TOK from Seloc
C64 gearbox 5th gear syncro failure / selector issue - no more 5th and 6th gear for this gearbox.
2005 S2 na exige with some fast road cams- and a lotus sport LSD.
If I read this thread anymore, Iāll end up driving like my grandma 
Yeah, I must admit I thought I was sitting pretty here in my NA, sub-220bhp version, safe from this problem - but if I read too many more of these Iām going to end up replacing my gearbox with a stronger version! (or am I just trying to justify the KAAZ close ratio gear set to myselfā¦ ahemā¦) 
Dave seriouslylotus groupbuy!
If we can get a group buy going at a sensible price, Iād be totally up for itā¦ 
This thread encouraged me to dig through loads of archived posts from this forum and others from back in the late 00ās when S2 tuning was probably a bit more of an āinā thing to discuss.
It was quite amusing to keep reading stuff like āThe jubu gears are a fine mitigation until someone comes up with a proper solutionā, but various abandoned projects to look at Honda and/or Audi box conversions etc all fell flat.
Thereās a great thread on exiges.com (here if interested: Exiges.com - Exiges.com - Lotus exige club) where many of these topics are discussed. At that time Jubu gears (and Kaaz) were new solutions and nobody was really sure how robust they had been. Perhaps the fact that many of these projects fell flat over the years is because the Jubu/Kaaz solutions actually worked out to be pretty dependable.
I have to say, from 2009 till when I sold my car in 2016 the gearbox was fine, I know Lee who bought it off me has not had any issues since either but he doesnāt really track it.
I probably did 20 or 30 track days after fitting the Jubu gears and the cooler. I am also quite nice to my cars and was always careful not to run curbs on full power so I didnāt shock load the transmission
From Darkslide over on SELOC
Year of car - 2008
Power of car - 240BHP
Original box? - Yes
Box fluid changed before failure ( time ? Day / week / month before ? ) - About a month before failure (recommended upgraded oil)
LSD? Factory or aftermarket? - Open standard diff
Speed when failed ( approx ) - 40mph ish
Went when changing from 2nd to 3rd at fairly high revs but not on the red line. No user error that I remember and all seemed fine until the fatal gear change. Was able to drive home but with 3rd gear missing and some interesting noises coming from the box. Not entirely sure what failed as managed to source a replacement with the broken one now in my shed awaiting stripping.
Also worth mentioning that the box didnāt grind at all even when cold and never gave any hint of the impending failure.
Iāve heard of loads going, I think every specialist in the land has a little pile of broken gearboxes sitting in a corner somewhere
Not sure how relevant this is, but thanks to my new ECU and gearbox temp probe Iāve got a bit of data.
My motorway cruising temp seems to settle at 80degrees celsius, recovery time is very slow so it will retain heat for a long time after parking up too compared to engine oil which I guess is dispersed over a wider surface area so can cool quicker.
On track I saw a peak temp of 103 degrees C. Not as high as I expected, but I do keep my sessions short at around 15 mins max. At time of cooling down laps, my temp was still steadily climbing up so Iāve got no reason to doubt that temps would continue going up and up had I stayed out on circuit longer.
I canāt find any details for what temps we should aim for in a gearbox, gearbox oil doesnāt need to worry about burning off/evaporating moisture like engine oil does as itās a closed system on the C64, but I guess you should still aim for a minimum temperature so that the oil is at its best. MT90 which most people (incl. myself) use donāt seem to advertise any optimum temperatures, at least not that I can find. The only temp related data I can find on their datasheet is:
Vis @ 100Ā°C, cSt 15.6
Vis @ 40Ā°C, cSt 86.2
That means absolutely nothing to me Should we infer from this that 40 degrees is the minimum operating/load temp we should aim for and 100 is the highest? Or are they just two arbitrary measurements?
Based on pretty much nothing other than guess work, I think Iād aim for a gearbox cooler that kicks in (via electric pump) at 85degrees so that itās not on the go constantly under normal/cruise driving in an attempt to keep the temps under 100 degrees.
Oh, and to add balance to the thread I survived a trackday with ~ 195ft/lbs on standard gears and no cooler. Letās see if I can get it to survive another one 
cSt is the units for the viscosity, centi Stokes. It shows the oil is much less viscose as it gets hot. They are not limits, just points on the curve. I think good synthetic oils can go to 150C before they start to break down. At about -40C they are like treacle so starting an engine is tricky.
True story, in parts of the USA where it gets very cold truck drivers would warm there engine with a small bonfire under the sump. Unfortunately, the new engine we were working on has a plastic sump, for a while!
Yes I figured the oil would be good for way into the 100s. I think some of the C64 theories are that the casing can get so hot that the gears start disengaging by a fraction of a mm - just enough to move the shear forces to a weaker part of the cog. Sounds far-fetched to me but wtf do I know?!
Iāve heard casing temps of 120+ being recorded on track, so it would seem that Iām a fair way under that but my sessions are short, it was a cool day and Iām not running āseriousā torque through it so all mitigating circumstances.
Interesting thread, which I have only just come across after #138 had its gearbox taken apart (to install a Quaife ATB diff), and then reinstalled without replacing anything else! 
No issues were reported though, so, fingers crossed, seems like there is still some life in the internals.
On another note, came across this thread about āwobbly JUBU 3rd, 4th 2006 Exige CUP 240ā: wobbly JUBU 3rd, 4th 2006 Exige CUP 240 | The Lotus Cars Community
āAs is It is difficult to say if it is too loose, but typical transmission are not press fit like the Toyota.ā
So, it begs the question as to whether Toyota boxes are too tight for circuit or hard road use?
Any thoughts on this?
Itās (yet another) theory I guess. I think itās fair to say that the C64 has no single weakness, everything discussed in this thread and elsewhere just contribute to the whole package being weak above 260bhp/170ftlbs sort of region (but not bullet proof below that either!).
- Temp
- 3rd/4th thickness/strength
- FD strength
- Input Shaft Strength
- Casing Strength
- Synchro weakness
- Driving style/aggression
etc, etc. If you address 3 of those 6 things (for example) then I imagine your chances of a failure start to come way down - so itās about picking your spending limit and choosing your compromises I guess.
Interesting thread
I had the 260 upgrade around 8 years ago, shortly after dropped a pulley size to match the 211 (275hp?) then I think a year later dropped another pulley size (canāt remember the sizes off the top of my head) because I just thought what the hell if it goes it goes.
Iāve had no issues in all that time using the Millers CRX 75x90 NT motorsport gearbox oil which I change every year without fail
I did however once try the redline gearbox oil for a year, really didnāt like it. After a couple of months changing gear became more notchy then when I changed it out the oil was white/Mayo coloured due to excess heat. Thankfully since going back to millerās Iāve had no trouble and the gear shift is smooth again.
Interesting most gearbox failures had the redline oil in at the time, maybe just a coincidence though
My failure happened with the standard factory oil, always ran redline after that.
Hello all.
Iām thinking about some preventative maintenance on my RGB and am looking at the Kaaz gear set from EP, canāt seem to locate the 4.3:1 final drive anywhere though, any pointers for suppliers?
Also, is the ACT clutch recommended?
Thanks 
Further to what weāve spoken about, Ade on here has been through this , as has Will, and Dave at Seriously has good advice on the subject ( ignore the ābuy a sequential chapterā)
Personally i would keep your money and repair or replace if needed. Box is fine for a 260 car unless your doing lots of standing starts and TDs and again i would not worry until something happened
4.3:1 FD can be had from Toyota. Sorry I donāt know part numbers, but Dave might - and Tom @ Datum Motorsport definitely does. I think he said about Ā£650 when I was talking to him about going that route.
For what itās worth, even though Iāve not gone as aggressive as the Kaaz ratios - changing my final drive as part of the reliability upgrades has been one of the most fun changes Iāve made to my car. 