LocostUSA.com

I know this has been discussed, but I was wondering if anyone has a definitive swap list to get the tallest gears possible in therx-7 rear. I don't want to waste money on a crap shoot. I've got the 85 GSL with 3.91.

So far I've read that the 82-84 B2000 Gas MT and Diesel MT (front) will swap in, but cannot find anything locally or online but a bunch of 3.6:1 and 3.38:1 for between $35 and $75, ring and pinion only. Not sure if these will bolt up to the lsd.

Anyone have these specs for the mystery Mazda?

Year
Engine
MT/AT
2wd/4WD
front or rear (ring & pinion)

I gave up and got a Toyota setup...

I'll send you a few files of everything I've been able to find out so far. One file lists the year/gear ratio/teeth counts/ and a lot of commentary that I've pasted it from various sites/forums.

Basically a 1982 to 1984 Mazda B2000 with standard shift transmission has a 3.308:1 r/p that will fit into the 1st generation Rx7 rear ends.

I just bought a complete 1984 Mazda B2000 stick shift carrier for $130 delivered from a Fla junk yard upstate.


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10/22/07 I finally took my RX7 differential apart and the B2k gears WILL NOT FIT! The gas engined B2k has 7-1/2" gears not the 7" that the RX-7 uses. Sorry for the misinformation but I was quoting information I found on several sites on the web.
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They have another one and I know of another yard that has the same carrier.
http://www.fadra.org/
will get you to the start of the search for the carrier. "A-1-A auto parts" and "Langston's used auto parts" are the two yards I know of. Shipping will be expen$ive though. The thing weighs 60+ lbs. [The standard "I'm not affiliated.........." applies.]

The B2k carrier itself will not bolt up to the RX7 housing since the number of bolts that hold carrier to the housing is different between the RX7 and the B2K. Also the B2k carrier housing is larger in diameter. But the r/p gears are supposed to fit onto the RX7 carrier from every thing I've been able to determine. Other parts in the carrier will also fit, spiders, bearings etc. so you'll have "spare parts".

I haven't taken my RX7 carrier off the car yet. I'm still measuring the B2k one and putting off changing things since it's so hot down here. But I'll start soon. I'm making a 7" micrometer right now to set the pre-load. When I get through with my change over I'll add the info to my website.

Something I just found the other day that is very interesting. Some '94+ 1.8L Miatas have a rubber "dampener" flange that goes on the pinion. The dampener is used on the rear ends that have LSD. NO, not -that- LSD!!!

That r/p will also fit into the RX7 housing. Unfortunately the ratios are all numerically high and won't help us. BUT the r/p gears are the same as the RX7 gears, so that means the flange should also fit the B2K. I'm not sure of the u-joint bolt pattern though. Once I get my RX7 flange off I'll go over the the local Mazda dealer and see if it's something usable.

Right now my shifts can be pretty harsh with no cushioning in the drive train similar to what the motorcycles have. Hopefully the dampener can cure that.[/b]

Rowdy. WOW. Thanks for the great reply. Awesome news all around. I'll monitor your site for updates.

The rubber bushing sounds like it might be useful for BEC, so I'm surprised I've not read of one being used. I've seen porsche's and a couple old brit cars with them but not a miata, and many old cars use them in place of universal joints on the steering.

I can't wait to see if that rubber dampener will fit on the r/p OK. Somewhere on the web I found a site that listed the bolt patterns for the various Mazda u-joint flanges. Naturally I can't find it again to check the pattern of the Miata vs the RX7. Shucks!

I just don't think my drive train will stand up to the harshness of some shifts as it is now. My car has Heims all around so there is no cushioning anywhere in the drive train. I wonder if the dwarf race cars use a dampener anywhere? I haven't seen it mentioned on the sites I've visited.

I think if I had built the rear suspension I would have tried to use the RX7 links with the rubber bushings. As light as our cars are the rubber would probably act as if it was harder than when it was on the heavier RX7.

One affect of the Heims, is that under acceleration the back squats as you take off but then the front is lifted another ~2" as the torque builds. That takes a good bit of torque to lift the -front- of the car with the torque reaction of the rear end.

It's rather strange because it is squat, a change in revs to 7K, then lift. Of course the G's are still building so some of it could be more squat. But the chassis sure seems to rotate about the rear axle when it happens. I think with rubber in the rear, the axle would just wind up some more.

The weight transfer, lifting and the LSD also tends to push the car straight ahead if you are just taking off too fast in the middle of a street corner. A slight lift of the throttle gets you headed in the "right" direction again.

I have two 1994 miata's. Neither have LSD but both have the damper.

The rx7 has a very long pinion compared to miatas and trucks. Solomiata has info on his site regarding these swaps. It isn't fresh enough on my brain to comment with confidence.

The FC RX7 is the one with the long pinon. The stick axle RX7s have the same ring and pinion as the 1.8 Miatas.

IIRC, the rubber isolator was found on ALL 1.8 manual trans cars, LSD or open. The only diffs without it were on automatic cars, which were supposed to all be open diffs. That may have led to the rumor that it was on all LSDs.

Not trying to start a war here but....... here's a quote from Solomiata;
"The 94+ Miata uses the same length pinion as the 1st gen RX-7 (and B2000/2000 trucks/RWD626) and the front housing is unique to the Miata to allow a mount for the PPF."

My 1st gen RX7 rear end and the B2K carrier have the same length pinion by eyeballing them while still assembled.

modernbeat
Do you happen to know how much [if any] the rubber dampener moves the u-joint mounting in relation to a non-rubber flange? I presume it would normally make the drive shaft shorter.

Also, do you know if the 1st gen RX7 u-joint bolt holes in the pinion flange have the same dimensions as the '94 Miata?

I was thinking that it might be possible for me to adjust my Heim links to move the axle back 1/2" or so [if my slip joint can't handle the change] and use the rubber dampener flange. As if a 92" w/b isn't long enough.

Before I decided to go the locost route I thought seriously about doing a V8 swap into an RX-7. I've owned several RX-7's and love the chassis, but they were always a little underpowered. A V8 seemed like a great alternative to a turbo'd NOS'd wankel that would go 30k miles between complete rebuilds.

As you might imagine a V8 has a hard time keeping up with a 3.91 or higher rear-end.

One of the swap shops was looking seriously at having some custom rings made to get the ratio down into the high 2's.

I haven't looked at the website for an age, but you might look/send an email to Grant and see if they ever followed through on it.

www.grannysspeedshop.com

That's the site I was looking for with the RX7 u-joint bolt patterns. TNX

This is the RX7 info.
...Rear pinion flange bolt pattern- (4) .325 holes on 2.946" bolt circle (rectangular pattern). 1.772" dia male pilot.

If anyone has one of those '94+ 1.8L Miata rubber dampeners could you please measure the u-joint bolt pattern and let me know what it is.

The "1.772 inch dia male pilot" is a projection on the RX7 u-joint that fits into a similar machined circle in the pinon flange. Does the Miata flange have this?
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After examining the pictures posted on one of the Miata.net links posted below it appears that the dampener is basically a hunk of rubber vulcanized onto the normal pinion flange. I don't think that is going to absorb enough shock from a BEC shifting gears. I was hoping that the power flowed through the device so it would act more like the motorcycles shock absorber setup in the rear wheel.
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"Exhilaration is that feeling you get just after a great idea hits you,
and just before you realize what's wrong with it." —Rex Harrison

A couple of threads on M.net

http://forum.miata.net/vb/showthread.php?t=164718
http://forum.miata.net/vb/showthread.php?t=163665

A damper in the system is a good idea. I think it will take some of the lash out of the launch and the 1st-2nd gear change. But past that, shifting should be so smooth that you barely notice a change.

If you very slightly (and I mean slightly) lift off the throttle and simultaniously shift up to the next gear, the transition is seemless and very fast. The first few time you do this, you'll crash the box. But as you learn how little to lift, it becomes easier.

JeffU goes up and down the box without the clutch. I'm not that good!

--Chris

I actually use the clutch going down most of the time. There were a couple of autocross events where I went down w/o the clutch, but pretty much use it always now...until I get one of those Trickshifters with the auto-blip feature...

Here is a good paper on shifting technique for dog boxes (same as MC transmission)

http://www.hewland.com/svga/help.htm

I don't make a habit of it but I can down shift w/o the clutch if I'm going slow. But I'd rather not take the chance.

Part of my problem with noise etc is that I think my [cars] rear end has way to much back lash. I can rotate the pinion flange ~1/8" back and forth. If I put more pressure on turning the flange it will move another 1/8". I'm not sure what that is. But it doesn't seem right.

The B2000 carrier I bought has about 3/1000ths on an inch of backlash measured at the ring gear. I'll probably start swapping the gears over next Monday.

Right now my effective 6th gear ratio - engine to rear axle is 7.7 to 1. After the gear change it will still be 6.5 to 1.

If you are really worried about absorbing driveline shock you can make a custom driveshaft flange that incorporates a Guibo inline before the differential.

The Guibos used on Alfa axles are about as small as you would want to use and are most likely the easiest to purchase.