How tight is too tight on a holding tanking fitting install?

[vmaks]

I had to replace me holding tank due to a decent size crack at the bottom. The tank was on an '86 Pearson. I had a new tank made, it is ready for me to take to the boat when she gets launched in May(Lake Michigan). I am getting confusing info re how tight do I thread in the npt fittings (x3- 90°) once I "hand tighten" and how many wraps of Teflon tape is suggested. The tank company said to tight as tight as I can go, other readings say if I tighten too much I can crack the tank, other forums say hand tighten than add 2-3 rotations with a wrench. Consensus??? Tank is polypropylene with heat sealed spun threads with backing plates and counter sunk into the tank as to make a flat surface on the outside(tank is a very tight, shoe-horned fit into the v-berth,there was little to none wiggle rm for me to modify the tank configuration).
1. Wrap 5-6 x's of Teflon tape around the threads starting at the end
2. Hand tighten the fitting onto the tank
3. Wrench tighten....HOW MUCH MORE TO TIGHTEN??????
4. Warm hoses (via warm water or hairdryer) + use KY lube to install on the barbed end, secure with x2 clamps which are opposite of each other by 180°.

 

[RitSim]

The purpose of tightening is to get the male thread to conform tightly to the female thread and provide some mechanical stability to the joint - no free rotation. The sealants (tape or dope) provide some lubrication while putting "stuff" into the gaps between the thread forms. So IMHO, just tight enough to assure no free rotation and a leaktight joint. I prefer to use dope on joints that are critical- a waste tank would qualify- unless you want to do this job again. If you choose dope, read the label. Some dopes are NG on plastic. Using tape is no guaranteed solution unless the tape in inside the joint gaps. Putting too much tape just gets pushed out as the pieces are assembled. Good luck.

 

[Peggie Hall HeadMistress]

The "rule of thumb is is "no more than 1/2 turn past hand tight." Reason: Threaded tank and hose fittings are NPT standard, which is slightly tapered--too slight to see with the naked eye to see in in a female tank fitting. Over-tightening male hose fitting will crack the female "collar"...maybe not immediately, but changes in temperature causes plastics to expand and contract. A hose fitting installed in cool weather will expand in hot weather and that's when you'll hear a "ping" coming from your holding tank--the sound the female "collar" makes when it cracks.

So you want to tighten hose fittings enough to prevent them from leaking, but not enough to risk cracking the female fitting.

--Peggie

 

[JoeWhite]

Can you install the connections while tank is outside and fill with water and let it sit overnight to check for leaks?

 

[Peggie Hall HeadMistress]

That would require plugging all the the hose fittings, otherwise the water would just run out of them. Installing the tank and toilet, then filling the tank with water before using the toilet would be easier and make a lot more sense, but still unnecessary 'cuz if the fittings are tight enough, they won't leak. If they do, tightening 'em a quarter turn more can be done with hoses on 'em.

--Peggie

 

[vmaks]

As you can see this tank has a really tight fit and a unique fit in the allowable enclosed area. These pics are of the old tank, old hoses,etc. We have a new tank, same fitting configurations on the tank, better hoses, larger vent + hose. I had to dismantle the entire v-berth enclosure to access this tank. Once I put e'thing back together, clearances for the front of the tank and the top are minimal. Without the enclosed wood structure the v-berth is in-operative as it provides the v-berth bed support. As Miss Hall suggests I will either use quarter mold or rubber blocks + webbing straps to keep it from moving.
Thanks all for the info, I will absorb the info do my best. Once completed my plan was to fill with water, wait a few days, see if there is any seepage, than pay the $10 for the pumpout. This will also allow me to reinstall my tank monitor tape and recalibrate the sensor for empty/full.
Miss Hall has clearly stated using Teflon tape. Others swear by goop. Is this one of those personal preferences things?

 

[Tally Ho]

I would use Teflon tape, 3-4 wraps should do the trick, and then once hand tight, go 1/4 to 1/2 turn more. 1/4 turn if it seems tight, 1/2 turn if it gets to 1/4 pretty easily.

When I pulled the water tanks out of my 1988 O’Day 322, I was surprised at how burly the threaded holes in the tank were…I was thinking they were just in 1/4” thick tank…but they were built up inside the tank quite a bit and probably had 3/4” to 1” of threads. I was more worried I would break the plastic male fittings I was screwing into the tank.

I think I used some TFE paste on mine, but tape would be less messy I think.


Greg