Big Gas Tank Dent Question

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Big Gas Tank Dent Question

Postby starcain » Wed Apr 21, 2021 1:43 pm

Anyone that's been around K Models or early Sportsters for any length of time probably has seen examples of big tanks that have a sizeable dent on the top rear section of the tank right in front of the seat nose. I have been perplexed by this for years not knowing what actually happens to cause this. I have little to no knowledge on buddy seats but have to believe they are rigidly mounted so they can't move forward. The solo seat has a pivot point at the front T bar bolt. If T bar is unlatched in the back it could conceivably make the seat rotate up and drive the nose into the tank? My problem is I can't come up with a scenario to why that would happen. Could it be that while riding with the seat unlatched for whatever reason and you came to a sudden stop the seat would rotate around with you on it??? Or, could someone be rotating the seat to get to the battery or whatever and rotate the seat up with enough pressure to dent the tank?? Either scenario really doesn't make much logical sense but I don't know what else to think. If anyone has the answer or has personal experience in producing said dent I would be curious to hear about it.

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Re: Big Gas Tank Dent Question

Postby Magneto Sportster » Thu Apr 22, 2021 11:26 pm

The buddy seats flex when the rider hits a sizable bump/pothole? That has always been my theory.

The factory did not leave much clearance when they mounted the buddy seats. I usually add a washer to two to give a little extra clearance to prevent damage to the tank and rear fender.
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Re: Big Gas Tank Dent Question

Postby chuckthebeatertruck » Tue Apr 27, 2021 4:20 pm

I once shared the same theory about the seat flex -- but I recently broke a welded on seat tab during a ride.

I didn't notice until I went to start the bike the next day.

The seat was sitting on the tank . . .and despite my 190 pound a$$ slamming on it for some miles, the only damage was some slight paint chipping.

So, I'm not so sure anymore . . .
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Re: Big Gas Tank Dent Question

Postby wz507 » Tue Apr 27, 2021 6:35 pm

Well, here’s one for you. I bought a “big” tank that was really straight a few years back that had Kreem sealer in it that had turned to crap (peeling/debonding) and therefore had to be removed. The tank also had heavy paint on the outside that also required removal so decided to bring it to a thermal stripper to clean both inside and outside simultaneously. Thought this would be a really nice way to prepare this “really straight” tank to a pristine condition for refinishing. When I went to pick up the tank after thermal stripping and got my first glimpse of it I was almost in tears. There were several significant dents in the tank, the bottom of the tank had fresh shiny lead all over it and major dents were present just ahead of the seat tongue area as described above. I suspect that somebody had likely done a beautiful job of filling all the tank flaws with lead, and now I had totally removed all the lead from the tank, revealing a totally different character.

Getting back to the subject at hand – tank dents from seats – I, like others was puzzling over the source of such dents. Then one day while trial fitting the tank and seat on a naked frame, the light bulb suddenly came on and I realized that when the seat was released at the rear, then raised and allowed to pivot on the front mount bolt, the square front nose of the KHK seat fit the tank dent like a glove. Moreover, there was one additional narrower dent further forward of the square-ended dent, and fitting a longer narrower nosed seat, found that this too fit the dent like a glove. My only take on the whole affair is that once the seat was freed from the suspension people must have felt compelled to grab the rear end of the seat and use it as a pry-bar against the tank top. Perhaps they were so preoccupied with looking under the seat they were unaware of what was going on up front?
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