GT6 Gearbox - First and reverse meshing

[Cadenne]

Hi,
I've had an issue with my GT6 MK3 OD gearbox where 1st/2nd synchro hub outer is meshing/in interference with the edge of reverse idler when in 1st gear.

I've taken the gearbox apart today thinking I'd find something obvious missing or broken inside mainly the split ring but there was nothing. All the thrust washers and split ring are there. I've taken all the measurement specified in the workshop manual and everything seems to be in spec.

The only thing that looks off is that I can move the 1st/2nd synchro hub up and down the main shaft by a few mm, which means I can make it interfere and clear the reverse idler when it's in first.

Anyone seen this before or got any ideas, I'm completely stumped.
Thanks

Link to a video showing the issue

https://www.dropbox.com/s/zhs1itanrvi15hm/Video.mov?dl=0

 

[RobPearce]

I've seen this a lot - enough that I consider it "normal". It doesn't actually cause much of a problem apart from the 1st gear synchro being of little use with a crash reverse idler. The clearance on a properly set up gearbox is extremely small and the tiniest bit of wear allows it to mesh.

[glang]

yes Ive seen a few posts on this issue either here or on the other site and it appears two possibilities are:

the reverse idler gear operating lever has been distorted so when not in use the idler isnt in the correct position;

or slight variations exist in idler gears and a different one from original has been installed.

[Cadenne]

Thanks both. It's good to know it's a common problem I thought I was missing something. It just seems strange that the 1st/2nd synchro has so much movement up and down the main shaft but I’m sure it’s designed that way for a reason.

I see what you're saying about it not causing a problem if everything in the gearbox is OK, my reverse idler has a tooth missing though so it was making a horrendous knocking noise in 1st every time the synchro ring caught where the missing tooth is. That’s why I got the gearbox out.

"the reverse idler gear operating lever has been distorted so when not in use the idler isn't in the correct position;" That was my initial thought, but the idler gear is right up against the distance tube. I'll replace the lever anyway just in case.

Hopefully rebuilding with new synchro rings will also give a bit of extra clearance. Is there a spec for Synchro ring gaps on this gearbox? I can't seem to find any.

Also, anyone got recommendations for where to get decent rebuild kits (bearings, layshaft, ring etc) or are all the usuals pretty good? Autogear seem to do a lot a gearbox specific stuff, anyone have experience using them?

[glang]

There is supposed to be some end float but limited and should be adjusted by selecting the correct components when rebuilding the gearbox. The free to download work shop manual is very good for this although unfortunately it doesnt seem to give the correct dimension of the distance tube (theres two part numbers for this item). 

As you say a newer synchro ring may help increase the gap but as far as I know theres no spec for them ie. as long as they dont butt up against the gear face theres life left in them. Obviously if not replacing them its sensible to put the least worn on second and third gears.

Replacement bits are a bit of a minefield as the only new bearings available are Korean so they and the synchro rings (unknown origin) are a bit of a gamble. If at all possible many people find NOS or reuse what they already have.... 

[Guppy916]

Try Tony on ‭01359 269777‬

or tony@kingstonsportscars.co.uk

he built my gearbox and o/d unit, and did a fantastic job