Mine is leaking like a seive. The dealer wanted $50 for a new one, and the service manager told me it's just a rubber hose w/ two clamps. So, what size is it? I'll go to autozone and get one tomorrow.

 

My oil return line is steel. It slides in to a rubber hose on the block under the oil filter with a hose clamp. If I had to guess I would say the rubber hose on the block is 3/4in.

 

I am not sure of the diameter off hand. I would say its around 1".

Do not just go and buy any type of hose. Usually places like Autozone want to sell you "heater" type hose. Oil will disintegrate heater hose.

A good piece of silicone hose rated for temperature is what you want. You might try your local Cummins dealer it should only run you $10 or so.