Isuzu G-series Twin Cam

hey there,

new member here but have been on the gem sites for a long time. good to see a forum dedicated to those with interest in the twin cam motors. Should be of much benefit!

im buying a motor on saturday, a g200w straight from a gemmi. Just had new head gasket and top end service to the value of $1,700. Complete with clutch, 5 speed gearbox, tailshaft, Microtech computer, exhaust, engine mounts. All the parts needed for the conversion into my wagon were included. For $1100

now only thing i found wrong with it was second gear crunch when down shifting. Owner told me he wasnt sure if the box had any oil still in it, but told it hasnt been a big problem and if worse comes to wear, apparently 5 spd gem boxes have the same guts for the rebuild?

Wiring will also be needed, it comes with the loom to suit the gemini but i gather theres a bit of difference in the acutal wiring of the piazza motor eg: starter motor on the other side, things like oil pressure etc.

overall im happy with what im getting for the price im paying. Ill get some pics up when i get it home and start cleaning her up for installation.

You dont need to rebuild box's, just swap the front half of the DOHC box onto the SOHC box. Easy as.

true you can
but if i were you jaz i would be looking at getting those gears replaced etc
wouldnt be that hard for a box joint to find something that will fit

well ill put some good oil in her n see what happens

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.

Hey Jazzza - nice buy! Sounds like a pretty schweet deal.

How complete is the exhaust system? Mine came with a header and midpipe (the big 2-1 bit that would stop about where the passenger sits) but I don't think I have any way of fitting this to my stock TX exhaust (although thatd be temporary as its a nasty 1.75" exhaust anyway) so I might have to try to bend the stock midpipe up to the twincam header - lol. It MIGHT work.

If you got a complete exhaust system right to the tailpipe that'd be sweet. Also I hope the ECU is tuned and running the engine properly, but hey, at least you don't have to break it in with an untuned ECU

If you want good torque down low, try to keep a reasonable 2:1 section - about 24 to 26 inches is ideal (that's measured from the stock DOHC manofold's twin outlet).

I'm selling DOHC 4:2:1 extractors for a Gemini if you're interested. It has a nice two-bolt flanged end. Around the$300 mark....?

PeterA

A great write up on the finer points of the MSG boxes PeterA.

I was thinking about setting up some graphs showing the different gear splits of Gemini five speed boxes. I'll add it to my list of things to do.

I use the taller ratio fifth gear diesel boxes and although third is a bigger step from second than it is in a petrol box, I like the overall feel of them.

many people also don't realise just how many ratio changes were made in the Gemini boxes. Nearly every model brought a different box ratio. Diesel ratios were different again.

IZU069 wrote:

I'm selling DOHC 4:2:1 extractors for a Gemini if you're interested. It has a nice two-bolt flanged end. Around the$300 mark....?

PeterA

Could you post pictures of the extractors, Im interested

well, heres the pics i promised:















i must say theres some weird things on this motor, pretty different from the g series motors. like the water arrangement, totally different. seem the intake manifold is dry bar the water pickup for the throttle body on cold starts.

im tempted to fiddle around with a few things, but i think ill get the motor in and running on my megasquirt (ECU now for sale ) and once its running ill take it back out again and start fiddling then

is there different models of the g200w? im not much of a twin cam man i must say...are they worth putting on lighteneed flywheel to get those revs up quicker, and what clutch do they run? standard gem??

so may questions lol

Just do the things you would normally do to a g motor to the twin cam. there are a few different versions but the bolt on bits are the same. You want a 2litre flywheel and clutch and lightened is better. how much power do you want to get out of it. i would also upgrade the throttle body it is very restrictive and put a set of extractors on it.

if your running an aftermarket computer you want need a wiring diagram. these motors electrical are very simple just get the wiring diagram of whatever computer you are running and re-wire it to that. injectors throttle pos sensor, temp, fuel pump and ignition wires and your done.

Two main types, early and late. Early used either twin carbs or the forward-facing throttlebody. They also require a diesel crossmember as the oil filter sits where the engine mount should.

Late model [what you have] has the ECGI plenum, oil filter at the front, bolts in, and from memory a different dizzy.

Both G180w and G200w should use the G200z clutch. Get yourself a Exedy Daiken HD, retail for 170 @ autobarn. Ive never driven a car with a lightened flywheel, just about to put one on my G200z in my daily but id say do it while its N/A.

If your using a megasquirt, make a custom hall-effect dizzy [speak to WFC hell show you how] so you can run fuel/ign and if you want to be a real trick DIY bastard, multi coil [if your smart about it it will cost you fook all.]

If you can get cams ground, go for the L1 race spec cams. Wheres Peter, hell tell you what to do...

Nick-

te g161 clutch is 170 but the g200z clutch is $255

and his is a I-TEC manifold...

i think i might grab some new bits for the box, spigot bearing, the gasket inside the housing bit, behind the spigot bearing (dunno its name?) some good oil, new clutch and lighten the flywheel (although i dont want to lose out on any torque)

anyone have a part no for the clutch? interested to see what its worth on trade...

also, with all the water piping, does all that need to be there? i mean there is pipes corssing the motor everywhere, seems a bit pointless but i guess ill leave it all rather than f*@k something up

and gemini coupe, apparently mine didnt bolt in? it was adapted slightly? maybe to have the right gearbox/tailshaft position im not sure, but ill see.

cams sound sweet, to suit turbo spec? and multi coil would be trick as

and his is a I-TEC manifold...

Thats what i meant, dont know why i wrote ECGI.

L1 cams are a high-rpm N/A race cam, no good for turbo

Multicoil would be well worth the effort in my opinion.

Nick-