Still don't know whats making my tranny grind...
 

Checked clutch pedal, its fine.
Replaced engine mounts today with poly mounts, still grinding.
It only happens when shifting from 3rd at high RPM. General driving is no problem. 1-2 is fine, little wheel spin. 2-3 is fine. But after 3 it just grinds, won't go into any gear until the RPM drops.
I haven't changed my transmission oil yet, apparently MT-90 makes a difference. I think thats the next thing to do.
Any ideas?

There is a clue in the above .... N/A rx8s don't get wheelspin unless you get on gas too soon (before clutch is out) after changing gear . This is really hard on the gearbox . My guess is your syncros are gone and it's time for a new/2nd hand gearbox.

I'm gonna agree with the above on synchros being worn out.

MT90 will help with but I doubt it will solve your issue. Have you ever changed/checked the transmission oil?

You are giving your transmission time to warm up before beating on it correct?

Changing the gear oil will not help. It does in fact sound as if you have a bad synchronizer set on 4th, 5th and 6th gears. The RX-8 trans is inherently weak when it comes to shifting quickly. Sounds as if its time to pull the trans and either replace it with a known good unit or rebuild it.

Yeah I always wait until the coolant temperature goes to the middle, then I wait another couple of mins for it to really come up to temperature. I've only owned the car for the past 1000 miles so i've never checked the trans oil or changed it. The car only has just under 40k miles, I asked all the questions to the previous owner and it seemed he knew what he was doing and always let it warm up too. Its not like its had a hard life. But maybe i'll have to start looking for a new tranny as you say. The car shifts fine at lower RPM's no problem, its just a bit annoying when i'm about to blast some ricer and sit there trying to get into gear.

An easy way to see if it's the synchros is try double clutching and see if you can get into the gears.

When you upshift, push the clutch in, put the shifter in neutral, release the clutch fully, give it a blip, then press in the clutch again, shift to the next gear, and release the clutch slowly. That's double clutching.

It's mostly used in the old days where synchros were not commonplace in manual trans. Double clutching eliminates the work synchros normally have to do.

I'll try that tomorrow. I usually rev match when downshifting. What would be the point in blipping the throttle on an upshift may I ask?

Id be interested to see the damage internally. Our Aisin transmissions are not know for these type of issues at such low mileage. Wouldn't hurt to verify that it IS full to proper level before you rip it out.

I'm gonna change the oil with MT-90 soon. If its just an oil level issue then it will be sorted with that.

If you are new, it takes a while to do the double clutching, enough for the rev to drop a bit too much even for the next gear.

Double clutched today. It shifts fine if I do that. But I got in my car this morning and reverse is gone... no matter how hard I pull with the car on or off I can't get it into reverse anymore. And 5th and 6th are really hard to get into. I think the tranny has finally gone. I'm looking at buying a used one now, they seem pretty cheap. I'll find one with low miles and have my mechanic put it on.

If you can't get in reverse, double clutch it while stopped. I had to do that on my old Corolla a lot since lots of older cars don't have synchros on reverse and in some cases, 1st.

But yeah sounds like the synchros are busted.

Tried that, I have tried everything to get into reverse. Its just gone. If I yank on it really hard I can get it half way in but when I come off the clutch I just hear grinding gears. Looking at new trannys now, not going to wait for it to get even worse so its completely useless.

Are the RX8 tranny's different over the years? Now is the chance for me to get a better one if possible?

S2 trans is supposed to be better. There are a few wires that have different plugs you will have to fix. A quick search will give you the info.

I'll look into it thanks. At least RX8's have cheap parts lol

On a flat level surface , when you put it in first with your foot still on the clutch .............does the car tend to creep forward ?

Nope. No movement at all.

Ok ...just wanted to check you had full travel of the release bearing . If it goes easily into first when stationary and doesn't creep forward ...then that wont be the issue.

I bought two trannies. £85 for one with 30k miles on it, and I paid about £100 for the other with 70k miles on it. Both perfectly working. Now I have spares! I also bought 1 gallon of MT-90 and an Exedy clutch kit. Might as well do it right! There is a small amount of play on the input shaft on the 70k one (haven't tested the 30k yet as picking it up later). Is this normal?

Damn that's cheap ! Would be between $700 to $1000 for that here!

Weren't there some complaints with the Exedy clutch?

I personally wouldn't mess with the clutch. If you want some improvement in responsiveness of the engine, get a lightweight flywheel(don't forget to include balances).

I think it was the Exedy Stage1/2/3 clutches that had problems, the Exedy oem replacement is fine.

I mean the tranny was coming off anyway, and I know the OEM clutches dont last that long. So it makes sense to replace the clutch now rather than pay more labor to drop the tranny again when the clutch starts to slip. I bought the Exedy oem replacement from LC Rotary. It was like £289.

^ That clutch is fine. It is the Stage 1 clutch that created problems for people. A few years ago, the release bearing that was included in that kit was fragile, and people were buying OE, Koyo, or SKF bearings to use instead, but I think it has been improved since then.

Clutch life depends on how you drive the car. I have had OEM clutches last nearly 200K miles on daily drivers in the past. I have a lot of track hours on my RX-8, and at 44K miles, I still have more than half the clutch disc material left and zero slippage.

Still, if you are going to the trouble to drop the trans, I always recommend changing the clutch and rear crank seal, while you are in there.