Oh sht - here comes another short post....
For gearboxes, it’s hard to compete with $84 (Pick-A-Part; or about $93 with warranty) for a Gemini 5-speed box. Mind you, you have to find one!
If you can do the work yourself, I agree with Nick (gemcoupe) – just swap the bell housing BUT note that there can be differences in the front cover (inside the bell housing that covers the front of the box which also carries the clutch throwout bearing).
If you can’t do it yourself, then I’d agree with Bradlze assuming it costs as much for a gearbox specialist to replace a couple of gears – or synchro rings (which used to cost $50 each new) – as it does to swap bell housings. But I’ll add that I often seem to hear of bad jobs done on engines and sometimes gearboxes. (How come people can hear my “allegedly reconditioned” gearbox from inside their houses?)
If neither of the above apply, then I don’t care – I will still agree with each of my esteemed colleagues above. (Or should that be “disagree with the other one”?)
Unfortunately 2nd gears are a traditional problem in Isuzu gearboxes, though they may have been fixed with fatter gears (this is mentioned in some Forum about MUA gearboxes – more later...)
If getting another box, some things to be aware of...
- Has either got a centre plate that is steel instead of alloy? This is not a problem – just noting that steel ones – typically fitted in Piazza turbos – are stronger. The “centre plate” (which actually carries all the gears) is where you split the bell housing from the “gearbox”.
- Is the front plate the same shape? Earlier gearboxes have a needle-bearing’d lay shaft; later boxes have a (stronger) roller bearing. The layshaft can take either, but the roller bearing has bigger outer diameter and bell-housing hole and a the larger front cover to cover it.
- Do they have the same clutch-thrust-bearing mechanism?. Not a problem – it’s just that if changing from a cable/pull to a hydraulic-slave cylinder (or mechanical linkage) push system, you need to swap the bits over. Pull out/off the required throwout lever and its pivot “mushroom” bolt. The mushroom/pivot bolt can be mounted in either position (side) on all gear boxes – the all have at least the blank cast etc – but some may require drilling and tapping. Check to that the mushroom bolt is the correct length. On my Isuzus (older Belletts etc), they can vary in length (and thread size).
- Check that new box has the same input and output shaft splines and diameters. Early gearboxes (Belletts & the earliest Geminis) had course spline input shaft (on to which the clutch plate mounts) and course output shaft (drive- or prop-shaft); both were 1” diameter with 10 splines. They then went to fine spline input shaft, and later to fine spline output shaft, and later to larger diameter output shaft. The front shaft is probably 17mm diameter into the flywheel bearing – on really old stuff they were 16mm. (Same 6203 40mm OD bearing – but specify 16mm ID or 17mm ID (inner diameter).)
- The thrust bearing carrier is probably 40mm long and 32mm ID, but they can vary in length and inner diameter. The ID is obviously critical – the carrier should slide smoothly over the front gearbox cover. But the length may not be that critical – it’s in combination with the length of the pivot/mushroom bolt. The rule of thumb that I use is that with the gearbox bolted to the engine, the throw-out lever should be approximately perpendicular when pushed against the clutch pressure plate.
And when replacing or fixing the box, replace the front and rear oil seals.
And it seems that silastic is an acceptable substitute for all the gaskets – but get rid of all oil and grease (strictly speaking, wash with thinners etc as opposed to petrol) and use that blue silastic stuff.
And maybe reline the clutch plate and replace thrust bearing whilst its out?)
Geez – that makes it all sound really complicated. But usually it isn’t. If the gearbox’s front cover is the same, and the in/out splines match (and they are “splitable” gearboxes – not the old one-piece box with bell-housing), the you are probably okay. And these are easy to verify visually – a big “obviously steel cast” plate versus a smaller, lighter (alloy?) plate, and 10 teeth course spline versus many (24?) teeth fine spline.
As to diameters, they often look the same to the inexperienced – and me!
The other factors (like output shaft diameter) are only an issue if using boxes from Rodeos and maybe Piazzas etc, but should be measured to be sure.
FYI - The auto boxes also had larger diameter output shafts (same spline and diameter as later gearboxes (Rodeo, Piazza?), but you shouldn’t confuse an automatic gearbox with a (5-speed) manual. Mind you, you won’t have the same gear crunching problems with an auto – hopefully! LOL.
(I mention it only because it’s possible to mix and match any prop-shaft and yoke to any gearbox if you have the right bits – eg, a selection of 3 (or 4?) yoke sizes/types that use the replaceable universal joints (UJs) (those with 1” outer diameter) – and this varies between individual cars – eg, I have 2 Piazzas, one has replaceable UJs, the other has the swaged non-repairable UJs.)
I mention all of the above because boxes mated to twincams can come from anywhere.
They may be Gemini (or other) boxes with a swapped bell housing.
They may be from certain diesels or early Rodeos (KBs) with LHS starter motors.
They could be from older Isuzu 117 coupes or early DOHC Piazzas.
Or they could be from Gemini ZZRs etc.
(If 4-speed, they could be from any old Florian, Bellett or Wasp which all have LHS starters, but these are one-piece boxes).
One consideration is gearbox ratios – although I spoke of this in another thread, but it may be on the Piazza (OzIsu) Forum ‘cos I can’t find it here. (Damn – should have searched for the above info too – I hate repeats and contradictions!)
I like the Piazza else Gemini (eg: 1984-ish LG LJ LS LT) 5-speed boxes because they have a 0.782 ratio 5th (Piazza is slightly better with 0.775) as opposed to the more common 0.855. It doesn’t sound like much, but I can hear the extra revving in top. (And that’s in a 1600cc OHV G-161 Wasp. I swapped from my Gemini with LH-starter bell housing to a Rodeo gearbox last year and man I miss the lower revving 5th! From 4th gear, it was a 22% drop in revs – now it’s only 14%.)
The other ratios are the same for may Isuzus (the above Geminis, ZZR, Bellett GTs) though first may vary slightly.
RodeoBob (who may be able to help you source gearboxes) has also written about boxes and ratios in the Piazza forum – see in particular
http://ozisu.com/forum/viewtopic.php?t=688, maybe also
http://ozisu.com/forum/viewtopic.php?t=993 and
http://ozisu.com/forum/viewtopic.php?t=198.
And also some crap from me herein at
http://poida.completelyfreehosting.com/ ... highlight= (Fri Jan 05, 2007 6:31pm).
So now that everyone’s asleep, I’ll discretely end this reply.
I wouldn’t want to take up too much space or time.
Cheers,
Peter.
IZU069 - Isuzu means a lot to me.
) and once its running ill take it back out again and start fiddling then
