Just noticed last week that I had a small leak coming from the oil pan drain plug on my 70-442 (yes, it is fully tightened). I mentioned this to a mechanic and he asked if the leak is seeping through the bolt threads or was it due to a worn out gasket on the drain plug bolt. I purchased a new drain plug and two versions of bolt gaskets (one is the white plastic type material and the other gasket is a copper ring-I understand the copper version is what originally was used as factory new?) The mechanic said if the leak is coming from the threads of the pan and not due to a faulty gasket that I might need to head to a Napa like parts store and ask for a bolt that is up sized one or two sizes, which sounds a bit different than simply a larger diameter bolt, not sure. Anyway, I will tackle it tomorrow afternoon, so thx in advance from anyone that has dealt with this before and can provide advice.

 

The gasket is what seals. Check the threads under the head of your bolt, are they full depth above the sealing washer when added to the bolt ?

 

Did your mechanic change the oil? If so, he most likely stripped the drain plug. He did give you the best (least expensive) advice to up the drain plug size though. This happened to me with my little Hyundai daily driver. It was cheaper to have Mr. Tire change my oil than do it myself. Long story short... got it done, got the car home and the next day there was oil on the driveway. The fools tightened the drain plug back on with an air gun and stripped it out.

 

As stated the oil seals at the gasket to pan-not the threads. The only threads that seal anything are pipe threads where the pipe actually gets bigger as it goes up. Anyway, if the threads are ok then I would make sure you have a nice flat bung on bottom of pan and there are no raised bumps or whatever. Maybe while plug is out take some sandpaper and flatten bung.Then make sure flat part of plug is nice and flat or get a new one and new gasket- should be ok. I would use a nylon/plastic gasket- not copper. Let us know how it goes!!

 

I would be wary of the advice from a "mechanic" who thinks the threads on a drain plug have anything to do with how it seals. They are straight threads. They do not and are not intended to be any sort of seal. That's why there is a gasket washer. If the threads are stripped or if there is a crack in the pan or a knick where the washer seals, that needs to be addressed.

 

GM # 14090908 is a metal/rubber combination drain plug seal that I use on all my cars, even foreign. Works well.

 

Adding a point of clarification/information.... these are called "tapered" pipe threads and designated NPT (National Pipe Taper). NPT pipe is more common. There also pipe that has the same "thread count" that is "straight" NPS (National Pipe Straight). Do NOT mix the two different pipe threads.

 

The drain plug is neither of those pipe threads, to further clarify.

 

McMaster Carr sells various one piece sealing washers with integral rubbed bonded to the washer. They work great!
https://www.mcmaster.com/standard-washers

 

A quick fix is to loosen the plug and wrap some twine around the plug and tighten it back down. Then you can replace the washer at the next oil change.

 

Well you answered my initial feeling. My 69 does this from time to time. In my case "fully tightened" is not enough it needs another half turn. My guess from reading all replies in this thread is my washer bolt combo is slightly worn out (50 years after all) and that extra half turn is compensating for wear out/issues...

 

Copper washers will work too. So will a replacement plug. Provided the pan threads are intact. Pilot point pan plugs are still available.

 

Fixed the leak at the drain plug today. I used a fresh plug/bolt and what is the more common white hardened plastic/teflon gasket. I used a ratchet to get it snug and now without issue. Thanks for the tips. The old gasket hard become a tad inverted and just worn out after 50 years.