- The general rule on alpine ski drill bit sizes is 3.5mm for non-metal skis and 4.1 for metal. There are exceptions, such as a metal binding plate in a non-metal ski, carbon fiber top sheets, etc
- The outside diameter of an alpine screw thread is just over 5mm. The inside diameter of the screw thread (shank) is just over 4mm which matches the tip of a 4.1 diameter bit. Softer materials such as wood or plastic will compress when driving a screw into a 3.5mm hole. Non-compressible materials like metal and carbon should be drilled with the larger diameter, 4.1mm hole so the shank does not compress and damage the material while being driven into the ski.
- The minimum depth for a screw for a binding mount is 6mm/1/4″. Measure the screw less the binding thickness will provide you minimum drill tip length.
- Make sure this will not exceed the thickness of the ski or snow board.
- The excess depth of a hole deeper than the length of screw will be filled with the glue.
- Flathead Screws are measured from tip to top. Pan heads, buttons and sockets are measured from tip to bottom of the head.
- Older skis probably used longer screws as the skis were thicker and newer skis typically require shorter screws.
- The thread pitch is unique to binding screws and optimized for skis and snowboards. A typical hardware store machine screw is NOT a recommended alternative.
- Tapping metal top sheets or mounting plates is highly recommended. Some also recommend tapping non-metal top sheets.
- Glue can fill in a deeper hole than screw length.
- Minimum screw depth is 6mm/4 thread bite into ski, Typical hole drilling depths required by ski manufacturer range from 7.5 to 9.5mm. Look for recommended drill depth on the ski or manufacturer’s literature.
- The alpine screw heads are 10mm
- Measure existing screw if available
- Measure existing hole diameters
- Measure thickness of binding at screw location
- Place screws in bindings to help determine binding thickness and screw projection
- Measure hole depths.
- Is the screw a flathead, tapered or have a shoulder (panhead)?
- Place non-alpine screws with similar heads types into bindings as measuring aids.
Alpine, Stainless Steel Inserts & Heli Coil Taps:
- Tapping skis with metal and carbon top sheets using a 12AB tap is recommended. Tapping all holes regardless of topsheet type or drill tip diameter is recommended. The same tap works for 3.5mm & 4.1mm diameter holes.
MISTAKE! Many ski touring bindings use different length screws for the toe and heel units, if you’re not paying attention it’s possible to use the wrong screws. Example being the use of heel screws in this Vipec toe, resulting in barely two threads engaging the ski. Key is to evaluate all screw lengths before you begin a mount. Keep a clean and well organized workbench so you don’t grab the wrong screws during the process, as once the binding is on the ski and you can’t see how far the screws are inserting.
The other day… a misfortunate soul I chance encountered at a trailhead had to ski out of the wild on one plank. The binding had popped off the ski like a champagne cork during your 2nd wedding anniversary. I got a quick look at the culprits. Not sure what truly went wrong, but the toe unit screws had about two threads engaging the ski. Suspicion is the mounting tech fired the shorter heel screws into the toe, or, just picked up random screws of the workbench. Whatever. Inspiration for a blog post.
Thing is, I realized we needed more exposition here on Wildsnow about binding screw length for those of you doing home mounts (and perhaps for shop employees as well?). More, I’m realizing that some of the binding pull-outs we’ve heard of over the years could well have been caused by improper screw length. I’ll add links to the post to all our home mount DIYs, check it out.
Normal protrusion of a ski binding screw is about 7 millimeters. BUT, always measure thickness of the ski and compare to the screws, some thinner skis require shorter screws.
Using screws that are too long is a common mistake. Sometimes doing so has no consequences, but you run the risk of damaging the ski if the screw presses against the base laminations, causing a “pimple” or blowing the layers apart.
Modern skis often include information about what size hole to drill. In this case, 3.6 mm diameter hole that’s 9.5 mm deep, commonly provided by dedicated ski mounting drill bits. Most of the time I use my 7.5 mm length bits, presumably the Volkl 9.5 spec is specific for mounting Kingpins as the Kingpin rear unit screws insert about 8 mm into the ski. It’s said that a CE regulation requires all skis sold in the EU to have this information printed on the ski, but we’re not seeing that so who knows…
Always measure ski thickness in the binding mount areas, and compare to screw lengths. In this case, a 7 mm screw insertion still allows for 4.5 mm of ski base layers, which is plenty. At a minimum I’d be sure your screw length allows for a couple of millimeters of material, as in the case of very thin skis. Shortening screws is easy but has to be done correctly. Don’t just blunt the ends, first shorten with a grinder, then slightly sharpen the end so the screws threads can ‘lead’ it into the hole without requiring excessive downward force on your screwdriver.
Dedicated ski drill bits. 3.6 x 9 at top, 4.1 x 9.5 at bottom. The 4.1 mm bit is used for skis with metal topskin. In my experience, if you don’t have a dedicated bit you can use a 5/32 inch (3.9 mm) twist drill bit to drill both non-metallic and metallic (with a depth stop and care with how much force you apply). It’s also possible to use a slightly smaller 9/64 inch (3.5 mm) bit in non-metallic skis, but starting screws or tap in this size hole can be tricky. Use epoxy with all mounts, and tighten the screws with care so as not to strip. Reality is that the “standard” bit diameters are just an average of what works ideally for different types of ski construction, so slightly varying the hole size is not a big deal. We’ve been doing so for years with no problems. More information about thread tapping skis for binding mounting.
Blizzard drilling info calls for a wider 4.1 mm diameter hole. The numeral “9” probably refers to maximum depth, as a 9 mm deep hole would be unnecessary for a screw only inserting 7 mm. In all, a good example of why you need to evaluate every factor specific to a given binding and ski mounting combination. Interestingly, the “max. 49 mm” spec refers to the maximum of how far apart your bindings screws should be, left to right.
Again, IMPORTANT, most ski touring bindings use different screws at the toe and heel. A good example is Vipec, which uses a fairly long panhead at the toe, and a shorter tapered head screw at the heel. Sort everything out before you begin.