1. As I've said I'm well aware of the problems trying to mate NPT and NPS threads. Groco has tried to get around this problem with it's NPS thru-hulls by taking the cheap
way out, cutting/ tapering down the male NPS thread peaks.
2. The appropriate way to create a Groco threaded thru-hull for a ball valve conversion it to use an NPT die on the top 1" of the Groco NPS thru-hull. You will then have an NPT thread EXACTLY the same as the Groco flange.
3. The wall thickness for a plastic Schedule 80 1.5" pipe threaded part (AT THE BOTTOM OF THE THREAD) is approximately .150" and on a bronze fitting possibly less.
4. The "bronze" used by Perko and Spartan and maybe on Groco's best seacocks is not technically bronze. It's leaded red brass or 85-5-5-5 (85% copper, 5% each of zinc, tin and lead.) It's a price
compromise material. I'm not saying it's a bad compromise, I've used them in my 50+ years of boating
experience with excellent success as have most boaters. The lead simply makes machining (especially the threads) easier and cheaper with virtually no loss in corrosion
quality. But it is soft.
5. I would have thought that if you attached a Groco thru-hull (properly machined with the top 1" NPT) to a Groco ball valve the assembly would break at the threads just above the thru-hull retaining nut. If that were the case then I would agree that the Groco flange and Groco valve combination would be stronger. The issue I thought would be simply one of a longer lever arm on the thru-hull combo.
6. However, that wasn't the case. The test showed that the assembly's weakest point was at the FIRST THREAD below the valve.
7. In other words, with the pressure coming from the top of the valve the greatest transmitted pressure was on that top thread just after the point where NPT threaded connection ended. Once that thread valley fractured the valve just sheared away at light speed. The result looked like someone had sawed the thru-hull fitting right below the valve.
8. The question is probably forming in your mind that the Groco flange NPT thread is different. The fact is that all these threads should meet the ASTM standard. If the Groco flange uses a more robust wall thickness than the thru-hull (which I doubt, as it would decrease the flow rate, and there would be a lot of diesels mfg'rs that would be mighty unhappy). The ASTM standard requires 11.5 threads/inch for a 1.5" fitting and 4.8 turns to tight. That leaves you with quite a few NPT threads of weakness on the Groco flange assembly.
I have nothing against Groco, no axe to grind. But, Groco has created two devices, the flange and the combo threaded thru-hull to end run a true seacock. Both products have the same exposed weakness that a true seacock doesn't have. In my mind the bronze seacock wins on safety
The question I've been trying to raise with anyone interested, is what is an acceptable standard regarding impact on a thru-hull (or Groco flange) ball valve assembly. In a way I'm advocating for the Groco product.
My number of hits question was not meant to be scientific, given the scenario I presented I was looking for boaters to give me a sense of their personnel comfort level. So my question is subjectively, is there a minimum number of hits you'd be comfortable with?