I have a 1 1/2" ID Conbraco bronze mushroom-head (NPS thread) thru-hull; the length of the threaded shaft is about 3". I also have a Forespar/RC Marine (Marelon) seacock. 1. West Marine catalog (http://www.westmarine.com/webapp/wcs/stores/servlet/producte/10001/-1/10001/12111/377%20710/0/904011/Primary%20Search/mode%20matchallpartial/0/0?N=377%20710&Ne=0&Ntt=904011&Ntk=Primary%20Search&Ntx=mode%20matchallpartial&Nao=0&Ns=0&keyword=904011&isLTokenURL=true&storeNum=null&subdeptNum=null&classNum=null) says threads on the seacock are NPS which should jive with the thru-hull. However, the Forespar website, if I'm reading this page right (http://www.forespar.com/onlineCatalog/2007/Marelon/06-flangeMountingSeacocks2007.shtml) suggests threads are NPT. What am I not getting right here? Could I have ended up with a seacock with BSP threads? 2. Dovetailing to above question, when I dry-fitted the thru-hull into the seacock, I can get the first 4 full-turn threads on, but then the fit gets increasingly difficult to turn by hand...to the point where I can't get the thru-hull to move at all. Do I have a thread mis-match here? (I'm thinking about exchanging the Marelon seacock for bronze...only about $12 more). 3. And this is the crux of my post. Our hull (1981 Oday 28) is about 1/2" solid fiberglass. The backer block I intend to install is 1/2" fiberglass which will be epoxied into position. This means I have 1" of "hull thickness" for the thru-hull's tube. But the depth of the seacock receiving end is about 1 1/2". Can I assume that I will have to cut off about 1/2" off of the threaded end of the thru hull (remember the shaft of this thru hull is about 3") so it can bottom out in the seacock (or get close) as the seacock fully engages to the backer block? Or should I plan to "shim" the backer block? What is the preferred method for installing these parts? In advance, thanks for your input. Best regards.