2GM20F Water Heater / Heat Exchanger

[cfoxcvg]

I am about to refit with a new heat exchange water heater. Does anyone know what size and thread the hose barbs need to be on a 2GM20F motor?
I have had 2 local shops tell me they are 3/8 but you need to tap the holes first, I would prefer to buy the metric size that fits and not tap out the pump housing. Yanmar has a kit but it is $80 for 2 plugs and well money is tight so I need to look at options.
Any ideas?
I am located in Ft. Lauderdale if anyone knows a good place to go.

[cfoxcvg]

My water heater is also going to be slightly higher than the motor. Do I need a surge tank? I don't understand the purpose since the water heater is a small closed circuit.
Can anyone enlighten me?

[never monday]

IIRC, it's 3/8" pipe thread. You'll need to remove 2 allen head plugs