Gearbox help please - this is driving me mental!

I know I have another thread on gearboxes, but that is bloated out to 8 pages now and will take trawling through so thought I would start a new one.

To cut a long story short, we originally took the box out because I ended up punching a hole in the box caused by clutch release bearing seizing - it was a sloppy box, jumped out of gear and all sorts so it needing sorting anyway. So got another box and set about doing a semi rebuild - so replaced anything that looked aboiously worn - we did the layshaft (series 3 later 1 piece layshaft), reverse idler and a few other service items.

After "rebuilding", box felt fine but started jumping out of 4th gear it seems to do it anywhere between 5 and 15 miles - there is no pattern to it - i.e it will do it randomly, not just on overrun, not a high speed/revs, not under load, etc. It will jump outof gear regardless of weather it is in overdrive.

Suspected it was the syncro unit and also shimmed up 3rd gear (now 0.16 which is supposed to be within toleraces). 1st/2nd selector fork was worn so did that at the same time as well as the mainshaft bush and thrust washer, 3rd gear thrust washer and shim....put box back in again, and it still jumps out of gear. Noooooo!!!! So thought perhaps was the detent springs - replaced - still jumps out of 4th gear. Nooo!!!! No strange noises, changes gear lovely, not sloppy etc just jumps out of bloodey gear! Mainshaft nut has been checked tight by the way.

So today have taken the box out again to see if I can see whats wrong, and I am really struggling. Bought and fitted gearbox pinion (pair) and replaced the primary pinion bearing and needle roller bearing, but to be honest, it had little wear or play - so I really doubt thats the problem. What i dont want to do is go to the effort of refitting (yet again) for it just to jump out of gear.

So....I guess my questions are

1)Any more suggestions to why it might be jumping out of gear still
2)The 3rd/4th Syncro clutch has a bit of play in it - about 4mm back and forth - is this normal?
3)The 3rd gear has 0.16 play - the workshop manual says tolerance is up to 0.18...so that should be OK? But would that make it jump out of 4th gear anyway?

By the way, it's a 1963 SWB Series 2a (2a/3 box)

have you got sufix d synchro hub or input shaft ,suffix ds have coffin shaped dog teeth earlier just straight 3rd and 2nd should have no more than 6 thou if possible,hub does have play

All the packaging etc is suffix A...but I would consider the teeth to be coffin shaped?

look carefully as it can be hard to tell if you havent the two to compare they will work together but give your symptoms,check input internal teeth and 3rd and on hub befoe d suffix paraell with triangle point on front d has triangle poin but tapers thinner behind it like a coffin shape and all 3 need to be the same

I can confirm all the stuff is suffix A

if theres no excessive endfloat or free play in gears and shafts .theres only springs in hub left and setting forks which is done by hold selector shaft square and central with interlocks,undoing pinch bolt on fork ,moving fork as far forward and back and marking each then tightening fork in middle of the two marks

theres only springs in hub

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What hub do you mean?

3rd/4th synchro hub

The whole syncro unit was replaced with new, inc springs, last time I took the box out (50 miles ago) but thanks anyway. I will take your notes with setting up the selector forks

was it genuine hub or pattern ,worth still changing springs even new ones can be faulty if not not evenly as strong it can make it jump out of gear

pattern :\ one of the few things I could nt afford to get genuine. But then...the chances of it failing and causing the same problem which I had before the synco was replaced...probably quite slim? But then I guess it has to be one of these daft little things which is causing it!

pattern springs can be poor, jumping out of gear is usually too much end float on gears and shafts worn dog teeth or not fully selecting

gears are all new. Could the float on 3rd cause fail on 4th?

Right....took clutch off...there is noticeable play on the input shaft when inserted - suggesting that the spigot bush is worn...so will get that replaced. I suppose I should have done that in the first place!

Also the 4th selector fork is very so slightly worn...doesn't look too bad tbh....but perhaps worth replacing anyway? Maybe.

My last concern really is the flot in 3rd gear being 0.16 when the max is 0.18 - could that make 4th jump? Possibly not but it does make we wonder why I have that amount of flaot, when I am using the largest thrust washer (.130 part 5073) - unless the thrust washer has been cut badly?

Anything else people can think of before I order the bits?

3rd wont make 4th jump 6 thou is okay ,shimming is done with with 3 x selecteable washer behind 2nd and brass bush and 3 x selectable washer in front of 3rd i would just change springs in hub,spigot shouldnt effect it as shaft is held by clutch friction while running spigot only comes into play when clutch pedal pressed,unless front input brg shagged or endfloat on main shaft ,dog teeth worn ,selector not set ,poor hub,should be allright

Yup I understandhow the shimming works - currently have the largest shim on the third gear - re-measured today and its the .30 shim. I was thinking that (considering its at the end of the tolerance - order the .35 shim which is designed for 2nd gear (but will fit) - bringing the float on 3rd quite a bit down (but not lower than the minimum recommended float). Fair enough if it wont affect 4th - but considering i have the box out, makes sense to make it right.

spigot shouldnt effect it as shaft is held by clutch friction while running spigot only comes into play when clutch pedal pressed,unless front input brg shagged or endfloat on main shaft ,dog teeth worn ,selector not set ,poor hub,should be allright

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Thats just it - the bearing did have some play - could noticeably see the difference now I have fitted the new bearing this weekend.

leave 3rd 6 thou is alright if you make the end float too small it will seize washer or gear or bush

Fair enough....but if I fit a .35 it will take down the play to .11 which is spot on according to the workshop manual? - thats gt to be better than having 6 thou (well, 6.3 thou I suppose which is very close to the maximum amount of float allowed?

Also...the syncro...is it normal for that to have play? What I mean by that is when fully assembled, I can move the syncro up and down the hub by about 4mm. Normal or not?

2 to 8 thou in old money,hub has 2 x baulk rings joined by 3 shafts the hub with dog teeth is supported by 3x spring it should feel tight in the springs,one end has recess in inner splines for snap ring for 3rd