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Old 07-12-2007
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Holding Tank: how to get a 4" hole threaded - FPT

I am ordering a high density polyethylene (HDPE) holding tank made.

I want to put a 4" hole at the top so I can use it as a clean-out port. Something I can put a stick in and poke around. I will use one of those standard pvc plugs as a hatch (same thing that is in your sewer line clean out that the roto rooter guy uses).

The maker can thread all the holes except that one. He doesn't have the tap and can't afford it for only one job.

Any ideas on who might be able to do this type of thing?

I sent out a few emails to plumbers, but without success so far.

thanks,
gh
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Many winery tanks, have an 8 inch handhold. Which is a 8 inch lid that screws into place. Maybe add wine tank to your search to pull up a few more companys.
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Umm.. why don't you find a deck inspection plate that you think will do the job, and have the tank maker leave a hole the size required by the deck inspection plate. Then you can attach the deck inspection plate deck ring to the tank and use the deck plate as the inspection hatch. I am using a clear Beckson 6" circular screw-in deck plate for the inspection hatch on my holding tank. The reason I went with a clear deck plate is so I can do a visual inspection as necessary, and only open the inspection hatch if absolutely necessary.
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Mayb something like this instead?
West Marine: Locking Deck Plates Product Display
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I used this instead... I think it seals better than the locking type ones do.
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Go to your plumbing supply house and get a 4" bolt up flange made of PVC. It'll gasket inside and out, and the plug screws in. Cheap.
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Quote:
Originally Posted by sailaway21
Go to your plumbing supply house and get a 4" bolt up flange made of PVC. It'll gasket inside and out, and the plug screws in. Cheap.
This seems like a good way to go. However, should for some reason you still need to machine the threads, there is a way to have that done without using a tap. You would have to take the part to a machine shop where they can thread mill on a CNC machine. I make a living helping people with threading many different materials. Should you want to pursue this route and would like more info, PM me and I will help you out.

Jeff
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Have a cover plate made of same material 3/4 inch larger than the opening. Then tap the tank with 10-24 screws and using a fuel resistant gasket of the same size ( neoprene )put it all together with washers and lock washers/ screws. Works great
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