SD1 Gearbox Rattle and Crunch

Hey y'all,
Looking for someone who may have some insight into the SD1 LT77 gearbox, or just gearboxes in general.
Ive got a P6 with an LT77 and it's making some unusual noises in 4th gear, like a rattling tapping sound especially at lower revs. It starts under gentle load, gets louder as the load increases then sort of fades out as you accelerate (probably just drowned out by other noise). It also makes a similiar noise briefly as you shift into 3rd from 4th, which makes me think it might be the syncro ring
The box is generally a bit hard work, notchy going into 1st, 2nd and 3rd, especially hard getting into 2nd but it's useable if you're patient and dont abuse it, but I am rather concerned about this rattling noise. It seems parts are difficult to find for this box so not sure whether its worth attempting a rebuild or just trying to source a half decent replacement.
 
4th gear is a direct drive so all the gears are "loose" and not been driven, which might explain the rattling sound if something is worn. I had problems engaging gears on a new Lt77 box in a Land Rover & that turned out to too much preload on the mainshaft, the bearings and one synchro ring were found to be shot when dismantled.
 
The type of lubricant has a big effect on box performance. If you have 80/90 gear oil in it then the shifts will be as you describe.
Auto trans fluid was a recommendation to follow on after Rover realised the problem.
Latest seems to be fully synthetic engine oil with molyslip or one of the expensive posh trans oils such as Castrol SMX or Redline MTL
 
4th gear is a direct drive so all the gears are "loose" and not been driven, which might explain the rattling sound if something is worn. I had problems engaging gears on a new Lt77 box in a Land Rover & that turned out to too much preload on the mainshaft, the bearings and one synchro ring were found to be shot when dismantled.
Yeh it does sound like a loose metallic rattling - that's why I kind of thought it might be the 3rd or 4th syncro ring, but I dont really know much about gear boxes. Can they come loose or get broken somehow and rattle about on the shaft? It still shifts into 3rd/4th quite well, just a bit notchy into 3rd
Hey Cobra, I've done quite a bit of research on the type of oil to use for this box and I've settled on Mobil1 5/w40, which definently made a big improvement.
Seems to mixed reviews on that Molyslip stuff, there's one guy claiming it saved his nasty gearbox from certain death 20 years ago and other guys saying that its sure to knacker all your bearings
 
SD1 Box
svr plimsol 005.jpg
I just wonder if too thick an oil has been used and sheared the oil pump drive. And on that subject don't tow the vehicle any real distance on 4 wheels ( or rear wheels) without disconnecting the propshaft first, as gearbox oil pump is only driven from engine not the wheels when in neutral.
 
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Wow I wonder how many people have wrecked their gearboxes not knowing that. Cheers for the tip! Do you know any way of testing the oil pump? And how is it driven from the engine?
 
The pump is driven off the end of the layshaft , but that in normal use only revolves when the engine is running. It is a little plastic crescent pump mounted at the back of the gearbox underneath the 5th speed synchromesh in the drawing I posted. You can also see the pick up pipe and the oil galleries, including down the centre of the mainshaft. (Dotted lines). I am not 100% sure but I believe the only way to check is take the back off the box and see that it is all intact.
 
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