Transmission leak

I have a Commando MK III that I am trying to put back in proper order. A previous owner butchered it in many ways. I have a transmission leak that I believe is coming from a large acorn nut near the bottom/front of the gear box. It looks like it has a flimsy aluminum washer that is allowing the leak. I think I have identified the acorn nut as holding up the "detent plunger" and spring. If I pull the acorn nut to insert a corrrect washer, what are the chances of getting a loud (and potentially expensive) clunk from inside the gear box.

That shouldn't cause any problems:
http://www.oldbritts.com/gearbox_info.html

Thanks for the site. I am very familiar with Old Britts, they already have a bunch of my money. I don't remember seeing that portion of their site. For those not familiar with Old Britts, they are very helpful. With some fear and trembling, I will try the new washer route.