901 gearbox advice, end float on Drive Shaft Options

[VEEMAN]

I'm currently rebuilding a 901 gearbox. I've fitted a new roller bearing onto the Drive shaft. On tightening the nut on the shaft to as far as it would tighten ( torque unknown, correct torque would be appreciated ). I noticed that I had a 9 thou gap between the 5th gear cog and the thrust washer ( see attached photo)

I have another 901 box and it also has similar play on the shaft at this point. I'm not sure if this gear box has been striped in the past.
So my question if any one can help. Is this play got to be there?
Also if the shaft has to be tightened further, how do you lock the shaft to get the desired torque.
Any thoughts would be welcome.
Thanks.
Veeman

[VEEMAN]

Thanks for the quick reply. I've shone some light at the back of the shaft, and there is a little bit of light shinning through. I had another go with the feeler gauges and an 8 thou one goes right in easy. So it 's between 8 to 9 thou.
I have been told that there should be this free play there, but was not sure. Can you at all clarify this for me.
Cheers
Veeman