If you can measure the outside diameter of the copper pipe above the pump and subtract 1/8" from that dimension (diameter) you will have the size of the copper pipe.
Then, the pipe at the bottom of the pump and into the boiler should be the same size but is black iron pipe schedule 40. Schedule 40 is standard weight pipe, the same as used for gas lines. Sometimes, the pipe into the boiler is one size smaller than the rest of the pipe, for some unknown reason especially if this an old boiler.( It was just how the boiler companies piped it).
If the system works and heats OK, I would buy the same size pump which usually comes with flanges and gaskets that fit the pipe size used. Today, the flanges and gasket set may come as a separate item depending on where you buy the pump.
The fitting below the pump looks to be standard black iron. You can buy either a reducing tee to accept a 1/2" or 3/4" boiler drain valve or a cheaper way may be to buy a standard tee and a reducer to fit the valve. Regular black iron fittings sold by Lowe's and Home Depot can be used on hot water boilers. They don't have to be a special material. Don't let anyone tell you that you need a special fitting of brass, galvanized, stainless steel or something. Boiler water is usually not corrosive.
Once you take the piping apart in the spring you will be able to better know the sizes of the piping and it's condition. As for the pipe going into the boiler, if it needs replaced, be careful when taking it out. If it breaks you will need someone who knows how to take out the broken piece. From the picture the pipe into the boiler looks like 1-1/4". You can "google " (standard pipe dimensions) to find the outside diameter of the steel piping.
This was a very helpful answer to find what I needed and the pipe sizes. After I used the advice above to figure out what I needed I finally found the manual online (totally forgetting the paper manual I have near the boiler) so included a diagram and the parts so I can accept this answer in case anyone in the future needs this info for a Weil-McLain CG boiler.
From bottom up:
- 1) 3/4" Drain boiler drain valve
- 2) 1-1/4" x 3/4" x 1-1/4" black iron tee (3/4" at bottom)
- 3) black iron nipple. 1-1/4" diameter. 5-1/2" length for Taco 007 circulator pump, 6" for Bell & Gosset little red pump (LR20 1/20HP), 5" for B&G100 or Taco 110
- 4) 1-1/4" black nipple close
- 10) bottom flange 1-1/4"
- 9) top flange 1-1/4" or different depending on size of pipe above
- 13) Pump gasket (came with pump)
- 6, 11 & 5) Bolts and Nuts 7/16-14 nut, 7-1/15x14x1-1/2" bolts. 4 nuts and bolts come in a pack from the pump manufacturer or can be bought at big box store in a pack with the flat style flange gaskets.