Bette,
There is 4 adjustment points for thread clearance on the Singer 500 bobbin area. Here is a pic I took with the adjustment points indicated:
[ATTACH=CONFIG]463558[/ATTACH]
Specs:
A & B= .012" to .016"
C = .015" to .023" ( between the spring and the hook race )
D = .024" to .045"
I know from experience with my #1 500 that if these are too tight the threads will sometimes catch and jam up.
You can check these easily check them with the small narrow gap gauges you can buy at auto parts stores.
If they are too loose I suspect it can affect the tension as well.
I have this info in big .pdf files (2.1 mb), so if it turns out you need it, ( it includes instructions on how to do the adjustments ) just PM me your email and I'll do a screen cap and send it to you.
Or, you can go to the Singer site or the Yahoo slantsingersewing group and download your own. That might be easier.
Joe