What speedo adapter does a 70 LT1 with 4:11 use ?

[Roger O.]

Looking in the GM parts manual I come up with #6477965 in one section but other sections don't support that number. Thanks.

[Joe L.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Looking in the GM parts manual I come up with #6477965 in one section but other sections don't support that number. Thanks.

Roger-----

First of all, understand that all information provided below depends upon the car having tires of near the original OD. If that's not the case, all bets are off.

It all depends upon which DRIVE gear is installed. If the transmission is an original 1970 M-21, it should have a GM #3924097 DRIVE gear installed. This is a BLUE NYLON DRIVE gear. This is the gear which mounts on the transmission mainshaft and which can only be removed and replaced by partial disassembly of the transmission.

If you have the above referenced DRIVE gear installed, then you use a 25 tooth GM #3860348 orange DRIVEN gear and NO adapter is required.

If the transmission has a GREEN NYLON DRIVE gear installed, everything changes. In that case you require a GM #3987921 21 tooth RED DRIVEN gear + a GM #6477965 adapter. The latter was replaced by GM #1565208 but it's also discontinued. In any event, both are a 0.8333:1 adapter and a speedometer shop should be able to supply such an adapter.

Rather than trying to "peer in" to the transmission to see which DRIVE gear is installed, we may be able to infer it by noting which DRIVEN gear was installed. So, what DRIVEN did you remove (tooth count and color)?

[Roger O.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Roger-----

First of all, understand that all information provided below depends upon the car having tires of near the original OD. If that's not the case, all bets are off.

It all depends upon which DRIVE gear is installed. If the transmission is an original 1970 M-21, it should have a GM #3924097 DRIVE gear installed. This is a BLUE NYLON DRIVE gear. This is the gear which mounts on the transmission mainshaft and which can only be removed and replaced by partial disassembly of the transmission.

If you have the above referenced DRIVE gear installed, then you use a 25 tooth GM #3860348 orange DRIVEN gear and NO adapter is required.

If the transmission has a GREEN NYLON DRIVE gear installed, everything changes. In that case you require a GM #3987921 21 tooth RED DRIVEN gear + a GM #6477965 adapter. The latter was replaced by GM #1565208 but it's also discontinued. In any event, both are a 0.8333:1 adapter and a speedometer shop should be able to supply such an adapter.

Rather than trying to "peer in" to the transmission to see which DRIVE gear is installed, we may be able to infer it by noting which DRIVEN gear was installed. So, what DRIVEN did you remove (tooth count and color)?

Thanks for the info Joe. I'm asking for a buddy , I'll shoot him an email and get back with the driven color and tooth count. Thanks again.

[Terry M.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

To the best of my knowledge all 1970 Corvettes with either 4:11 or 4:56 used the external right-angle adapter based on my observations of original examples.

Joe,
You will have to decide if the parts lists support my observation, but I am pretty convinced that is the way it was. Of course that may not matter to Roger's friend if his focus is an accurate speed indication.

[Joe L.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

To the best of my knowledge all 1970 Corvettes with either 4:11 or 4:56 used the external right-angle adapter based on my observations of original examples.

Joe,
You will have to decide if the parts lists support my observation, but I am pretty convinced that is the way it was. Of course that may not matter to Roger's friend if his focus is an accurate speed indication.

Terry-----


4.56:1 would have required an adapter. However, 4.11:1 would not have required it IF the GM #3924097 speedometer DRIVE gear was installed in the transmission.

GM says that all 1970 Corvettes with M-21 transmission had the 3924097 DRIVE gear installed. Whether they actually did, I do not know. If they actually had the 3924098 DRIVE gear installed, then an adapter would have to have been used for 4.11:1.

[Wayne M.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Joe L, Terry M. -- Being a C2 person, I need an education on these adapters (reducers / increasers). First; were all C3 adapters [where used] the 90-degree type ? Did any of the right angle type have just a 1:1 ratio (to at least reduce the load on the speedo cable) ?

Looking at that example of a '70 LT1 or L46 with a close ratio and 4.11 final drive, agree by my 1970 P&A30B that no adapter is required. But if the engine is an LS5 with close ratio and a 3.08 or 3.36 diff, then the 6477966 adapter is required with the 3924097 drive gear on the trans mainshaft. As the same drive/ (orange) driven gears are called for both 3.36 and 4.11 rear axles (the first with an adapter, the second without), then we can deduce that the adapter is acting as a speedo cable increaser with a ratio of 1.223 (ie. 4.11/3.36) for the 3.08 and 3.36 gearsets.

Similarly, for wide ratio 4-spd, the adapter 6477965 is a 0.832 reducer (as Joe says) [ie. same 19-tooth natural color driven gear for both 3.08 (w/o adapt.) and 3.70 (with), and both with drive gear 3924098].

Finally, adapter 6477964, used for wide ratio with 4.56 axle (although this combo is not found in 1970 power train charts; would be quite a stump puller ) with a reducing ratio of 0.736 or therebouts, deduced from same blue 20-tooth driven gear being used for the 3.36 axle (w/o adapter) and 4.56 (with).

Below is pic of a non-angled Stewart Warner with a reducing ratio of 0.9743 (ie. 39 turns of the speedo driven gear at the trans will spin the speedo cable 38 turns -- don't even know why they'd bother for such a small adjustment, although it does provide some degree of freedom for locating the end of speedo cable). I assume these were never used in a Corvette, although they could be, for fine-tuning, depending on adapter. ratio required.

All the above is for factory tires F70 by 15. Joe, are any of the other two adapters ...966 or ...964 still available thru GM ?

[Joe L.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Joe L, Terry M. -- Being a C2 person, I need an education on these adapters (reducers / increasers). First; were all C3 adapters [where used] the 90-degree type ? Did any of the right angle type have just a 1:1 ratio (to at least reduce the load on the speedo cable) ?

Looking at that example of a '70 LT1 or L46 with a close ratio and 4.11 final drive, agree by my 1970 P&A30B that no adapter is required. But if the engine is an LS5 with close ratio and a 3.08 or 3.36 diff, then the 6477966 adapter is required with the 3924097 drive gear on the trans mainshaft. As the same drive/ (orange) driven gears are called for both 3.36 and 4.11 rear axles (the first with an adapter, the second without), then we can deduce that the adapter is acting as a speedo cable increaser with a ratio of 1.223 (ie. 4.11/3.36) for the 3.08 and 3.36 gearsets.

Similarly, for wide ratio 4-spd, the adapter 6477965 is a 0.832 reducer (as Joe says) [ie. same 19-tooth natural color driven gear for both 3.08 (w/o adapt.) and 3.70 (with), and both with drive gear 3924098].

Finally, adapter 6477964, used for wide ratio with 4.56 axle (although this combo is not found in 1970 power train charts; would be quite a stump puller ) with a reducing ratio of 0.736 or therebouts, deduced from same blue 20-tooth driven gear being used for the 3.36 axle (w/o adapter) and 4.56 (with).

Below is pic of a non-angled Stewart Warner with a reducing ratio of 0.9743 (ie. 39 turns of the speedo driven gear at the trans will spin the speedo cable 38 turns -- don't even know why they'd bother for such a small adjustment, although it does provide some degree of freedom for locating the end of speedo cable). I assume these were never used in a Corvette, although they could be, for fine-tuning, depending on adapter. ratio required.

All the above is for factory tires F70 by 15. Joe, are any of the other two adapters ...966 or ...964 still available thru GM ?

Wayne------

According to GM, for 1970 all M-20 Muncies were equipped with the 3924098 DRIVE gears. This DRIVE gear is used with 18-22 (silver) tooth DRIVEN gears. All M-21 Muncies were equipped with the GM #3924097 DRIVE gear. This DRIVE gear is used in conjunction with 22(green)-25 tooth DRIVEN gears.

If the above was actually how it was, then the following applied:

L46 with M-20 and 3.55:1 required adapter GM #6477965 (0.833:1). I expect that very few of these were ever produced;

LT1 with M-20 and 3.55:1 or 4.11:1 required adapter GM #6477965;

LS5 with M-21 and 3.08:1 or 3.36:1 required adapter GM #6477966. I believe this is an overdrive adapter but, at the moment, I can't find the ratio, although it was likely about the 1.22:1 ratio you mentioned.

The GM #6477964, 6477965, and 6477966 were all 90 degree adapters manufactured by Stewart-Warner. GM #6477964 was discontinued in October, 1976 and replaced by GM #1565202. It is now also discontinued.

GM #6477965 was discontinued in March, 1976 and replaced by GM #1565208. It is also now discontinued.

GM #6477966 was discontinued without supercession in July, 1980.

I don't know of any adapter used by GM on a Corvette with a 1:1 ratio.

By the way, for 1971 and later ALL Muncies were equipped with a DRIVE gear compatible with only the 18-22 (silver) tooth DRIVEN gears. So, any rear gear ratio which could not achieve speedometer calibration with the 22 tooth silver gear would require an adapter. However, very few were made with such numerically high gear ratios, so I expect that not too many adapters were used.

[Dick W.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Stewart Warner no longer manufactures these adapters, but S S White has taken over the manufacture of them. Either a speedometer shop or an older HD truck dealer should have either the complete adapter of a kit to assemble one for you.

[Ralph E.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Roger,
If you need an adapter, try PATC 1.888.877.1008 or

http://www.partshp.com/speedometer_gears.htm

If your friends speedo is slow or fast they can provide an adapter that will make the correction either way.

Ralph

[Dan P.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Finally, adapter 6477964, used for wide ratio with 4.56 axle (although this combo is not found in 1970 power train charts; would be quite a stump puller ) with a reducing ratio of 0.736 or therebouts, deduced from same blue 20-tooth driven gear being used for the 3.36 axle (w/o adapter) and 4.56 (with).

Great info Wayne. Your data is correct. Here's a sample of the "stump puller" variation you are talking about.

These adapters were used in 1970 on 4.56:1 cars with close ratio gearboxes - though only on the ZR1's (the only way a buyer could spec. 456 in '70 was with the ZR1 package). Here's a detail photo of the adapter mounted on the M22 from my car.

The photo shown earlier in this thread shows a square body assembly. I've seen many of that type. I've never seen another round housing body style like the photo below. Were the round and square housing units used interchangeably?

[Roger O.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Roger,
If you need an adapter, try PATC 1.888.877.1008 or

http://www.partshp.com/speedometer_gears.htm

If your friends speedo is slow or fast they can provide an adapter that will make the correction either way.

Ralph

Thanks Ralph,they have everything speedo related !
Roger

[Terry L.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Joe,
Here are the the trans numbers on my 70 LT1, P0C16B, the judges manual states this trans is a M21 close ratio. does this trans with a 4:11:1 rear diff. require the speedo adaptor GM part # 6477965 with a 21 tooth driven gear?

All this info is great but I want to make sure I am looking at this correctly

Thanks,

Terry



[quote=Joe Lucia (12484);495183]Wayne------

According to GM, for 1970 all M-20 Muncies were equipped with the 3924098 DRIVE gears. This DRIVE gear is used with 18-22 (silver) tooth DRIVEN gears. All M-21 Muncies were equipped with the GM #3924097 DRIVE gear. This DRIVE gear is used in conjunction with 22(green)-25 tooth DRIVEN gears.

If the above was actually how it was, then the following applied:

L46 with M-20 and 3.55:1 required adapter GM #6477965 (0.833:1). I expect that very few of these were ever produced;

LT1 with M-20 and 3.55:1 or 4.11:1 required adapter GM #6477965;

LS5 with M-21 and 3.08:1 or 3.36:1 required adapter GM #6477966. I believe this is an overdrive adapter but, at the moment, I can't find the ratio, although it was likely about the 1.22:1 ratio you mentioned.

The GM #6477964, 6477965, and 6477966 were all 90 degree adapters manufactured by Stewart-Warner. GM #6477964 was discontinued in October, 1976 and replaced by GM #1565202. It is now also discontinued.

GM #6477965 was discontinued in March, 1976 and replaced by GM #1565208. It is also now discontinued.

GM #6477966 was discontinued without supercession in July, 1980.

I don't know of any adapter used by GM on a Corvette with a 1:1 ratio.

By the way, for 1971 and later ALL Muncies were equipped with a DRIVE gear compatible with only the 18-22 (silver) tooth DRIVEN gears. So, any rear gear ratio which could not achieve speedometer calibration with the 22 tooth silver gear would require an adapter. However, very few were made with such numerically high gear ratios, so I expect that not too many adapters were used.[/quote

[Dick W.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Great info Wayne. Your data is correct. Here's a sample of the "stump puller" variation you are talking about.

These adapters were used in 1970 on 4.56:1 cars with close ratio gearboxes - though only on the ZR1's (the only way a buyer could spec. 456 in '70 was with the ZR1 package). Here's a detail photo of the adapter mounted on the M22 from my car.

The photo shown earlier in this thread shows a square body assembly. I've seen many of that type. I've never seen another round housing body style like the photo below. Were the round and square housing units used interchangeably?

Dan, the square body adapters are definitely vintage Stewart Warner. The radiused corner bodies, I have seen, but cannot remember who made them, and if SW did, what vintage they are. Being involved with trucks for the first 35 or so years of my career, I saw a lot of those adapters. Used to have a kit to make any adapter ratio that you needed.

[Joe L.]

Re: What speedo adapter does a 70 LT1 with 4:11 use ?

Joe,
Here are the the trans numbers on my 70 LT1, P0C16B, the judges manual states this trans is a M21 close ratio. does this trans with a 4:11:1 rear diff. require the speedo adaptor GM part # 6477965 with a 21 tooth driven gear?

All this info is great but I want to make sure I am looking at this correctly

Thanks,

Terry

Terry------


With an M-21 and 4.11:1 rear gear ratio, you should not require an adapter. There should be an ORANGE, 25 tooth DRIVEN gear installed in the transmission and that should suffice. If you have any other gear installed, advise for further information.

As mentioned previously, an accurate speedometer calibration also depends upon tires of or very near the original OD.