Well, Well, Well Nuts

In one of your articles on holding tank probes you offered the idea of using "well nuts" by drilling a 3/8-inch hole. Great article, by the way. What size well nuts would you recommend buying and does this move the actual probe far enough off of the tank wall to insure it will take awhile for the buildup, etc., to really be a factor?
Bill, (Trussville, AL)

Thanks for the compliment Bill. I use 3/8-inch well nuts; that is, the hole size is 3/8” in diameter. Be very careful drilling into polyethylene, ABS or any plastic holding tank. The drill bit has a tendency to grab the soft plastic and create an off-round hole. I recommend using a Forstner-style drill bit. You’ll want the holes for each probe to be perfectly round to avoid any chance of a leak. Simply push the well nut through the hole all the way up to its flange. There is no need for any sealant behind the rubber flange of the well nut itself; however, it will be necessary to apply sealants to the threads of the machine screws. The screw passes through the well nut and is threaded into and through the embedded nut and into the tank. Thus the machine screw becomes the actual probe. If no sealant is used on the screw, liquid will “follow the threads” and leak down the outside of the tank. I also recommend using a washer with an outside diameter a little larger than 3/8-inch so solid pressure remains against the outside wall of the tank.

Tightening the screw bulges the nut section inside the tank and creates a good seal. Do not over-tighten the well nut. If the monitor panel wires have ring connectors attached, be sure to insert the machine screw through the ring connector first before threading it into the well nut. Be sure also to use stainless steel machine screws and washers. Unfortunately, paper and waste can still hang onto these probes because they stick into the tank a little further. 

Update! Since this question was posed, a new product came onto the market; Horst Miracle Probes. The RV Doctor is now recommending that only these probes be used as replacements for any through-the-wall monitor probe. Read about Horst Miracle Probes on the Product Spotlight Page.