Knobs won't sit straight

[Jono Bolton]

I bought some new flat-top knobs to replace the wee dome-tops that were on my P Bass but I can't get them to sit straight. They're split, splined shafts on the pots and they're push-on knobs but I'm guessing that when putting the knobs on one side of the pot shaft inward and causing the knob to then sit at an angle. Are there any fool-proof tricks to getting them to go on straight?

[Prosebass]

A nice snug piece of matchstick in the split on the pot shaft.

[BigRedX]

There are two different types of spline shafts - coarse and fine - see [url=http://blog.philadelphialuthiertools.com/2013/05/16/choosing-the-correct-knob-for-your-guitar-or-bass/]here[/url].

I suspect that you have the wrong knobs for the spline of your existing pots.

[bootleg]

[quote name='BigRedX' timestamp='1454629051' post='2971579']
There are two different types of spline shafts - coarse and fine - see [url=http://blog.philadelphialuthiertools.com/2013/05/16/choosing-the-correct-knob-for-your-guitar-or-bass/]here[/url].

I suspect that you have the wrong knobs for the spline of your existing pots.
[/quote]

I've just refitted the original Spector Tonepump to my Euro LX with the original knobs, but having problems with them sitting straight and staying on. The pots are coarse spline and buttons press on according to this link. Didn't help that the mounting nuts and washers were missing. There doesn't appear to be enough of the pot shaft to get a grip on. Do I need washers between the guitar body inside and outside?