Cooling System Pipe Fitting Sizing?

For several years I have had a small coolant leak in my custom hose reducer. The coolant from the custom heat exchanger exits from an integrated fitting that takes a 5/8” hose. The manifold intake needs a 7/8” hose, so I had to cobble together a strange fitting to match the hose IDs that I just cannot get to seal properly.

I am planning to fix this with two hose-hose barb adapter fittings and a small bit of 1/2 hose. One fitting is 7/8-1/2” and the other 5/8-1/2.” My question is whether the 1/2 hose fitting 3/8” ID) is too small to circulate enough coolant? There is a similar size fitting on the raw water side but that water is always replaced by cool water so I doubt it’s an apples to apples comparison. I wasn’t able to size anything higher than 1/2” since 5/8 and 7/8” fittings are so rare and I couldn’t find Metric equivalents.

Engine is 2 cyl Universal M18 14hp.

Actually, 3/8" ID with 1/2" ODfor your engine will be fine. You will have a hard time finding 5/8" OD fittings with 1/2" ID because they do not make them--it jumps to 3/4" OD. You might want red brass instead of bright brass fittings if you can find them. Less zinc in red brass, I think and preferred for salt water. But if you monitor your system annually, either will work. This size issue is a quirk of the machinist world.

Thanks for the confirmation. I did some more digging into McMaster-Carrs website and finally sourced some stainless fittings with bigger ID’s. I’ll report back when installed.