Thread chaser for rusty drain-pipe cleaning hole?

My house has a drain pipe for the kitchen sink, and the pipe gets plugged sometimes. Then you have to remove the plug or bung from the cleaning hole and run a snake down the pipe.
Last time I had to unplug the drain, I wanted to replace the (60mm plastic) plug. The drain hole is in a fitting that looks like cast iron, and the house is about forty years old.
I had a lot of trouble getting the new plug to engage the threads of the cleaning hole because the threads were rusty. I took a stainless-steel wire brush (and I think some Liquid Wrench) to the threads, but that didn't seem to do much.
So I just put the old plug back in---that wasn't too hard to do. But I am afraid of cross-threading the plug and ruining it sometime.
Next time I have the plumber over to do some work I thought I might ask him to do something about those threads.
Would he have some kind of tool to clean out the threads without much risk of messing things up?
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Probably the easiest thing to do would be to get *two* new metal plugs for the hole - preferably iron. Take one, and with your 4" angle grinder, cut slots across the threads so it looks similar to a very large tap. Thread that into the hole and tighten it well, letting it clean the threads of the old pipe. Then remove it and install the second, unmodified one as normal with pipe dope.
I don't even want to think about what a tap would cost for that size of a pipe thread; I do have a thread chaser that works on large/ unusual threads but it was about $100, and only works on straight, external threads.
nate
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wrote:

Probably the easiest thing to do would be to get *two* new metal plugs for the hole - preferably iron. Take one, and with your 4" angle grinder, cut slots across the threads so it looks similar to a very large tap. Thread that into the hole and tighten it well, letting it clean the threads of the old pipe. Then remove it and install the second, unmodified one as normal with pipe dope.
****************************************************************
I recently did exactly that on the thread for a sink drain. It cleaned them up a bit, but notthe heavier rust further in. Maybe I should try it again, but try hardening the modified thread before use.
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Matt wrote: ...

Sure, but might mention it specifically when you call so he's sure to have the threading tap w/ him on the truck -- it's likely it's in his standard kit, but for a routine call threading a fitting would be pretty unusual so might not carry them all the time.
What you need to do while it's fairly recently been out is take it back out and use white lead or a non-drying plumber's thread dope on it liberally.
That will help keep the threads from "growing together" w/ time/ corrosion.
I have several which have the plastic plugs in cast as well and they do tend to gall. The nice thing is they can always be destructively removed w/o hurting the cast if it ever does come to that, but by using a dope or grease liberally you can prevent a goodly amount of the problem.
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take it apart thread in a plasic extension fitting or new permanent metal fitting. so you have a new fitting to work with
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snipped-for-privacy@aol.com wrote: ...

This is cast drain line, not galvanized...
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threads. Be sure to wear protective glasses so you don't get wire or debris in your eyes. If you want to take the time you can use auto valve grinding (lapping) compound and the plug to clean the threads. Just apply the compound to the threads and screw the plug back and forth while slowly working it in. Renew the compound when it quits cutting. This works for any kind of threads but can take a long time.
Don Young
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Matt wrote:

Rather than looking for a way to make removing the clogs easier, why not look for a way to keep the drain from getting clogged in the first place? The best thing you can do is stop putting junk down the drain that doesn't belong there. If you don't have a garbage disposal, stop putting food down the drain. Scape leftovers into the garbage can. Scrape as much grease and fat out of pots and pans as you can and toss in the trash before washing the pots and pans. Hot grease can be easily drained into an old tin can where it will congeal, and you can throw the can in the trash.

Baloney. You have the wrong plug. If the old plug screwed right back in there is nothing wrong with the threads so much that a clean new plug would not screw in. The new plug should fit easier.
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snipped-for-privacy@rochester.rr.com wrote:

Maybe about once in five or ten years.

Oh, but I do.

garbage, not trash

Did it register with you that the old and new plugs are plastic?
If the hole is not 60mm, what size other than 60mm would be about 60mm?
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Matt wrote:

A 2" NPT pipe thread is approximately 60mm in diameter.
See, the NOMINAL diameter of the inside of a 2" iron pipe is 2". The wall thickness is approximately 3/16" thick. 60mm is approximately 2-3/8". You do the math.
You say the new plug is plastic? You won't hurt the cast threads a bit if with a plastic plug if you happen to cross-thread it. The plastic threads will simply strip out.
I'll bet if you compare the two plugs closely you will see they aren't quite the same...
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snipped-for-privacy@rochester.rr.com wrote:

Yes, and so is the old one.

Right, well, I can't get the new plug to stay in. I get the threads to barely engage, but the plug pops out when I turn it. Then I look at the threads and see that they are deformed. Isn't this explained by rust filling the threads of the drain hole?

I don't much want to take the plug out just to check. My explanation of the old plug going in easily and the new one going in hard is that the plastic threads of the old one are deformed (somehow crushed and/or shaved) so as to conform to the rust-reduced size of the threads in the drain hole. It seems basically that the end threads of the old plug are smaller than the other threads, as if the plug is tapered more. The new plug is not so deformed and the threads are too big to go in.
I come back to the question of whether there is some size close to 2 3/8" that isn't 2 3/8". I don't believe anymore that there is such a thing as a 60mm nominal plug.
In what way(s) do you think the old and new plugs might differ---I mean how might they have differed when they were both new?
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The most simple solution would be to install either a Benjamin cap or a test plug:
http://www.mobilemart.com/product.aspx?sku 981
http://www.plumbingsupply.com/testing.html (scroll down to see 3 types)
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On Fri, 24 Oct 2008 13:31:17 -0500, Matt

Simple. You probably can't find the correct tap for the pipe and if you did it would cost more than you want, but you don't really need that particular tap.
Instead find or borrow any smaller tap with the same thread pitch and use it as a thread file. You'll have to manually hold the tap in the proper orientation and scrape the rust from the threads.
Use the link below to determine the pitch of the tap http://www.plumbingsupply.com/pipethreadsizing.html
Example: If you pipe is 2" then you need any 11-1/2 pitch tap. A 11-1/2 pitch tap is only common to pipe threads, not screw threads.
http://www.engineersedge.com/screw_threads_chart.htm
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