Unable to Identify Gearbox Suffix and Parts

[Turnips]

I'm having a little difficulty identifying the suffix of my Series 3 gearbox. It is a 1976 ex MOD vehicle which had a Sherpa van engine in when I bought it.

I have stripped the gearbox but cannot find the stamped serial number anywhere on either the gearbox or transfer box casing. I have cleaned most of the carp off but haven't washed them in the dishwasher yet, I need my mother-in-law to go out for the day...

The only place I can see a serial number is on this conveniently located plate on the bellhousing:

I am cautiously thinking yes, as the part number 576730R is for a recon four speed box. It states B05403 as the serial, so this would be a B suffix box.

I have been following Gazzars useful thread on disassembly, and using the workshop manual also. I am a little confused around a part on the mainshaft. 

On the mainshaft front end, between the third speed gear and second speed gear, there is a bronze bush. Mine is pictured below.

The workshop manual also references this, but calls it a distance piece - however it is clearly one single part! I cannot find this part in the parts book.

There is reference to two bushes which look like a bit like it, but in two parts: FRC4076 and FRC4077. I can see on Syncro Gearboxes website they claim a two piece or one piece is the same outcome - can anybody please confirm this is definitely the case?

Apologies if I am being overly cautious - I have never rebuilt a gearbox before and would quite like to make a decent job of it.

Thanks in advance.

[Snagger]

That is it, but it was also evident from the shape of the teeth on the parts in your other thread that this is an A, B or C unit, so you need the parts in the diagrams with the 3 in the circle.

[Turnips]

51 minutes ago, Snagger said:

That is it, but it was also evident from the shape of the teeth on the parts in your other thread that this is an A, B or C unit, so you need the parts in the diagrams with the 3 in the circle.

I see, thanks for that.

It's a little frustrating but the constant gear which is damaged is only available as the full unit with the pinion shaft as well. Any idea what the reason for this is?

[Snagger]

That is normally the case.  The reason is that those two gears run together for so long that they bed in together and replacing just one would subsequently (likely) cause a lot of noise and premature wear.  That is tolerable on gears 1-3, because they spend relatively little time engaged and are usually idling, but as the pinion and input gear are under load any time 1-3 are engaged, and still under some load when 4th is engaged (bypasses the lay shaft, but the lay shaft will still be turning and stirring oil and spinning all the idling gears on the main shaft, so has some residual load), the noise and wear generated by ill-matched gears could be considerable.

[Turnips]

That makes sense. I think I need to watch a video of how the gearbox and transfer box actually work. I can't tell by looking at it, and the book doesn't tell you how it works - only what you should expect to be there.

[Snagger]

Main gearbox

In a nutshell, the gear box is all driven by the primary pinion.  That is in constant mesh with the lay shaft, so they are both always turning at a speed related to engine RPM - the pinion at engine RPM and the lay shaft at a fixed ratio depending on the ratio of the gears on the pinion and the front of the lay shaft ( not far from 1:1, and possibly is 1:1 - count the teeth to work it out).

The lay shaft gears are fixed on the shaft, so all turn at the same rpm.  The main shaft 1st, 2nd and 3rd gears are positioned in sequence from the back end of the box and are free to spin on bronze phosphor bushes on the main shaft.  They stay in the same positions and should have no movement along the shaft except for tiny endfloat to allow oil ingress and prevent seizure.  There is no 4th gear; the primary pinion and main shaft get locked together with the other actual gears all idling freely on the shaft.

All the main shaft gear wheels are free to spin on the main shaft unless selected by the lever mechanism.  This does not move the gears but moves the synchro hubs which engage on splines or dog teeth on the gear wheel faces, locking that gear wheel to the main shaft through internal splines on the synchro hub and shaft.  In the case of 4th, the synchro hub slides forward to engage on the dog teeth inside the primary pinion, so drive is transmitted through those teeth to the synchro hub to the front end of the main shaft.

Reverse gear is the exception in these boxes, where the selector shaft does move a gear forward from a stowed position to engage on the straight cut gears on the lay shaft and the outer circumference of the 1st/2nd synchro unit (with that synchro in its middle “neutral” position).  As a tangent, while this the case for all LT76s, it not for LT77 and R380 and many other units, which use pre-engaged gears and synchro units for reverse like the forward gears.  

All this means that at any time the engine is running and the clutch engaged (pedal up), the pinion, lay shaft and main shaft gears will all be turning.  If the vehicle is in motion, the main shaft will also be turning.  If the vehicle is in idle or 4th, the torque load on the lay shaft and the unlocked gears on the main shaft will be low, but those gears will be spinning on the main shaft, each at a different speed because of their different ratios.  In all circumstances except when reverse is engaged, the reverse idler gear should be retracted out of tough of the lay shaft and 1/2synchro teeth and so is isolated and stationary.

The lateral rods and balls inside the top cover and the sliding pin through the middle selector shaft (1st and 2nd) act as interlocks to ensure that the other rods are in neutral before any rod can slide fore or aft to move a synchro hub and engaged a gear.  If the pin in the middle selector shaft or either lateral rounded rod between selector shafts was missing, the interlock system would be disabled and simultaneous gear selections could occur.  Remember that - removing them and selecting two gears at the same time is a good way to lock up the box for loosening or tightening the nut at the back of the main shaft.

The bronze baulk rings run clear of the tapered cones on their respective gears in general operation.  They should only be contacting when selecting that gear, when lever pressure on the sliding part of the synchro unit will apply pressure through springs against that baulk ring in the direction the synchro is being pushed.  It grips the gear cone and matches the speed of the synchro to the gear to prevent crunching and grinding - it is a speed matching clutch system.  If the springs are missing, like on your 3rd/4th, then that pressure can’t be applied and the grip will be very poor.  Likewise if the rows of ridges inside the ring are worn, they won’t cut through the oil film and poor grip will again lead to a poor gear change.  The coils springs and balls in the 1st/2nd synchro and the leaf springs on the 3rd/4th hub are not gear selector detents to hold the box in gear (that is done just by the springs and balls in the top cover near the stick), but are there to apply that baulk ring pressure on gear change.

 

Transfer box 

The transfer box does two jobs - it has two different ratios of reduction gear (high is 1:15 on all non-1-Ton and low on SII suffix C and later standard units is 1:2.35, 1:2.8 before that).  Think of it doing the same job as the front gears of a mountain bike with two sprockets.  The other function, done after the gears, is the selection of 2wd or 4wd.

The gearing is done by having a single output gear on the end of the gear box main shaft drive a second shaft, the “intermediate gear cluster”, as a fixed ratio.  That intermediate cluster has two gears on it which can mesh with two matching gears on the output shaft of the transfer box.  In this case, one gear is in permanent mesh with the intermediate cluster (high range) and is locked to its shaft by a sliding collar.  This collar happens to be the low range output shaft gear, which when slid to its engaged position will slide along the shaft and disengage its hub from the high gear and move into line with the matched gear on the intermediate cluster.  Move it to neutral and the gear de-meshes from the intermediate cluster but has not moved far enough to bridge the shaft and high range gear splines.  Move it further aft and it will move even further from the intermediate cluster low gear and will bridge the high range gear splines, acting in the same manner as the gear box synchro hubs.

Because low range increases torque to the axles so much, LR decided that low range would always be 4wd to spread that torque across both axles.  So, the way 4wd mechanism in the front casing works, the sliding dog clutch locking the forward output shaft (only inside the forward housing) to the aft output shaft (inside the transfer box main casing and into the back of the forward housing) is moved into locked by moving the red lever aft.  When in high range, the ratio selector rod doesn’t push that collar back, so a second mechanism for the collar was added to allow high range 4wd, and that is what the yellow knob does.

 

Overdrive

This can have two configurations.  There is the Fairey type (and the less common Toro) that use a main and lay shaft system and the Roverdrive/Roamerdrive which uses epicyclic (concentric) gears (as does the GKN available for Defender and a lot of 80s BL cars).  Both do the same job in a similar fashion, though the logic is easier to see on the Fairey type.  Both are sited behind the transfer box, but mechanically speaking, they are sequenced between the gear box output and the transfer box input - the gear box turns the overdrive and the overdrive then turns the transfer box.

I think I remember seeing you have a Fairey.  I have a rebuild guide for that on my blog.  It operates much like the main gear box, the gears all being in continuous mesh and a sliding synchro unit (very similar to that of 1st and 2nd) selects engaged, neutral or disengaged.  As before, it locks respective gears to the shaft via its dog teeth and hub splines.

The difference is that in disengaged, it locks the outer output shaft to the inner input (main) shaft through the hub splines on the main shaft and the dog teeth at the back end of the output shaft, so it has a 1:1 ratio and the lay shaft is just idling.  In “engaged”, the input and output shaft are unlocked from each other and the main shaft gear meshing with the lay shaft becomes locked to the main shaft, so torque is now directed through the main shaft gear to the lay shaft, and then the lay shaft front gear to the output shaft.  The lay shaft’s two gears are different sizes and have different ratios, so they step up the output rpm by 28%.  With neutral selected, the synchro unit outer ring stays clear of the dog teeth on the main shaft gear and the output shaft, so neither is locked to the main shaft and drive is lost.

My rebuild guide is here, and the photos may help make sense of the above.  http://www.nickslandrover.co.uk/overdrive-overhaul/

 

 

 

 

[Turnips]

Thanks @Snagger, I appreciate the time you have put into writing that up for me. It explains everything very concisely. I always thought that gearboxes were too complex for me to work on or understand - however the theory of them is quite simple in a way. I imagine designing them is anything but simple though! Once again, thanks for taking the time to help me out  - time is a valuable commodity. I owe you a drink, although Dubai is a bit of a stretch for me.

Anyhow, back to identifying the suffix of my box.

I washed the casing today in pursuit of a stamped serial number. All I found were some random stamps who's meaning is known only by the chap at Solihull who placed them there - unless anyone can tell me differently!

 

According to Syncro Gearboxes' website; the layshaft which needs replacing could be an A, B or C & D. I will quote some of their text below:

Suffix A Layshaft - Teeth: 27, 22, 13, 16. Used with 31 tooth 1st/2nd Hub and sleeve.

Suffix B Layshaft - Teeth: 27, 22, 13, 16. Used with  30 tooth 1st/2nd Hub ad sleeve, and 35 tooth 1st speed gear.

Suffix C & D Layshaft - Teeth: 27, 22, 13, 14. Used with 30 tooth 1st/2nd Hub & Sleeve, and 31 or 32 tooth 1st gear.

The teeth on my layshaft are 27, 22, 13, 16. This indicates Suffix A or B. My 1st/2nd hub and sleeve has 31 teeth. This indicates a Suffix A gearbox.

So there we are, I believe that mystery is solved: my gearbox is a Suffix A.

[Snagger]

Have a look at the reverse idler gear.  If it has a plain bronze bush inside rather than a needle bearing, then it is A.  Mine has that.  They improved it because the shaft tends to get worn with a taper, a bit bell shaped on one side, which puts a thrust force on the gear when engaged and forces it to jump back to neutral.  Unfortunately, the later shaft is different (thinner, I think), so can’t be retrofit.

[Turnips]

You are correct, mine has just a bronze bush inside the reverse idler gear.

Hopefully I won’t discover any more big question marks as I strip the transfer box!

[Snagger]

Take your time and ask whatever you need.  It’s a good learning experience, and as you put it back together, you’ll understand exactly how it all works.

[Turnips]

I'm back in the workshop today and cleaning up parts. I thought I would upload this picture while I eat my lunch.

It seems that, just like Snagger's, my factory recon box has had it's serial number crudely removed. They kindly stamped the suffix (which I now know to be correct) underneath. This thread could have been avoided had I washed the top cover first! Anyhow, at least I know for definite what I have.

[Snagger]

Sod’s law, isn’t it? Then again, with all the other letter stamped here and there around the casing, would you have trusted that to be a suffix denominator, and for that to accurately reflect the spec of the innards?

[Turnips]

Knowing what I know now... no chance!