Today the Worecester Wizard shows us how to properly mount a flat-bottom Frisco-style Sportster gas tank using rubber bushings. The cool thing about Metal Shop features with Jay is that the old pros can still learn something by watching someone else's work and noobs can get a serious jump start with a little digital mentoring. Read on, and then go out in the garage and melt some shit together. Here's Jay...
The great solid vs. rubber mounting debate will go on forever. “My tank has been solid mounted for 100,000 miles with no problems man” or “The factory solid mounted tanks and motors for years”. Well I’m not here to argue that it can’t be done correctly; I’m just a fan of rubber mounting a gas tank when you’re able to do so. In this installment of Metal shop we’ll be tackling such a project with a flat bottom a friscoed sportster tank.
To rubber mount a gas tank the way we do in this how-to, it helps to have a tank with a nice flat bottom. Other tanks can be rubber mounted; you just have to get creative with the mounting points. On a flat-bottomed sportster tank, I like to use a 3-point mounting system, one mount at the rear and two at the bottom front of the tank.
The first thing I’ll do is to place some tape on the backbone of the frame so I can mark where the front and rear of the tank ends up once I determine where I want it to sit (front to back).
I’ll be using standard steel mounting straps that use the rubber mount isolators, cutting one in half for the tail section of the tank. Before sliding the rear mount into place to locate/mark where you’ll need to weld the 5/16” 28 tpi top hat bung, I like to shape the rear mounting strap a bit so it fits in tunnel contour a bit better.
Mark, center punch, drill the appropriate sized hole in the frame backbone. Step drills work really well for this step until you have to drill bigger holes, so here’s a little trick. Use the newer step bits to drill the first few hole sizes. I use a step drill that’s had the first few steps trimmed off when drilling the hole to final size so the tip doesn’t bottom out on the other side of the tubing. Fit and weld the rear mounting bung in place.
Because I’ve melted too many rubber isolators while tacking mounts to frames/tanks, I used the almighty lathe to spin up some solid mount aluminum spacers to mirror the thickness of the isolator in the mounting strap. Bolt down the rear mount and aluminum spacer to the rear mounting bung, place the rear tunnel of the tank onto the rear mount strap making sure tank is centered and sitting the way it should be. Tack rear mount well and remove tank to finish welding the entire mount.
I have a piece of aluminum that I use as a fixture to mark the tank bottom for drilling and bolting to the steel 5/16” 18tpi blind top hat bungs while tacking them into the bottom of the tank. After tacking, remove the fixture and finish weld the bungs.
Once the tank has cooled, bolt the rear mount up with the solid spacer in, make sure the tank is sitting straight, and measure for the mounting tab lengths you’ll need to weld to the backbone for the front mounts.
Depending on how the tank sits in relation to the backbone, you may need to shape the ends with a file or angle grinder to fit the radius of the backbone tubing well. Once you have a fit you’re happy with, bolt the mounts to the tank with solid spacers and tack the mounts to the backbone in a few places. Remove tank and finish weld the front mounting straps to the frame.
Once the welds have cooled, install the three rubber mounts with inner spacers and bolt the tank to the frame.
Your tank is now rubber mounted… now go ride your bike.
See more of Jay's handiwork at: Special 79 and Barnstorm Cycles