I would check the opening in the fittings with a paper clip and see if there are any burrs inside the holes that you can feel.

OPS fittings are kinda on the small size for the drilled hole diameter. Maybe new fittings with better hole diameters?

You may even be able to drill out the ones you have but if your not careful you could weaken the neck of the fitting, so you would have to measure the stem size and go with probably a number drill to get the right drill size going to stay with a good wall thickness.

The thread sizes that are in my OPS 45 are M4 for the fittings, I dont know if they are the same as yours. You can safely drill out the M4's up to a M5 size if you wanted to go that route too, but check to see if the hole size gets larger with the threads. There wouldn't be a point it doing that if it didn't. Sometimes the thread size changes but the hole size doesn't. Go figure.

That "Tough Racing" picture is one I had on my computer. Looks like nice stuff and they have other boat stuff too. Here's the [link=http://www.toughracing.com/]link[/link] to that and they even state the hole diameters on some. Not the M4's though, of course...

You might want to investigate into [link=http://www3.towerhobbies.com/cgi-bin/wti0095p?FVSEARCH=pressure+fitting&FVPROFIL=++]pressure fittings[/link] or [link=http://www3.towerhobbies.com/cgi-bin/wti0095p?FVPROFIL=&FVSEARCH=nipple]nipples[/link] at Tower Hobbies. You might find another brand that has the same thread size that has a bit more robust inside diameter.

Most descriptions dont have thread sizes though, so you would have to ask or write them to find out, or look into the manufacture's specs. One thing I do know is OPS is wimpy on the water fitting diameters for some reason.

Another thing to look out for too when you want good flow is staying away from 90 degree type fittings.

One thing I should mention also is the inside hole cant be any larger than the bottom root of the thread, so when looking at fittings, look for the ones that don't have an undercut at the end of the threads at the shoulder. This usually indicates the hole diameter is on the larger size, or if not can be drilled to very close the minor thread diameter. Don't be fooled by the barbed end looking large.

Look for the fitting with the hole as close to the same size as the bottom of the threads. Hard to do sometimes with just pictures though. I think this is the drawback of the OPS fittings. They undercut the thread clearance near the shoulder too much and it restricts how large they can drill the hole.