My old GM400 gearbox, the autopsy.......warning , graphic im

[Brakebuster]

well,

i got to that bored stage again , so i thought i would perform an autopsy on my failed GM400 gearbox

after nearly giving myself another hernia, i lifted the box onto the metal bench , and the mop up began , no matter how much you drain these things they insist on arterial bleeding when you take them apart !!

so , i know the box was in its last throws of being almost reliable, but to what condition had to be investigated , when i removed the sump pan i was faced with THIS

IMG_2312 by MyPix on Talk Photography

TERMINAL CONDITION

there was a round magnet in the bottom , which i removed and found lots of metal filings on , this was before the picture was taken , ( it actually prompted the picture diary of the strip down )

the fluid was deceiving, as i did a partial flush a few years ago , but this was still a very dark colour and very viscous , the sump gasket came off in a thousand parts and was completely deteriorated , and the filter could be seen to be dirty

the main body of the box isn't any cleaner , and there is a black slime deposit covering everything

IMG_2315 by MyPix on Talk Photography

on taking the valve body apart all the metal parts are fine, but the separator plate gaskets just fell apart when touched , gain , terminal

on the good side............

i think a good clean and rebuild with new service parts from a £150 kit will make it good again

now, don't get nightmares as crime like this is rare , oh , wrong programme,,,,,,,,,,,,,,

BB

[poprivet]

That is in a terrible condition.. the Thunderbirds mug that is

Timely maintenance indeed, and the whole project has made really interesting reading. Another item for Rich's XJS problem list?

[Brakebuster]

sorry , meant to have the Pokemon mug on show, not the TB one............


the old gearbox has now been fully stripped and jet washed, and fully dried and cleaned up

bare case

IMG_2321 by MyPix on Talk Photography



Valve block fluid channels

IMG_2322 by MyPix on Talk Photography

right down to the other end , the arm at the end is the park brake lug

IMG_2323 by MyPix on Talk Photography



will drive out some old bushings tomorrow, and start to build her up again

just like the other one

BB

[BigCatXJS]

Vince - I'm amazed that you did not have a more severe injury lifting the GM400 box; that's a two man job especially if its loaded with fluid! Mine is stored in the "parts shed" and it claimed a victim getting there when the lad helping me narrowly avoided having his hand crushed!

When you have a spare moment, if you could draft something-up I / we'd be grateful

I've preemptively created a dropbox folder in the usual called "gearbox" and a sub-folder called "Pics"

Separately, apart from Vince's gearbox woes, I've not heard of anyone else having a major issue with the GM400 (this could just be my naivety!).

Cheers, Rich

[Brakebuster]

Thanks Rich

i used to do weights....... , and i even carried it about ten meters to the bench

as for the reliability, well, they are legendary for their endurance and build quality , that said, its an older design and does have some wear points apart from the normal friction material

regular fluid and filter changes help immensely , but the XJS design of the V12 gearbox mount prevents it being done quite so easily , as the back two bolts on the sump hold on the mount as well

i suppose the line to take is , if aint broke don't fix it , but mine was broke, and compared to how the rebuilt one feels and sounds, i am surprised at how i got used to the old noisy one rumbling away all the time

i will compile a few ( ) pages for the magazine

BB

[Brakebuster]

right

the diagnosis is, ITS STUFFED , lol

on taking apart the gear hubs where the drive gears mech with the main shaft, there is clear evidence of wear and sloppy thrust faces allowing excessive movement thus making lots of noise

though nothing would actually fall apart, this was allowing lots of noise and wear creates more wear killing the box from the inside out

in this picture, the smaller planet gears can be seen inside the carrier, these have very worn teeth to the contact point of the sun gear making the leading edge thin and polished on the contact points



this is tricky to see , let alone photograph , so another shot from the other side

IMG_2326 by MyPix on Talk Photography

the sun gear, is worn , but not as bad, but to be honest shouldn't be used again, the contact points can be seen by the polished section on the teeth on the left

IMG_2327 by MyPix on Talk Photography

and on top of this, the planet gear inside the carrier move around inside the frame by 1mm each , there should be next to zero movement , well, about a tenth of what there is

also , the locating stub that goes to the final drive was worn , though this is a normal service replacement for a gearbox rebuild

IMG_2335 by MyPix on Talk Photography

so this particular box is not going to be rebuilt using these components, so I'm on the look out for a cheap gearbox that i can make one good one from

BB