The flat gaskets will replace most all o-rings for that particular size.
2 inch is for most flow thru heater tube ends
See sizes in the more info and photos tab area

2 Split Heater Nuts
We also have split nuts for pumps
Part# Description
89-238-1040 Fits 2"" heater tubes
3-1/16"" across threaded side
2-1/4"" across back side

Please look at the part # on your old fitting
Nuts:2-1/2 is 4 inches OD
2 is 3-5/16 OD
1.5 is 2-7/8 OD

1.5 Split Heater Nuts
We also have split nuts for pumps
Part# Description
89-238-1015 Fits 1.5"" heater tubes
2-7/16"" across threaded side
1-15/16"" across back side

These fittings are called tailpieces
Found at the end of most flo thru type heater manifold housings.

Good generic brand of heater and pump fittings

Part # | Description | Manf. #
89-395-1300 Heater union half w/split nut, 1-1/2"s 0611-15
89-395-1302 Heater union half w/split nut, 1-1/2"s-2"spg 0611-18
89-395-130

Used on low flow heaters
3-5/8" OD across threads
Available in various output sizes
o-ring /gasket not included

Snap nuts are used in place of the older style split nuts with screws.
Part # Description
89-238-1037 1-1/2" snap nut (metal only)
89-238-1039 2" snap nut (metal only)*
*no longer avail, use a split nut