8M sediment bowl threads

We recently bought an 8N that has the threads messed up where the sediment bowl screws into the fuel tank. I bought a new sediment bowl and can't seem to find a proper tap to chase the threads in the tank. What size should I get to do this with. I know it's not n.c and it doesn't look like pipe threads. 9/16 is too small and 3/4 is too large so I'm pretty stumped. Looking for what I need to get this old girl going again. Thanks guys, Keith

3/8 pipe thread is what it originally was...Tom

You may have to drill and tap it for a bushing to get it back to 3/8 P.T. But make sure the stem for the sediment bowl will go up through the bushing first. Assuming the gas tank is dry, rig up a small hose to a vacuum or use some sort of a magnet to get out any metal chips. Sorry you ran into this with your newly bought 8N.

Thanks guys, I was thinking it probably is 3/8 npt so now I need to get a tap and chase the threads. I'm pretty sure there is plenty of meat for them, someone has just cross threaded the ones in there now. ONCE AGAIN, THANKS Keith

Keith, maybe I misunderstood, I thought you said 9/16 was too small. 9/16 is larger than 3/8 I think.

Thanks guys, I was thinking it probably is 3/8 npt so now I need to get a tap and chase the threads. I'm pretty sure there is plenty of meat for them, someone has just cross threaded the ones in there now. ONCE AGAIN, THANKS Keith

NO!!! You don't want to chase the threads on the fuel tank, especially if you don't know and understand threads. If you or someone else has already mucked up the tank port with the incorrect tap, or cross-thread the sediment bulb intake port, the tank is likely ruined. Attempting to 'band-aid' fix it is suicide. Do you want to be nervous every time running the tractor worrying about if the fuel is going to leak? NPT Threads are TAPERED. They are different than standard UNC and UNF threads. 3/8 isn't the nominal OD size like on a standard 3/8-24 UNF thread where .375" is the MAJOR DIAMETER of the nominal thread size. 24 designates threads per inch (TPI). On a 3/8 NPT thread, the 3/8 is defined as the minor diameter of the pipe or tube the thread fits. When an NPT tap is screwed in, the deeper it goes, the larger the top of the hole becomes, it's tapered small to large, and if screwed in too far, will destroy the hole size completely. The purpose of an NPT thread is to tightly seal the connection. As the male part is screwed into the female part, it naturally creates its own seal at each revolution. If it is enlarged beyond the largest portion of the tap, then the hole is ruined. There are GO-and NO-GO thread gauges for NPT threads just like standard UNC and UNF threads. If your 'new' sediment bulb is not going in, be aware that some mew/aftermarket units are junk out of the box -I've had one in my life, and if not careful they can ruin your tank. It matters where you buy parts from. Also related to the fuel line and sediment bulb is the special thread size used on the 1/4" steel line, fittings, and ports, both on the sediment bulb and the carb brass inlet elbow. The thread is a special pitch, used on brake and fuel lines, 7/16-24 UNS and anything else used will only booger it up and cause leaks. Forget Teflon tape, silicone, or any gasket material.

Tim *PloughNman* Daley(MI)