Filler plug diameter - Impala Tech
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post #1 of 4 (permalink) Old 01-16-2020, 10:43 PM Thread Starter
Join Date: Jan 2007
Location: Toronto
Posts: 81
Question Filler plug diameter

Looking to add a filler plug with posi tag on my 8.2 What is the diameter of the plug? The threaded end...
On some vendor sites, there are 3/4" and 1" plugs that hold the red tag. The tags also have the corresponding hole size
There is also a generic non posi tag plug that is 1/2" in diameter.
Why the different sizes. All are for either 10 0r 12 bolt differentials...
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post #2 of 4 (permalink) Old 01-17-2020, 02:17 AM
Join Date: Jan 2006
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1943 through 1964 Chevy made their own rear end.

Starting in 1965 Chevy (GM) bought their rear ends from Dana corp Spicer division. The Spicer ten bolt has an 8.2" ring gear and the 12 bolt has 8.875" ring gear; and all of the posi units were made by Eaton (posiTraction is a registered trade mark of Eaton Corp: which also makes the TruTrack, Detroit Locker and four other limited slip rear ends.

Starting in 1972 GM bough only one axle for all of their rear wheel drive cars from American Axle Works (the folks that make the 14 bolt rear end for three quarter and one ton trucks). That rear end called the "Corporate" because it was used across the whole fleet of GM cars has an 8.5" inch ring gear. AAW uses Borg-Warner limited slip rear ends. They eventually to make things cheaper changed the rear out for a 7.5" inch ring gear rear end that was first used by Chevrolet under the four cylinder Vega. When that proved to be too cheap, the factory raised it up to a 7.6 " ring gear. The 8.5" was reserved for cop cars and light trucks.

In 1994 Chevy brought back the Impala SS (basically a luxury interior cop car), and that car had n improved 8.5" rear end with a 30 spline axles (same size as the 12 bolt Spicer). So with a 1994-'99 8.5" rear end the only parts inside that are not as strong as the 12 bolt Spicer is the size of the ring gear. The 8.5" size has proven to be quiet strong (though I still prefer to use a Dana 60 with a 9.75" ring gear and 35 spline axles equipped with a Detroit Locker).

When Spicer first started making the rear end to GM specs they had a one inch NPT fill plug. By 1968 that had dropped down to a 7/8th inch NPT pipe plug (to save a fraction of cent on each car knowing they made 1.3 million cars a year). The AAW has a half inch pipe plug from the start. GM always starts out with an engineer designing the cars and car parts according to the designers goals. Before they are through with the first year's production the accounts are cutting costs on everything they can as that is how they justify their existence.

PosiTraction rear ends will usually have a metal tag on the right lower cover bolt with a two- or three-digit axle code and/or production date. The tag on the filler plug (usually white lettering on a red plastic tag) warns to use special lubricant additives for a PosiTraction and was attached to the pipe plug fill hole.

Big Dave
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post #3 of 4 (permalink) Old 01-24-2020, 01:19 AM Thread Starter
Join Date: Jan 2007
Location: Toronto
Posts: 81
Dave, why the 2 different hole size on the tags to fit the corresponding plug end diameters. Retailers claim that one or the other
is for either a 10 or 12 bolt differential.
Shouldn't they all have been 3/4 or 1" Weird!
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post #4 of 4 (permalink) Old 01-24-2020, 03:50 PM
Join Date: Jan 2006
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It was done to save money on tooling and machining labor. even if they saved only a third of a cent per car by doing that they put a bunch of money for dancing girls and booze to pay for the annual stock holders meeting at 1.3 million cars a year.

Every decision made at GM after 1964 was made by accounts who got a bonus if they cut costs. When engineers made the decisions from WWII through 1963 GM was the biggest company in America, but once the bean counters made the decisions GM went into bankruptcy.

Big Dave
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