I haven't been to the forum in ages, but then I popped in tonight to look at one of my own prior posts (ha ha, does anyone else do that?). My Velvac mirrors, which I had tightened back in 2014, finally started slowly creeping in again on blustery days, so..... found the post describing how to tighten them. I guess they need it once a decade
This thread caught my eye because a while back I suddenly had a mystery grey tank leak. Well, that sounds like it was completely without reason but actually I had slightly touched the tank on a grass mound while driving into a campsite. Nothing happened to the tank but suddenly water was spilling off the top of the tank instead of going into it.
I rigged up a video camera under there, then poured water down the sink....
mendodave.... look familiar?
And mine actually does have a support block above that whole hub/hub joint. But old ABS gets brittle... and the break is right where the strap presses. The hub being in the way of the whole shebang settling a bit more levelly into the grommet doesn't seem like it helps either.
The red arrow points to the crack around the upper half of the pipe (which probably goes all the way around but it's gapping at the top). The yellow line is just above the support block.
I also see yours has another issue that they probably all have (at least mine does). That is that the "hub" part of the elbow that leads into the rubber grommet/tank hole has partially mashed down into the grommet. That doesn't seem to help the overall geometry. I looked and looked to see if there was a "street" elbow that had a longer arm on the "street" side.... to no avail. Thought about fabbing up something (laying up fiberglass) but then came up with a (tentative) plan involving a Fernco and some flashing.
Interesting to see your fix and hear that you dropped the tank. Your repair looks a lot like what I have in mind. I've been thinking about how to tackle this for awhile. It's tricky (as you know!) because the two hubs are practically on top of each other, and there's a whole "system" of branching pipes and etc. on the incoming side.
I had purchased a couple of Fernco couplers - one Hub to Spigot and one Hub to Hub. I had another type of coupler in the past that also had a metal wrap around it, but these Ferncos didn't, so I cut a piece of flashing to act similarly.
A couple of things I have not decided:
1) I thought about going to a tight DWV elbow for that last elbow just above the tank. Now normally that would not be cool on anything but a vent, because the tighter elbow can't handle as much water flow. But then on the other hand, my Chinook doesn't seem to have a washing machine (ha ha, I wish!) and it's never going to see the sudden large volume of wastewater that a house would. The advantage would be that I could then have a section of straight pipe coming out the grommet (bottom) end of that elbow which would keep the elbow's hub from mashing into the grommet. I would then end up with a bit more space for the Fernco to cover, but it has that much adjustment. (Because the overall length of the tight vent elbow is less.)
Edited to add: Another reason I thought about this change is that -- at least on mine -- that plumber strap almost seems to pull the whole contraption upward..... right where it broke. It can't "level out" because the hub of the more gradual elbow they used hits the grommet (as you can see it's actually bulged the grommet as it tries to self-bury). With either a custom street elbow (no hub but longer thin section on the smooth side) or the tight vent elbow with some pipe glued into the hub side, I think that could level out a touch (possibly adding say 3/4" to the support block and hence the plumber strap not pulling up so much. Of course the Fernco being rubber may alleviate that stress/crack problem anyway -- like your fix likely did.)
2) I thought about going in through the floor from above. I have removed the carpet, but have not put down any sort of amazing "permanent" new flooring. Instead I have those interlocking foam puzzle pieces "for now." So I thought about making a closable access panel (sort of like you would have on the backside shower plumbing in the adjacent room). It might never need to be opened again, but then if not, you'd never know it was there (would add internal cleats so it could be screwed back in place. I have the parameters marked (it's 90% in the hallway).
Partly I thought about #2 because I might be doing it solo; but also I wondered how easy/hard/impossible it would be to get the elbow to shove into the new tank grommet (without breaking more of the plumbing!) just by pushing up the grey tank from below. With the proposed access panel, it could possibly all be done from above, but even if not (might be too cramped) then at least there could be a person above (reaching in through the access) pushing down on the elbow (not hard but basically just stabilizing it) while another person pushed the grey tank up into position.
So I'm very curious how your grommet reinstall will go (or has gone if you have already done it).
Funny that I just popped in after so long and your thread about this exact same break was posted within the last week or two.
PS: I also had a break/crack (for no apparent reason but age and time - never knocked into it) in the 1-1/2" ABS crossover pipe that connects the grey dump valve with the black dump valve. It was right between the hanger strap and the hub coming off the grey valve. That's fixed --- but of course it's super accessible so not that hard.