Craftsman 1.5HP air compressor: Drain plug upgrade and oil change

     It happened again. A slight nudge with the bumper of a car and my Craftsman air compressor model 921.152100 is on it's side with a broken drain plug. This fragile part had broken before in a similar fashion. Although this is not a costly part, I was tired of dealing with this deficiency in an otherwise stout and reliable machine. In this post I will show you how I upgraded this part from fragile to unbreakable. And I'll throw in an oil change.

                                                     The job at a glance

• Tools: Electric drill, 5/16 Allen wrench, Optional: channel-lock pliers and 15/16 socket.
• Materials: 1/8 drill bit, Air compressor oil, pan for drain oil  
• Parts: Steel set screw for plug: size 5/8 - 16    5/8" long    
• Cost of parts:  $.75
• Time: 45 minutes    

The arrow points to the missing drain plug

Preparing the plug

1) Find the correct size set screw for the plug. I took the broken part to a hardware store and used their thread matching jig to find it. The correct size is 5/8-18  (The "18" is the threads-per-inch count). The plug I found was 5/8" long but this dimension is not critical. 

2) Drill the hole for ventilation. The original plastic plug had two approximately 1/8" holes in it. We must vent this plug. Just use a sharp bit. This plug was recessed so I didn't need to center punch it.

The upgrade is ready

3) Wrap the plug threads in teflon tape.

Changing the oil

1) Lift the compressor over a drain pan and drain the oil. It won't take long, there isn't much in there. The manual omitted the oil volume required so I determined that by weighing the used oil in the machine. I measured it at 38 grams in a one time attempt to establish exactly how much oil to add.

2) Optional: The owners manual instructs to remove the sight glass to drain the remaining oil. With channel lock pliers and a 15/16 socket drain the rest of the oil.  I measured a mere 4 grams. This is hardly worth the trouble but the manual recommends it so I did it. Reinstall the sight glass.  

3) Fill with fresh oil. I weighed 42g of oil and drew it into a syringe to determine volume. It is 45cc or 1 1/2oz.  Fill and make sure it is level with the center of the sight glass. I put a sticker on the top of the compressor with the correct volume of oil to add to assist me in future oil changes. I try to change the oil once a year.

4) Install the ungraded drain plug. Don't over-torque.

A 5/16" Allen wrench will now tighten our plug

5) Test. Air does flow out of the plug. No leaks and good to go. Let's inflate some tires!