Do tub drains of the same diameter ever have different thread-per-inch counts?

Question

My tub's drain flange has had the little cross piece inside it broken out, which means I cannot screw a grid drain into it. Right now, it's totally open and an invitation for my toddler to drop stuff down. Not to mention lots of hair, etc.

I believe I need to replace the drain flange and put a grid drain on top of that (I want a grid drain like a shower instead of a typical tub stopper).

"Drain flange" terminology from this image.

I have measured the diameter across the drain at just under 2" which I believe means a pipe thread size of 1.5" (from http://www.plumbingsupply.com/pipethreadsizing.html). However, I thought I remember learning that drains may be either coarse or fine threaded.

Before removing existing drain flange, is there any way to know if it is coarse or fine threaded, or are all 1.5" drains the same as far as thread count goes? I want to be sure to order the correct part, whether I do the work or hire a plumber.

Answer 1

I have never come across flanges that weren't compatible, and haven't seen manufacturers specify TPI on their kits or replacement parts. There are plenty of oddball pieces here and there but upon coming across those (on rare occasions), I'd be replacing the entire assembly.

There is really no way to know the TPI without removing the flange, but chances are you will be dealing with coarse threads.

Answer 2

For the record, there ARE different threads. I just bought a new flange (only one type was available for sale in my neighbourhood) which did match the thread count on the existing drain shoe. Particularly annoying in that I was unable to purchase a new drain shoe.

Actually, the drain shoe is what I was really trying to purchase as the existing one had broken. I only bought the new flange only because the old one suffered mior damage to the cross piece during removal. In the end, I had to clean up and repair the old drain shoe and then reuse the old flange.

Caveat: My house is over fifty years old, and the drain shoe and flange in question are probably of similar vintage.

Answer 3

Use a thread TPI tool that comes with a tap/die set to find the correct TPI, and yes they can be different depending on age of the tub/brand/style.

Answer 4

I just replaced mine in a late 80s vintage house and thought the common 1 1/2 by 11.5 tpi size would work. When I removed the the old one, they looked the same. As is recommended, I tried a "dry" fitting to make sure the new flange would fit. It started fine, but after six or seven turns without any resistance it started to get very tight with the flange a quarter inch above the tub mating surface. I thought the receiving threads might be dirty, so I removed it and tried my best to clean them. Same result the next time. Turns out the old flange was 12 tpi. Thankfully I discovered the difference before assuming it was just cruddy. I was unable to find a 12 tpi flange anywhere. Ended up using a 1 3/8 flange and the included plastic adapter with 11.5 tpi. The adapter is less than a inch long and got tight low enough in the fitting to allow the new flange to screw all the way down to the tub surface. There are absolutely different thread counts in 1 1/2 inch drain flanges and not only in very old houses.