Bad threads on the water tank. Ideas? - Page 2 - Fiberglass RV


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Old 04-22-2012, 05:44 PM   #15
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At your local autosupply there are items called "Self Tapping Drain Plugs" that are useful when your local $.50 oil change place cross threads your pan plug. These are designed to enlarge and rethread the hole at the same time. You might be able to use one of these to open and rethread the existing hole to the next thread size and use a new fitting.
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Old 04-23-2012, 04:57 AM   #16
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Thanks all. Lots of good ideas. Not ready to cut new threads or go the Deck plate route yet. Floyd gets the prize for asking "What's the tank made of?" Turns out RV water tanks are made of polyethylene, similar to milk jug plastic. One of the reasons polyethylene is used for milk jugs is that it doesn't bond to other things. For that reason it can't be glued. Well almost. There are some products out that will stick to it. I am still researching. Raz
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Old 04-23-2012, 07:24 AM   #17
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my guess is thats thermomolded plastic. epoxy might,,,or might not stick.
to properly repair it you would weld it. this is not very hard, and harbor freight sells a plastic welding tool pretty cheap.
another option is an rv dealer,,,i'm sure they have had this issue and may have the perfect adhesive for this repair.
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Old 04-23-2012, 08:05 AM   #18
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Turns out RV water tanks are made of polyethylene, similar to milk jug plastic. One of the reasons polyethylene is used for milk jugs is that it doesn't bond to other things.
Yes, gluing to polyethylene is a nightmare. Epoxy won't work on that - been there, tried that.

I've lost track of what the material you're trying to bond to it is, but if it's also plastic then I would go to a modelling (as in scale modelling) store as they have very good solvent glues that will work with most plastics. It's a very thin liquid so you apply it to the join between the assembled parts (yes, assembled) and capillary action draws it into the joint.
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Old 04-24-2012, 05:06 AM   #19
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Here's what I have found out. There are several companies that make a adhesive that will stick to polyethylene. The only one I found that does not require special pretreatment or equipment is made by Loctite. Unfortunately it won't work in my application as it doesn't gap.

After reading everyones ideas, here is what I am doing. I tried Thomas' idea of adding a hose clamp around the outlet and it did indeed tighten things up. To fill the threads I am using a pipe joint compound paste (with teflon) suitable for plastics rather than Teflon tape. I let it cure over night. I am replacing all the pex pipe with vinyl for flexibity and will add some foam board on the sides of the tank to lessen its movement.

I suspect this is a short term fix. At some point in the future I will add an access hole (deck plate), drill out the fitting hole and add a brass nipple. Thanks again for all the ideas. Raz
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Old 04-24-2012, 06:26 AM   #20
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Raz, I probably missed something here along the way, but couldn't you find a new tank replacement and cut the old tank out ?
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Old 04-24-2012, 06:40 AM   #21
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Raz, I probably missed something here along the way, but couldn't you find a new tank replacement and cut the old tank out ?
Tank is very easy to remove as it just sits in a tub molded in the floor. What has happened is the outlet was leaking. I believe the threads were damaged because of movement of the tank and a lack of flexibility of the pex tubing. I am trying to repair the leak on the cheap! It's an interesting problem and I am learning about water tanks and there repair.
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Old 04-24-2012, 07:28 AM   #22
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.............I tried Thomas' idea of adding a hose clamp around the outlet and it did indeed tighten things up. .......... Raz
Did you heat the plastic at the same time? I think that would make a lot of difference. I use an electric heat gun like this:
Heat Gun - Dual Temperature Heat Gun, 1500W
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Old 04-24-2012, 07:33 AM   #23
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I agree I think the heat gun would help a lot.
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Old 04-24-2012, 08:26 AM   #24
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Prior to putting on the paste I tried a dry run if you will. Bearing in mind that plastic threads are delicate creatures, the clamp made hand turning the fitting quite difficult. No heat. Next, I put the paste on the fitting, screwed it in and tightened the clamp. I let it cure over night and the tank is currently sitting outside on a bench. It is full of water. So far so good. It won't leak until it is back in the trailer. Murphy, you know. Raz
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Old 04-24-2012, 10:56 AM   #25
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Maybe there is more you can do to prevent a recurrence - like use a very flexible drain hose like latex (like they use on sling slots).
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Old 04-25-2012, 01:57 PM   #26
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Morning Paul if it was me I would get a hole saw 2" put the hole where it needs to be over the original outlet then drill a hole through centre of the piece you remove put in a 3/4" brass valve then take the tank down to a local motorcycle shop with a plastic welder and weld the 2" piece with the new valve back in hook up the apex and for the price of a box of beer your back in business after riding more than 800 000 km on motorcycles I can tell you plastic welds hold and are much cheaper than replacement plastics , some rv shops also have plastic welders jmho , cheers mike
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Old 04-25-2012, 02:31 PM   #27
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Might consider that this RV tank repair kit uses epoxy. AP Products Plastic Tank Repair Kit found that kit listed at two RV suppliers. Along with this repair how-to that suggests one uses epoxy/resin How to Repair Your RV's Freshwater Tank | eHow.com
Can always test small spot with epoxy at the top where you could plug if epoxy eats tank.
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Old 04-25-2012, 03:05 PM   #28
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Melt the two fittings together. Get a plastic welder from harbor frieght.
Or heat a screwdriver with a torch a use it to weld the two pieces together.
Don't have the stomach to melt together? Slip a piece of hose over the fittings and double hose clamp. Use a ratchet to tighten instead of a screwdriver.
Mike
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