C2-67 L79 Harmonic Balancer

[Mike Swanson]

Can anyone help me ID this balancer? I just took it off my 67 L79. I can't find any part number on it but I believe that the correct balancer should be GM# 3817173. I started looking at this because I could not get the car timed where the timing mark was anywhere near the tab on the timing cover. When I put #1 cyl at TDC, the timing mark is at 12 oclock position and should be at ~2 oclock to be near the timing cover tab. I keep seeing references to the correct balancer having a timing mark @ 2 degrees before keyway centerline and this appears to be what I have if you look at the photos.
Am I missing something here?

Thanks guys for all your help as usual.

Mike

[Michael H.]

Re: C2-67 L79 Harmonic Balancer

Mike,

The correct balancer is the 3817173 that you mentioned. It's an 8" unit with the timing mark lined up almost exactly with the C/L of the center hole and the keyway, as is the one that you show in your picture. I'm not sure what balancer you have but the timing mark does nearly line up so this is not your problem, even if it is an incorrect balancer for the car.

It's really not possible for #1 piston to be at TDC when the timing mark is at the 12:00 position unless something is very wrong inside the engine. If the original keyway on the crankshaft is still used, it will be positioned at 45 deg when #1 is at TDC. Since the balancer timing mark is in line with the key/keyway, it too has to be near 45 deg, or somewhere between 1:00 and 2:00 clock position.

I would try to establish a genuine TDC on #1 and then see where the keyway winds up.

It is possible, and likely, that you have a pre 69 balancer but a post 69 timing chain cover with the timing tab moved 10 deg closer to the top. This common mismatch of chain cover and balancer is quite common and will cause the problem you describe. Establish a true TDC and let us know what you find.

[Mike Swanson]

Re: C2-67 L79 Harmonic Balancer

Michael, That is what is so confusing. I have checked #1 TDC first with pulling the plug and rotating engine till #1 blows and also with a piston stop and the timing mark on the balancer is at 12 oclock.
The car runs great, like a scalded ape but this is very confusing to me. Maybe the crank is keyed wrong? Maybe I should pull the timing cover and check cam and crank gear alignment? I'm kinda at a loss here. Even if the timing cover tab is incorrect that would only account for ~10 degrees not 45 or so.

Mike

[Michael H.]

Re: C2-67 L79 Harmonic Balancer

The most accurate way to establish TDC without removing a cyl head is with the positive stop method that you describe. The correct way to do that is, rotate the C/Shaft until #1 contacts the stop, then mark the balancer at the "0" line on the timing tab. Now roll the C/Shaft around in the opposite direction (360 deg) until it again contacs the stop and mark the balancer at the "0" location at the timing tab. The distance exactly between your marks will be exactly TDC. The deeper you install the stop, the easier it is to get an accurate center between your marks.

If the location between your two marks is at or near the original factory timing line, the balancer and keyway are correct. The problem is in the location of the timing chain cover timing tab.

If the original timing mark and the center of your two marks are not even close, you may have a sheered C/Shaft key and the balancer has actually rotated on the C/Shaft nose. If the balancer in your picture is the actual one off of your car, you should be able to inspect the C/Shaft key and keyway fo damage.

I'm pretty sure all sbc engines had the C/Shaft keyway on the same C/L as the #1 cyl rod journal. If the cyl centerline is at 45 degrees and the timing mark is at 45 degrees, the journal and keyway must then also be on the same 45 deg.

[Mike Swanson]

Re: C2-67 L79 Harmonic Balancer

I have looked at keyway on the balancer and also the c/shaft key. All looks good. No signs of wear or slippage.
I gotta go out of town today for the week so when I get back Friday night I'll go through the process again and report back. I'll be scratching my head all week on this one.

When the car was running last week, I tried to get it to run when I rolled the balancer timing mark back to the timing cover tab and no way it would run. It runs perfectly when I have the balancer timing mark at 12 oclock.

Thanks for all you help...Mike

[Michael H.]

Re: C2-67 L79 Harmonic Balancer

I have a stupid question. You are checking the timing with the vac advance line disconnected, right? If by chance it's checked while connected, the timing mark would probably be in just about the 12:00 position that you reported.

Also, is the distributor cent adv returning to zero or near zero at idle?

[Mike Swanson]

Re: C2-67 L79 Harmonic Balancer

Michael, Yes, I was timing with vacuum advanced disconnected and plugged. Yes, the cent adv is at zero when idling.

Since I removed the distributor, I have replaced the vacuum advance. I guess I will go through the process again this coming weekend and button everything all back up and see what happens. I may have to wait a bit since I am waiting for some other parts as well. I am also replacing the intake manifold at this time and will like get the balancer rebuilt. I really don't like the sight of rubber badly protruding in various spots around the balancer.

Thanks again for all you help and sanity checks.

I will be posting again when I have more news to report.

I've gott to fly off to LAX now...Mike

[Michael H.]

Orig Balancer

An original showing the in line C/L, keyway and timing mark

[Jim S.]

Re: C2-67 L79 Harmonic Balancer

Mike,

I can't help you with your timing problem, but I can tell you the correct balancer "is" stamped with the #3817173 on the outter edge of the inner piece,just before the rubber, on the front of the balancer. It should also have 12 cooling fins in the back as well. That is, at least correct for a 66 ..I am assuming it is the same for 67... Also as Mike stated the timing mark is practically even with the key .

[Joe L.]

Re: C2-67 L79 Harmonic Balancer

Mike-----

This balancer is apparently not an original balancer as it lacks the reinforcing ribs on the internal portion of the balancer hub. That pretty much means that it's a SERVICE replacement, since later GM SERVICE balancers, although of the original part number, lack the ribs.

Curiously, this balancer does APPEAR like it might be original and, personally, I've never been 100% certain that all original L-79 balancers were of the "ribbed" design. However, judging standards call for it and whether or not a ribbed balancer was originally used on your engine, you'll need one now for "judging considerations".

You can purchase a reproduction of the "ribbed" balancer. Or, if you want to go with this style, you can still get a brand new one exactly like the one you have from GM under the original GM #3817173 part number.

[Robert Willis]

Re: C2-67 L79 Harmonic Balancer

I had a crank that had the keyway in the wrong place. Changed the crank and it ran alot better. It's the only time I have ever saw that one myself.

Bob

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