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6 speed neutral drag & destroyed diff

 
Old 07-08-2019, 11:42 AM
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Default 6 speed neutral drag & destroyed diff

I have been hearing a loud clunk for the last few weeks when releasing the clutch in neutral. It has gotten so bad that sometime the car lurch forward a bit when I release the clutch in neutral. Other than that (and reverse that was sometime hard to engage) I was able to shift fine and the driveline was not noisy at all.

I pulled the transmission and rear diff yesterday to inspect them and I found 2 things

First, it looks like the LSD part of my rear diff is shot. The case was full of metal sludge and it looks like all the tabs of the diff clutch plates are broken (I broke the bearing inner race while removing it)




Secondly, I haven't had time to open the transmission yet but I noticed that there is a lot of drag when it's in neutral. If a manually turn the input shaft it takes a lot of effort to stop the output shaft from turning as well.

Does anyone know if it's possible to buy new clutch plate for the diff? I looked but I have't found anything

Also, what could cause the transmission to drag? I am thinking a bad bearing but usually they get noisy when they wear and the transmission was silent

Thanks
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Old 07-08-2019, 11:53 AM
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I'm assuming that diff is a Tochigi-Fuji diff based on the parts in the picture. If so, if parts are available, (which isn't likely) then its not worth the cost to rebuild. They commonly fail with hard abuse. You're better off finding a good Torsen diff or upgrading to a clutch type.

As for the transmission drag, that could be a number of things. If you have the time and tools, I'd pull it while the diff is out and play with it to see if its just a drag in neutral. Tip: use a clutch disc on the input shaft when spinning it to better gauge the force required to turn it.
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Old 07-08-2019, 12:58 PM
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4th gear is a two piece pressed together assembly. Higher power can force the two parts apart, "un-pressing" them. This makes them too wide and they then drag against the surrounding spinning parts, making the car pull forward in neutral. You need to either replace the trans or find someone who can disassemble it and press the two parts of 4th gear back together and tack weld them. It is repairable.
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Old 07-08-2019, 11:44 PM
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I would look for either a torsen pumpkin or an obx diff if you want to do it on the cheap. There's a mod for the obx that makes it pretty solid.
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Old 07-09-2019, 05:03 PM
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Thanks, I have ordered an OBX with a washer & bolt kit
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Old 07-09-2019, 05:22 PM
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I had time to disassemble the transmission today. It looks like 3, 4 and 5 gear are made of 2 parts

3 and 4 looks fine




5th gear has a gap of 0.016 between the helical gear and the dog teeth



Is it normal? I am tempted to put in it a press and close the gap then weld it

Last edited by p0staldude; 07-09-2019 at 06:13 PM.
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Old 07-09-2019, 06:12 PM
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I am pretty sure that this is the issue, the synchro of reverse and 5th gear are out of spec (worn).

Does anyone have a broken 6 speed with a good reverse / 5th gear and would be willing to sell me:

-5th gear & reverse synchro
-reverse gear
-5th gear
-5th gear / reverse clutch hub

I could also buy the whole inside of the trans

Thanks
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Old 07-09-2019, 10:10 PM
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Someone posted a video link here on the forum regarding the un-pressing of the gears and your symptoms sounded just like mine and his. Maybe mine isn't 4th either, but it surely drags forward in neutral and shifts poorly. Could be mine is just like yours and not the one in the video.

I think it's the broke and boosted YouTube guy Greg Peters.
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Old 07-10-2019, 12:23 PM
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Yes, he even has a thread on here with pictures. It's the same problem than I have but in his case 4th gear separated instead of 5th.

https://www.miataturbo.net/suspensio...problem-95561/

I will press the 2 parts of the gear together and weld them. I will also disassemble the main shaft and weld the 3rd and 4th gear just in case.

I ordered new synchro from the dealer, they wanted 400$ for the reverse gear and the clutch hub so I decided to keep my old one even if the teeth are in bad shape.

The new synchro and the OBX diff should be here by Friday, hopefully I will have time to put the car back together this week end.
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Old 07-10-2019, 01:47 PM
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Just a quick little tack. Don't take The Temper out of the gears.
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Old 07-11-2019, 10:22 AM
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I've got a bunch of 6 speed parts in a box (I've taken apart at least 3 to build one). Shoot me some pictures and measurements of what you want/need and I'll dig through to see if I have any thing good. As for the syncro's, I would just buy new ones from Mazda. No point putting used wear parts in something you have to do a major tear down to replace. When I did m build, I replaced 3,4,5,6,R syncros. 1-2 syncros were triple cone and expensive. Measured mine and they were good.
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Old 07-12-2019, 12:15 PM
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Update: I was not able to press the 2 parts together with a 20 ton press... They did not move

Midtenn: I am looking for the main drive gear, the 5th/reverse clutch hub and the reverse gear.

The diameter of the reverse gear is 3.9" and it's 0.6" thick
The diameter of the clutch hub is 3.33" and it's0.8" thick

Here are pictures of what I am looking for:


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Old 07-12-2019, 12:26 PM
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OK. I will drag the tubs down this weekend and see what I have. I'm pretty sure I've got a good input shaft. I'll get pictures of what I have.
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Old 07-12-2019, 01:09 PM
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I'm sure that Midtenn has what you need. But, if not, I've got a 6sp that lost reverse (I'm sure that it was a problem with the fork, and not the gears), that's been collecting dust for 3-4 years. I took the tailpiece off and stopped because I found another 6sp for cheap.

You're welcome to the insides for next to nuthin'.
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Old Yesterday, 09:05 PM
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I installed the OBX diff yesterday, it makes a big difference in how the car drives. I temporarily installed a 5 speed to test it

@Midtenn Did you find anything in your tubs of parts?

@rwyatt365 If you can take the insides of that trans and put it in a box I will gladly pay for shipping + what you think that these parts are worth
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