gearbox selector forks

[Peter Naulls]

A couple of years ago I rebuilt a gearbox for the Ruby, choosing the one with the best synchromesh. It works very well but is unbearably rattly, especially at higher revs in 4th. So much so that I've given up and put a rough old box back in instead.

I stripped it completely for a second time but couldn't find anything amiss, now I'm wondering if it is excess play in the selector forks - the gear end, not the gear lever slots. There is about 20 -25 thou free play along the length of the selector shaft and considerably more when rocking it rotationally. Is this excessive? They do look quite worn. If so has anyone tried welding them and re-machining to reduce the slack?

[A G Wood]

Yes, it is possible to build them up.
Assuming A7 ones are aluminium-bronze this may help you:

https://lotuselan.net/forums/viewtopic.p...4993&start=

I have done tractor selectors (the 'gate' where the gear lever is supposed to slot in) with the arc welder and high nickel content sticks, but I suspect that might be a bit much for car selector forks!

Edit: I have just checked several (modern) service publications and I note that the standard clearance for a lot of them is in the order of 0.10 - 0.40 mm (~ 0.004 - 0.016 ") and some allow up to 1mm/0.040".
A lot don't specify, I suspect someone will pop up with an empirical maximum clearance for an A7

[Terrytuned]

To reduce side clearance of the forks gently squeeze the sides of the forks in a vice, the amount of rock dosn't make any difference, gear rattle is caused by either excessive tooth clearance or to much end float on the layshaft. Layshaft end float can be set by shims behind the front bush, cleanup the face of the bush and front layshaft support plate, assemble in box no gasket on shaft rear.#, check measure clearance front bush to plate. I set zero clearance free rotation see attachment for bush removel. Use gear oil  75/90 GL4 suitable for 3 or 4spd boxes.        

[Peter Naulls]

Thank you both, very helpful replies. I had not considered the possibility that they might be aluminium bronze, just assumed they were phosphor bronze.

The layshaft endfloat is absolutely minimal so I'm happy with that but how would I know if the noise I'm hearing is gear rattle or selector rattle? One thing I find difficult is knowing what clearance (on gear teeth for instance) is normal. I suppose it's just experience.

Assuming it is selector rattle I had wondered about squeezing the sides to narrow the gap; I think this would work on the 1st/ 2nd gear selector where the contact area is on the edge but the 3rd/ 4th one is much shallower and seems to bear against the sides of the gear all the way down the depth of the selector, so I'm not sure how long it would take to wear down again if I squeezed it.

I use the same oil as you suggest, much better performance than straight oil.

   

   

[JonE]

Does 75/90 oil give any difference in feel/operation?

[Peter Naulls]

It seems less prone to thinning as it gets hot. I find that these gearboxes are inclined to baulk more and more often as they get hot and the multigrade oil reduces this considerably.

[Terrytuned]

The contact area is only at the ends of the selector forks, if not adjusted properly will rub constantly on one side wearing the contact patch. Gear rattle is more noticeable at idle when the box is hot,if you have ever run a straight cut gear conversion they all rattle. The gear shaft centres vary and so tooth clearance can vary, on changing one gear set from a damaged casing into another case the difference was quite noticeable. Terry.

[Peter Naulls]

Thank you Terry, much appreciated. Just out of curiosity are they aluminium bronze or phosphor bronze?

[Terrytuned]

Not sure Peter but they don't appear to be pure bronze. Terry.

[JonE]

It seems less prone to thinning as it gets hot. 

there were a couple of GL4 bottles going cheap on ebay so I pressed the button. Made me wonder, on feeling the consistency, what is the difference between the gear and engine grade scales i.e. is this always "thicker", or much the same as 30 at room temp?
Presumably Peter's comment above also means it might stay behind any form of seals, I'm hoping...