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!