Shell Modifications for Dummies

Contrary to what some ski boot fitters will tell you, the skills and tools necessary to modify ski boot shells are not scarce and you do not need a doctorate degree in engineering or material science to modify a ski boot shell. I have proven to myself that anyone can do it with a fairly rudimentary set of skills and tools.

To get started modifying ski boot shells, here’s all you need:

  1. A heat gun ($20–$25) (The concentrator nozzle accessory, which is sold separately, might also be helpful)
  2. An infrared thermometer ($25–$40)
  3. A plank of wood (1″ x 6″ x 8′ common board) ($5)
  4. Tools to modify said wood into roughly the dimensions of your foot, or into the shape of a “punch” for modifying specific spots
    • Hand saw ($10–$20)
    • Mitre box (with mitre saw) ($20–$30)
    • Dremel tool (basic model) ($35–$75) (Optional, but extremely helpful if you’re intent on creating a blank of your foot’s dimensions)
  5. Leather work gloves
  6. Heavy duty aluminum foil (optional, but seems like a good idea)
  7. A pair of very cheap ski boots to practice with (I found pairs for $10 at a Savers Thrift Store)

Here are the steps I used to modify my first ski boot shell:

  1. Create a wooden blank of your foot, or a wooden “punch”, or any other shape that can be jammed into the shell to stretch it for your needs, or just to prove that a stretch of any kind is possible
  2. Heat up the shell with the heat gun
  3. Frequently check the temperature of the plastic using your infrared thermometer
  4. When the temperature of the shell area you’re stretching reaches the appropriate temperature, jam the wood into the boot’s toe box to stretch the shell’s width. Make sure you’re wearing your leather gloves so you don’t get burned. (This step takes quite a bit of force. If it is too hard and your block won’t go any further, but needs to, the shell is probably not warm enough, yet.)
  5. Leave the block in the boot and let the shell cool to the ambient temperature (use snow, water, ice packs, wet paper towels, etc. to quicken the process)
  6. Remove the wood and admire your work

Here are some additional considerations:

  1. Line critical parts of the boot you don’t want to stretch that are near the areas you are heating with aluminum foil
  2. Shell temperature: 250° seemed too low. 400° seemed too hot, but didn’t cause any problems other than making the plastic change color and appear “burned”. (Important note: This temperature consideration applies only to the boots I practiced on, which may be made of different plastic than yours. I have no idea what type of plastic my test boots are made of.)
  3. Be very careful with the heat gun. It is not a hair dryer, even though it looks and sounds like one. Be cognizant of items and surfaces that are near the heat gun’s output.
  4. The shell stays warm longer than I expected. No need to frantically shift from heating to stretching.
  5. The stretch will not hold if you remove the stretching implement (in this case, a block of wood) before the plastic has cooled

I have only attempted a shell modification once, so far. The method I describe above is merely a rudimentary “proof of concept” test. Do not just jam a block of wood into your boots and expect that the result will be acceptable for skiing. I am working on refining my methods to work for my needs. I will be sure to post updates regarding my refinements and future attempts to share with you what I have learned about the process.

Ski boot shell stretch test result
Ski boot shell stretch test result

Disclaimer: Do not try these methods on your current ski boots, or expensive new ski boots. I suggest practicing stretching ski boot shells with used boots you are able to obtain for very little expense. Also, professional boot fitters often know what they are doing, and they know how not to ruin a ski boot when stretching shells. Until you are confident in your methods, consider visiting a professional for your shell stretch needs. If you’re consistently frustrated by bootfitters’ inattentiveness to your needs (like me), however, you might want to consider figuring out how to stretch your own ski boot shells!

One reply on “Shell Modifications for Dummies”

No follow up post about your success? I hope it worked well and you’ve been too busy enjoying your work.

I have just finished doing my own shell modifications to my new boots and now I’m very happy with them. My local shop didn’t do what I asked for, despite me clearly marking and explaining in detail exactly what I wanted 😦 And England doesn’t have too many ski shops. Good job that heating makes the plastic return to its original shape.

Your information was helpful to me (temperature values especially), so thanks for that.

I found that I needed to hit 150°C (300°F) for the plastic to soften enough to move. I didn’t go over 160°C and there was no melting/burning/discolouration. But I did find that the boot cooled quite quickly – dropping to 120°C (no longer soft enough to reshape) in less than a minute.

I made a tool to use inside the boot to do the stretch. I used a nut and bolt that fitted between two suitably shaped pieces of wood, and screwed out the nut to the right distance. I found that using this tool, I didn’t need to heat and then screw it out, but could just screw it out a bit while the boot was cold, deforming the boot slightly, then heat it. The parts around the punch area retract back, leaving the punched part high. Maybe best to use this technique only for small stretches in places where the boot can naturally deform a little when cold, otherwise cranking up too much pressure might cause the boot to crack.

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