The Stovebolt Site Forums The Shop Area Driveline Factory speedometer adapter

There have been a few posts recently about speedo adapters, so I thought that it might be helpful to dig the factory original out of my parts bin and post some pictures. To the best of my knowledge, this adapter came from the factory on my 1957 3600 3/4 ton with a 4 speed SM420, Eaton H052 4.56 rear end and 17.5" rims. I replaced it with one from Jim Carter when I switched to a 4.10 rear end and 19.5" rims.

The adapter is stamped--

AC
1563901 (must be the part number)
18-14 (ratio)

I was surprised the truck had an adapter. I assume the reason that it came with an adapter is that the factory had 'standard' SM420's being installed on the assembly line with one certain speedo gear combination and installed an adapter if a particular truck required something other than the standard. (This is just an assumption...not sure of the real reason.)

The speedometer was very accurate according to my GPS with the adapter. Doing math with and without prior to changing the rims and/or rear end, the adapter increased the speedo reading by 28%. I assume that is what the 18-14 stamp is for on the adapter...for every 14 revolutions on the input side, there are 18 revolutions on the output side...and increase in 28.6%.

The factory adapter feels much more heavy duty than the new one that I got from Jim Carter. I assume that it has metal gears in it. It has an oil cup for lubrication.

Anyhow, thought that some might find it interesting to see a picture of an adapter from the factory. Here is a link with pix--

http://s844.photobucket.com/user/ma...0856_zpssebb9wxp.jpg.html?sort=6&o=0

Matt

Very interesting. Which end of the cable does it attach to?

It screws into the tranny at the cable connection, then the cable screws into the adapter.

Matt

Here's a couple link with more into on your adapter


http://www.gmpartswiki.com/getpage?pageid=61813

http://www.ebay.com/itm/NEW-Genuine...71315702?hash=item541d72ccf6&vxp=mtr

Craig, this additional info is very interesting.

Given how well built the factory unit is compared to my new adapter, I wish I would have had this info back when I ordered my custom adapter so that I could have tried to match ratios to part numbers and then gone on a hunt to find an AC unit that was close enough. I might not have been successful, but I sure would have tried!

If the ebay link is indicative of the market price for a NOS one, I'd have rathered paid the $50 for NOS instead of $80 for aftermarket.

Plus there would have been the thrill of the hunt that this hobby gives us whenever we need hard-to-find parts!

Thanks,

Matt