Rebuilding tips

The main problem with this box is it's age, it was designed a long time ago and even with the recent modifications it remains a bad design by today's standards. It's pretty simple to throw together like a jigsaw puzzle, but getting things right will save ripping it apart again.  Hopefully.

Mainshaft low gear. Ok, so you have rebuilt the gearbox and have it on the bench, you turn the kick start over and it's tight as anything going down but the kick start returns fast. Chances are that the Loctite that you used on the big (main shaft sleeve / output shaft) bearing has contaminated the low gear (25 tooth) just behind it. This will leave you ripping it apart and starting again. A sign of this problem is an incredibly sluggish tight box and the main shaft turning when in neutral. Also, before you build your box, take the output shaft (main shaft sleeve) and fit the low gear (25 tooth) to it and fit the bearing. Push them tight together and keep the pressure up and turn the gear. If one turns or scrapes on the other you will need to shim the shaft to keep the bearing back, if you do not it will rub when in use (using up horse power) and the gearbox case could be scrap if the bearing starts to spin in it's housing. Late model shafts have a larger area to keep the gear and bearing apart, also a larger area for the bearing to seat.

Layshaft case end bush

Use Loctite, and bang the bush right down making sure that it is all the way home. From the outside of the case a bronze bush should stick out quite proud. This may seem obvious, but I have seen quite a few damaged bushes due to this, where the small gear rubs against the bush and the layshaft end cuts a circle inside the bush. With a badly fitted bush you can expect a tight box (which will drop your horse power), possible bad gear change, and it's probable that the kick start will not return as it should. 

Lay shaft / false neutrals

We will always get false neutrals but understanding why may help.

False neutrals are completely down to half changing, the lay shaft sliding gears not engaging the lay shaft nodules. This is caused by the box not being set up correctly, or a  partial movement of the gear lever.

The gear change has a double stage movement per gear, as in, move the lever up (or down) and keep pressure on it until it moves a second time. If you didn't wait for the second movement then you have only half changed and the box would be in a false neutral.

This is due to the sides of the lay shaft moving gears banging in to the lay shaft nodules, and having to wait up to a quarter of a turn until the cross slips into the nodules, and so get the next gear. However some times the crosses on each sliding gear are perfectly in line with the lay shaft nodules, which is why some times we get a swift and sweet single gear change.

It's totally unpredictable, other than there's more area to the side wall of the moving gears than crosses, so we have a greater chance of a double stage.

Ratchet

The gear change ratchet mechanism has to be set up correctly of course; the round barrel on the outer ratchet that sits in side the lever has to be exactly in the middle of the two small nuts holding it all together.
Obviously use a ruler etc.

The ratchets have been known to be a millimetre too small on some of the ratchet faces due to bad manufacture.

The old debate about bronze bushes being eaten away by the gearbox oil is true, depending on what oil you use.

Bronze is an alloy and has zinc, lead or aluminium in it. I have been talking to a bloke that is a bush supplier, and he recons there are different types of bronze and one that is less resistant to attack from modern oils. He told me the model number of the metal, but I forgot what it was, not that I'm going to make bushes.

 I used a British output shaft that had bronze bushes in side of it, the bushes were not new and probably well old as they did have slight signs of being eaten away, but still usable. It could be my imagination, but these bushes seemed to get worse in just three years. The tiny holes that were here and there seemed to be bigger and more of them. The bronze Hitchcock's lay shaft bush and kick start bush were still in very good condition, this leads me to believe that they may be made from the better type of bronze. May be.

When choosing the oil make sure it's no higher than a GL4 rating, as above GL4 the oil can be corrosive to yellow metals. Thanks to Coachgeo from the Dieselbike net forum for that bit of the jigsaw puzzle.

Owners of Enfields have been talking for years about this bronze and oil topic, and that of oil or grease, and what oil etc. EP80 is thicker and so will eat up more horse power than a thinner oil. A while back I had an early 1960's  Greaves Silverstone racing gearbox, and the oil that was in it was incredibly thin. This maybe worth looking into, sometime, maybe. I'm sure someone will come up with some facts and figures on this and I'll post the info here when or if it comes available.

I will never use grease as this does use up quite a bit of horse power, and if you think that when the gears are moving around they will throw off the grease and make a hole, an air bubble if you like. In this case what is lubricating the moving parts? I have ripped apart loads of these boxes mainly from India and this is exactly the case, so if your going to use grease I recommend topping up with ep80 gl4 oil to make the grease sloppy. Of course this does bring in to question, how does grease affects bronze, and what grease to use? I  have heard that apparently Hitchcock's use engine oil to top off the grease, so if we are going to do this we have to know what specification engine oil to top off with.   I know some owners have chosen to use the Indian iron bush which isn't as good as the bronze, so they can use what ever oil they like.

British - Indian differences

My recommendation is always use an Indian gearbox and the newer date of manufacture the better. Never mix Indian and British parts as they are not the same even though they look the same.

On the British box the gear teeth design changed from the more fragile H type to a more robust HG type in 1959. 

The Indian gearboxes had design change's from the early 1980's. The small main shaft bearing got bigger, 17mm inside diameter and the main shaft to match it, also the main shaft had a better design to combat some weak points. Of course the bearing housing had got wider to accommodate the new bearing size, also the wall behind the bearing got a lot thicker, and so the oil thrower behind the bearing got wider.

In 2002 the design changed for the better with parts designed to stop the oil leaks. Unfortunately they just didn't think it through well enough as it still leaks oil.

Diesel mainshafts

Diesel main shafts come in two lengths 12 1/2 & 13 1/4 inch.

The quality of shafts differ considerably, it really is pot luck as to what you get, but usually the splines are so tight you have to use emery cloth to get the small gear on, and the clutch splines are sloppy as hell. So if the gear will not fit your new shaft, emery those splines (not the grooves) until you get a not overly tight and not loose fit, tap them home with a rubber hammer etc.

Also, you will almost certenly need to open up the clutch push rod hole at the clutch end, they seem to have a bur or a narrowing just under the threaded area. I used a drimmel type hobby drill with a small grind stone, this does the trick but remember to wd40 the hole at the other end and flush it through before light greasing it.

Do not make the mistake of using a narrower clutch pushrod, as Enfield pushrods do mushroom. It would be better to use a pushrod from a different make of bike with a better track record of reliability. Or use a stainless steel rod if you can get one.  

Never hammer the gear on as this can crack the gear, and under load it's bye bye gear box (be warned, this has happened). Also do not heat the gear to expand it to get it on, as you will at some time need to get it off again The trials gears are softer metal than the original Indian or British gears, so never hit them with a hammer etc.

The reason there are two different lengths of diesel shaft, is that they are from two different factories, and the factory that makes the long shaft just got the measurements wrong. Which is damn good for bikes that use a wider twin engine.