[Blazes Boylan]

I pulled the tank off my 1983 Honda today and discovered something large rattling around inside.  It's definitely nothing mechanical.  It's amber-colored, though lighter and more opaque, hard as a rock, and too big to get out through the fill hole.  I know old gas forms a sludge when left to sit for a long time.  Can this sludge crystalize and harden?  Has anyone seen this before?  There are signs of rust and I'm guessing the tank liner probably went south decades ago.  The motorcycle definitely spent a lot of time--years possibly--outside through freezing winters and hot humid summers.  I'm going to clean the interior with baking soda and vinegar and see if this has any effect on the rock.

[NVDucati]

I pulled the tank off my 1983 Honda today and discovered something large rattling around inside.  It's definitely nothing mechanical.  It's amber-colored, though lighter and more opaque, hard as a rock, and too big to get out through the fill hole.  I know old gas forms a sludge when left to sit for a long time.  Can this sludge crystalize and harden?  Has anyone seen this before?  There are signs of rust and I'm guessing the tank liner probably went south decades ago.  The motorcycle definitely spent a lot of time--years possibly--outside through freezing winters and hot humid summers.  I'm going to clean the interior with baking soda and vinegar and see if this has any effect on the rock.

It is possible that it is from a past tank coating session. It might have puddled up in a corner, formed a "rock" and eventually broke free.
If you do a search for "removing old gas tank sealer" ... you will find that you are not alone.

[Bilgemaster]

I once scooped out several spoonfuls of fossilized 50:1 2-stroke fuel and ethanol snot from a long-stored outboard motor, and while it was indeed vaguely amber colored, its form was decidedly and consistently "granular" similar to that of demerara sugar or that Sugar-in-Raw stuff found in Starbucks coffee shops here in the States. Accordingly, I might also guess the big lump you've found is more likely some blob of tank sealant, as others have already suggested.

[Blazes Boylan]

I'm inclined to agree that it's a remnant of a clumsy repair job.  If that's the worst surprise lurking inside this '80s Honda I'll consider myself lucky.

[Bilgemaster]

My hunch, borne out by the fact that you managed to coax it all the way back to Brooklyn under It's own steam, is that it's a good bike. Even if you find the occasional boulder rolling around in the tank, you still did well there.

[Jack Straw]

I'm not sayin' it's aliens........but it's aliens.

[supercub]

Must have been a stone that didnt wash out when someone did a tank cleaning. Commonly done with vinegar and driveway stones. That last stone is the hardest one to shake out of a tank.

[Blazes Boylan]

Must have been a stone that didnt wash out when someone did a tank cleaning. Commonly done with vinegar and driveway stones. That last stone is the hardest one to shake out of a tank.

I'll let you know when I get them out.  I'm starting on the assumption that they're fossilized pieces of tank coating and will try acetone to dissolve or at least soften them up.  (There are also sheets of actual tank liner.)  Unless I'm wrong, there was one large piece to begin with--that's what it felt and sounded like--and it's now broken into several smaller pieces.  Smaller but still too big to extract through the fill-hole.

[supercub]

Acetone will loosen dissolve the liner if you fill the tank and let it sit for a week.

[Blazes Boylan]

I poured a gallon of acetone in the tank and left it stewing for a week, with mixed results.  I was hoping to use a hammer and chisel (long flathead screwdriver) to crack said mystery rock into extractable pieces but the acetone, while melting it down some, softened it up and rendered it more or less impervious to cracking.  (I was also worried about puncturing the tank.)  Off to Home Depot for another gallon of acetone, unless someone can recommend something better.

[zimmemr]

I poured a gallon of acetone in the tank and left it stewing for a week, with mixed results.  I was hoping to use a hammer and chisel (long flathead screwdriver) to crack said mystery rock into extractable pieces but the acetone, while melting it down some, softened it up and rendered it more or less impervious to cracking.  (I was also worried about puncturing the tank.)  Off to Home Depot for another gallon of acetone, unless someone can recommend something better.

If it were me, and I wasn't overly concerned about the finish, I'd look for an old school radiator repair shop. My guess is you'll find a few of them in left in Brooklyn, maybe down around Hunts Point where there's some old truck repair shops or over on RT17 in Carlstadt NJ. Ask them to hot tank it or clean it with caustic soda. It's not going to do the paint any good but it should remove most if not all of that tank sealer. I doubt acetone is strong enough to do much more than make a mess. We use to use acetone to clean and soften the tires on our race bikes when we raced them indoors on concrete during the winter and it'd barely remove the surface crud on them.

[AzCal Retred]

An 1983 Honda? Maybe it's a piece of "Honda-gris", similar to that whale byproduct ambergris that washes up on beaches occasionally. Possibly a precursor to a lucrative line of biker parfumes? 

[Blazes Boylan]

If it were me, and I wasn't overly concerned about the finish, I'd look for an old school radiator repair shop. My guess is you'll find a few of them in left in Brooklyn, maybe down around Hunts Point where there's some old truck repair shops or over on RT17 in Carlstadt NJ. Ask them to hot tank it or clean it with caustic soda. It's not going to do the paint any good but it should remove most if not all of that tank sealer. I doubt acetone is strong enough to do much more than make a mess. We use to use acetone to clean and soften the tires on our race bikes when we raced them indoors on concrete during the winter and it'd barely remove the surface crud on them.

That's an excellent idea.  What will the hot tanking or soda do for the rust?

[supercub]

Acetone is the best really. It has to be full and sit for some time. If it is an epoxy like Caswell, the acetone may not dissolve it. In that case, look online for Caswell tank sealer applucation instructions, they will tell you how to remove old Caswell.

[zimmemr]

That's an excellent idea.  What will the hot tanking or soda do for the rust?

It'll remove all the rust as well, but like I said it'll probably remove the paint unless they're really careful, which most radiator shops aren't.

I didn't think of it before, but you might also want to check out what aircraft mechanics use to remove the sealant in airplane tanks. Those are sealed with a similar (or maybe identical) substance and it's my understanding, (which maybe not be accurate), that they routinely remove the old stuff and apply a new coating. I suspect they might use MEK, which is pretty toxic stuff, I got some on my hands once and it turned my fingers blue and numb.

You might even be able to find someplace that specializes in small planes to do it. I'm thinking of Danbury Airport,
but I'm sure there's something closer to you. A boatyard might also offer the same service, they seal their tanks as well.