1993z32

Here's the story. I've done a ton of searching and haven't found anything like this.

6-Speed #1 - Miles unknown. I did have to remove the bellhousing to fix the reverse pivot after removing the "donotremove.jpg" bolt. Drove for 1000 miles with max of 230wtq with no issue, until it started getting the symptoms listed below. Tuned car @ 415whp/343wtq (all pulls in 5th), did a total of maybe five 4th STREET gear pulls, then while cruising, complete failure.

6-Speed #2 - 102k miles. Worked perfect for ~200 miles, no hard shifting, turned boost down to roughly 300whp/270wtq level to be nice to it. Maybe 5 pulls total, no wheelhop etc. Symptoms begin, to complete failure within 50 miles.

Symptoms -
1) Will not go into any gear without unreasonable persuasion. Normally a clutch fault but when in gear, clutch has at least 1-2" of travel off the floor until the car budges.
2) Trans in neutral, not touching the clutch - car tries to roll forward and sometimes even kills the engine from so much resistance in the trans. It acts as if I'm pushing really hard on the shifter and the friction of the synchro is pushing the car forwards.
3) Following along with #2, if I'm rolling 60mph, and put the trans in neutral + release the clutch, revs will come up to 2-3000 and car will slow down. Again like I'm pushing really hard on the shifter against a synchro.
4) Rear wheels fully lock up while car is in motion (luckily 15-20mph both times) even with clutch depressed. Usually followed by some sort of loud bang and the wheels free up again.
5) HERE'S THE KICKER. First 6-speed, when I got the wheel lock, trans goes back to normal and I drive it home like nothing happened.

When I drained trans #1, it was full of gold synchro dust.

The only part that remained the same was the neutral switch. Clutch is a brand new Clutchmasters twin disc, driveshaft is a 97' that's been in the car for years, shifter is a MR angle shifter, fluid was Redline in #1, Motorcraft in #2.

Please, for the love of cats, can anyone lend any advice to how I've wasted $1300 in 6-speeds in a month, one of them after complete sane driving that even the oldest, crappiest 5-speed would have no problem handling.

Let me know if you need any more details. I have a broken trans in the yard and one in the car. I'm going to attempt to disassemble one to see if I can find anything.

DaWaN

To me all the symptoms sound like the gears are not (dis)enaging properly.
Locking up the rear wheels would only really happen at low speeds if more than one gear gets engaged.
Only common part between these boxes is the MR shifter or the installation of that shifter, I would look into that direction.

sixshooter

iTrader: (12)

It sounds like you broke them. The only common denominator is the operator. Change that part and get back to me, lol.

So did number one have gears sheared off?

Rear wheels locking up is what happens when teeth come off of Gears and get wedged between other teeth.

thumpetto007

iTrader: (15)

Even cryo treated and coated gen2 six speeds have under a 300wtq rating.

Too bad you didnt just save the 1300 towards the trans you know you need. (Kmiata bmw kit)

bahurd

iTrader: (8)

There are 2 versions of the MR Roadster 6SP shifter depending on the year according to his web site.

MiataRoadster shifter kits MAZDA - MiataRoadster - High-performance service...and parts for Mazda Miata Roadsters

Zsanz

Weird how #1 broke while cruising.

similarish high torque failures seen here:
https://www.miataturbo.net/suspensio...d-46575/page7/

The hard engagement leads me to believe you may have jacked up the pivot ball(snapped) or fork due to increased pressure plate loading from the twin disc? Take a peek at the one you took out. else ref all aforementioned because if you broke the ball it wouldn't just go back to "normal".

1993z32

Or, multiple gears engaged at once. It's like there's a gear that's constantly trying to engage, and when the in/output shafts hit the perfect speed it pops into gear, despite already being in another gear.

Maybe true, but like I said the first one started showing symptoms while the car was in the low 200's, and the 2nd trans never even saw over 14-15psi for the 3 days it lasted. Would hate to spend $3k+ just because I can't figure out a problem.

1993z32

#2 same deal. Never even knew there was an issue until I was in a slow intersection and BANG. Split second halt then back to normal again.

Morello

Is returning to the stock shifter not an option?

Chiburbian

iTrader: (1)

Dumb question - are those two transmissions trash now, or is it possible to rebuild? I know that maybe right now the availability of $600 transmissions says that it's just cheaper to swap, but if you wanted to pay someone reasonable money (whatever a trans rebuild normally costs plus 20%) would fixing a trans blown out in this way be possible? I am just curious because at some point the market for 6-speeds might make this a worthwhile option.

eurojulien

iTrader: (1)

could it be crank walk? I don't know much about the phenomena or if it even happens to BPs.. but a guess for you.

1993z32

Sure, the MR unit is just worlds better and I've never heard of any problem with them. Waiting to see what the problem is, then might consider trying a stock shifter... although I really can't imagine how the shifter could cause this, especially when I was so nice to the 2nd trans.

Pulling one apart as we speak. May just be a single broken part, I hope to be able to report my findings later.

Ruled out; 100% transmission related issue.

eurojulien

iTrader: (1)

I could use a good 3rd gear synchro for my 6 speed if you end up with spare parts!

patsmx5

iTrader: (16)

I'm guessing, but will ask a couple questions.

Do you have the dowel pins installed? Did you have all the bolts for the bellhousing installed and torqued? Were the flywheel bolts cleaned/thread loc applied and were tight upon removal? Same for PP bolts? Is the flywheel balanced? Is the clutch/pressure plate balanced? Is the pilot bearing installed and not trashed? Is your crankshaft bent?

Misalignment or imbalance are my guesses as to what could possibly be causing two transmissions to quickly fail with similar and yet odd failure modes.

sixshooter

iTrader: (12)

I'm going to speculate that it's bent shift Forks and that at rest there is a misalignment of the shift Forks such that as the op pointed out two gears are able to be accessed simultaneously at times.

99Racer

Likely clutch is not the issue of you have had lockup. Don't assume this is normal at all.... It broke.

So,... There is LOTS of stuff on transmission failures at 300+ HP. I guess you need to search a little harder.

https://www.miataturbo.net/suspensio...es-dust-75506/
https://www.miataturbo.net/race-prep...riences-73618/

1) The Miata 5 speed is 1:1 in 4th gear so, its a good choice for dyno pulls. No transmission gears involved in the power path, NO power loss in transmission so more accurate HP reading. First Motions Shaft (Clutch shaft) and Third Motion Shaft (output to Drive Shaft) are coupled end to end
2) The Miata 6 speed is 1:1 in 5th gear (NOT 4th) so, 5th gear is a good choice for dyno pulls. Same reasoning as above for the 5 speed above.
3) The 4th gear on the 6 speed transmission has the gears located fairly far away from the bearings that support the counter shaft and the bearings that support the 3rd motion shaft. So, when in 4th gear there is more likelihood that the shafts are starting to push away from each other and 4th gear starts to fail. bbundy demonstrated this at 360 hp.

I suspect in your case, you have a mechanical failure like: Broken gear tooth jammed between a couple of gears to force a lock up. Failure due to axial movement causing the side of one gear jam against the side of the adjacent gear. bearing failure, ... I also suspect the primary failure is not a sychro but synchro ring (brass filings) is likely a side effect of something else more severe failing.

In the suggested threads, read the bbundy posts, I have a few also on transmission theory and a few suggestions too 99Racer.

LOTS of people looking at options: T5 is mentioned in several build threads (G-Force gear sets are available to make a better ratio set for racers). Others have tried Quaife with it seems about the same poor reliability at high HP.

1993z32

I want to lean this way as well, definitely something up with the shifting mechanisms.

Cool, but I've seen a million threads on stripping 4th gear. Skipped them because that's not my issue. Also - all dyno runs were performed in 5th gear. I did a few 4th gear street pulls on trans #1.

Update: disassembly complete. 100% of the gears are 100% in tact as I suspected, because I know I didn't do anything to strip a gear nor is that what the symptoms were leading me to believe is going on. Patsmx5 - All is torqued/loctited/pins in place. Clutch was not balanced, the rest of the motor was. Had a last minute clutch change that didn't allow for it to be balanced. Car does not have any drivetrain vibration that would lead me to believe something that large is out of balance. As far as the rest of the trans, according to the FSM and AllData all detents, springs, *****, etc were in place. Nothing stands out as even having any wear on it. All bearings are butter smooth. As far as the shift forks and other alignment devices, to my untrained eye everything LOOKS straight, nothing stands out as being bent, etc.

Current trans in the car - whilst sitting, the shifter does not freely move from 1/2 - 3/4 - 5/6 selector positions. (Aka just moving the shifter all the way left to right in the neutral position). This is true for the MR and stock shifter.

Yes I did it quick and dirty.

Savington

iTrader: (31)

How many miles on the clutch+trans combo?

Colipto

+ follow to stop breakage of my 6spd

thumpetto007

iTrader: (15)

Honestly man, you are wasting your time with this thread

It doesnt matter why they are breaking, wjether its something in the shift linkage, clutch, transmission... Becausr you are at a torque and power level that easily breaks the miata 6 speed.

Even if you fix the current issue, you WILL have a torque related failure with the next miata 6 speed, within a few months, if you actually drive the thing.

Get the k miata setup, it replaces the linkages too.