L79 Distributor Rebuild

[Kenny H.]

I'm rebuilding my original L79 distributor (#1111196) and have a couple of questions. An assembly procedure I have says to fill the oil well cavity with SAE 20 weight, then install the seal and saturate the felt washer with SAE 20, which is easy enough. However, there is no mention of a specific lubricant to use to fill the two grooves on the main shaft that the weight base / cam slides over - it just says use a high quality grease. Is white lithium grease with a minimum 350 deg F drop point ok? Is that same grease also ok to use on the tach drive gear and the tach worm gear on the shaft?

Also, can someone recommend a source of OEM centrifugal advance weights and springs for the L-79?

Thanks.

[Joe L.]

Re: L79 Distributor Rebuild

I'm rebuilding my original L79 distributor (#1111196) and have a couple of questions. An assembly procedure I have says to fill the oil well cavity with SAE 20 weight, then install the seal and saturate the felt washer with SAE 20, which is easy enough. However, there is no mention of a specific lubricant to use to fill the two grooves on the main shaft that the weight base / cam slides over - it just says use a high quality grease. Is white lithium grease with a minimum 350 deg F drop point ok? Is that same grease also ok to use on the tach drive gear and the tach worm gear on the shaft?

Also, can someone recommend a source of OEM centrifugal advance weights and springs for the L-79?

Thanks.

Kenny-----


I'd skip the 20 wt motor oil and fill the cavity and grooves with EP chassis grease.

I know of no source for OEM-equivalent weights and springs other than NOS.

[Duke W.]

Re: L79 Distributor Rebuild

Search under my name, thread I started (there aren't that many), for a report on a L-79 distributor blueprint/overhaul posted December 2012. Since you probably won't have the GM Distributor and Starter Motor Lubricant I used, use a high quality synthetic grease like Mobil 1. White lithium will dry out too fast.

Assuming you have the OE weights use them and buy the spring kit and a B26 VAC. Also, buy sufficient shims to bring the end play to the desired two to seven thou and buy and install the breaker plate ground repair wire, even if yours is okay.

Also search the archives for a write-up started by me on breaker points (also published in The Corvette Restorer) to select the proper set.

DO NOT USE ANY GREASE ON THE CENTRIFUGAL ADVANCE MECHANISM! Grease will gum it up and make the mechanism sticky. These hardened parts are very lightly loaded with low relative velocity and don't need any lubrication. Just wipe the parts with WD-40 to provide some corrosion resistance.

Duke

[Kenny H.]

Re: L79 Distributor Rebuild

Thanks Joe and Duke. I had bought some EP grease as Joe suggested and it was Mobil 1 synthetic so I should be good. I measured the end play on my distributor at 0.062" - wow. No side play on the shaft / bushings so in good shape there. I've ordered a kit with some parts like the shims, new ground wire, etc. I had previously bought a B26 VAC and will install in when I re-assemble the distributor. I had set the total timing at 36 deg (all in by 2,700) with vacuum disconnected, resulting in a 16 deg initial idle timing. When I connected the manifold vacuum to see what the total idle timing was the numbers didn't make sense if it were a B26 VAC (I'm not at home at the moment so I don't recall exactly what that number was), so I dug a little deeper and found a B1 VAC had been installed in the distributor. At that point I decided to pull the distributor and return everything to factory specs. I'll do a search and find the articles. Thanks again both of you.

[Duke W.]

Re: L79 Distributor Rebuild

Best total WOT advance is 38-39, not 36. Also, OE L-79 distributors offer 30 degrees total centrigual at something like 5000, but check the CSM/AMA specs for your year. Based on that initial should be about 8-9, so maybe you are not revving it high enough to bring in all the centrifugal, even with lighter springs, or maybe it's not the original distributor.

The B1 VAC is a boat anchor - not appropriate for any Corvette engine, but unfortunately the B1 in listed in many parts books due to GM substituting their number that was equivalent to the B1 in lieu of the 201 15. The B1 may take more than 18" to fully deploy, so it doesn't even pass the Two-Inch Rule for a base engine.

Most mechanics/"engine builders" are clueless about why the VAC is there, what it does, and how to select a proper one for the application.

End play in the range of .040-.090" is typical for factory assembled Corvette distributors on carbureted engine. (FI distributors are usually a lot better.) Chevrolet did not follow there own blueprint tolerance range, and all that end play results in about 2-4 degrees spark scatter that you can see on the timing light, which sure doesn't help engine performance and smoothness, and excess end play can also lead to premature point bounce. With 2-7 thou end play as specfiied in the 1963 Corvette Shop manual the timing mark will be dead steady at any RPM.

This little detail can have a signficant impact on engine performance and smoothness.

Duke

[Kenny H.]

Re: L79 Distributor Rebuild

Thanks Duke, good information on the best WOT total timing I've always been operating under the premise that most chevy small blocks like 36. When I set my total at 36, and the resulting idle was 16, I noted the centrifugal advance stopped increasing at 2,700 but I revved it to 4,500 and still no increase in total timing so I assumed I had all the total available. Then I set the initial idle timing at 10 deg like one of the restoration guides I have recommends. That resulted in a total timing of 34 deg, again all in by 2,700 and revved it to 4,500 with no additional timing. It appears my distributor only has 20 deg in it, not 30 deg like your Dec 2012 distributor article suggests. Maybe my distributor has the wrong bushing in it limiting the total centrifugal advance? Also, the springs appear to not be original (the weights appear OE) so is it possible the springs are stiff enough to not allow more than 20 deg centrifugal? Lots of old, sticky grease on the underside of the advance weights and weight plate. I'll clean everything, install new springs, set the correct end play, and install the B26 VAC and start with 38 total timing and see where that puts me on idle timing.

[Duke W.]

Re: L79 Distributor Rebuild

While its apart check the numbers on the parts and compare to what I found and reported on the L-79 distributor in the Dec 2012 thread. It's possible that the grease is causing problems, but you should do more investigation to find and correct the problem. With only 20 centrifugal too much initial is required to get 38 total, which could cause detonation. Sometimes the lower limit is when the weights contact the football and sometimes the weights contact each other and don't retract all the way to the football. Posting some photos might help along with reporting all the numbers you find. You may have an incorrect cam assembly or incorrect weights. You can use a protractor to make a cardboard template to measure the actual angular rotation of the mechanism, which should be nominally 15 degrees for the L-79

The Chevrolet Power manuals from the seventies recommend total WOT advance in the range of 36-40. In the Tale of Two Camshafts article published in the Fall 2010 Corvette Restorer, I reported that 38 total yielded about one percent more torque/power across the range than 36 on a chassis dyno test. I don't know why everyone thinks 36 is best. It's just another internet myth.

When setting total WOT advance I target 38-39, and if it doesn't detonate, it's good to go. Recently I did so on a '65 FI that still has the Flint-installed .018" head gasket and "11:1 CR" along with the aggressive 24 degree centrifugal curve that is all in at 2350, and it doesn't detonate on 91 PON California fuel.

Duke