I am trying to drain the engine block to get the thick sludge out but the drain plug is a square female plug of about 5/16ths. Never seen one this size.Is this a bit that can be purchased?Thanks,Chris

 

If it's close to 5/16 then it is most likely actually 3/8. Stick a short extention on a 3/8 drive ratchet and then stick the square drive end into the hole and take the plug out.

 

I haven't tried to remove the block plug on our Cherokee, so I don't know for sure what size it is but I do know there are both 3/8" and 5/16" plugs. 5/16" square bar stock is available at most hardware stores and I've used a short piece of it along with an adjustable wrench to remove those pesky 5/16" plugs that look like 3/8".

 

Bar stock, brilliant! I know for a fact that this drian plug has not moved in 15 years so maybe I'll temper it a bit before using it .
Thank you