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Description: Fastback Standard Trunk Gas Tank Underlayment
Year: 1970
Model: Mustang Sportroof 63A, BOSS 429 63B
Model Options: All (excluding MACH 1)
Engine: All
Transmission Type: All
Factory: San Jose, Dearborn, Metuchen

Accuracy Rating: 9
Best Repro Available: YES

Jack's Comments:

All Mustang fastbacks (excluding Mach 1) came from the factory with standard sound insulator trunk underlayment that covers the gas tank and a small amount of the rear floor. This underlayment goes under the trunk mat. Ford engineers designed one piece of underlayment for cars with or without staggered rear shocks.

We made our template from original surviving examples including cars from the Bob Perkins collection. All factories installed underlayment that had the California emissions equipment knockouts for the larger gas tank filler neck and the gas tank vent snorkel (verified by Bob Perkins). The color of the paper side will vary as it did from the factory. This underlayment piece should be installed paper side up for best fit.

Common Questions:
Q: Will your underlayment crack and fall apart like the originals did?
A: No, we used binders in the tar that hold it together so you can put it in and out without it ever breaking apart.

Q: My original underlayment is a lot darker than some of your pictures. What gives?
A: The darkness of the paper varied in our original examples from very light to almost black. Our experience is that slight changes in the tar and the our shops ambient temperature during manufacturing effects the final color. Cooler temps will make the paper lighter and hotter temps will make the paper darker. Take a look a the last picture showing the two extremes in color variances. We shoot for the somewhere in the middle but you could get a piece that is closer to one of the extremes. Good news is all are correct.

Q: My original underlayment seems thicker than your reproduction. Are you making it the correct thickness?
A: Bob Perkins said the same thing when he first got his first piece of underlayment. If you look at an edges of original pieces they are swollen at the edges from years of heat cycles so they look thicker but when you cut down the middle of it you will find ours is the same approximate thickness.

Q: Wow, thats a lot of money for a piece of tar. Why does it cost so much?
A: Yes, we totally agree it's expensive but its not cheap to make. We make it ourselves in California with our own custom production equipment in small runs. The materials are not inexpensive and our partially automated manufacturing process is labor intensive. The size of the die and the equipment to stamp it out is another reason. If we could source the material itself from a mass production manufacture it would be a whole lot less but its just not available.

Install Instructions: Lay paper side up in the sun for five minutes before installing. Pick it up with both hands with the gas tank filler neck closest to you and let it bend in the middle (make sure the paper side is up facing you). Curl it up a little so it can fit through the trunk opening, insert into the trunk, center the fuel filler neck hole and then square it up over the gas tank. If you have staggered rear shocks you will need to push down on the area between the gas tank and staggered shock reinforcement (see pictures). It will settle into place just like the originals did with time. Enjoy!
*California Emissions Equipped Mustangs: Remove the fuel filler neck hose opening and vapor snorkel knockout before installing. Place the snorkel knockout next to opening as the factory workers did.

Tip: Don't forget you need the anti-rattle board that goes on the gas tank before you install this piece and there is also another piece of underlayment that covers the rest of the trunk and into the backseat.