I'm trying to create a standpipe for my laundry tubs drains to prevent water
back up after heavy rains, but I'm having trouble getting PVC fittings to
work.

The inside-the-tub threads are slighty to small for a 1 1/4 in MIPS adapter
and too wide for a 1 inch adaptor. Any suggestions as to how I can get this
to work.? A check valve in the tub drain line and a back-water valve are
down the road, permanent fixes
..

Thanks Ken B

Ken,

Fix the reason for the backup... When it rains, why does it backup? Are
you on a septic system?

Randy