Im gunna go out on a limb here...

1 you used a clutch that isnt recommended or built for a miata. (Told you so on youtube mannnn) measure the thickness and conpare it to stock clutch, maybe you need a bellhousing shim or something. (Not sure what is necessary in this case, other than buy a quality clutch that everyone knows works without issues from 949 racing)

2. You tried forcing the shift lever, at least one time, into gear, and bent a shift selector, linkage...

Not saying the clutch isn't the problem, but for arguments sake if I were to space the bellhousing off the engine, the input shaft would not go all the way into the pilot bearing. I already verified that the pilot bearing does sit in the exact factory location as far as fore/aft position.

I'm leaning towards buying a 949 setup and starting fresh.

I have Y8S's twindisc setup if you want to save some dollars and give me some shelf space back. I got a smaller Quartermaster/Prather setup i'm going to use, and binned my turbo project so i don't need anything as strong as the Tilton.

I guess when the trans is busted and you're not putting it back together, there's no reason to disassemble it in an organized fashion! :rofl:

If you're talking about the Quartermaster 5.5" twin disk, the strength comment is funny. They're rated to 600 ft/lbs.

That's the setup I run. Great for the track, sucks huge balls at everything else.

I'm super excited then.

Don't really care. It's lighter than the Tilton setup, and ease of driveability is less important to me in an n/a car than a turbo car.

What's the health of your clutch hydraulics like? I'm curious whether the slave cyl might have shit out, or can't cope with your clutch. Just trying to eliminate the obvious here.

Update: 949 Twin Disc purchased, currently hunting for 6-speed number 3 :burncash:

Hey greg,

first of all: i love your videos and watch every episode.

To the 6-speeds and clutches: I remember a guy here with a nearly as strong setup as you, who came from a FM2 clutch (full face sprung) to a 949 Twin Disc clutch
and he broke his gearbox in 2 weeks. He said, he now only uses sprung clutches for keeping the gearbox alive.
Not saying, that the 949 clutch isnt any good, it sure is, but maybe those light weight twin discs tend to brake gearboxes just a bit earlier...

As you said, we all know that we are pushing our luck with that kind of powerlevels and the stock miata components, but two broken ones in two weeks is just too much.

I really hope it's a clutch related problem :)

Might be way too far for you but I have a 6 spd fs in San Jose.

Greg, you contribute a lot of awesome material for the Miata community. Some personal thought's based on your problem description:

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. -I think that this is the first/best thing to keep in mind when I read through the problem description. When the trans was say even on the ground when you first got them were they fairly easy to move into gear?
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. -Sounds like internal components are bound.
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. -Same as above
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. -Teeth getting stuck and ejected?
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. -Stuck components broke free potentially
When I drained trans #1, it was full of gold synchro dust. -Quite possible if shifting without the clutch functioning 100% and it's trying to make it in gear with resistance of a non fully depressed clutch.

My theory is that the transmissions are having true mechanical failures, synchro dust & loud bangs are never good. Some of the symptoms could happen with differential failure however the one a few that stands out is the car moving forward in neutral. I would measure the new clutch versus the old and see if there are any differences before you lose the chance to once you install the 949 Unit.

Best of luck.

Foooor sure. I have one sitting on my workbench that I don't know if I want to install because the Exocet occasionally sees stop and go traffic.

I would leave it right where it is until you have a trailered track car to put it in. Stop and go traffic will kill it awfully fast.

I have completed disassembly down to the core of the failure. See pics below and as well as my theory at the bottom as to what I think could have caused it to happen.

First, let's have a look at the countershaft. This houses the selector for 3rd and 4th gear only. Anatomy outlined below.

https://preview.ibb.co/dNRYKw/IMG_20171231_175242.jpg

Below is also the countershaft but with the gear selector sleeve removed so you can see the hub. Important note: there's supposed to be a specific distance between purple and yellow, so that 4th gear is free floating.
https://preview.ibb.co/cG54Rb/IMG_20180101_124016.jpg
This appears to be the failure. The synchro has slid off 4th gear slightly, causing it to bind with the hub, and therefor the countershaft, causing the car to pull forward in neutral.
https://preview.ibb.co/g61NCG/IMG_20180101_123416.jpg
This is what it looks like after being tapped into place:
https://preview.ibb.co/gEWdmb/IMG_20180101_123709.jpg
Unfortunately once the ring had been dislodged from its home, the damage occurs quickly, note the heat ring from the friction as well as the lip that would hold the synchro ring on the gear:
https://preview.ibb.co/dExJmb/IMG_20171231_175419.jpg
As well as the 4th gear side of the hub with lovely gouged out surfaces:
https://preview.ibb.co/bC2UsG/IMG_20171231_175448.jpg

So, with my new in-depth understanding of what's held in place with clips and what's actually able to move around inside our 6-speed boxes, I present the ONLY theory that makes any sense to me:

Edited out nonsense.

Edit - plausible scenario: The selector sleeve has a death grip holding the toothed synchro to the hub (while in 4th). The torque (or a hard shift, especially without the damping of a spring hub clutch...) then pulls 4th gear off the synchro (the steel ring, not bronze) due to the direction of the helical cut. This theory also coincides with the 4th gear mis-alignment failures I've heard of. I guess long story short, if you're pushing mad JDM power through your 6-speed, 4th gear is the one to take it easy on.

In the first picture there, what's the direction of rotation of the countershaft while the car is moving forward, and which end is towards the engine? If it's moving the way I think it is, big power is putting a huge load on the gear in the direction of the shaft itself, it's a downfall of helical gear sets and it may just be tearing itself apart.

My initial thought is that the shift fork is more prone to bending before you could actually apply enough force to separate the gear and the syncro from each other like that.

Probably should get a track car first...

+1 and +1.
Re: the helical gears - once Savington explained that the 6-speed has an issue with misaligning 4th gear under power it all started to make sense after really digging into the trans and finding the exact cause of the damage. Although I wasn't super hard on the 2nd trans, I did do a few pulls, albeit at lower boost, and maybe I just got real unlucky. And I did verify that under load, 4th gear does want to pull away from the hub/synchro.

Re: the shift fork - Since the shift fork would actually be pushing the ring back into place, there's no way it could be dislodging it on a hard shift. However, I don't think the forks would be strong enough to slide 4th gear on it's splines anyways.

Eyyyyyyyyy what do you know.... A torque related failure....

So another vote for lowering the final gear and only have full torque in 5th (lowering the final gear to spend more time in 5th than before).
Kia 4.77 and 6sp is a good track choice, but not the nicest on the street :).

Considering you'd only have a top speed of 120mph I really don't think that's an option for most tracks. Hell at summit point (fairly tight track) I only used 4th gear twice with a 3.9 final gear ratio.

Your next video should be titled "Greg Peters, the six speeder eater"

I am curious the results post clutch/flywheel swap and with a bearing that presses in like I'm used to. Would be sad if the trans is failing due solely on a slightly larger power increase. You aren't making unbelievable power to where you expect the trans to instantly fail due to mechanical restrictions. You're making respectable #'s for what you have invested and nothing seems obscene. Other Japanese mfgs I work with are designed to support the original >200whp/tq yet are problem free making 600/400 +whp/tq with an upgraded clutch in countless scenario's as well as many more in the 800-1000 range. Would be unfortunate if this is the case as there aren't many affordable bolt on options for the BP. Friend of mine runs the Turbo2 on his 1.6 (he's had this combo for quite some time now) however it was far from a bolt on affair and required fork to be modified, trans adapter plate to be made from scratch, trans mount, etc.

P.S - I don't want to make the move over to a BMW trans myself either. Maybe 3rd time is a charm.

Have there been any other reported cases similar to yours Greg? You didn't even get a chance to truly enjoy the thing before failure.

Would it still be worth tossing the 5 speed in just to troubleshoot any other possible scenarios/issues before possibly inadvertently eating another much more expensive 6 speed? I know it's a lot of extra work to put the 5 speed in, but from watching your videos you can certainly do it pretty quickly it seems. I would just hate to see the same thing happen again because the clutch and trans just don't play nice together for whatever reason.

EDIT: Just kidding, I missed the post where you said you bought the more well-known 949 set up. Disregard...

Quigs I was thinking the same however if he loses the latest 6spd on the new parts list....it's time to scrap the idea of using the 6speed to retain that power as there would be no other contributing factors - hopefully Greg tries it in stock configuration with the OEM shifter if it's sitting in the turret area when he purchases. Since there is no rock solid proof of cause, just damage, starting with a static OEM-like configuration would be nice and if everything is vetted then re-add the short shifter back in and let her rip. For what it's worth the MiataRoadster shifter setup does look like a quality configuration but I would still test without so the only aftermarket component is the clutch/flywheel. From what I've read he is replacing the clutch/flywheel combo along with a new pilot bearing + master/slave (I would replace the throwout with OEM as well if it isn't already-I personally never use aftermarket brand bearings, oem NTN Bearing quality etc)

Greg, I don't think there should be a gap between the parts of the 4th gear assembly. I don't have a picture of a NB1 assembly, but here is one of the NB2/MSM 4th gear.

https://images.imgbox.com/2a/6d/NYdDuRA3_o.jpg

Im not familiar with the miata gearbox specific but the gap has to be normal. When you shift u move the syncros to the gear. They work like a little clutch to match their speed.

The gap between the bronze synchro and the gear is normal (its actually specified in the repair manual what the minimum gap between them should be), but that's not the gap Greg is showing:

https://preview.ibb.co/gEWdmb/IMG_20180101_123709.jpg

See the "smooth" area on the left? That is where the synchro rides on the gear assembly. The gap he shows shouldn't be there at all, possibly. Those two parts should be flush to one another. I've got shots of my NB1 internals, but nothing clear enough that I can say there isn't supposed to be gap. I'll try to get some pictures this evening.

Correct. And what happens once it slips down is the steel cone on the left contacts the main hub between 3rd/4th, and it's not supposed to be able to. Once I tapped the synchro cone fully onto 4th gear, and reassembled the countershaft, 4th gear became free floating once again.

Also - I will be running my old faithful 5-speed for a while until I figure out what my next move is.

I agree with Greg's theory about the synchro gear being ripped from the gear because of the force applied from the engine... for the most part.

Usually, when people shift gears, they are off the throttle. You should be able to easily slip out of a gear with the clutch engaged and 0% throttle. This shouldn't matter if it was a 100 hp or 400 hp engine, especially if they are the same engine, so it doesn't make sense as to why it would happen. To me, it would only make sense if you are flat foot shifting and the clutch wasn't 100% disengaged when you went to shift, however, why would it only happen in fourth then?

I was originally thinking the short shifter, but really thought about the helical gears.

Greg, in this picture, the way the gear is oriented, the input shaft gear for 4th should be pushing 4th gear away from the selection hub. Meaning, fourth gear is rotating up, as in clockwise from your hand, as in your hand turning right (clockwise).

https://cimg0.ibsrv.net/gimg/www.mia...26caec4cb1.png

From this next picture, it seems that the gear is over the synchro and that the teeth mesh there, then the synchro gear meshes with the counter shaft.

https://cimg4.ibsrv.net/gimg/www.mia...ef0d2f4521.png

I'm thinking that, under power, the gear itself is pulled away from the selection hub and slides on the synchro gear, and since the clip is holding only the synchro gear against the other gear directly connected to the countershaft (in the bottom of the picture), the synchro gear is able to stay in place as this happens.

Then, when you let off power, the gear tries to return to it's original position thus pushing the synchro gear into the hub. The reason the clip isn't worn is simply because the synchro gear is going the same speed as the counter shaft, so there is no pressure on the clip besides the single force pushing the synchro gear into the clip, which is just pressure, no friction.

While, yes, the hub can push against the synchro gear, it may not be enough force to push the synchro gear all the way back into the main 4th gear. If it could, then every stock Miata should've failed after a few hundred miles from wear on the hub and the synchros automatically engaging 4th and 3rd.

As a result of all this, the transmission just constantly grinds away the hub while even in neutral with the clutch engaged. It would be more extreme the farther away you get from fourth (first and sixth) as the input speed (hub) will be spinning far faster than the output speed (fourth gear). As far as the rear end locking up while in neutral with the clutch disengaged, my statement about two gears being engaged should be true in the instance that the synchro may have pushed over so much, that fourth's synchro pushed the selector onto third's synchros a bit and got itself caught in the hub as well.

Also to note, this pulling motion of the input gear on the counter gear under power should happen in any gear, not just fourth, since the two gears for fourth (input and counter shafts) are always spinning. If anything, the force should be more in fourth gear since the synchros can hold the synchro gear to the hub.

I hope this helps. There may be more to it, but, considering all the contact points of the gears based upon the pictures you supplied and what I found through searching combined with my knowledge of High School physics, I don't think there is any more elevated math required or hidden forces. Granted I have never worked on a transmission and there could be more to it that I don't know, and as a result my opinion may be very wrong, this is what makes sense to be from my knowledge of Physics, what information I have, and the information that I checked against other sources. I hope it at least gives you guys more ideas to work with.

The solution may be to get gears (helical or straight) where the gear and synchro gear are one solid piece, or one where the interface between them isn't just a straight gear, perhaps tapered like a helical gear, or one that literally screws on and has some sort of locking pin or lock nut on it. As a last resort, some red thread locker or some killer bonding agent between the gear and synchro gear may help.

As far as why this didn't happen to any of the other gears is a mystery to me. Third gear should technically be pushed all the way on the synchro gear in this case, I think.

I hope this helps.


Edit: Have you checked if third had any extra wear on it similar to fourth? I think third still could be exposed to similar error, but 1st and 2nd and 5th and 6th appear to have more than just a little ring to push against on the main gear that allowed them to stay in their correct positions.

Also, I'm sure you know this, but the reason the car was able to move a bit in neutral is definitely from that friction in fourth.

Updates on new clutch? Did you give hell to clutch masters?