Gearbox Cover

[T Robinson]

Hi Chaps,

 

I am looking to modify a gearbox cover and add overdrive isolator switches.

I have a four syncro gearbox with the raised bosses to accept overdrive isolator switches. The bosses are un-tapped.

I am led to believe that drilling and tapping the holes in the centre of the raised bosses is not necessarilly the correct position for them.

Has anyone any dimensions I can use in order to drill and tap the holes in the correct positions.

 

Thanks in anticipation,

 

Tony

[Don Elliott]

This is how mine looks on my 1958 TR3A that I bought brand new. There are two isolator switches. Some early TR2s only had overdrive on 4th gear and later TRs had another switch which is for the back-up light.

[Steve Knight]

I think this would be possible. BUT only providing there are already some manufacturers notches in the selector shafts. This allows the switch to activate by dropping the plunger into this cut out. Gearboxes made without this facility may well use solid shafts, BL in those days would not think of standardising, every part would be re-worked no matter how much it cost. I guess buying another remote would be one answer, a tap that size in NPT or is BSP? and getting the exact alignment ( + the switches ) could be expensive. Some had three switches, the reverse lights not being used ! but while its apart find out what does what by seeing what shaft effects which gear. I would have thought the furthest near side would be reverse.

[Ian Vincent]

Hi Tom,

 

I had a saloon Type A gearbox with overdrive that I wanted to use on my TR3a and I wanted the overdrive to operate in TR fashion rather than using the saloon switches.

 

The cover from my original gearbox was non overdrive so it had the bushes but was not drilled and tapped. I acquired a s/h gearbox cover which was already drilled and tapped (and looked exactly like Don's) and fitted it to my saloon box.

 

The switches are not activated by detent notches in the selector rods, they are activated by raised profiles on the selector forks. My 2000 sallon box had forks with the approporiate raised profile but if I recall the notch on one of the forks was in the wrong position but that isn't a problem, all you need to do is to take the cover of the box and check the position of the relevant fork for each gear. The only thing you need to remember is to make sure the box is in neutral before you lift the lid.

 

In my instance I swapped the fork that was the wrong way round with one from my old box because everything else is pretty much interchangeable and dismantling the lid and selector forks is a pretty straightforward exercise if you follow the workshop manual. If you haven't got the luxury of a spare top cover complete with forks and rods, you may have to mark and drill in a different place to through the existing blank boss but it won't be difficult to work out where to drill, the actuating bit of the profile is quite large so you don't have to be overly precise.

 

So to summarise - start by taking the top of your gearbox (in neutral) and work out how it all works - if you change your mind its easy enough to replace the cover using with a bit of instant gasket.

 

Rgds Ian

[ianc]

Just to clarify which switch/boss is for a given operation, I have pasted here a response which I made recently on the Forum to a chap called Duncan.

 

There can be 1, 2 or 3 bosses on a top cover. If there are 3, the one on the left (looking forward in the vehicle) is unused unless a reversing light is fitted.

Think about the gear lever: to engage reverse, one moves the lever to the far right, which engages the leftmost selector, which lies beneath the boss which has no switch. So, there's no switch for a reversing light - normal on a TR2/3/4.

With the gear lever in the central position, one can operate on 3rd & 4th gears using the central selector fork, which lies beneath the switch on the centre boss.

With the gear lever in the leftmost position, one can operate on 1st & 2nd gears using the rightmost selector fork, which lies beneath the switch on the righthand boss - this switch will operate on 2nd gear only.

All this can be checked very easily with a multimeter or a bit of wire from the fusebox and a bulb.

 

Think of it this way: as you select 2nd gear, the gearlever is pushed to the LEFT and back, and this (because there is a pivot point on the gearlever) operates the selector fork on the RIGHT of the gearbox.

Ian Cornish

[T Robinson]

Thanks chaps,

 

I will give it a try,

 

Tony