.756 5th Gear Tranny

[mbuskuhl]

I'm looking to get a spare tranny and have it rebuilt since the one I last had rebuilt seems to be going out again after 2 weekends. Probably should have a fresh one on hand at all times.

I hear the .756 5th gear tranny is the one to have, which according to Mazdatrix came in the 89-92 Non Turbo w/LSD RX7's. I do have the 5.12 rear.

There's a local guy who's got a few trannys but he's not sure which exact trim package and year they came from since they are on a shelf. He parts out RX7's and will hook me up for $75 so I'm not too concerned if it's a turd after it's opened up and have to scrap it.

Question: Is there anyway to tell if a tranny (w/o tearing it apart) has the .756 5th gear when you don't know the donor car?

Thanks!

[dickita15]

Put a mark (sharpie) on the input and output shaft. Put the tranny in fifth. One full turn of the input will give ¾ of a turn in the output shaft. Any less than that and it is the wrong ratio.

[seckerich]

Very little chance of finding a tranny with the .76 gear. Only the GTUs and a few oddballs ever came with it. Call Mazdaspeed and have the gear put in when you rebuild the trans. Not very expensive and you know the condition.

[dickita15]

Put a mark (sharpie) on the input and output shaft. Put the tranny in fifth. One full turn of the input will give ¾ of a turn in the output shaft. Any less than that and it is the wrong ratio.
[/b]
oops I got that backwards. 3/4 turn of the input shaft is one turn of the output shaft.

[C. Ludwig]

The other ratios are .711 and .697. If your confident you can tell the difference between .756 and .697 of a revolution with the amount of backlash in the teeth then count the turns. Otherwise it's a crapshoot. Basically, you're never going to find that gear in a boneyard by chance.

[its66]

Counting turns will work, but you have to make more than one turn. 7.5 turns=10 turns(approximately)...There is too much slop to do it off of only one turn.

Like Steve and Chris said though, it is highly unlikely that you will find one in a junkyard. There weren't many of them made. That said, with the price of hubs and input shafts(if you can still get them), I would get a couple of those $75 transmissions for spares or parts. It is a good deal.

[mbuskuhl]

Thanks everyone. No mystery codings on them, too bad.

Good idea on 10 turns Jim, I'll have him do that. That should be pretty easy, put a line on each input and output shaft on the top. In 5th gear, turn the output 10 revolutions, if the input line is still on the top, it's a .711 or .697. If the input line is on the bottom, it's the .756 gear.

Looking at the specs and features sheets for the 2nd Gen (originals from RX7club.com) I was not able to even find a single year/trim pkg that came with the .756. Even the GTU said .697. I'm not sure where the Mazdatrix data came from, I guess these are a lot more rare than I thought and may just have to get the gear from Mazdacomp.

I'll probably pick up two spares at that price, seems that we have a bigger supply of parts than those of you in the Southeast.

[Eagle7]

... and may just have to get the gear from Mazdacomp. [/b]

Just to be clear, I'm pretty sure it's two gears - overdrive (5th) and it's countershaft gear.

[Scooter]

What you might be missing is that that gear only came on one model: The GTUs. Which was really rare, which is why you'll almost certainly not find it in a junkyard. If you're looking to pull it off of a car though, there is writing on the fender that says GTUs on it.

I bought my 5th gear from Mazda Motorsports and had the tranny shop throw it in there. It's simple and reasonably cheap as well.

[C. Ludwig]

Counting turns will work, but you have to make more than one turn. 7.5 turns=10 turns(approximately)...There is too much slop to do it off of only one turn.[/b]


That's pretty darn sharp right there. :)






Just to be clear, I'm pretty sure it's two gears - overdrive (5th) and it's countershaft gear.
[/b]


It is. You need both gears. I'd need to go back and count teeth but from what I remember the .756 RX-7 gear is the same gearset as one of the Miata OD gears. The Miata ends up with a different output ratio because the input ratio is different. Seems like the same set used in the Miata box ends up being taller. But the point is that if you can find the right Miata trans there are probably inexpensive gearsets to be had.

[sirchadd]

Hook a poor racer up, I have no spare tranny. I will give you 75 bucks for a tranny. Would be nice to have at least one.

Mark these RX-7 transmissions are definitly the weak link in them, especially if you are loading up third gear and second gear on your downshifts maybe. I trashed my first transmission in my 88 model, rebuilt it myself and changed my driving habit. I have 8 races on mine, works great still.

[tbtapper]

I'm not sure why folks keep saying that the transmission is a "weak" link in the RX7's.

I've raced my ITS car for going on 5 years now on the same transmission.

Still works great, no synchro problems or issues.

Naturally now it will break at the next outing but the transmission and clutch, actually all the running gear has been flawless. That's why I race a Mazda.

Ricky once showed a bunch of us a video of him in his IT7 at VIR and he power shifted every shift. Guess that's where he gets some of his lap times from but under those operating conditions I could see the tranny getting tired really fast.

I don't slam the shifts but use a little more "finesse" (not implying ANYTHING negative toward Ricky just a 11/10th perspective vs a 9/10's perspective).

Personally, I run Redline Shock Proof and can only extrapolate from my experience that in a racing setting the Redline lube provides that little measure of shock resistance that has been effective for me.

Travers
ITS 19

[sirchadd]

Yeah, I have no tranny problems either, but finese is the key. I two finger my shifts and neer slam it, but I see a lot of people rough with them and they are replacing their gears often. I can't say how many times I see someone go out of the race cause they lost second or third. Maybe we have been driving ours for so long we just know how they want to be treated.

I ran redline, it worked good, switched to mobil 1, have had less trouble with it. Tried Royal purple, not pleased with the gear lube but the motor oil has been excellent. Everyone say run valvoline, but I have Royal purple proiding me with oil so I run it, and I'm pleased.