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Miata input shaft cut

Old 06-22-04, 05:54 PM
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Miata input shaft cut

OK, How much do I have to cut off of the miata input shaft to get it to slide into my rx 7, yes the center section is the same and bolts up to bell and shift housing.
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Old 06-22-04, 09:25 PM
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you can use an s5 rx7 input shaft too
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Old 06-22-04, 11:02 PM
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Do I have to cut it, someone told me it is to long comming out of the miata and needs to be cut off to fit properly.
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Old 06-23-04, 03:46 AM
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I have a couple of each in the garage. If no one posts by tonight I'll measure them for you. You can not use the RX-7 input shaft as the input ratio is different.
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Old 06-26-04, 11:07 AM
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Originally posted by GT3_racer
Do I have to cut it, someone told me it is to long comming out of the miata and needs to be cut off to fit properly.
we had a guy who had us put the fc gears into the miata case, and we had to put the miata input shaft on cause the fc was too short. it fit like it was made to go in there
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Old 06-26-04, 01:22 PM
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You will need to shorten the pilot bearing area and reduce it's diameter. Then you need to cut back the spline some. To do it right it should be ground and then rehardened. I have a shop that does it for me but it is expesive, around $200 each.
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Old 06-26-04, 04:29 PM
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My initial post was based on this chart.
http://members.ij.net/mrmazda/tranchrt.txt

After counting teeth on the various Miata and RX-7 trans I have in the garage I see that they both have 21 teeth on the input shaft. It should then be like j9 says a drop in swap of input shafts.
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Old 06-27-04, 10:34 AM
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"After counting teeth on the various Miata and RX-7 trans I have in the garage I see that they both have 21 teeth on the input shaft. It should then be like j9 says a drop in swap of input shafts."

The number of teeth on the splines are the same but are the forth gears the same?
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Old 06-27-04, 12:48 PM
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The "4th gear" isn't really a gear. When you engage 4th gear the input shaft locks to the output shaft creating the 1:1 drive ratio. This effectively bypasses the counter shaft which is just along for the ride at that point. Assuming everything else is the same (case center line, tooth pitch, etc) the tooth count on the counter shaft that meshes the input shaft would have to be the same between the two. I'll count teeth tomorrow to verify this.

Try to better explain 4th gear? The input shaft drives the counter shaft which in turn drives the output shaft for every gear except 4th. The counter shaft is actually underdriven off the input shaft. The input shaft in this case has 21 teeth and the gear that meshes the input shaft on the counter shaft has something like 37 teeth. You look at all the other gears and how the trans transmits power through the box and all that's left is that gear pair and you say that has to be the 4th gear set. But it's not. Like I said before when 4th is engaged the input shaft is locked to the output shaft directly driving it. The counter shaft is still spinning with the input shaft but the rest of the gear sets are free to spin on their needle bearings at different speeds than the output shaft. The only thing the "4th gear set" does is transmit power from the input to the counter shaft for every gear except 4th gear.

The only question that would remain at this point would be whether the RX-7 input shaft matches the gear set of the Miata box. Since the Miata box uses different ratios for 1-3 you need to use the Miata counter shaft. If j9 says it fits and the tooth count is the same I'd say it fits. Someone just call Pineapple and ask Rob what's involved. He advertises the swap on his site.
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Old 06-28-04, 05:36 PM
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I've done the swap also, complete Miata set in the RX-7 box. I just had the input shaft machined down to RX size because I assumed that they were different counter shaft drive ratios. Geez I'd hate to think I did the extra work for nothing...
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Old 06-29-04, 12:30 AM
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Well, I need to revise my earlier statement. I counted teeth again today and the chart I gave the link for is indeed correct. The tooth count for the two shafts are different.

That being said I'm going to sit the RX-7 input shaft in an RX-7 case tomorrow with the rest of a Miata gearset and see if j9 is correct. It looks like it will "work". But working and being correct and lasting are two different things all together. I'll keep everyone posted.
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Old 06-30-04, 12:28 AM
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All right. I made some measurements and tried to assemble a hybrid box today using an RX-7 input shaft and a Miata gearset.

The initial problem is that the diameter of the input gear of the RX-7 shaft is about .100" larger than that of the Miata shaft. I did not measure the diameter of the gear on the counter shaft because I do not have a mic that large. I figured I'd try to put everything together, see what fit and how it all mated. I measured 3 RX-7 shafts and 2 Miata shafts and all the numbers were spot on among the lot.

The tooth count is different between the two input shafts and couter shafts but the pitch appears to be the same because once they are together they mesh very well and the assembly initially rotated very freely.

I assembled the whole front side of the box (1st-4th gears) and everything looked great. The problem occurred after reverse gear was in place and I began to add the OD gear. The OD gears would not go on. There is enough of a side load on the shafts from the diameter difference of the input gears that the clearance is ruined on the backside of the tranny.

At this point I became frustrated from being so close and left the project sit. I will go back at it tomorrow and measure the input gear on the counter shaft and the various OD gears I have available to see exactly where the problem is coming from. At this point I have to say it's the larger diameter of the RX-7 input shaft.
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Old 07-01-04, 09:45 PM
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http://myweb.cableone.net/currie/tom/rx7/trans.html

Some info and pics. The guy (mouse) that is writing this is also selling them already built. Look for his add in the parts section.
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Old 07-02-04, 04:18 PM
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Thanks for your help guy's

Thanks so much for your help, found someone in the local area that did the shaft grind and reassemble for me. Same guy that is doing a bridgeport 12A for me, thanks again!
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