C2 Transmission and Differential Case Drain Plugs

[Mark F.]

I'm wondering why C2 Transmissions (other than M22s) and Differential Cases didn't have drain plugs on the bottom...I believe later generations did have them (?) ...
anyhow, a few questions:

❶ Is suctioning old fluids out thru the filler plug holes sufficiently effective at removing crud and gunk that may have accumulated in there ?

❷ What currently-available gear oil is recommended for an M21 trans ?

❸ What currently-available gear oil (and additive ?) is recommended for a posi rear end ?

Thanks in advance for any help...

[Duke W.]

My SWC's T-10 has a drain plug, but the elimination of drain plugs on later Muncies was likely a cost cutting measure.

The current modern replacement for the original specified Mil-L-2105 (Check your owner's and service manuals if you don't believe me.) gear oil for BOTH synchromesh transmissions and hypoid rear axles is SAE 80W-90 GL-5, which is commonly available in auto parts and big box stores like Walmart. Brand makes NO difference. Just make sure whatever you buy has the API "donut" with this spec.

The mil-spec was cancelled and replaced by the commercial API spec in the seventies, and you will see the API spec listed in owner's manuals beginning in the mid-seventies, but I'm not sure of the exact year.

Siphoning from above will likely not get the typical heavy sludge buildup at the bottom of Positraction axles, which is wear particles from the Positraction clutches for old axles that rarely or often never have seen an oil change. If a freshly rebuilt Positration axle gets an oil change every 30K miles the sludge buildup will be greatly reduced, and as long as you're changing the axle oil you might as well do the synchromesh transmission, too, using the same oil as the axle.

When changing, the oil should be warm and change it as soon as possible after the car's last move to catch as many particles as possible that are still suspended in the oil so they can be removed with the oil rather than being allowed to drop out to the bottom of the housing.

All clutch types LSDs use the same "friction modifier" to prevent clutch chatter. You can buy this from manufacturers' dealers that offer clutch-type LSDs (GM, Ford, Mercedes...), and you can also buy it from NAPA, part number NCB 4285V, and any other parts chain should be able to cross reference this number to whatever "brand" they sell.

You'll need at least one 4-oz. bottle for a C2/3 Positraction axle, which has a total capacity of 3.7 pints. Start with one bottle and fill with oil to no more than a quarter-inch below the opening. Test drive and if no clutch chatter is present, top off with oil. If chatter is present add 2-4 more oz. of additive and retest.

T-10 and Muncie 4-speeds oil capacity is 2.5 pints. Oil level should be checked with the fluids warm and should be between the bottom of the fill hole to one-quarter inch down with the car on a level surface.

Duke

[Mark E.]

1. Pumping the oil from the fill port is how they are serviced. Do it with the transmission warm to suspend particles.

2. GL-4 SAE 80 or 80-90. For years I used GM #12345977. Last time I used Mobil 1 GL-4/5 Synthetic Gear Lubricant which noticeably made the transmission quieter and shift smoother.

3. Same as above plus a bottle of GM Limited Slip Differential #1052358 or equivalent.

UPDATE: I just saw Duke's simultaneous post. Great advice and perspective; just remember to use positraction additive.

[Mark F.]

...The current modern replacement for the original specified Mil-L-2105...gear oil for BOTH synchromesh transmissions and hypoid rear axles is SAE 80W-90 GL-5, which is commonly available in auto parts and big box stores like Walmart. Brand makes NO difference. Just make sure whatever you buy has the API "donut" with this spec.

Siphoning from above will likely not get the typical heavy sludge buildup at the bottom of Positraction axles, which is wear particles from the Positraction clutches for old axles that rarely or often never have seen an oil change. If a freshly rebuilt Positration axle gets an oil change every 30K miles the sludge buildup will be greatly reduced, and as long as you're changing the axle oil you might as well do the synchromesh transmission, too, using the same oil as the axle. I intend to do both.

When changing, the oil should be warm and change it as soon as possible after the car's last move to catch as many particles as possible that are still suspended in the oil so they can be removed with the oil rather than being allowed to drop out to the bottom of the housing. Excellent suggestion - didn't think of that...

All clutch types LSDs use the same "friction modifier" to prevent clutch chatter. You can buy this from manufacturers' dealers that offer clutch-type LSDs (GM, Ford, Mercedes...), and you can also buy it from NAPA, part number NCB 4285V, and any other parts chain should be able to cross reference this number to whatever "brand" they sell.

You'll need at least one 4-oz. bottle for a C2/3 Positraction axle, which has a total capacity of 3.7 pints. Start with one bottle and fill with oil to no more than a quarter-inch below the opening. Test drive and if no clutch chatter is present, top off with oil. If chatter is present add 2-4 more oz. of additive and retest.

T-10 and Muncie 4-speeds oil capacity is 2.5 pints. Oil level should be checked with the fluids warm and should be between the bottom of the fill hole to one-quarter inch down with the car on a level surface.

Duke

Thanks much Duke for all the advice...

It may be a while until I can get to this, but I'm overdue and want to do it as soon as I can...

[Mark F.]

Thanks Mark E. for your post, too...

[Duke W.]

A lot of internet "advice" says use GL-4 in our old design manual transmissions. GL-4 didn't exist in the sixties, which is why all those transmissions whose basic designs went back to at least the forties were factory filled with the same lubricant as hypoid axles, Mil-L-2105, of which the modern equivalent is 80W-90 GL-5. Both have a special sulfur-based EP additive, which is what gives these oils their pungent odor. You can't miss the "aroma" of GL-5. Back in the day is was just called "gear oil" or "90-weight".

Beginning in the seventies transmissions were designed for "lighter" lubricants to reduce windage losses and ATF was often specified, which is a 70W-75 gear oil and at some point GL-4 came along that is usually specified for transmissions and transaxles with helical final drives like what is typical in FWD transverse drivetrain layouts. and synthetics are usually rated 75W-90.

Engine oil and gear oil "weights" not interchangeable. Engine oils range from zero to 60 and gear oils 70-140. This is so they won't be confused since they have different additive requirements. For comparison, typical 70W-75 ATFs have a viscosity range that overlaps SAE 10 to 20 engine oils and 80W-90 gear oils overlap 30-40 engine oils.

I've never seen any long term test data on using GL-4 in older design manual transmission, so I continue to use and recommend the modern equivalent of the original Mil-L-2105 in these old design transmissions, which is 80W-90 GL-5.

I don't know what GM 12345977 is. Does the package list an API gear oil spec? Also I've seen internet "advice" that says it's okay to use GL-4 in hypoid axles, which lacks the hypoid gear EP additive in GL-5. I think you can guess what I think of that.

Duke

[Joe L.]

A lot of internet "advice" says use GL-4 in our old design manual transmissions. GL-4 didn't exist in the sixties, which is why all those transmissions whose basic designs went back to at least the forties were factory filled with the same lubricant as hypoid axles, Mil-L-2105, of which the modern equivalent is 80W-90 GL-5. Both have a special sulfur-based EP additive, which is what gives these oils their pungent odor. You can't miss the "aroma" of GL-5. Back in the day is was just called "gear oil" or "90-weight".

Beginning in the seventies transmissions were designed for "lighter" lubricants to reduce windage losses and ATF was often specified, which is a 70W-75 gear oil and at some point GL-4 came along that is usually specified for transmissions and transaxles with helical final drives like what is typical in FWD transverse drivetrain layouts. and synthetics are usually rated 75W-90.

Engine oil and gear oil "weights" not interchangeable. Engine oils range from zero to 60 and gear oils 70-140. This is so they won't be confused since they have different additive requirements. For comparison, typical 70W-75 ATFs have a viscosity range that overlaps SAE 10 to 20 engine oils and 80W-90 gear oils overlap 30-40 engine oils.

I've never seen any long term test data on using GL-4 in older design manual transmission, so I continue to use and recommend the modern equivalent of the original Mil-L-2105 in these old design transmissions, which is 80W-90 GL-5.

I don't know what GM 12345977 is. Does the package list an API gear oil spec? Also I've seen internet "advice" that says it's okay to use GL-4 in hypoid axles, which lacks the hypoid gear EP additive in GL-5. I think you can guess what I think of that.

Duke

Duke-----

GM #12345977 is a GL-5 rated 80W-90 gear oil. It was discontinued and replaced by GM #89021669 which is also now discontinued without supercession.

[Mark E.]

Duke-----

GM #12345977 is a GL-5 rated 80W-90 gear oil. It was discontinued and replaced by GM #89021669 which is also now discontinued without supercession.

I have an inventory of this stuff purchased on sale at a Chevy dealer during a club event in the '90s.

It sounds like GM got out of the gear oil business?

This probably means my GM limited slip additive (purchased the same time) may also be obsolete.

Duke- thank you for the GL-4 vs 5 advice; good to know.

[Mark F.]

Thanks much for all the knowledge shared here !

I have all the service manuals for my '67, but knowing what has changed over those years and what's recommended (and/or available) today almost 60 years later is very helpful.

[Duke W.]

I have an inventory of this stuff purchased on sale at a Chevy dealer during a club event in the '90s.

It sounds like GM got out of the gear oil business?

This probably means my GM limited slip additive (purchased the same time) may also be obsolete.

Duke- thank you for the GL-4 vs 5 advice; good to know.

Back when C2s were new the Positraction additive was commonly called "whale sperm". It wasn't really that, but the primary ingredient was whale oil-based. Don't ask me how whale oil was found to be effective at controlling LSD clutch chatter. I've never been able to find out.

Whale products were outlawed in the USA back in the seventies, so the industry had to find a substitute. A "synthetic" product was developed that did the job, and I expect this product is more temperature-stable than whale oil, so it won't break down as fast. Most animal and vegetable oils are more subject to thermal breakdown than petroleum-based or "synthetic" oils.

I don't recommend stockpiling more than a year or two of engine oil because over longer terms the additives can drop out, and this is indicated by a thin dark film at the bottom of the bottle. Gear oil has additives too, but I'm not sure they are subject to long term drop out. As far as the LSD additive is concerned, I'm not sure if today's product is the same as when the first synthetics were developed in the seventies, but it's possible it has been improved over the subsequent decades.

GM was never in the "gear oil business". They just contracted with an oil company that packaged a product that meet the GM spec, which is the same as Mil-L-2105 and later SAE 80W-90 GL-5 to supply GMPD with a product that dealers could order through the parts system rather than going local purchase. But since our vintage Corvettes specify generic products for the engine, transmission and axle oils, I just buy the least expensive products that meet the API specs, which is usually Walmart's house brands.

Duke

[Mark F.]

...Back when C2s were new the Positraction additive was commonly called "whale sperm". It wasn't really that, but the primary ingredient was whale oil-based. Don't ask me how whale oil was found to be effective at controlling LSD clutch chatter. I've never been able to find out...Duke

Here's what AI says when I type in:
"What are the unique properties of whale oil ?"

Whale oil's unique properties stem from its high wax ester content (especially in sperm whale oil), which made it exceptionally stable, resistant to oxidation and gumming, and stable across a wide temperature range, making it an excellent lubricant for delicate instruments like clocks. It also burned brightly and cleanly, producing little smoke, which made it a superior lamp fuel, and could be refined into high-quality soaps and candles. While general whale oil (train oil) was different, the specialized nature of sperm oil's wax esters and glycerides gave it properties not found in petroleum or most vegetable oils.

[Mark E.]

Now I'm feeling guilty that my old LSD supplement may be whale oil.

[Joe L.]

I have an inventory of this stuff purchased on sale at a Chevy dealer during a club event in the '90s.

It sounds like GM got out of the gear oil business?

This probably means my GM limited slip additive (purchased the same time) may also be obsolete.

Duke- thank you for the GL-4 vs 5 advice; good to know.

Mark-----


The positraction additive, originally GM #1050428, has gone through several part number iterations and remains available from GM and ACDelco dealers under GM #88900330.

[Mark E.]

Mark-----


The positraction additive, originally GM #1050428, has gone through several part number iterations and remains available from GM and ACDelco dealers under GM #88900330.

Thanks Joe.

Is it the good stuff with whale oil?

[Joe L.]

Mark——-

The whale oil ended with the first iteration.