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engine plug size confusion |
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cbr1000dude 
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Joined: August-23-2011 Status: Offline Points: 330 |
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 Topic: engine plug size confusionPosted: June-20-2015 at 3:28am |
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My 90SN has two brass plugs I remove each winter to let out the water, but they are almost impossible to rethread, and only screw in about 3 threads. because of the taper. Today I couldn't get either started, and after a bit of measuring, a 1/4"-18NPT tap went in cleanly about 1 1/2 inches.. A straight 1/4" pipe plug went in easily, and bottomed out flush. I used RTV on the threads and it does' not leak..
Was the wrong plug used at the factory? |
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8122pbrainard
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Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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Posted: June-20-2015 at 8:21am |
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A 1/4 NPT plug should be in the 1/4 NPT drain. You said the plug is straight thread? Are you sure and is it factory?
You chased the treads with a tap and now the plugs go all the way in flush? Sounds like you went too far and now the taper is larger? By the book, normal thread engagement by hand on NPT is 2 to 3 turns. Get rid of the RTV It's not the proper application. Brass does not need any thread sealant but if you want (I do) use teflon tape or a pipe thread compound containing teflon such as Slick-Tite. |
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cbr1000dude
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Joined: August-23-2011 Status: Offline Points: 330 |
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Posted: June-20-2015 at 9:49am |
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The pipe plug I used is steel, and has no taper. Sounds like I went too far with the tap to use a stock brass tapered plug, but the threads were gone for the first 2 or 3 to work. in any case.
Thanks for the info.. Since it's not leaking, I'll take it out in November and use pipe dope next time with a new plug. Although, I've never had anything leak using RTV black ultra sealant. left to dry overnight. |
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8122pbrainard
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Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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Posted: June-20-2015 at 3:58pm |
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Sorry for the confusion but I was following your first comment.
Since you misunderstood the concept of taper pipe threads and went too deep, I suggest drilling and tapping to 3/8 pipe. I also suggest using drain cocks. I have them on all my boats as well as other members. They are not the cheap radiator drains but rather brass with removable stems which allow you to remove the stem only to probe the water cavity for sediment.  McMaster has them. |
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DrCC
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Joined: April-12-2004 Location: at home Status: Offline Points: 2867 |
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Posted: June-20-2015 at 5:30pm |
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Yes, I agree. Those drain cox are wonderful. MerCruiser has been using them for years. And, if you are among the "leave the plugs out" over the winter group, there's no need to clean threads in the spring, which causes the issue in the first place, over time.
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cbr1000dude wrote: