I just purchased a American Standard 1640.305.002 Deep Soak Bath Drain for a bathtub/shower.The specs say the tailpeice pipe is 1 1/2"OD. Last night I slipped this tailpeice into a 1 1/2" abs pipe & it was too small by maybe an 1/8" or less around the abs pipe so maybe it is 1 1/4". This tailpeice also screws into the overflow tube assembly.I also noticed there is a male thread on the outside of the overflow tube assembly where this tailpeice screws in. I have few questions.1) Is there an adaptor to fit this male thread on the tube assembly so I can connect it to the 1 1/2" abs pipe? Is this tailpeice necessary if I find an adaptor to fit on the male thread? What would be the right technical name to use for this adaptor if I go to the local hardware store. I have an option of using 1 1/2" or 2" abs pipe since I'm running a new line out to the existing pipe? Any help would be much apprieciated. Thanks

1. Yes, an 1 1/2" ABS Female adapter. No need to use the 1 1/2" tubular tailpiece if you use a female adapter. You could use an 1 1/2" trap adapter glued onto your ABS pipe, and the tailpiece drops down into the trap adapter and is secured by a beveled washer and nut.

using the tailpiece makes it easier to install. you don't have to have a perfect measure as the tailpiece slides down inside a slip adapter.

Would a 1-1/4" to 1-1/2" abs increaser(tailpeice to 1-1/2 abs pipe) work, then a 90 degree elbow (1-1/2 abs)with 3 feet of 1-1/2 abs pipe to the main sewer work

there is nothing about that tub drain that will work with 1 1/4 inch pipe.
you need a trap under the tub and that trap must be properly vented to prevent syphonage..

Your tailpiece is 1 1/2. Measure its OD and you will find it is exactly 1 1/2. 1 1/2 ABS pipe ID is more than 1 1/2 in. Tailpiece is tubing and specified by OD, ABS pipe is pipe and specified by nominal ID. Actual ID is more than nominal ID for the smaller pipe sizes.

Why are you asking about 1 1/4"? The tailpiece is 1 1/2" and the piping is either 1 1/2" or 2", so there is NO 1 1/4" anywhere.

I bought a threaded adaptor to screw on the drain so I can glue a small peice of 1-1/2" abs pipe to it & attach my p-trap with a clean out plug on the bottom of it. Another question: I have a run of less than 18" with my 1-1/2" abs pipe to the main 3" abs line. I have a 1-1/2" Tee to vent. Should I install this after the drain or just before the 3" line or does it matter.The higher end of the 3" abs pipe runs from a toilet. The toilet is a new install as well. The toilet is not vented, so I was thinking of venting the tub & toilet together from that 1-1/2" tee just before where the 1-1/2" ties into the 3" line. From the toilet to where the tub drain would tie into the 3" line would be less than 2'. Can I vent the toilet together with the tub or do I need to vent the toilet separately?