2002 Buick Century

The last time that the oil was changed was in a shop (Sears). That plug is in really tight. The odd part is that I can't seem to get a wrench on it good enough. 5/8" is too big, so that an open end 5/8" wrench can move about 40 degrees or so before it locks on the plug's hex nut end. With alternatively trying a 12-pt box end wrench or a 12-pt socket, there is also enough movement that I'm afraid to crank hard on it for fear of rounding. But a 9/16" box or open end or socket is too small to even get on.

15mm seemed too big, and 14mm too small to get on.

From what little I can see using a mirror and flashlight, the plug does not look rounded.

So I tried an adjustable wrench and set it to a nice tight fit, but I can't get myself in close enough to hold my free-hand thumb on the thumb-adjuster wheel -- so when I push down hard on the wrench the jaws open a little. I did not use any kind of extension on the wrench. After two attempts like that, I quit for fear of rounding.

So what should I do? Try to find a 19/32" wrench? Would a 6-pt 5/8 socket, if I can find one, be the approach least likely to cause rounding? (I've removed drain plugs dozens of times in the past lying on the ground and using an adjustable wrench, but there was never one this difficult.) Thanks.

(Advance Auto's site is no help as to size. I suppose it's 5/8", but why would an open end wrench be loose before I even put any force at all on it?)

  • 1
    It's most likely a 15mm (for the bolt head). Nov 1, 2018 at 21:31
  • 1
    A 5/16 whitworth?
    – Solar Mike
    Nov 1, 2018 at 21:41
  • @SolarMike Exactly. I hate when BSA makes parts for Buicks. In 1971. Just wait until he needs to add dashpot oil in the dual Stromberg or Standard Union carbs.
    – SteveRacer
    Nov 2, 2018 at 10:17

1 Answer 1


According to rockauto.com, your drain plug should be an M12x1.75 with a 15mm head. If a 15mm wrench is not fitting onto it, get a new drain plug, then get the old one off using whatever method which will work (short of breaking something you still need!). Using a pair of finely tuned Vise-Grips should most likely do the trick. You might also try a 15mm 6-point socket.

NOTE: I have no affiliation to RockAuto.com ... it's just a convenient source of information on parts.

  • That's seems to answer that, thanks. I'll try the 15mm 6-pt that I just bought, when I have an available rain-free day. You don't think that Vise-Grips would tend to round the plug's hex-head more than the 6-pt socket could? What about using a short pipe as an extension on Vise-Grips, is it possible? ty again.
    – Bob3411
    Nov 3, 2018 at 21:21
  • @Bob3411 - The idea of the Vise-Grips is to get the bolt out, not to reuse it. Vise-Grips, if used correctly, will take it out, I have no doubt. It will, however, need to be replaced, which is why I stated to get the replacement bolt before attempting. Using a 6-point socket should do the trick, though, so go that route and use the Vise-Grips as a last resort. The plugs usually aren't in there too tight, but who knows what the arm-strong mechanic used to tighten it. Nov 3, 2018 at 21:43

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