speedometer turtle

Walt....it refers to the number of gear teeth on the little driven gear inside the turtle. The speedo drive gear (on the driveshaft just below where the turtle sits) will only have 7 teeth. Ford used the same 7 tooth drive gear (B-17285) for all Ford passenger cars thru 1948, Merc cars from 39-40, and pickup trucks thru 1941. For 1941-42 Mercs they show an 8 tooth gear. The 1942 and later pickup trucks had the open drive trans so they have the 21C drive gears which are altogether different.

The 19 tooth driven gear was most common since it was for the 3.78:1 rear end ratio. The driven gears were available in a small handful of tooth counts, depending on rear end ratio and tire size. The 21 tooth driven gear was used with the 4.11:1 rear end. These charts are in the back of the green bible.
 
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Walt, Good thing about this site is when you post something wrong like I did someone like Mac corrects it or gives the right info. Sorry about my mis-information . Kerk
 
Kerk,

It's funny but I didnt really start out to do any correcting. I was simply going to add part numbers to the discussion, but in the process I saw that the 19T gear was the correct one for the 3.78 rear end. Believe me.....I've been wrong before too! :)
 
How interchangeable are these gears in the turtle? For example can you install an 18 tooth gear in a 21 tooth housing and if so how do you get them apart?
 
I don't know this for a fact, but reason leads me to believe there must be slight height differences in the turtle housings, to allow gears of different size to mesh properly with the one size fits all drive shaft gear.
 
Russ/40 said:
I don't know this for a fact, but reason leads me to believe there must be slight height differences in the turtle housings, to allow gears of different size to mesh properly with the one size fits all drive shaft gear.
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You are correct; they are all different heights HOWEVER an 18-T gear can be used in a #19 (9-34=3.78) turtle and the gears will mesh. (Credit Dan Krehbiel for this revelation) The 21 is too tall, probably a 20 (uncommon) is too tall?

Someone posted on the Barn quite awhile back that he had set up and machined the 21's to shorten them so they would essentially be an 18. Maybe he will see this and post...
 
Ford parts books have the tooth count chart for all the turtles giving correct ones for different ratios and tire sizes...in the 17,000 parts section. Corrections are given for a small number of tire variations.
 
Stroker Coupe said:
How interchangeable are these gears in the turtle? For example can you install an 18 tooth gear in a 21 tooth housing and if so how do you get them apart?
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You can install a 18 Tooth Gear in a #19 Turtle. Do not use a gasket when you instakll it, just use a little sealer of some sort. I do not think a 18 would work in a #21 Turtle, too tall. I just did two of them in the past month. I think Mac may have the different Turtles, I know they have the individual gears.

CW
 
These turtles and gears frequently turn up on Ebay, you just have to watch for them. They usually go fairly cheaply. Of course when you are needing one they never seem to show up!
 
Stroker Coupe said:
Does anyone know how to get them apart if you want to change gears?
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There is a this formed sheetmetal piece that is pressed in the threaded end of the turtle. The inside sleeve of this piece retains the shaft from comming out. You can sometimes, by using a knife blade, seperate the sheet metal retainer from the casting, where the threads are and work it out. If it is really stuck, you can drill a hole in and remove the welsh plug in the other end of the casting. You can now drive out the shaft and retainer with a pin punch or similar tool. The Plug is 7/16 and sometimes you may find them at a good hardware store or I think Mac has them.

It is no big deal once you see how the shaft is retained. The gear just slides on the shaft and is not pressed on.

I hope this helps,

CW
 
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