One leading cause of stripped threads is over-tightening of the drain plug. This is often done to try to stop a leaking plug. There is a specified torque, which should never be exceeded. This specification varies widely from one vehicle to another and even on the same vehicle with different engines.  Drain plug torque can be found in a service manual for the vehicle.

Over tightening the plug will not make it seal.  Instead it distorts the threads and crushes the seal.  Once the threads are distorted they quick tear away at the threads in the oil pan.

A seal is always used on an oil pan drain plug.  This is because bolt threads themselves do not seal.  No matter how tight, there is always a gap between the plug and the threads of the oil pan. This is by design and the reason an additional seal is always provided.  On some designs the seal is an integral part of the drain plug.

On other designs a separate seal is used. Where separate washer type seals are used, they should be replaced at every oil change. They are designed to crush in order to seal. Once crushed they will not seal again at the proper torque. More torque destroys the threads in the oil pan.

In time integral seal wears out as well. They can be torn, get hard or crushed.  When this happens additional tightening of the plug will only damage the threads, not increase sealing.  Replacing the plug with a new original equipment plug can prevent a very expensive problem.  If you change your own oil, buy an extra plug before you need it and keep it in the glove box.  Keeping an extra drain plug on hand is a wise precaution.  This can help avoid the temptation of reusing a bad plug. 

The drain plug shown on the right is not loose, in fact it could not be removed with a wrench.  The problem is the plug is cross-threaded into the pan and has seized.  Both the pan and plug had to be replaced.  An oil drain plug should be started by hand and will normally thread several turns into the pan with hand effort only.  If the plug does not start straight it will get very tight.  At this point replacing the plug and chasing the threads with a die may save the oil pan. 

With oil pan drains, observing a few simple precautions can prevent a lot of grief. 

Oil pan drain stripping can be prevented with a few simple precautions:

Wipe the threads clean before reusing the plug.

Inspect the threads and seal, every time a plug is removed.

Replace any plug with distorted threads or a damaged seal.

Replace washer type seals every time the oil is changed.

Hand tighten the drain plug as far as possible.

Replace any plug that is hard to remove or reinstall.

Always torque the drain plug to the proper specifications.

Always buy an OEM drain plug, aftermarket plugs are sometimes not of the same quality.