When measuring for bulkheads, you will need a hole in your tank that is slightly larger than the "C" measurement for the corresponding bulkhead but no larger than the "A" measurement.
1/2" ABS Bulkhead Slip X Slip Dimensions (see diagram)
A - 44mm/1.72"
B - 43mm/ 1.69"
C - 27mm/1.06" (Installation Hole Size)
Use our 30mm Diamond Coated Glass Drill Bit for DIY installation
3/4" ABS Bulkhead Slip X Slip Dimensions (see diagram)
A - 52mm/2.04"
B - 50mm/1.98"
C - 34mm/1.33" (Installation Hole Size)
Use our 35mm Diamond Coated Glass Drill Bit for DIY installation
1" ABS BulkheadSlip X Slip Dimensions (see diagram)
A - 68mm/2.67"
B - 63mm/2.48"
C - 42mm/1.65" (Installation Hole Size)
Use our 45mm Diamond Coated Glass Drill Bit for DIY installation
1 1/2" ABS Bulkhead Slip X Slip Dimensions (see diagram)
A - 86mm/3.38"
B - 73mm/2.87"
C - 59mm/2.32" (Installation Hole Size)
Use our 60mm Diamond Coated Glass Drill Bit for DIY installation
2" ABS Bulkhead Slip X Slip Dimensions (see diagram)
A - 98mm/3.85"
B - 73mm/2.87"
C - 72mm/2.83" (Installation Hole Size)
Use our 75mm Diamond Coated Glass Drill Bit for DIY installation
Example: If you have a 1.75" diameter hole currently in your tank, then you would want to use the 1" ABS Bulkhead.
Looks to me that you actually need a 3/4" bulk head for small hole and 1" bulkhead for larger hole. Realize the size reference in the bulkhead sizing is for the I.D. Inside diameter of threads. Not the O.D. Outside diameter of the threads that are on the outside of bulkhead (used for the nut only to lock bulkhead to glass).
The outside diameter of the pipe would be the inside diameter of your bulkhead. ie inside diameter is 3/4 then you'd need 3/4 outside diameter pipe.
Also, may have been stated already but if using PVC you'd need an inside diameter bulkhead that's slip for PVC to slip in not a threaded inside. I did t ha e time to catch up on the rest of the posts.