[Latitude 28]

We replaced our rear toilet with a SANI-FLUSH macerating toilet and the mid bath toilet with a Dometic Gravity flush toilet.  The aft toilet works great.....but the front gravity toilet does not flush all that hot given the original piping is 1 1/2" ABS.  I wanted to increase the piping to 2" but the thought of removing the black tank to drill out the original 2 5/8" hole to 3" for the new 2" UNISEAL (https://www.amazon.com/dp/B072QR9KR6?psc...ct_details) was not on my list of things to do this summer.  So with some pondering, I came up with a method to increase the hole size while leaving the tank in place.  

I took a 2 5/8" hole saw (this is the bore guide for the 3" hole saw) threaded it on to the arbor of the 3" hole saw.  The thought of loosing a hole saw into the tank would mean pulling the tank anyway.  As a primary method of retaining the hole saw I put a 6" disk to keep it from falling into the tank.  A back-up, on the neck of the arbor there is an area that I twisted two separate pieces of wire in case I needed to retrieve the hole saw.  This neck area is smooth allowing the wire to stay stationary as the arbor rotates.  BTW the tank is empty and flushed in case I had to do the dreaded black tank dive. 

Now to rotate the hole saw in a restricted area....I do not have an angle drill and the ones available are too big to fit between the black tank and floor so I took a six point socket on an air ratchet that fit the arbor to spin the whole contraption.  Cutting the hole was a piece of cake and as predicted the hole saw did go into the tank but no further than the blue safety disk. 

The UNISEAL popped right in without any issues.  BTW the tank is empty and flushed.

Installing the pipe will be for another day perhaps tomorrow. 
                           

[Jack Houpe]

Ingenious Steve! Where there is a will there is away. Nice job

[Richard]

I love it, thanks for posting.

[Latitude 28]

Yesterday I tried to install the assembled drain pipe into the new UNISEAL..... two hours with no joy!  As with most things we do, sleeping on it creates the solution.  I needed to put vertical pressure inline with the UNISEAL  So I made up this contraption to install a stub pipe that used the compartment ceiling as a back stop and "extrude" the stub pipe into the UNISEAL.  I mean "extrude" as it will take some effort, even with this contraption, to install the pipe.  From the stub pipe I can build up the rest of the drain pipe.  But with that effort you will not have any leaks from the UNISEAL.

I have included a picture of the old piping next to the new in order to give you an idea of the cross sectional difference. 
                   

[BusNit]

Wow, that is great! I was fortunate to have a 3" pipe from the toilet flange to the tank. You are going to love the gravity flush. So much simpler and reliable. One less thing to worry about.

[Latitude 28]

We have about a 12" offset to the Black Tank so its not a straight run (no pun intended) so I did investigate using a 3" pipe but did not have the room.  You're right, we love the gravity flush toilet for the reasons you described just did not like the low flow drain with the 1 1/2" pipe.  Plus, if we have a problem while on a trip it's easy to get one shipped in from Amazon for half the cost of a rebuild kit for the air toilet.

[Latitude 28]

Completed the 1/2 bath Dometic gravity flush drain improvement project this morning.  Took a 3" stainless steel toilet flange and with some creative PVC fitting modifications I was able to neck it down to a 2" PVC down pipe.  Attaching it to the cross over pipe to the black tank with  stainless steel backed rubber coupling. 
   

[BusNit]

Wow, that looks like it was a pain to get done. I had to tighten the 3" coupling over my tank and it was a test of contortionism while doing it blindly. (Aka, left cheek of face up against bus gazing at the bushes while feeling my way around!

[Gnawrocki1]

Steve,
Nice job! I will have to commit to memory your stacking hole saw idea, never saw that before.

I hate plumbing.

[Latitude 28]

Thanks!  The stacking hole saw idea..... I've been trying to sort out the solution for this project for several months now.  Any idea I was coming up with was not going to be easy.  Until, I just recently saw someone on FB enlarging an existing hole using this method.....an eureka moment ensued.  So the stacking hole saws credit goes to someone on FB.....It certainly made this project work out!