1989 Grand Marquis oil drain plug size

[GMUE03]

Hey guys, recently I bought an 89’ Grand Marquis. When I went to do an oil change I found that someone previously stripped either the plug or the pan on the rear sump, and put one of those rubber expandable plugs in :/. I did notice that there were some threads still in the pan so I went and bought a 1/2 - 20 drain plug to see if I could get lucky. The plug starts to thread about maybe 2 full turns then starts to get some resistance. I think I can run a thread chaser through the pan and fix my issue.

My question is what size drain plug is on the 89 panther 302s? I was doing some reading on the forum and someone said that 1989 is the only year with a metric drain plug. Is this true? I don’t want to use the wrong size tap if that’s the case.

Thanks.

[sly]

It should be 1/2-20. If it's a later pan from a 90/91, it could be M14-1.5.

[GMUE03]

It should be 1/2-20. If it's a later pan from a 90/91, it could be M14-1.5.

Ok good to know. Pan looks original to the car. Thanks!

[Tiggie]

I'm nearly certain my 88 and 90 are both standard thread, my 94 Mustang with the 5.0 does have the metric.

They make one and two step oversize plugs. I have had good luck with them.

[WagonMan]

Take the front drain plug out and compare the threads to the new aftermarket plug you bought. WagonMan

[GMUE03]

Take the front drain plug out and compare the threads to the new aftermarket plug you bought. WagonMan

Good idea. I noticed some of the threads on the front plug are actually missing too. Looks like I’ll be making another trip to the parts store

[GMUE03]

They make one and two step oversize plugs. I have had good luck with them.

Ok interesting. To my knowledge these oversized plugs keep the same thread pitch but are a slightly larger diameter? Do you run a drill through the pan or just let the new plug cut the threads? Asking just in case the original threads are complete FUBAR, but fingers crossed!

[friskyfrankie]

I believe the entire theory of the oversized plugs is to simply run them in without having to tap the hole.

[GMUE03]

I believe the entire theory of the oversized plugs is to simply run them in without having to tap the hole.

Ah ok. Followed by a nice flush of oil too. That’s gonna be plan B it seems.

[Tiggie]

I believe the entire theory of the oversized plugs is to simply run them in without having to tap the hole.

Yes, no tap needed. They are just barely oversize and fill out the threads that are a little overstretched.

[GMUE03]

Update: I ended up carefully running a 1/2 20 thread chaser through the pan and installed a new drain plug. I was able to tighten it, no leaks so far (fingers crossed).

To answer the original threads question; yes, an 89’ Grand Marquis has a 1/2 20 oil pan plug on both the front and rear. Thanks for the responses everyone!

[sly]

Yeah... I think it was 90.5 that went to the metric pan.

[gadget73]

kinda wonder if someone got the metric plug and tried to jam it in there.

[WagonMan]

I always wondered why threads that are always oiled can get buggered up. I have seen oil pans ruined from oil change places that use impact tools to remove/replace drain plugs. WagonMan

[friskyfrankie]

I always wondered why threads that are always oiled can get buggered up. I have seen oil pans ruined from oil change places that use impact tools to remove/replace drain plugs. WagonMan

You've answered your own question. When you have "mechanics" that never made the jump from ape to human, this is just one of the negative issues to be expected!