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Low miles hard use r32 gtr synchros issue

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I'd like to hear opinions on this (cross post from the end of a different thread)

I ran a standard r32 gtr 1991 5 speed on track for years. it had heavy shockproof in it that whole period. No problems at all.

I bought a brand new series 3 box from nissan via rhdjapan and my garage put "normal" Millers CRX 75w90 NT oil in it. It ran fine for a few track days and changed the oil.  now after around 5-10 days on track (so 10-20hrs of hard use) it crunches on all upshifts over 4500rpm or so. Only double de-clutching stops the crunch on high rev changes. Downshifts are ok.

When it started crunching I got the garage to inspect the clutch (os twin plate) and bleed the master and slave cylinders. They couldn't see anything wrong. They feel if the clutch wasn't fully disengaging (which was my first diagnosis) it would have problems at low speed, which it doesn't. They feel the synchros are toast and it needs a rebuild.

What killed the new box so quick?

Was it the wrong oil? The first fill on the new box and the oil change were both Millers CRX 75w90 NT which are described as for "Manual transmissions, synchromesh ... which require API GL4 or GL5 performance"...

I'm going to throw the old high mile box back in, with heavyshockproof as before.

But I'd rather not ruin the synchros my old box too, so what would you suggest as diagnostic steps to make sure it isnt an issue with the clutch or something else that is the root cause?

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I think you need to pull the box and have a specialist for a view about what happened.

I race a 32 GTR and have never had a problem with the synchros, using redline lightweight oil. I have swapped or rebuilt the box every 2-3 years as a precaution. On the other hand, Mark that I used to race with killed his synchros every year or 2. I'm pretty sure the difference was I shift slower and rev match better (good for box, bad for last tenths in lap time). If you have a data logger you should be able to tell from your TPS log how fast you shift.

In the end synchros only have the time you give them between gears to work before forcing them. The slipperiness of the oil would be a factor but the time is the important part. If you need to change gears faster reliably you need dogs, not synchros.

Of course, it could be anything else, like an initial assembly error too....that's why I'd get the box checked.

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Thanks Duncan, that's some good experience-based advice, I really appreciate it. Not many genuine specialists left in the UK sadly.

Driving style is in the back of my mind as a cause. I generally shift fairly slow and I have a fair amount of mechanical sympathy. I don't religiously heeltoe though, and might have started to get a bit quicker shifting recently, with a bigger turbo now, trying to keep it on boost.

I have logs going back a few years I could see if I've got a bit keener over time. What would you look for in the data logs? I dont log the clutch switch but maybe could look at the time it takes for the revs to drop for the next gear?

I actually have a sequential on order, long lead time though, so the 5 speed will need to see me through at least the next few months.


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My experience with the transmission is that synchros are very slow. I can literally count to 2 in my head for high RPM shifts. The other thing to keep in mind is that pretty much every transmission these days needs an oil with friction modifiers in it to help the synchros work correctly. This is a common issue the Miata people run into, they stick to Motorcraft manual transmission fluid because otherwise people get crunchy shifts and gear grinds on worn synchros.

For a track car I would definitely rebuild with dogs. For a street car I'm not skilled enough and smoothness matters a lot so I'm sticking with synchros.

Edited by joshuaho96
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On 8/25/2021 at 6:35 AM, alexj said:

What would you look for in the data logs?


On 8/25/2021 at 5:57 AM, Duncan said:

your TPS log

Throttle off to back on time. If it's faster lately, your shifts are faster lately.

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Yeah the throttle position is a reasonable indicator of gear shift speed. Not saying that is the only potential cause, the could be a bent selector or some other weird gearbox magic...that's why a gearbox shop should look at it.

Back when we did have an excellent skyline gearbox specialist in Sydney, they recommended high quality mineral oil for new gearboxes and diffs, changed regularly, rather than synthetics because the synthetics tended to have more modifiers which can glaze the gears. 

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  • 4 weeks later...

Small update. Had a track day on Monday after swapping back to the old box running heavy shockproof. Good as gold. So it's not the clutch mechanism or hydraulics at least.

I can't seem to see any difference in the gear change speed going back in the logs but i need to look at it a bit more methodically.

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An old box even with clapped out loose or damaged synchros will feel fine with shockproof. I personally hate shockproof, but I will say it can completely hide problems with almost fully destroyed synchro baulk rings or missing coupling teeth.

A new GTR box with tight synchros using the wrong oil like a GL-5 oil will absolutely crunch - there seems to be very little tolerance for slightly incorrect oil types when new.

Is there a reason you were using that Millers stuff, as that appears to be a 75w-90 GL-5 diff oil with lsd friction modifiers - Nissan synchros will hate that. I know it "says" gl-4, but if it says gl-5 too it won't be good for it.

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Thanks BK. That millers stuff was what my workshop chose. I should have researched it myself. Thinking about it I had similar issue ages ago when I put Mobile 1 SHC in my old RB20 box - that's GL4/5 oil too.

So how much work is it to strip this suspect box the the point where I could see if the synchros where actually OK?

Does it make any difference that its a series 3 spec box?

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Stripping wise not any different between the early and late boxes, it's just in how the synchros are assembled that changes. You'll have to pull the gearset out on the centre plate for a start to inspect, but ideally complete disassembly from plate if you want to check everything.

I wouldn't bother though as being a brand new box I highly doubt there is a problem. I found out years ago how much the gl-4 vs gl-5 oils can make a difference with the first OS giken gearset upgrade we did on an early box.

OS giken geasets require you upgrade to the late 97 -98 synchros so it was essentially a brand new box inside. Decided to use gl-5 Castrol Syntrax 75w90 for better gear protection and on the Drag strip the box absolutely refused to shift at high rpm, especially on the 2nd and 3rd gear changes.

Though I'd lost an energiser clip or something so the oil was dropped - nothing. Put new oil back in which was Castrol GL-4 75w85 syntrans, as that's what we already had, which I was using in the other GTR with an early box at the time which had no problems with high rpm shifting. Guess what happened - box started shifting properly.

Switch to a gl-4 without lsd friction modifiers.

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