cancel
Showing results for 
Search instead for 
Did you mean: 

Black tank repair?

turbojimmy
Explorer
Explorer
Hi all -

My Allegro was starting to sink a bit so I pulled it out of its parking spot to reposition the ramps. I have pretty tall ramps in the back, but I didn't think they were tall enough to contact the black tank which is directly behind the left rear wheel. Maybe they're not - I don't know. In any event my black tank has a sizable crack in it and it's leaking. I never noticed before, but it had been repaired in the very spot it is now leaking. It's never leaked before so I must have aggravated that repair somehow.

Anyway I see there are plastic tank repair kits available. Has anyone used them and/or have recommendations?

EDIT: I put it back in its spot and leveled it and it's still leaking. It wasn't leaking before so something happened when I moved it just now. It wasn't the ramps - there's no way they'd reach it. I don't even see any signs of new damage to the tank. But whatever, I have a problem now.

Thanks!
1984 Allegro M-31 (Dead Metal)
28 REPLIES 28

wildmanbaker
Explorer
Explorer
Thanks for the update Jim.
Jim
Wildmanbaker

turbojimmy
Explorer
Explorer
Back from my Columbus Day Weekend trip. We had full hookups but as per usual I let it fill up before dumping. No leaks! Winter is coming with some extreme changes in temperature. I guess time will tell if it holds up. What didn't hold up was the GM HEI ignition coil. I moved the rig forward to get a better angle on the sewer hookup, dumped, and then it wouldn't restart. A quick check revealed no spark to the plugs on the 454. I got a ride to the auto parts store, got a new coil (which I also happen to have in my garage at home) and it was good as new.
1984 Allegro M-31 (Dead Metal)

time2roll
Explorer II
Explorer II
Nice to hear it. I expect the repair to last as long as you have the MH.

turbojimmy
Explorer
Explorer
Update:

I put 2 coats of the resin higher up on the tank on Friday. I put another 2 on it yesterday. I'm now officially out of Plasti-Mend.

I filled the tank (to the brim) earlier today and let it sit for a few hours. No leaks!

So I guess now the question is how long will it last? My next trip out is Columbus Day Weekend. We'll see how it goes after some miles on it.
1984 Allegro M-31 (Dead Metal)

turbojimmy
Explorer
Explorer
Okay! I filled the black tank halfway up and....

...it still leaks.

But it's not the repair - the repair is awesome. I didn't get the resin up high enough on the side of the tank. It's really tough to get in there. I might have to grind out some metal in order to get up in there to clean and apply the resin (I have enough left to patch it). Ironically, that's not where it was leaking but in the process of preparing the area for the patch, I ground out the old patch. So now it leaks where it didn't before.

So I'm going to let it dry out for a couple of days and hit it again over the weekend. It's a tiny leak - like a drip every few seconds. But any sort of black tank leak is a bad leak.

My next trip out is in a couple of weeks, but I'll have full hookups. Worst case is I won't let the tank fill up past 1/4 (where the leak is).
1984 Allegro M-31 (Dead Metal)

turbojimmy
Explorer
Explorer
time2roll wrote:
By the look of the crack I would have recommended some sheet ABS to add support. Although getting the sheet on just right with the Plasti-Mend goop also adds complications.

Looks like good work to me.


Thanks I ordered some ABS but like you said getting it just right looked difficult so I rolled the dice with the mesh.

And I will definitely let everyone know how it goes - good or bad. This is a learning experience for me.
1984 Allegro M-31 (Dead Metal)

wildmanbaker
Explorer
Explorer
Jim, please do let us know how it turns out, good or bad. All prior fix kits has not worked for such repairs. Could be some new recruits for this process.

Jim
Wildmanbaker

time2roll
Explorer II
Explorer II
By the look of the crack I would have recommended some sheet ABS to add support. Although getting the sheet on just right with the Plasti-Mend goop also adds complications.

Looks like good work to me.

turbojimmy
Explorer
Explorer
I put the 5th coat of resin on it this morning. The kit says not to do more than 2 a day, and that a minimum of 3 is required. Since I have some resin left I'm going to put a 6th coat on it this afternoon and then call it done. The instructions say that it will remain flexible for a few days, so I'm thinking I'm going to wait until the weekend to fill the tank.

What's most interesting about it (to me anyway) is that it does appear to be melting into the tank like the instructions say. It's very thick and you glob it on as thick as you can without running or dripping. But after sitting overnight the repair is flat and smooth. This is what it looks like this morning (it's not actually 2 different colors, it's the sun hitting one part of it but not the other):



The true test is going to be if it has really bonded to the tank or is that giant patch just going to break off under a load. I prepared the surface per the instructions so fingers crossed....

The instructions say the stuff can be used for cracks, holes, refastening valve assemblies that have been torn from tanks, altering tanks or custom-fabricating tanks. The repair is stronger than the original tank. The instruction sheet goes on to say it was invented in 1983 by an RV Tech that had a need for a quick fix for holding tanks. After leaving the business he forgot about the stuff until he bought an RV with a broken holding tank. Being retired, and needing cash, he started to market the stuff. It has a money back guarantee, too.
1984 Allegro M-31 (Dead Metal)

turbojimmy
Explorer
Explorer
I haven't called Tiffin about the tank yet.

Whatever repair they used before held for at least a decade. I don't know what it was - it was a hard plastic that filled the crack and bonded to the tank. I had a helluva time getting it off, and had to leave a lot of it there. This Plasti-Mend stuff appears to be bonding to the ABS. They say not to coat it more than twice in a day because it will deform thinner tanks as it melts into it.

The gray in the circle was the first coat. It dries pretty much the same color as the original tank. I'm going to put 2 more coats on it tomorrow, then fill the tank to see what happens.

1984 Allegro M-31 (Dead Metal)

dougrainer
Nomad
Nomad
I am going to give you the CORRECT answer you will not like. Based on 38 years and still working on Allegro motorhomes and have replaced at least 100 of the old 1976 thru 1990 underbelly Allegro tanks. THERE IS NO REPAIR YOU CAN MAKE ON THESE OLD ABS TANKS THAT WILL HOLD. THE ABS IS TOO WEAK AND FLEXES TOO MUCH TO ALLOW ANY TYPE GLUE/WELD/PATCH TO STICK. EVEN IF THE TANK WAS ONLY 1 YEAR OLD. YOU have the time and luxury of attempting to do the impossible. But, you are slowly learning. It takes a professional less than 4 hours to replace either tank on the old Allegro's. Have you called Tiffin to see if they still have your model tank? Doug

turbojimmy
Explorer
Explorer
wildmanbaker wrote:
Jimmy, the only way to fix a crack that big is to plastic weld it. It may be the age of the tank may make it unrepairable. I know its a major pain to have to remove and replace the tank, but if you are keeping the rig, this is probably the only real fix.
Good luck.


Thanks for the reply. I wouldn't have it welded - I'd replace it. It wouldn't be a horrible job but I'd like to avoid it if I can.

That being said, the Plasti-Mend instructions say it's made for situations like this - even holes. I put the first coat on this morning and it's pretty amazing stuff. The instructions say to get it inside the crack, which I did. It melts the existing plastic (almost like a weld) while becoming hard plastic itself. I then put the mesh they sent into the resin to cover the crack, and then covered the mesh with the resin (much like working with fiberglass). It looks to me like it's adhered well and covered the crack completely. I'm hopeful....the only real issue is that the friggin sun swung around to where I was working and blinded me as I was getting to a crucial part of the repair. I'm not sure I got the mesh in the right spot, so I'll have to goop that spot up pretty good with subsequent coats.
1984 Allegro M-31 (Dead Metal)

wildmanbaker
Explorer
Explorer
Jimmy, the only way to fix a crack that big is to plastic weld it. It may be the age of the tank may make it unrepairable. I know its a major pain to have to remove and replace the tank, but if you are keeping the rig, this is probably the only real fix.
Good luck.
Wildmanbaker

turbojimmy
Explorer
Explorer
Okay here's what I'm dealing with:



The crack is about 12" across and goes up 2 sides of the tank an inch or two. The area in the yellow circle is tricky because it's some sort of indentation. And then there's trying to get an area clean enough for the resin and ABS to stick. Whatever they used appears to be pretty good stuff - it's like a rock. I ground it down the best I could without grinding through the tank. I'm hoping if I go wide enough of the messy area that the Plasti-Mend will stick.

I looked at new tanks and came up with a couple that I could make work. No exact fits, though. It would be a couple hundred bucks and a whole lot more hassle to swap it out, but I may have to go that route if the Plasti-Mend doesn't work.
1984 Allegro M-31 (Dead Metal)