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Index > Trucks or Parts WANTED > Thread: wtb: speedometer drive unit from 74 4-speed
Thread: wtb: speedometer drive unit from 74 4-speed
inboost


Newbie
Posts: 10
posted October 04, 2010 08:39 PM
Edited By: inboost on 4 Oct 2010 20:41

WTB: Speedometer drive unit from 74 4-Speed

Hi All, just got my first REPU. It's got a speedometer problem, the driven gear for the speedo in the tailshaft housing is broken. The transmission is an '84 GSL-SE 5-speed box with a 75 RX-4 tailshaft housing. The speedometer drive unit was RX-4 with an RX-7 driven (green, 17 tooth) gear in it. It managed to snap the gear at the neck where the retaining pin held it. I think there may be an alignment issue and would like to have a look at a proper REPU part from a '74 4-speed box. I have a brand new white 21 tooth driven gear for '74 REPU (p/n 1011 17 441)but it's shaft is larger than the RX-7 and RX-4 version so it won't slide into the driven unit housing that I have from the RX-4.

Thanks for any help!

       
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Brad


Rotorhead
Posts: 1672
posted October 06, 2010 12:50 PM

You might be able to drill the shaft out of a 1st gen one. There's not much going on in there. The roll pin would need to still retain the shaft though.

Don't have a spare repu speedo gear housing but I'm sure someone here does.
____________
-brad-
74 REPU Lawn Green
81 Rx-7 racecar. 12a J-
Bridge

       
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inboost


Newbie
Posts: 10
posted October 06, 2010 04:42 PM

Thanks for the reply Brad. I'm likely going to bore / hone for the larger shaft but I'm unsure which housing should be the donor. I've found the RX-4 drive unit I have in the tailshaft housing now to be about 0.080" off center line of the body of the sender unit in the direction away from the drive gear on the output shaft. I seemed to be on the vertical centerline which makes sense (the worm gear in the trans is wide, no need to make a precision fore/aft alignment, just mesh is critical). From what I have found it seems the RX-4 may have used a smaller gear such that this 0.080" offset may not be suitable for the larger diameter RX-7 and REPU gears.

From Felix's website:
http://fm.no-ip.com/sgpartno.txt

RX-4 has a 19 tooth red gear in 'small' diameter
RX-7 has a 17 tooth green gear in 'large' diameter
REPU has a 21 tooth white gear in 'large' diameter

So I believe I either need a REPU housing or an RX-7 housing bored out to accept the REPU gear.

Is your transmission still out such that you could make some observations for me?

I'd like to know:

The main body diameter is 1.102" (28mm) on the unit I have, is the REPU unit the same?

I'd like to know the clocking of the mounting tab on the REPU sender unit(the RX-4 tailshaft housing has it's mounting bolt hole at 3 o' clock from driver's side view (tab towards rear of truck).

I've also observed about 0.83" from center of the mounting bolt hole to the center line of the sender body. Is this about right for REPU?

If you could estimate any eccentricity in the vertical direction that would be even more helpful. I would expect it to be greater than 0.080" given the data from Felix's website.

       
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inboost


Newbie
Posts: 10
posted October 06, 2010 09:47 PM
Edited By: inboost on 6 Oct 2010 21:48

Some photos for reference:

Here's the alleged RX-4 sender, looking at the part fische I'm guessing it's not.


Here's where it plugs in

       
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Klaus42


Rotorhead
Posts: 1877
posted October 07, 2010 09:52 AM
Edited By: Klaus42 on 7 Oct 2010 09:56

I'd be surprised if anything other than the gear itself is any different for REPU vs. rx-4, especially for 1974, and most likely 1975...(?)

(BTW Simon, you might want to start over with an earlier 1st-gen rx-7 5-speed, since the GSL-SE units were not known to be as durable.)

       
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inboost


Newbie
Posts: 10
posted October 30, 2010 09:35 PM

Just a few closing notes on this project:

I believe that the driven gear housing pictured and in use in my truck is actually NOT an RX-4 piece but rather something from a 12A RX-7.

I built a test piece that allowed me to load the new REPU gear on-center with the housing bore. When installed it felt like the mesh was a bit tight. So I decided to re-bore the housing I had from one of the RX-7 transmissions with the offset center-line. I ended up killing the seal trying to remove it but found one at my local dealership to replace it in stock! Once loaded into the trans, the gear rotated freely (which became a bit tight when the new seal went in). I then removed it again and marked all the teeth with a sharpie to watch for wear. I then re-installed, connected up the cable, and ran it on the jack stands for a minute. I should mention that previous to this I pulled the speedo and cable to give them a through cleaning and applied fresh lubrication. All systems were go and I got proper speedo and odometer function. I checked it at 50 miles and then 200 miles as well. The gear shows no wear and the sharpie marking was hardly even disturbed. I'm calling this one done. The speedo and odo record about 6% faster / farther than what my GPS shows. This ends up with a displayed 70MPH when I'm traveling at 65MPH. Not too bad, especially since the truck doesn't have original rims & tires on it (205/65/R15).

       
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Brad


Rotorhead
Posts: 1672
posted November 01, 2010 01:02 PM

Sorry I totally forgot about this thread. Glad it worked out. If you can find a 79-82 rear wheel drive Mazda 626, it has a 22 tooth speedo drive gear that will slow down your speedo about 5%. It is red in color and the larger diameter, coarse tooth like what you've been working with and would be plug and play into a 1st gen rx7 speedo gear housing

       
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