Sewer Backup to Laundry Tubs

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