Hey all,

First off this is going to be about my gearbox problem and then about some mods to "race prep" a gearbox (make it shift smoother, harder and faster).

1. I've rebuilt my 5 speed, new seals, bearings, circlips and roll pins.

2. When rebuildling I checked/corrected any shimming issues to be withing factory spec.

3. All the syncros have the grooves nice and clean and have a good gap between the blocking ring and the gear.

Im a fitter machinist by trade and I know how to work with bearings/seals/moving assemblys/etc so the gearbox was put together without abuse or force and using proper fitting practices - press the bearing race that is resisting, heat ali centre plate and drop the bearings in, new circlips and roll pins and all that crap.

HOWEVER, (very big however) despite all this attention to detail and care, the box crucnhes in a moderatley hard 2nd to 3rd gear shift (yes clutch is adjusted correctly and I know how to use it!) This isn't a long buzzing grind untill it goes in gear, but more of a very short and very sharp crunch like the dog gear skips say 5 or so teeth before engaging.

So after a think about it, in a 2nd to 3rd upshift the syncro has to slow down the clutch fibre, input shaft, layshaft and the 3rd gear - quite a job if its a 6.5 - 7K RPM!!!

Soooooo, I determined the very short crunch noise is due to the syncro not slowing down the shafts quick enough - ie. not enough friction (as the syncro has a about 1.6mm gap, pulls up square to the gear and has good bite - meaning its in good nick).


My thinking lead me to beleive that over 30 years, the syncro cone has been nicley polished, and the detent springs and ramps have worn. This leaves us with a super smooth and slippery syncro cone with a weak spring applying force to it - ie reduced "braking" performance.


I was wondeirng if our residient gearbox experts (Revhead? anyone else?) can confirm what I have worked out to be right?

My idea of fixing this is when rebuilding a gearbox, sligthly stretch the syncro springs to preload them more (or wind a new one in a larger diameter piano wire) to extert more force and improve the braking effect.

Secondly, very finley grit blast the syncro cone surface. The factory new syncro cones are surface ground, not mirror shiney. The grit blasted surface will allow the syncro to bite it harder and allow faster shifting.



More gearbox mods are (that Ive done - wish I did the above ones too!, but it's my first gearbox rebuild ever) loctite the shift forks. I've retained mine with the new roll pins but also used a high temperature, high strength and high viscosity loctite to stop the mintue play in the shift forks. This gives a more rigid shift performance - at the expense of making them a PITA to remove later - loctite needs to burnd out/pressed off.

Secondly, completey rebush the shifter. I made my own bushes that when installed into the shifter and yoke, they JUST slide in with slight resistance (tap fit) on both the pin diameter and the width between the yoke. This is to remove slack and tighten up the shifitng performance.

Lastly is a new ball end bush for the shift lever - I couldn't buy one so I made one that is tight on the ball end and a light push fit into the shift rod in the gear box - again this removes slack and tightens it up.


With this work the gear box is as smooth as butter and a joy to go through the gears - untill the crunch from ther super smooth (less friction) 3rd gear syncro cone.

What are other peoples thoughts? Am I correct in my diagnosis of the syncro cone not "rough" enough - seeing as the syncro is mint and so is everything else?! It doesnt leave the problem to alot else other than that.

Lastly, I run Castrol Syntrans (GL-4) oil - being GL4 is has the correct amount of friction modifiers, but being synthetic do you think it reduces the friciton on the syncro cones??? I dont think it does (oil boasts superior syncro performance) but I'll let others chime in!

I was previously running a castrol product and my fresh 60 series was experiencing similar to what you have described. I switched to penrite 75 weight [recommended by my gearbox guy] and I have not looked back. Also I have noticed with these particular boxes, I find they perform much better once they are up to there operating temp. The castrol product i was using was terrible on cold mornings.....

That is my first step (last is pulling the box apart again!).

This weekend I will look into a good mineral oil and see how that goes.

+1 for penrite 75. Reccomended to me by benny.

Very very, happy with the results.

How would you guys rate the Penrite 75 against Redline shock proof?
Just wondering cause the Redline oil is sooo expensive at $66/litre!

The purpose of speeding up (or slowing down) the gear is not entirely left up to the synchro ring on it's own, but also the energising lugs and springs play their own part as well. What series gearbox are we talking about here? 1.6mm sounds fairly high for a 60 series, more like a 71 series, either that or they are like new.

Roughing the surface of the dog cone will certainly help but it will be short lived. It will polish out soon enough. Some cones have their own machined grooves that allow a place for oil to travel to allow a harder grip between the synchro and the cone. It offers a lessened surface area for oil at a reduced syncro life as the trade off. The problem you have may not lie with the synchro or cone itself but maybe with the energising lugs. As they get older they wear the square shoulders off the tab and offer less resistance to the selector ring as it applies the pressure to the energising lug to the synchro. The energising lugs release earlier while the gear is still being selected and it can cause some short lived grinding as a result of the gear speeds not being matched correctly.

It's a tough one to call over the phone but the oil is the easiest way to try and sort it out, before pulling the box apart.

Yes the roughed up cone will work but like you said it will soon polish out - but for a super light rotating assembly motor, it would allow real quick shift speeds - at reduced syncro life.

My thoughts too were about the energizing lugs, they wear and round over allowing less force being exterted - so I said wind a stifer spring/or you can replace them.

I can tell a slight difference in selecting 3rd then say 4th or 2nd - tiny bit less resistance (3rd has more wear on the lugs) unless thats just me thinking it is.

On a positive note, it seems that more driving is fixing the problem! It "sounded" like the dog gear skipped 5 or so teeth and now it "Sounds" like its only skips 1 or so - the crunch turned to a more of a instant dull click - hard to explain but its definately better.

Oh and the 1.6mm sounds huge but when I opened the box I noticed some else had been in it before (sealant, bolt heads) and theres new syncros all round! It may have crunched before and they tried to fix it, I dont know. They could have changed the energizing lugs for me too, sheesh...lol.

Nah, I was stoked with that and I did the bearings and seals.

Did you check the surface condition of the synchro cone on the gear? I find they can get nicely glazed and the cones then lack real bite on the gear. Spinning it up slowly in the lathe and breaking the glaze with a very fine very flat stone does the trick. Agree that a mild bead blast to the friction surface of the ring is well worthwhile. It helps to have a few rings to choose from as sometimes the mating angles are not perfect.
Get it all right and they really bite hard. Also last a lot longer as they don't slip near as much.
Hope this helps

While we are on the topic, how much oil does a 60 series box take?

I run Redline Super Lightweight (yellow) in my racecar and will not use anything else now. Did use Syntrans in the early days but it ate my first close ratio 260Z box ......
The Redline works well for us - and at about $25 - 30 a litre it won't break the bank for a small volume box like an A series.
Cheers.