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I've recently purchased my first car (2004 Subaru Legacy), and though I sent it to the shop soon afterward for routine maintenance, I want to learn and do as much as possible myself. 3,000 miles have rolled by since then, and I'm sending it to the shop again to have maintenance done (and a few details checked on).

However, I changed the oil and filter myself today, so they won't need to this time. My question arises because in doing internet research before the job, I found that it's often recommended to change the oil drain plug gasket. So far so good. But the options stump me. So far I've heard fiber, nylon, rubber, copper, aluminum, and steel referenced. There are "crush washers". Some say metal is better, some say it's worse. Some say nylon melts.

My current washer is metal and seems in good shape. The contact with the pan and plug head is smooth, and the thread of the plug is very good. I guessed it would be fine to leave it in this time.

I want to do the best thing here, but I don't know what that is.

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2 Answers 2

I've been changing my own oil since 1975 or so, Daniel, more often than not, several cars at a time (it's been a long long time since I've only had one vehicle). I've never changed a drain plug gasket - I've never seen one that leaked significantly enough to warrant replacement.

Do wipe around the drain hole, and wipe the existing gasket, before putting the plug back in. That helps prevent dirt from getting in there & preventing a good seal.

EDIT: Oh, Crap. Subaru engineers in their infinite "wisdom" elected to equip that car with a real live crush washer, original is copper, domed side to the pan flat side to the plug head. They recommend replacing it at every oil change. Somehow that saves or earns them a little money, as compared to the nylon washer (if any washer at all) that everyone else uses.

If it were mine... I'd replace it (dealer price something like a dollar US each) only when necessary to make it stop leaking, and carry a couple of spares in the glovebox... and very seriously consider migrating to a nylon washer instead.

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I did this, and like I said, I was satisfied that the seal was sound. I'm just wondering why people change it, and by extension, why I might want to. –  Daniel Aug 23 at 19:44
    
I'd think it would need to be replaced only if it was found to be leaky when you started - you'd know that if you discovered your oil level was dropping between changes and you found the plug to be wet with oil and crusted with dirt, really pretty nasty. –  TDHofstetter Aug 23 at 21:05
    
@Daniel, see my edited answer... 8( –  TDHofstetter Aug 23 at 21:15
    
OK. Why nylon? Like I said in my question, I've seen people worry they melt. Is there a benefit that outweighs this? –  Daniel Aug 23 at 23:25
    
The melting point of nylon is in the range between about 425*F and 500*F, both of which are considerably hotter than the oil should ever be in your engine... especially in the oil pan, where so much surface area is exposed to ambient air. Some internal engine parts are made of nylon, for examples timing chain tensioners and sprockets. In both cases, the parts are fully surrounded by oil with no cooling ambient air at all. If it's good enough for that, then it's plenty good enough for a drain plug gasket. –  TDHofstetter Aug 23 at 23:32

Assuming you're buying the OEM filter from a Subaru dealer, they should be giving you a new washer with each one. It's an aluminum washer that's sort of folded over on itself so that it crushes.

My understanding of the crush washer on your drain plug is that it's not a gasket -- i.e., not to keep fluid from leaking out -- but serves as a lock for the plug, and also as a mating surface between the steel of the pan and the steel of the plug, so that neither damages the other. Given that, and given that it deforms in order to fulfill its locking purpose, I believe it's best to replace it, and I'd suggest using the Subaru washer.

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I'm using a Wix filter, not OEM. And I don't think my washer is a crush washer, or aluminum. It looks like a regular washer, with no ability to deform and no evidence that it has deformed. Is my washer more likely to leak than an aluminum crush washer? I don't mind replacing if that's the best thing to do, but if my choice is between a crush washer that needs regular replacement and a washer that doesn't need replacement, I would go for the one that doesn't need replacement. –  Daniel Aug 23 at 19:49
    
The job of a washer that doesn't deform is just to spread load out over the workpiece. I don't know if that's necessary, and in any case it would only happen if the washer's outer diameter is larger than the plug's. Out of curiosity, what metal is the current washer? –  Josh Caswell Aug 23 at 20:09
    
Not sure, but it's not as light as aluminum, and not copper colored. I think steel. Right now it is in its place, and I would need to turn my car upside down to get it off without spilling the new oil... :) I'm the third owner of this car and it's now at 116K miles. –  Daniel Aug 23 at 23:22

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