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.

 

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.

 

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