I notice many lav drain whether pop-up or push in or lift n turn, even the drain assembly is specified as a 1-1/2" drain, the tailpiece typically reduces to 1-1/4" in size.

This means you need a 1-1/4x1-1/2 reducing washer to connect to a 1-1/2" tubular p-trap. If you go with a 1-1/4" p-trap, you will still need the reducing washer at the trap adapter inlet anyway.

I personally do not like to use this reducing washer because with a slip joint the trap easily shifts when accidentally bumped and even more so with this reducer.

Is there a tubular plastic fitting that will transition from 1-1/4" to 1-1/2", such that one end connects to a 1-1/4" tubular pipe by cementing, and the other end is a 1-1/2" tubular spigot that can be inserted into a 1-1/2" p-trap inlet? Does something like that exist? Or a "rubber sleeve" of some sort I can slide over the bottom of the 1-1/4" tail piece (below the reducing washer) where it is inserted into the bell of the p-trap to restrict any lateral movement better?

Finally, technically speaking, isn't this against code? You are taking a 1-1/2" drain, reduce to 1-1/4" in the tail piece section, going back to 1-1/2" downstream of that. I know it's insignificant but isn't this against code that says you cannot reduce sizes as you go downstream?

I believe I’ve seen the fitting your looking for at the big box store
The tailpiece for a lav sink is 1-1/4 inside diameter and back in the day plumbers would install 1 1/4 piping
Today 1 1/4 pipe is pretty much obsolete
I think keeney makes the red rubber reducing washer that holds pretty tight to the tailpiece
The plastic one seems to hold tight as well
Although you will have horizontal movement on your trap with any washer
The union sch 40 trap may be a better option if your looking for minimal movement for some reason we refer to it as an L.A trap ( possibly better to hold up during an earthquake)?

The drain is not 1-1/2", it's 1-1/4" so the 1-1/4" tailpiece is not reducing the drain. I've found it's better to use the 1-1/2" trap and use the reducing washer at the end of the tailpiece rather than using a 1-1/4" trap with the reduction going into the trap arm. As bsipps says, the rubber washers that come with the big box "trap-in-a-bag" are pretty good. I just installed one and the whole assembly is pretty solid and doesn't move when bumped.



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