What size washer for truss rod?

I don't know the size, but... If it isn't a pressing issue, then next time you restring the bass you could take the truss rod nut out (you have to remove it anyway to put the washers in) and take the nut to a good hardware store with a large selection of nuts/bolts/screws/washers (that would be Ace Hardware in my neck of the woods - huge selection of nuts/bolts/fasteners). Simply match a washer that doesn't exceed the diameter of the nut, with the center hole being large enough. That's how I found my washers.

 

I recently tried this with my P-bass, which has a '62 Jazz RI neck. It didn't work, but that's because I'm a technical moron. Other people have done this operation with success. (Read about it here.)
I bought some of the standard #8 and #10 size washers, and they were both too big in diameter to fit down the hole. When I went down a size to #6, the hole in the middle wasn't big enough to fit over the truss rod. There doesn't appear to be a standard size that will fit without a little bit of extra work. If you read the thread I mentioned above, people talk about drilling out the hole in certain washers so they fit, or prying apart lock washers and using those.

 

take that nut to your local hobby shop, radio control cars, helis, planes use all sorts of small, non-standard hardware.

show the guy behind counter what your looking for, chances are he's got a drawer or box of assorted unused hardware you/he can pick through.

 

I've found washers that fit in a hardware store. Take the nut with you. Not all hardware stores have all sizes of washers.

 

If your truss rod has a #10-32 thread as mine does, a sure fit is McMaster-Carr #93574A438.

http://www.mcmaster.com/itm/find.ASP?tab=find&context=psrchDtlLink&fasttrack=False&searchstring=93574A438

It has the right inside and outside diameters and each is 0.020" thick. Two of them on the truss rod will give you just over 1 additional turn of the nut. They're expensive for washers at $8.49 per pack of 25, though.