1968 L79 Harmonic Damper

[Jeff P.]

Re: 1968 L79 Harmonic Damper

Joe, are all 68 8" dampers date coded? If yes could you please give an example. Thank you, Jeff

[Gene M.]

Re: 1968 L79 Harmonic Damper

I just keep thinking about this mid year balancer configuration. The 327 higher horse was for the most part the same from '62 to '67. The '68 had bigger journals. Then in '69 along came the 350 with longer stroke. Why would GM change the balance efforts on 327 engine they had been making for so many years.

Corvette phased out the 327 after 68 and most others were going to the 350 too. I don't know much about '69 and up. Was the added weight balance continued with those models? All cars, or only engines destine for Corvette? If so it is not really an all internal balanced engine.

When rebuilding a 327 I've never heard of the balancer being involved in the engine balance. The balancers I was involved with were separately rebalanced to insure they were neutral. The engine remained what I always thought of as internal balanced. I'm under the impression 350's are treated the same..... no?

[Terry M.]

Re: 1968 L79 Harmonic Damper

Gene in the C3 era (with the possible exception of 1968, although I think they were dynamically balanced also -- I just am not 100% sure) engines (both BB & SB) were final DYNAMICALLY balanced while running on the hot-test stand. Weights were added to either the harmonic balancer and/or flywheel/flex plate as indicated by the test instruments. This is a DYNAMIC (moving) balance operation and has nothing to do with internal or external balance.

ALL GM engines were STATICLY balanced by control of component size, therefore control of component mass. This STATIC balance can be either internal (all 50s to 70s Small Blocks and 396 & 427 engines) or external (all 454 engines). The long stroke of the 454 prevented the engineers from adding enough mass to throws of the 454 crankshaft to achieve STATIC balance, so off-center mass was added to both the harmonic balancer and the flywheel/flex plate in order to achieve STATIC balance.

Think of STATIC balance as using a bubble balancer to balance your wheels and tires. Those of us of a certain age will remember that. Now think of DYNAMIC balance as using a machine that rotates the wheel/tire to achieve balance.

[Joe L.]

Re: 1968 L79 Harmonic Damper

Joe, are all 68 8" dampers date coded? If yes could you please give an example. Thank you, Jeff

Jeff-------


I really can't say, for sure. Generally, when what appears to be date codes are seen, they are seen STAMPED on the outer ring (usually) or STAMPED on the hub (sometimes). In your case, the "B8" appears to be in the casting. However, it could be that it represents a casting date. If your car was built after February, 1968, I'd definitely be inclined to think it represented the date. That much of a "co-incidence" would be hard for me to accept.

Also, I've had some second thoughts about your balancer being a SERVICE balancer. I have now found that these "M-C" embossed balancers were definitely used in PRODUCTION. Just how common they are, I don't know. But, they WERE used in PRODUCTION. Given the fact that the "M-C" balancer was used in PRODUCTION, the apparent 1968 date, and the balance weights, I'd now say it's a very good liklihood that this is the original balancer.

One more thing: note the "tri-spoke" design of the hub with the "spokes" extending all the way to the very perimeter of the balancer. This design feature changed in later SERVICE and the "spokes" became much shorter. As a matter of fact, I think this feature change occurred not too long after 1968.

The only thing I do not now understand is the fact that the balancer is fully painted on the rear. That strongly suggests that the balancer was painted prior to installation on the engine. In the old-days SERVICE balancers were usually painted out-of-the-box but I always thought that balancers installed in PRODUCTION were bare and painted with the engine. Since they were definitely on the engine when it was painted, I don't see how they could have avoided being painted with the rest of the engine.

[Joe L.]

Re: 1968 L79 Harmonic Damper

Gene in the C3 era (with the possible exception of 1968, although I think they were dynamically balanced also -- I just am not 100% sure) engines (both BB & SB) were final DYNAMICALLY balanced while running on the hot-test stand. Weights were added to either the harmonic balancer and/or flywheel/flex plate as indicated by the test instruments. This is a DYNAMIC (moving) balance operation and has nothing to do with internal or external balance.

ALL GM engines were STATICLY balanced by control of component size, therefore control of component mass. This STATIC balance can be either internal (all 50s to 70s Small Blocks and 396 & 427 engines) or external (all 454 engines). The long stroke of the 454 prevented the engineers from adding enough mass to throws of the 454 crankshaft to achieve STATIC balance, so off-center mass was added to both the harmonic balancer and the flywheel/flex plate in order to achieve STATIC balance.

Think of STATIC balance as using a bubble balancer to balance your wheels and tires. Those of us of a certain age will remember that. Now think of DYNAMIC balance as using a machine that rotates the wheel/tire to achieve balance.

Terry------


Absolutely correct and well-stated. Just a few comments: I think that the dynamic balancing of the engines began prior to 1968---probably for 1966 but at least for 1967. Why do I think that? Well, the balance weight holes were added to small block balancers for the 1966 model year (1965 for big blocks). Certainly, the addition of these holes had a cost. GM didn't do that for no reason.

Also, there was one small block that used external balance. That was the 1970+ 400 cid small block.

[Gene M.]

Re: 1968 L79 Harmonic Damper

So once one rebuilds an engine the "DYNAMIC balance" is lost. But I never had an engine not run just as smooth rebuilt as previous. Interesting.......

[Joe L.]

Re: 1968 L79 Harmonic Damper

So once one rebuilds an engine the "DYNAMIC balance" is lost. But I never had an engine not run just as smooth rebuilt as previous. Interesting.......

Gene------


Not necessarily, but usually. If one does a "standard rebuild" the dynamic balance is lost. No question about it.

However, if during the rebuild one has the reciprocating components CUSTOM-BALANCED, the dynamic balance can be maintained. In a custom balance, the individual components are balanced separately. Then, fully assembled, the entire reciprocating assembly is final balanced. This mimics what the factory did at final start and balance. In fact, it can be even more precise because a narrower tolerance range is usually observed, at least by competent balancing shops.

[Terry M.]

Re: 1968 L79 Harmonic Damper

Terry------


Absolutely correct and well-stated. Just a few comments: I think that the dynamic balancing of the engines began prior to 1968---probably for 1966 but at least for 1967. Why do I think that? Well, the balance weight holes were added to small block balancers for the 1966 model year (1965 for big blocks). Certainly, the addition of these holes had a cost. GM didn't do that for no reason.

Also, there was one small block that used external balance. That was the 1970+ 400 cid small block.

Thanks Joe. I had forgot about the 400CID small block. External balance was necessary for that engine for the same reason as on the 454 = no room internally for more mass on the crankshaft counterweights.

I wouldn't disagree with the dynamic balance starting earlier than 1968. I just don't know enough about the mid-year build practices to become involved there.

[Jeff P.]

Re: 1968 L79 Harmonic Damper

Many thanks again Joe . Great reply. Thanks everyone for input to another part conclusion.