed as Damper Assembly for 1967-1968 350 engine only. 1969-up had a different number.

What I don't understand is the meaning of "GB" and "2" near the casting number and the stamped "F K 6". Can anybody help out here? Is this unit for '67-'68 350 non-corvette engine only? I know the 350 was introduced in 1967 for the Camaro SS only and Corvette did not get the 350 until 1969.

What I would like is to confirm the application since it is not correct for my 1967 L79. Below are photos of the balancer.
[ATTACH=CONFIG]109787[/ATTACH]
[ATTACH=CONFIG]109788[/ATTACH]
[ATTACH=CONFIG]109789[/ATTACH]
[ATTACH=CONFIG]109790[/ATTACH]
[ATTACH=CONFIG]109791[/ATTACH]

Rocco------

This balancer is, as the part number embossed on the hub indicates, GM #3896966. Usually, this type of embossment indicates a casting number for the component on which it is found. However, in this case, it's the actual finished part number for the balancer assembly. Why? Well, I expect that this was a very low volume piece that GM outsourced the manufacture of. I'm sure that manufacturer is identified by the "GB" casting mark. That manufacturer decided to emboss the finished part number, as required by GM, on the hub section of the balancer.

The GM #3896966 balancer was applicable to 1967-68 SS350 Camaros. I don't think there were any other 350 cid engine applications during this period but, if there were, I have not been able to identify them. The GM #3896966 was discontinued without supersession in July, 1983.

The 1967-68 Camaro SS350 engine was rated at 295 horsepower. It was essentially identical to the 1969-70 Corvette 300 hp 350 with the primary difference being the exhaust manifolds. However, the balancer used the pre-1969 timing mark location. Otherwise, I believe the 3896966 balancer was identical to the GM #3947704 used for 1969-70 Corvette and other 300 HP engines EXCEPT for the timing mark location.

[Rocco S.]

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

Joe & Joe,

Thanks for your replies. Both of you confirmed what I thought. 1967-68 Camaro SS 350. I sure learned a lot about these balancers. Especially the relationship of the keyway and timimg mark. Now that I know the application I will be listing this on eBay since I have no use for it.

[Joe L.]

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

Hello All:

My research into the question of suitable replacements for the 1962-1968 SHP #3817173 "big" balancer has revealed a lot of interesting information, and the research is still ongoing. However, I thought I would provide an interim report for those who are interested. Following is a short summary of available options for getting a GM balancer:


NEW GM SERVICE REPLACEMENT

GM discontinued the original 3817173 part long ago. For 1969-up, the #3947712 balancer used on the LT1 was almost identical to the #3817173, *except* that the timing mark was moved approximately 10 degrees. GM made this change to the balancers (and the corresponding timing covers) to increase visibility for timing the engine. So, if you want to purchase a brand new GM service replacement balancer, you will have to add a new timing mark in the correct location and use that for timing your 1962-1968 Corvette.

With help from Joe Lucia and some web research, it appears that the original LT1 balancer #3947712 has gone through a long succession of GM replacements. Following is a list, in approximate chronological order, that has not yet been checked by Joe Lucia:

3947712
6272224
12551947
12555879
88960604
19260269

The last one, #19260269, does not show up at gmpartsdirect.com, but it is still available at some web sites. For people who want a brand-new GM balancer, this might be worth considering. Below is an add clipped from the web.

IMPORTANT NOTE: The ad below says this balancer is for "externally balanced" engines, but I think it was supposed to say "internally balanced engines." The 400 small block was the only production small block where a counterweight in the balancer helped to complete the engine balance. All other small blocks were "internally balanced" and used a "neutral balance" balancer. This aspect should be verified before purchasing a 19260269 balancer for use on a 327 or 350.


[ATTACH=CONFIG]109786[/ATTACH]



HAVING AN ORIGINAL BALANCER REBUILT

There are at least three companies that will rebuild a balancer with a new rubber isolator between the hub and the inertia ring:

Damper Doctor (https://damperdoctor.com/)
Damper Dudes (https://www.damperdudes.net/)
Dale Manufacturing (www.hbrepair.com)

The cost for this service averages about $170. My guess is that when re-assembling the balancer, these vendors would be happy to put the timing mark in the 1962-1968 position.


FINDING A REBUILDABLE CORE

Provided that a balancer has not been damaged by incorrect installation/removal, the only two things that need to be renewed during a rebuild are the rubber isolator between the hub and the inertia ring, and the wear surface on the nose for the seal in the timing cover.

Over time, the timing cover seal tends to wear a groove in the nose. A well-accepted solution is to install a very thin sleeve on the nose to provide a new surface for the seal. An alternate solution is to use a different seal whose lip is slightly offset from the stock location where the groove has been made.

On ebay, a search on the older GM part numbers 3817173 and 3947712 typically yields several used and NOS balancers at prices around $400 and up. Most used balancers should definitely be rebuilt. While a NOS balancer might seem like a safe choice, keep in mind that if it is a very old NOS unit, the rubber isolator may be more than 50 years old.

HERE IS A USEFUL TIP FOR EBAY SHOPPING: Many ebay sellers think that the part number for the balancer is the one that appears on the hub casting, but that that is just the casting number for the hub. The most common hub casting used for the big balancers is casting number 3942538. If you search on this number, you will likely turn up many candidate balancers at about half the cost of the balancers listed under the part numbers 3817173 and 3947712. Yet, most of the "3942538" balancers are actually the 3947712 or its replacements. Typically, the last four digits of the part number, such as "7712" or "2224" are stamped somewhere on the face of the balancer, but it's often not visible in the ebay photos. The key thing to keep in mind is that if the hub casting number is 3942538, the balancer is one of the "big" balancers in the 3947712 balancer family.


OTHER CANDIDATE CORES

There is a very common 8" GM balancer that looks like the "big" 3947712 balancer, but is only 1-5/16" thick rather than the 1-11/16" thickness of the 1962-1968 "big" balancers. In the ebay ads, it is very difficult to detect this difference if the seller has not stated the thickness. However, the hub casting number is usually 3896966, and many ebay sellers list these as GM part number 3896966. So, a search on 3896966 will usually turn up lots of used balancers for about $50 that are 1-5/16" thick. The 1-5/16" balancer is a good budget compromise for replacing the stock 1962-1968 "big" balancer because it is the same 8" diameter. When installed, the primary visual difference is that there will be a gap of about 1/4" between the back edge of the balancer and the timing cover. The original balancer was almost touching the face of the timing cover.

It's worth noting that the GM part number for this thinner, 1-5/16" thick balancer was originally 3947704, and later superseded by 6272222. The AC Delco 6272222 is still available new for about $130, so there is little incentive to purchase a used core and have it rebuilt.

However, there is a "mystery balancer" that shows up on ebay when searching on the 3896966 hub casting number for the 1-5/16" thick balancer. This balancer uses the 1-5/16" 3896966 hub, but the inertia ring looks wider than 1-5/16". On close inspection of the photos, it appears to me that the inertia ring has an outer flange that extends the back edge of the inertia ring. When installed on the engine, this would give the appearance of being one of the "big" balancers, although it could be detected from looking at the balancer from under the car. I've purchased one of these "mystery balancers" on ebay for $34 just so I could study it. I will report what I find when it arrives.


REPRODUCTION BALANCERS

The above summary is limited to GM balancers. It should be noted that one or more reproduction balancers are available for the "finned" 3817173 used from 1962 to early 1967, and for the "un-finned" version that was used after mid-1967. Typical prices are $400 to $500. I have not researched these to find out where they are made.

Joe------


It's a popular misconception that all small blocks except the old 400 cid are internally balanced. That's not the way it is, though. Beginning in 1986 most small blocks converted to a one piece rear seal and, thus, the crankshaft rear flange counterweight was removed. So, these engines became externally balanced and require a counter-weighted flywheel. As far as the balancer goes, I do not believe they are counter-weighted. However, they may be otherwise specific to these engines. I do know that the GM #88960604 and its successor, GM #19260269, are balancers that are used on the ZZ4 crate engine and that NON-PRODUCTION engine is definitely externally balanced.

I cannot confirm that the 88960604 and 19260269 are actually supersessive to the last PRODUCTION-qualified balancer, GM #12555879. They may be but, as I say, I cannot confirm that. IF they are not actually supersessive to the 12555879 then that tells me that GM, for whatever reason, does not consider them equivalent even though they may actually be a usable replacement for the 1969-80 SHP balancer,

The GM #19260269 remains available from GM.

[Frank H.]

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

as an aside, does the nose of the crank being tapped for a retaining bolt have any impact on which balancer is used?

[Joe L.]

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

as an aside, does the nose of the crank being tapped for a retaining bolt have any impact on which balancer is used?

Frank------


Let's put it this way: there was never a GM balancer that was used for both center bolt and non center bolt crankshaft applications. So, that strongly implies there was a difference. What the difference was I do not know but I surmise it might have had something to do with the interference fit on the crankshaft.

[Joe R.]

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

Joe------


It's a popular misconception that all small blocks except the old 400 cid are internally balanced. That's not the way it is, though. Beginning in 1986 most small blocks converted to a one piece rear seal and, thus, the crankshaft rear flange counterweight was removed. So, these engines became externally balanced and require a counter-weighted flywheel. As far as the balancer goes, I do not believe they are counter-weighted. However, they may be otherwise specific to these engines. I do know that the GM #88960604 and its successor, GM #19260269, are balancers that are used on the ZZ4 crate engine and that NON-PRODUCTION engine is definitely externally balanced.

I cannot confirm that the 88960604 and 19260269 are actually supersessive to the last PRODUCTION-qualified balancer, GM #12555879. They may be but, as I say, I cannot confirm that. IF they are not actually supersessive to the 12555879 then that tells me that GM, for whatever reason, does not consider them equivalent even though they may actually be a usable replacement for the 1969-80 SHP balancer,

The GM #19260269 remains available from GM.

Hi Joe:

Thanks for the correction about external balancing of 1986-up small blocks that used a counter-weighted flywheel. That's useful to know.

Just based on a visual inspection, the balancer's components appear to be neutral-balanced, but it's possible that the balancing holes that GM drilled were actually targeting something other than neutral balance. The balancer rebuilders probably know was the balance spec was.

[Ronald L.]

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

Guys - you talk about GMParts and autozone - why aren't you looking at the Corvette Restoration Part suppliers?

[Joe R.]

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

My investigation started with an email from a friend who said he had contacted "all" the Corvette Restoration parts suppliers and none of them had stock. I did not independently verify that because my initial interest was in finding him an available GM part.

I agree that it would be useful to check with the Corvette Restoration parts suppliers if the goal is to do a complete survey of available options.

[Ronald L.]

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

They didn't try Tracy's whose been in business over 50 years, and a month ago had ten of the current state of best repro out there.

Those are made by the individual that bought the machines and has the prints, to the extent they follow the Original GM print remains up to you to verify, but I bet its a lot closer than an autozone part.

[Frank H.]

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

hi joe, do we know when Chevy went to the center bolt crankshaft?

Frank------


Let's put it this way: there was never a GM balancer that was used for both center bolt and non center bolt crankshaft applications. So, that strongly implies there was a difference. What the difference was I do not know but I surmise it might have had something to do with the interference fit on the crankshaft.