1971 Base Engine Harmonic Damper

[Michael B.]

The Judging Guide states that 1970-71 base engines had an eight inch harmonic damper installed. I've got a 1971 coupe I'm disassembling that has a 6 3/4 inch damper I'm pretty sure is original. The car is serial number 194371S114736, engine assembly date 0325, suffix CJK, body build date April 15th.

I pulled the damper to check for part numbers and I found a 7701H stamped into the face of the balancer above the keyway slot. About an inch below the 7701H the numbers 11 are stamped. Other threads have mentioned date stamping but I could not see any on this damper. However, on the rubber between the inner and outer sections, on the inside of the damper, there is a raised number HT1583.

Anybody else have the 6 3/4 inch damper on their 71?

Thanks,

Mike

[Joe L.]

Re: 1971 Base Engine Harmonic Damper

The Judging Guide states that 1970-71 base engines had an eight inch harmonic damper installed. I've got a 1971 coupe I'm disassembling that has a 6 3/4 inch damper I'm pretty sure is original. The car is serial number 194371S114736, engine assembly date 0325, suffix CJK, body build date April 15th.

I pulled the damper to check for part numbers and I found a 7701H stamped into the face of the balancer above the keyway slot. About an inch below the 7701H the numbers 11 are stamped. Other threads have mentioned date stamping but I could not see any on this damper. However, on the rubber between the inner and outer sections, on the inside of the damper, there is a raised number HT1583.

Anybody else have the 6 3/4 inch damper on their 71?

Thanks,

Mike

Mike------


I am not aware that any 1969-71 Corvettes with base engine were originally built with a 6-3/4" balancer. In addition, GM says that all were built with an 8" balancer of GM #3947704.

The number found on your balancer, 7701H, refers to GM #3947701. This is a 6-3/4" balancer used on many 69-71 low-performance small blocks with 2 barrel carburetors. It was also a predecessor to the GM #6272221, 6-3/4" balancer used on 1972 Corvettes with base engine.

Where is the timing pointer on your engine? Is it close to the balancer or some distance away? Is it welded to the cover or is it retained by 2 timing cover bolts? Even if it's close, as it should be, it's very possible the timing cover and/or pointer was also changed.

[Michael B.]

Re: 1971 Base Engine Harmonic Damper

Joe,

Thanks for the response.

The timing tab is mounted to the timing cover with two bolts, exactly as it is on my 72 base engine car. It is very close in to the balancer.

I guess it's possible that this damper could have been replaced at some point, but this car was parked long ago, sat in a field, and rusted away. The rust is so bad there are portions of the frame completely missing. I purchased it for parts. If the damper was replaced, it was done so many, many years ago and I would think that original dampers would have been available at the time. Why pick a damper that's too small and then have to change the timing tab as well?

Based on the amount of built up grease and sludge, I'm guessing this thing is original and I'll bet there are other April or later cars with the same.

Mike

[Patrick H.]

Re: 1971 Base Engine Harmonic Damper

You just want me to go out and check yet another car, don't you?

I think my balancer is actually in the basement. Send me an email if you don't hear in a couple of days. My 71 car was made about a month after your parts car.

[Alan S.]

Re: 1971 Base Engine Harmonic Damper

Hi Michael,
I'm the original owner of my 71 base motor, 4-speed, no p/s or a/c car. It was built in mid-January 1971.
The balancer on the CJL motor, which was assembled January 7th, 1971, is 6 3/4" in diameter.
I mention the lack of p/s and a/c because the 2 groove pulley on the crank shaft is 6 3/4" in diameter too.
The pulley is mounted at this time so I can't see any markings on the balancer.
Regards,
Alan

[Michael B.]

Re: 1971 Base Engine Harmonic Damper

Good Morning Patrick,

I can always count on you.

It seems the more I learn about these cars, the more questions I have.

It seems doubtful to me that an owner would intentionally replace a heavier balancer with a light duty one, especially if that also necessitated changing the timing cover and timing indicator. However, I'll admit that is certainly a possibility.

I think a more likely explanation could be that GM had already made the decision to put the smaller damper on the 72 engines and had either started using them at the Flint Engine Plant in April 71, or had authorized Flint to use them on an emergency basis whenever the inventory of the eight inch dampers fell short.

Hopefully we'll get some responses from other 71 owners to find out if this is the case, and if so, approximately when these smaller dampers started to appear.

Mike

[Michael B.]

Re: 1971 Base Engine Harmonic Damper

Thanks Alan. At least now we know they were used as early as January 71.

Looks like another revision to the Judging Guide may be in order.

Speaking of the JG, does anyone know when the new one will be available?

[Patrick H.]

Re: 1971 Base Engine Harmonic Damper

Speaking of the JG, does anyone know when the new one will be available?

Late 2016?
We're still working on revisions. 2 of the 4 sections are essentially done. I would be a bit surprised if it is ready Sept 2016 (they always come out after the Convention) but it may yet be in the next year.

[David L.]

Re: 1971 Base Engine Harmonic Damper

I have a balancer stamped "7701" and "1SO" (or "1S0) that I removed from a salvage yard 1971 Chevrolet engine back in November 1992 that was stamped "K0217CGC" with a "1 B 12" casting date. I would assume that the "1" in "1SO" was for year 1971 or possibly the month of January.

I also have a balancer stamped "7704" and ""ASO" (or "AS0") that was removed from a 1970 Monte Carlo as well as a "7704" balanced stamped "B9F" (B9 = Feb. 1969 with "F" being the manufacturer).

Dave

[Joe L.]

Re: 1971 Base Engine Harmonic Damper

This is extremely interesting and news to me. GM never acknowledged or specified that any balancer other than the 8" was used for 1971 270 HP. Relative to the balancer, there was never even a "1st design" and "2nd design" shown. In addition, to this day if one was to go into a Chevrolet dealership parts department seeking a balancer for a 1971 Corvette with base engine, one would be sold the GM #6272222 balancer (the 8" balancer that superceded the 3947704) which is still available from GM.

It will be interesting to see what others with 1971 base engines report. That's the only way we're going to get information on when the change occurred and/or how prevalent the use of the 6-3/4" balancer actually was for 1971.

[Joe L.]

Re: 1971 Base Engine Harmonic Damper

Hi Michael,
I'm the original owner of my 71 base motor, 4-speed, no p/s or a/c car. It was built in mid-January 1971.
The balancer on the CJL motor, which was assembled January 7th, 1971, is 6 3/4" in diameter.
I mention the lack of p/s and a/c because the 2 groove pulley on the crank shaft is 6 3/4" in diameter too.
The pulley is mounted at this time so I can't see any markings on the balancer.
Regards,
Alan

Alan------


There's another anomaly I see here. Irrespective of balancer, the 1971 base engine was supposed to have used crank pulley GM #3850838. That pulley is supposed to be 7.33" OD. Can you see any part number on the pulley? I can't see it in the photo. Also, every 3850838 pulley that I've ever seen from that era was phosphate-finished.

[Alan S.]

[Robert H.]

Re: 1971 Base Engine Harmonic Damper

I have a 71 Base Coupe, CJK, 270HP AT, and it has the 6 3/4 inch balancer. My serial # 114705. My engine was built March 24, 1971. My engine.trans/chassis and body are numbers matching. I will try to get some numbers off the balancer.

[Joe L.]

Re: 1971 Base Engine Harmonic Damper

Hi Joe,
I removed the pulley.
It measures 6 3/4".
It's stamped……. GM 3858533 BJ…. on the front side.
You're right, it's dark, (maybe manganese), phosphate.
Regards,
Alan

Alan-----

GM #3858533 is a deep groove pulley usually used with SHP applications. However, the 3858533 pulley was specified for the 1972 base engine application. So, apparently the 1972 configuration came into PRODUCTION well prior to the outset of the 1972 model year. I expect the reason that the 3858533 was used was that the 3850838, being 7.33" OD would "overhang" the 6-3/4" balancer. Still, I would not think it would align properly with the standard groove waterpump pulley.

Your pulley appears to be painted to me rather than phosphate-finished.

[Alan S.]