Just started doing some of the maintenance on my bikes myself. It’s fun, greasy work, and I seem to be going through a lot of towels I’m stealing from the gym.
How do I clean shop towels without ruining my washing machine?
Take them back to the gym. 
Dude, go to Home Depot or Lowes and get yourself some shop towels. -No wonder the darn gym membership fees are so high!! You bad man.
In my defence, I always take the really crappy threadbare towels they were going to chuck anyway.
Do you just throw the towels out when you’re done with them? Or is there a way to clean them?
I steal my from work and then throw them out when I’m done. No wonder the price of cars is so high.
jaretj
old t-shirts, old towels, old sheets, etc. torn or cut into smallish or medium squares works great for me. i throw them away when they are black and dirty.
washing them in your washing machine not a great idea as the grease, dirt, and oil can contaminate your machine and possibly the other clothes you wash there.
Brilliant. Sometimes the answer is right in front of you and you can’t see it.
Also, wash them at the laundry mat.