Help :/ TWO Broken 6-Speeds in 1000 miles (Weird Problem)
 

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.

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.

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.

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)

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

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".

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.

#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.

Is returning to the stock shifter not an option?

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.

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.

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.

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

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.

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.

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.

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, balls, 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.
https://preview.ibb.co/d0dZ4w/IMG_20171227_204101.jpg

How many miles on the clutch+trans combo?

+ follow to stop breakage of my 6spd :likecat:

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.

How did i know just by the title that craig jeters had eaten a second six speed?

I was pretty sure everybody in the world knew that you did Dyno pulls in 5th gear with a Miata 6-speed, fwiw. By the way, your horsepower and torque numbers being reported may be different than actual because you were using a lower gear on the dyno.

Saw this thread and wondered it is was our boi @ TCPC :( It was indeed.

I don't think it sounds like a clutch problem, but idk. Do you have a spare 5 spd to throw in? Maybe that will help identify the problem if you use the same clutch setup. As others said it could be the shifter, but the fact that the first failure occurred while during cruise kind of throws that out the window.

PM me if you need a 5 spd. I can send one out today, just pay shipping.

DYNO puls were all in 5th. A few street pulls in 4th on trans #1.
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1st trans unknown, but purchased from a personal friend and it was working fine when pulled. 2nd trans 102k. Clutch/flywheel 1400 miles.

I feel I am not wasting my time with this thread. As stated, the 2nd trans was not subjected to any driving or power level that should have caused it to fail. 270wtq max and a few 5th gear pulls, no hard shifting, mostly cruising. and 150-200 miles later complete failure? I see plenty of 6-speeds fail at the 300+whp level in road racing conditions, stripping gears. But that's not what's going on here. I also wanted to start this thread so if/when I do figure it out the information will be available for others, instead of just saying "yep u got 300hp u need $3000 drivtrane" when there are tons of cars running 300-350whp or higher on 6-speeds for long periods of time (especially street cars). Not saying I'm not pushing my luck, but I feel this level of failure is out of the norm for what they were subjected to.

There are two things in common between the two failures. Flywheel/clutch ane the MR shifter. From the symptoms, I agree we can rule the clutch/flywheel out and I know you checked the endplay on the crank (I watch the videos as they come out. Waiting for the junkyard to pull my VVT so I can dump it in my 90!).

This leaves the MR short shifter. The fact you are saying it is sticky moving between gears while sitting on the second one tells me something is up with the gear shifting mechanism. It takes 5min to swap in a stock shifter if you have one. Do that and report back so we can at least eleminate that as a variable.

I put the stock shifter into trans #2, although I figured it wouldn't do much. Even with no shifter in the turret, the car is pulling forward in neutral, so much resistance inside the trans the engine will nearly stall with clutch out + neutral. And still feels identical moving left to right in the N position.

My guess is it's related to your clutch/flywheel in some way. Spacing is off, balance is off, something is loading the input shaft in a way it shouldn't be. Is the sheet metal spacer thingy that goes between the engine and trans installed? If you have a stock clutch/stock flywheel, perhaps check a few dimensions to verify the new clutch is spacing the disk in the same spot on the input shaft as a stock clutch.

To clarify, do you have any mileage with this particular clutch on another transmission where it didn't fail, or was this clutch installed along with the 6sp during your 1.8L build?

My thought process:

• Two identical, uncommon failures means it's likely an external force acting on the gearbox. There are not many of those (pilot bearing, clutch, shifter, driveshaft, ?)
• Hi-power 6sp failure mode is a gear failure, you don't have a gear failure (verified by visual inspection) so we can set that aside for now
• Driveshaft is OEM, we can rule that out
• Hundreds of people using MR shifters in 6speeds, this is not a common failure, so we can rule that out
• Clutch is an uncommon twin-disc that I've actually never heard of before, so.... ???

This has to be a clutch issue and seems like the only reasonable explanation for the failures.

I'm surprised some people are acting like a 6 speed can't reliably handle ~300hp on a street driven miata. I've got probably 80k miles on high mileage NB1 6 speeds over 300hp and I've only killed one after a whole lot of abuse.

Just for educational purposes, what sorts of things could be wrong with the clutch that might cause this? I am just trying to think this out...

this picture gives me nightmares. :inout:

Even a seized throw out bearing could cause it to try to pull

I agree. It's either an issue with the clutch disc/flywheel combo, clutch fork flex, or weak hydraulics. I'd put a flywheel and clutch of known history in with the reassembled #2 transmission and see if that fixes the issue.

My thoughts exactly. Most documented failures at the 300whp level are after long periods of heavy street and/or track use.

A clutch dimension/design problem seems like the most likely issue at this point. I'm just trying to wrap my head around what component could put a force on the input shaft could cause this kind of behavior, at least that way I have some ground to stand on when I present the issue to the company I purchased it from.

With the clutch pedal out, it wouldn't matter if the TOB was seized. And it's not because I just had it all apart and check it.

Considering this or putting the 5-speed back in for now to see what happens.

These are both irrelevant while the clutch is out though.

There are documented failures of 250wtq track work killing 6 speeds.

Most of the "300-350whp street cars running for a long time" is not a good data point, because they dont actually drive with that setting. Ive never been in a community like this that knowingly builds for decent power, then turns it way down like a bitch.

You are running 350wtq, i fail to comprehend why you are trying to make a 6 speed that is basically designed to have its shafts pushed apart under 300wtq, work.

And we all fail to comprehend why we've already determined that it wasn't a power failure, yet you still keep posting about it.

T H A N K Y O U

You sure do.

This can be pure FUD, but there is another thing in common with the two failures. I assume it's the same mechanic?
Regarding alignment i have always wondered about my own ignorance about the alignment between the trans and the PPF, I just slap it together in whatever angle the engine/diff mounts make sit possible and torque it down,

I have no clue about if/why/how the prop shaft can make any effect on the possibility to get syncros mangled up och pressurized, just though i'd throw in another question mark to get straightened out.

Have you been able to find pictures on shift forks (bent and ok) to see if yours are truley ok (the pictures in the transmission manual only focus on wear and clearances, not straightness)?
Could it be the "shift rods" where the forks sit, making neutral pressing either forward or rearward (on one or all forks depending on what's being misaligned)?

The whole shifter assembly from a 6sp with stripped 4th gear might be the spare part you are looking for?

Maybe the MR shifter promotes shifting abuse in some way, I know I love mine. :)

.

I see in your video that your pilot bearing hole was too large, and that you used bearing retaining compound to hold it in.

A badly machined pilot bearing hole is pretty suspicious given that you're having weird resistance when the clutch is disengaged. It also looks like you installed it below flush, though that is probably less of an issue.

The hole was a couple thousandths off, and both CM and a couple others with experience in the field recommended the retaining compound. Again with the trans in neutral a seized pilot bearing or malfunctioning clutch should not be causing the car to creep forward. Although, I am leaning towards ditching the clutch entirely at this point. As Savington mentioned it's the most reasonable cause for the problem thus far, despite not having any idea why it could cause such a problem.

I've never once, ever, on any car, for any reason, have had to use retaining compound on a pilot bearing, nor have i ever been told to do so.

And i've done a clutch or dozen in my time.

I didn't know such a substance existed.

The tricky thing is that a single application of "unreasonable persuasion" can add a bent shift fork (and partial synchro engagement, and creep in neutral) to a different problem.

Believable. But not seeing how that could cause the problem. I realize we're all theorizing here, but the more most reasonable explanations are going to help me choose my next move. If spending $1000 on another clutch + flywheel setup plus another $600 on a 6-speed is the most reasonable fine, but at the point that that 6-speed dies in 200 miles I'd hate to have wasted $1700. And after further research the BMW trans swap is $4300... and that doesn't include the differential or the transmission. I'm happy at 300whp until I can afford stronger drivetrain, which I still think is a reasonable power level for a street driven 6-speed.

By "unreasonable" I'm still talking less force than shifting quickly during a normal pull. More like a pull towards 2nd gear then shooting into first. With the car rolling at 1mph it should take a pinky finger to get it into 1st or 2nd.

Well. You have two broken 6 speeds, so you need another.

And you have a pilot bearing that didn't fit right, so what you do there is probably up to you. :)

Good info that soothes my mind... Savington knows :likecat: - Hopefully removing that clutch will resolve your issues.

I think most of us know what its like to take your time fixing something and then breakage :vash: :vash2:

I vote crappy or incorrectly installed clutch and or flywheel

Greg, do you have a Patreon account? You have a bunch of viewers at this point and I'm sure enough would pitch in to help you get this resolved. Hell, I've learned enough from your videos and enjoy them to the point where I'd pitch in $20 to help you get a new clutch/flywheel and trans. Get something reliable from Trackspeed or 949 this time. Those guys know their stuff and I'm very curious to see this get resolved.

You don't have to do the diff. You can just do the transmission swap for under $3k all in.

And have ratios even shorter than the Miata 6-speed. If I'm spending thousands more dollars, I'm doing it proper.

They should be releasing the getrag 5 speed kit soon which would have very similar ratios.

Two main things stick out to me.
1. You mention that in neutral with clutch engaged, it will try to roll forward as though it is pushing against a synchro. This is obviously transmission.
2. When it did fail, the car still was locked when the clutch was pushed in. This could be anything from the flywheel to the hubs, but if you can clarify that the diff is good and that the engine was still running when you clutched it, we can narrow it down to the transmission.

On top of that, you opened up the trans and it looked 100% fine, and the only thing was synchro dust.

To me, though you have a widely used shifter, I figure maybe it's too tall from the pivot point to where it meets the little cup that actually changes the gears (don't know what it's called). And as a result, it relaxes in a position where it is trying to go into a gear.

The only other thing is likely the clutch and trans not mating properly and the input shaft wreaking havoc on the internals.

VERY strange.

Edit: I read back through your symptoms. It absolutely has to be the shifter or linkage. The only thing that doesn't line up is the fact that it locked up, but I guess two or three gears could somehow both be engaged by accident and lock up the transmission (from the linkage being screwy) and then pop out and return to normal.

Perhaps the power does just pull things apart or move them around and just mess up everything. Is there a way to access the gears and shafts but leave a part of the housing off? I bet even if you lock up the output shaft and use a torque wrench somehow to apply 300+ ft lbs of torque to the input shaft, you can listen or see if anything weird happens.

Edit 2: I figure you could even cut a hole in one of the transmission to look, lol.

Reading up (time zones...) makes me wonder. Would pressure applied to the input shaft (by to thick clutch assembly) have an effect?

I made the mistake once of not installing the pilot deep enough, which resulted in an engine not turning over, so a pressure hard enough to move the input shaft would probably have seen immediately.

Honestly ive seen people put the clutch disc in backwards and it locks the clutch setup so that its engaging all the time. They get the movement while in neutral. Not saying you did that but it just makes me think the clutch setup is suspect.

Hey Greg, I'm a fan.
More speculation here, maybe the overall engine vibrations at certain RPMs ranges are killing the transmission boxes?
You are running two clutch discs with no springs at all, although the one is fibrous, I doubt that is enough to dampen all the harmonics from the power waves.
So my take is that vibrations push the transmission internals to a state they shouldn't be, and bang.

Mount a sacrificial 5-speed to drive around and diagnose if the clutch itself has manufacturing/tolerance errors.
Until it dies, start collecting parts for a transmission swap! No no, not the BMW..

At your power level, a stronger transmission box is a must IMO.
There are known partial failures of the 6-speed at 390hp at the flywheel.
Although I believe you don't want to daily over 400whp power, it makes sense to have a better solution even at 300whp.

I would like to suggest an RX-7 box.
Turbo FC ribbed version for a notchy feeling or FD version for smoothness. I went with the FC version, but car isn't running yet so can't offer a first hand experience.
The turbo FC is what mattman runs, with a twin plate clutch. Hopefully your torque won't be able to kill it :P

Basically, I believe it's the most cost effective option for you.
In Greece it is a pretty common swap for high power miatas:

• You sacrifice a 5speed for a hybrid welded bell housing. Shifter remains where it originally was, and is compatible with the miata shifters, so you can retain your MR. You can use an adapter plate, but may end up being more expensive and pushes your shifter backwards.
• The transmission is plate mounted to the ppf or whatever other custom fabrication you prefer.
• You can use NC-6speed single clutch discs, or you get to keep your twin clutch and exchange the discs for 23teeth centers.
• The driveshaft needs to be cut and welded to an RX-7 spline. You may balance it DIY style to make sure it's smooth. You could buy a custom shaft.. but underneath the car is your natural habitat.
• You keep the miata starter/slave/bearings
• Turbo FC boxes go for around 150-200€, I expect them to be similarly priced there too.

My point is, for about the cost of another good 6-speed, you can have a known robust solution, as well as cheap enough replacements, should you end up destroying RX-7 transmissions which I doubt.

Yes. All is was well for hundreds of miles prior to any issues. Also verified last night that pilot bearing depth is correct, the grease mark on the input shaft shows the bearing sitting right at the end of the shaft.

Verified both discs faced correctly. Also, neutral movement has been pinned on the trans itself, a soon as it exhibited symptoms in neutral/clutch out.

Update: Trans #2 is out. Inspected clutch/flywheel, everything looks fine, including the pilot bearing being in place and butter smooth. Trans itself is definitely jacked up. The only way I can see the clutch causing the problem is if the pilot bearing hole isn't perfectly centered which would cause vibration in the input shaft that I couldn't feel, and then somehow spawning unicorns inside the trans to rearrange the shift forks. I suppose at this point I'm going to try a different clutch and flywheel, and put my 5-speed back in, and none of the mysteries will be solved. Unless trans #1 miraculously seems fine after I put it back together, I suppose I'll try to run that.