Note: I have no mechanical or plumbing experience, this is just a documentation of my experience replacing a Dometic 4-bolt style toilet with a Dometic 2-bolt toilet using a floor flange 4-bolt to 2-bolt adapter.
Why? Just bought an used RV, previous owners bought a new toilet but did not install. I later realized that there was a fresh water toilet leak, so I called an RV service place and asked if they could swap the toilets. I gave them the model # of the toilets, and they said that this was not possible. So I decided to do some research and complete this job myself.
Old toilet: Dometic, SD 511H (low profile, ceramic, foot flush on left side)
New toilet: Dometic, 311-SS (low profile, ceramic, foot flush on right side)
Step 1: Drain and Flush Your Black-water Tank
Before starting, be sure to drain and flush your black-water tank before starting. With the toilet removed, your nose will thank you if the tank is empty and clean.
Step 2: Old Toilet - Remove Mounting Bolt Nuts
Simply remove the 4 mounting bolt nuts which secure the toilet to the floor flange. Once the nuts are removed, you can lift the toilet off from the upright mounting bolts (after plumbing is removed also).
Step 3: Old Toilet - Remove Water Hose Plumbing
My old toilet had plastic pipe running from the water pump, through the bathroom wall, and into the left-side flush mechanism of the old toilet. I couldn't detach the old pipe, so I cut it (as pictured). This wasn't a concern because I knew I would need to replace the tubing anyway, in order to route it a bit farther down, to the right-side flush mechanism of my new toilet. In this case, I'll need to remove that 90' elbow connector in the picture, and just use a 180' straight coupler to extend the hose about 8" down so that I can reach the right-side of the new toilet.
Step 4: Lift Out the Old Toilet
With the floor bolts removed and the plumbing detached, you can lift the old toilet straight off of the bolts. Thar she is, upside down on the floor. Aw.
Step 5: Remove the Old Gasket & Clean Area
Your new toilet should have a new seal/gasket, so I removed this old one. This could have been a possible cause of the leak. With the old toilet removed, clean the whole area.
Step 6: Design New Plumbing & Buy Parts
With the old toilet removed and the new one not installed, you have plenty of space to plan out your new plumbing. You can even temporarily set your new toilet in place (resting it on the floor flange), so that you can measure the length of tubing you'll need.
I was planning to replace the tubing with the same material as previously used, straight-length PEX pipe/tubing (polyethylene pipe). However, I decided to try flexible clear tubing instead as it seemed much easier to install.
I brought the old fittings to the hardware store and bought new supplies and extra screw-down hose clamps.
Step 7: Drill Pilot Holes for Adapter
The 4-bolt to 2-bolt flange adapter I used:
Dometic 385311719 310 Mounting Adapter Kit - White
Purchased at Walmart for about $50 (you can probably find a cheaper version online).
The adapter came with instructions that I'll also outline here.
* Insert 2 of the old bolts into the original floor flange. This is only to hold the adapter in place while you drill the pilot holes for the mounting screws.
* Place the adapter on the original floor flange as pictured, and use 3mm drill bit to drill pilot holes into the 8 screw mounting holes on the adapter. The pilot holes should be no more than 1.5" in length. You will be drilling through the original floor flange and into the floor board.
Step 8: Add Silicone Caulk to the Adapter
As specified in the instructions that came with my adapter, add silicone caulk around the inner ring as shown.
I used silicone sealant since I had some on hand.
Step 9: Mount the Adapter to the Floor
Lining the adapter up carefully in place, making sure you've added silicone caulk to the underside, screw in the 8 mounting screws into the pilot holes.
Step 10: Position 2 Bolts & Mounting Plate
The new toilet came with 2 new set/mounting bolts. Those simply slide into place as shown. In the next step, you'll pick up the new toilet and set it down into place, with the 2 mounting bolts inserted into the toilet base.
Be sure to set the frisbee-sized mounting plate over the adapter. This part is needed, otherwise the toilet base will NOT be flush with the floor. You've added about an 1/8" of height by adding the adapter, so this mounting plate is needed to essentially raise the floor by that 1/8" (I'm guessing on the actual height of 1/8" here).
Step 11: Connect Plumbing to Back of Toilet
While the new toilet is not installed yet, I found it easier to add the plumbing to the back of the toilet, yet not connect the hose coming out of the toilet to the "wall" yet, as shown. I used a female swivel adapter to connect the back of the toilet plumbing to the incoming water pipe/hose. Tighten down the screw tubing clamps now, and make sure the screw is facing up in case you need to tighten or loosen it after the toilet is installed.
Step 12: Set New Toilet Into Place
I thought this step was tricky. I picked up the new toilet (heavy!) and simultaneously tried to hold the bolts into place. This took me about 6 attempts before I finally got it. I held the toilet in place and felt for the mounting holes with each hand, guiding each side's bolts into the mounting holes in the toilet base.
If you had some clay, you could use that to stabilize the upright mounting bolts so that they don't fall over when you're trying to center the toilet over them.
Step 13: Mount New Toilet to Floor
Alternating sides, compress the squishy base seal of your new toilet so that the toilet becomes flush with the mounting plate base. Don't completely tighten one side at a time. You will want to alternate sides, tightening a bit on the left side and then move over to do the same on the right. Repeat alternating tightening until toilet base is flush to the mounting plate, and don't over-tighten.
Step 14: Complete Plumbing
At this point, the toilet has a loose "tail" of pipe/tubing coming from the toilet. I used a pipe/tubing coupler to join the pipe coming from the water pump at the "wall side" of the bathroom. Tighten the screws on the tube/pipe clamp and make sure the screw head is facing in a position that you can access later.
It was really challenging to get the tubing onto the coupler. It may help to use some oil or vaseline to lube the inside of the hose and the outside of the coupler, and "mush" it on at a 45 degree angle.
Step 15: Fresh Water Test
When finished, you may want to add water to your fresh water tank, turn on the water pump, and do some test flushes. You should be able to repress the flush pedal 1/2 way to fill the bowl (let up a little if the ball seal is opening; water should not exit the bowl). Repress the pedal completely to flush. Look for leaks at 1) the base 2) the back of the plush pedal 3) any parts where you've connected hose/pipe.
Note: You may want to consider cosmetic implications. In this case, the mounting plate that surrounds the adapter is 1) the wrong color (white, not 'bone') and 2) wrong shape; extends the original toilet floor step.