i have a gerber 21-310 floor mount back outlet toilet and i need to replace its gasket.

i found the gasket here (3rd one from the top):


as you can see, they come in same size but in 4 different thickness.

i called them up and they advised me to measure the thickness of horn of the waste outline, then add 1/8" and that's the thickness that i should get.

i am trying to find out the thickness of the horn of the waste outlet without pulling the toilet out, so i called gerber customer service and the person didn't know the answer and that he will forward my question to their engineers and he said it might take a while for them to get back to me.

i called several gerber dealers but they don't have the bowl on display so they can't really take the measurement for me.

in the meantime, i found the specs sheet of the bowl online and i can't definitively pinpoint the measurement of the waste outlet horn from the diagram. but i do see a measurement of 7/8" on the diagram that seems to indicate the distance between the finished wall to the back of the toilet, but i am not sure what part of the back of the toilet it is based on. if that is indeed the thickness of the waste outlet horn, and if i add 1/8" to it as suggested by the plumbing company, then i would need to order the gasket with 1" thickness. but i don't know if that is what the measurement is on the diagram.

here is the link of the specs sheet with the diagram:


can someone please take a look at the diagram and see whether or not that 7/8" measurement is indeed the thickness of the waste outlet horn?

better yet, does anyone have an access of the actual bowl and be able to help me take an actual measurement of it?

any help and advice would be greatly appreciated!