NB trans grind

99 Miata 5 speed, 81k miles, developed a 5th gear grind after the first track day I did last year. Then 2nd and sometimes 3rd. I've adjusted the clutch pedal and then yesterday I replaced the trans oil with synchromesh. The condition doesn't seem to have improved. I noticed when I was under there that something looks hinky. PO disclosed that the engine was replaced at 50k miles with a Mazda crate engine, but didn't mention anything about the transmission. However, the big bracket that connects the diff to the trans, where it bolts to the trans there's a machined flat spot, the shape of the machined area doesn't match the bracket at all. I don't know if the case design changed over the years, but there's also red RTV sealant along all the seams. Sure looks like a reman transmission, possibly from a different year, and it sure feels like I've got 3 synchros going out. Is that all there is to this mystery? I wasn't expecting to have to deal with this at 81k miles.

Sure sounds like the trans has been apart. I have a 99 I am building as an Exocet. If you can take pics of the PPF mount to the trans people can comment on if there is an obvious issue or alignment but it sure sounds like the synchros are going/gone. The sealant on my 99 is grey. This car has 180k+ miles on it with no issues shifting.

I don't understand what "replaced the trans oil with synchromesh" means. What fluid did you put in it?

^^What Keith said.

I also wouldn't expect the trans having been apart at 81k either.

ACDelco Synchromesh MTF

I've never tried that fluid, so I don't know how it plays with Miata transmissions. I'd probably throw in some Redline MTL (street) or MT90 (track) and see if that helps. I've seen a lot of shifting problems fixed with good fluid.

A dragging clutch will also cause some grinding. This is most obvious at rest when you try to shift from neutral to first.

Maybe the slave/master cylinder is starting to go? Maybe worn engine/diff mounts are allowing the transmission to rock too much? In either case, do yourself a favor and put the Ford XT-M5-QS fluid in there. That stuff is magical. It remedied my 1st to 2nd gear grind.

I have essentially the same trans in my RX7 race car. We started having synchro issues right away. We went through 3 transmissions in 3 races.

WE went to idemitsu gear oil on the recommendation of Mazdatrix. We are on the same transmission for the last 10 races (about 140 race hours). No more issues.

Treat the trans as if it is made of glass. No SUPERQUICKMADTYTEYOJDMMEGASHIFTS.

Update, I read through the documentation from the PO and the trans was rebuilt in 2010; I suspect that really means replaced with a rebuilt unit from some other year/model.

I also had a friend drive it and he insists any 2nd/3rd gear problems are all in my head, but that the 5th gear isn't a worn synchro but a flat out busted one.

New question -- replace/rebuild the transmission (my idea), or keep driving on a busted 5th gear synchro (including track days) until the transmission is completely destroyed (my friend's idea)? I don't want to get stranded at a track 4 hours from my house when a gear tooth breaks loose and lunches the whole thing, but I'm also not too keen on spending a whole weekend acquiring a new unit and swapping it out.

Preventative maintenance is always more fun than repair.

I have found, particularly in Japanese transmissions, that 5th gear is quite often hung out back in the tailhousing, and can be changed without removing the trans.

A low-mile Miata 5-speed is, what, $300? The synchros aren't going to get better on their own, and there's no point in rebuilding it. Swap it when it's convenient, the replacement will probably last the life of the car.

I've had a slight grind going into 5th for maybe 20,000 miles; If I shift slowly 4-5 it doesn't grind. I picked up a used trans ages ago to replace the one in the car, but it's never gotten any worse. Could something as simple as a fluid change really make a difference?

Yes. The synchros rely on controlled slip to work properly. If they have too little or too much friction, they'll have trouble matching speeds. Get a good fluid in there and you'll maximize their odds of getting it right.

I had a 3rd gear grind in my 6-speed. Replaced unknown fluid with Ford Motorcraft and it went away after about 1500 miles.