73 M-20 tranny speedo gear and TSC switch

[Steve L.]

Does anyone repo a right angle speedo adapter at the tranny? Mine is a Hobbard 6478879 1:.882 stamped on the body. The gear was junk when it was new- kept shearing the plastic speedo gear teeth every 25k miles like clockwork. Decades later I saw a tech bulletin with this exact problem. Mine is now totally seized. It doesn't look like it is easily rebuildable since there is a number of crimped sheetmetal plugs. I'd like to get a replacement then much later take a stab at rebuilding it.

Paragon seems to sell a straight version.



Also, the TSC switch hanging off the 3-4 gear shifter shaft- anyone repo that?

[Joe L.]

Re: 73 M-20 tranny speedo gear and TSC switch

Does anyone repo a right angle speedo adapter at the tranny? Mine is a Hobbard 6478879 1:.882 stamped on the body. The gear was junk when it was new- kept shearing the plastic speedo gear teeth every 25k miles like clockwork. Decades later I saw a tech bulletin with this exact problem. Mine is now totally seized. It doesn't look like it is easily rebuildable since there is a number of crimped sheetmetal plugs. I'd like to get a replacement then much later take a stab at rebuilding it.

Paragon seems to sell a straight version.



Also, the TSC switch hanging off the 3-4 gear shifter shaft- anyone repo that?

Steve-----


Does this car have a 3.70:1 rear end ratio? That's the only 1973 application that should require an adapter.

I don't know what a "Hobbard 6477879" is. Your application, if it's an original 3.70:1 rear gear ratio, originally used a GM #6477965 which was a right angle 0.883:1 adapter. This was replaced quite some time ago by GM #1565208 which is also a right angle, 0.883:1 adapter. It was recently discontinued but your dealer should be able to locate you one in dealer stock. The problem is that this piece GM lists for about 200 bucks. Otherwise, you should be able to get a similar adapter from a speedometer shop.

The TCS switch does not "hang off" the 3-4 shifter; it screws into a fitting on the 3-4 shifter lever boss on the side cover. The original switch, GM #3996286, was available until very recently. It's now GM-discontinued but your dealer should be able to find you one of these, too. You may also be able to find one on eBay. It only GM listed for about 12 bucks.

[Steve L.]

Re: 73 M-20 tranny speedo gear and TSC switch

The Hobbard is stamped on the sheetmetal plugs so maybe Hobbard is only the supplier of the plugs. The P/n 6478879 along with
the 1: .882 is stamped onto the body.

My paperwork from GM shows my diff is:Option GV4 3.36 ratio.

I've seen a number of on-line sources for GM 1565208 for around $110. From the pictures it looks a lot different than the original.

Looks like the TCS switch is available as well.

thanks for the update in partnumbers

[Larry H.]

Re: 73 M-20 tranny speedo gear and TSC switch

Steve,

I think to replace your #6478879 speedo adapter you will need #1565211, which is GM discontinued though your local dealer can probably find you one. The GM parts book says that to use the #1565211 in place of the #6478879 you will also need to replace the key with #6476970, which after a couple of number changes is now #88961013. The key is still available from GM.

Larry #24649

[Michael W.]

Re: 73 M-20 tranny speedo gear and TSC switch

The Hobbard is stamped on the sheetmetal plugs so maybe Hobbard is only the supplier of the plugs. The P/n 6478879 along with
the 1: .882 is stamped onto the body.

My paperwork from GM shows my diff is:Option GV4 3.36 ratio.

Steve,

I've got a 3:36 ratio also and the same adapter on my '73. Checking the indicated speed vs GPS shows very little error at 60 MPH.

[Joe L.]

Re: 73 M-20 tranny speedo gear and TSC switch

The Hobbard is stamped on the sheetmetal plugs so maybe Hobbard is only the supplier of the plugs. The P/n 6478879 along with
the 1: .882 is stamped onto the body.

My paperwork from GM shows my diff is:Option GV4 3.36 ratio.

I've seen a number of on-line sources for GM 1565208 for around $110. From the pictures it looks a lot different than the original.

Looks like the TCS switch is available as well.

thanks for the update in partnumbers

Steve-----


If you have a 3.36:1 ratio you definitely don't want the adapters I mentioned. They are for a 3.70:1 rear gear ratio which, incidentally, would be extremely rare with an M-20 (but was theoretically available).

I show that the the only application for 1973 which was supposed to use an adapter was cars with 4 speed and 3.70:1 rear gear ratio. However, since you and Mike both have them, I've got to conclude that my information is wrong.

The GM #1565211 is the successor to the 6478879 as Larry mentions and does require the different key. However, it's very possible that this adapter has the same configuration differences that you noted for the 1565208. I expect that the 1565211 and 1565208 are "birds of a feather" except for ratio.

By the way, I think you could eliminate the adapter and change your speedometer DRIVEN gear to either a blue 20 tooth or a red 21 tooth and still have acceptable speedometer accuracy

Note: I came across why I had the wrong information on the speedometer adapter. Apparently, initially, 1973 Corvettes were scheduled to use FR-70-15 tires and the speedometer information I have was based on that. However, no 1973 Corvette ever did use that tire size as far as I can tell. With the GR-70-15 tire size the 6479979 adapter was used with 3.36:1 gear ratio.

[Harmon C.]

Re: 73 M-20 tranny speedo gear and TSC switch

73 four speed cars need the adapter. I got one from GM but they have no ends to connect to so you need three parts. Input adapter,output adapter,and the main 90 degree adapter.

[Steve L.]

Re: 73 M-20 tranny speedo gear and TSC switch

What does the key do--is that the flat strip of metal that screws to the tranny housing and holds the adapter in place/

The old '75 service bulletin lists this key as 6476870 while Larry above lists it as 6476970?--neither of which looks readily available from anyone.

The 90 degree adapter 1565211, looks available on line for around $80.

Also from Lyle, I see 3 pieces-- the 90 degree adapter,the input adapter that threads onto the 90 degree adapter and the nylon gear. What is the output adapter?

[Harmon C.]

Re: 73 M-20 tranny speedo gear and TSC switch

Both ends of the 90 degree adapter need a small fitting (key) to drive the cable.

[Steve L.]

Re: 73 M-20 tranny speedo gear and TSC switch

Thanks Lyle
I see the cable key on my original, now that I know what the key is.