Watch Screwdriver Sizes for Casebacks
Direct answer: Match the screwdriver tip to the screw slot. Most watch case screws take a flat tip; start with a tip that fills the slot wall-to-wall without overhang, and step up only if it is too narrow. The wrong size — especially too small — strips the screw head. Do not open waterproof or gasketed casebacks unless you are qualified to reseal and pressure-test them.
Author: John / founder
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Size is fit, not a single number on the box
Watch screwdriver “sizes” are tip widths (and blade thickness) that must fill the screw slot. A tip that rattles in the slot will cam out and tear the head. A tip that is too wide will not seat and can scar the caseback around the screw.
For desk organization once you own a range of tips, see Screwdriver Carousel vs Cup and How to Use a Watch Screwdriver Set. For the wider beginner kit context, see Essential Watch Tools for Beginners and Best Watch Repair Tools.
Flat tips for most case screws
Many caseback screws, bracelet screws, and movement screws you will meet as a collector are slotted (flat). Phillips exists on some hardware, but flat is the default assumption until you look at the head.
Practical habit:
- Look at the slot under good light (loupe if you have one).
- Choose the smallest tip that fills the slot end to end.
- Seat the tip fully before you turn.
- If it still feels narrow, step one size wider — do not lean harder on a skinny tip.
“Start small and step up” protects the screw. Starting oversized and forcing the blade into a short slot is how you chip finishes.
Why wrong size strips
Stripping is geometry plus force. A tip that only touches the middle of the slot concentrates load on two soft corners of the head. Under vertical pressure that is slightly off-axis, the tip climbs out and rounds the slot. Once rounded, every later attempt needs more force and does more damage.
Prevention is dull but reliable: full seat, vertical shaft, slow turn. Technique belongs in How to Use a Watch Screwdriver Set; size selection is what this page answers.
Flat vs Phillips (and other heads)
| Head type | Tip to reach for | Notes |
|---|---|---|
| Slotted / flat | Flat tip matched to slot width | Most common on case screws you will see |
| Phillips / cross | Correct Phillips size | Less common on fine watch screws; still match size |
| Hex / other | Dedicated bit | Do not grind a flat tip to “sort of fit” |
If the head is not flat or Phillips, stop and identify the drive. Improvising with the wrong blade is a one-way trip to a ruined screw.
Magnetic tips and tiny screws
Light magnetism can help hold a screw on the tip during placement. It is not a substitute for a parts tray. Keep screws parked in one place; a caseback screw lost in a carpet is a longer afternoon than choosing the right tip.
When not to open a waterproof case
Screw-down casebacks and gasketed waterproof watches are not practice pieces. Opening them without the right case tools, gasket care, and pressure-test path can leave the watch non-waterproof even if you get the screws back in.
- If the watch is rated for water resistance and you are not set up to reseal it, leave the case closed.
- Strap, bracelet, and non-sealed caseback work are a different category from cracking a diver open on the kitchen table.
- When in doubt, use a qualified watchmaker for case opening.
This size guide assumes you already have a legitimate reason and method to access the screws you are sizing for — it is not permission to open every caseback you own.
Where to park the tips you own
A range of flat tips only helps if you can find the size you just matched. A screwdriver carousel base (base only — tools sold separately) keeps sized tips upright on the desk. A zip screwdriver case keeps the same set from rolling loose in a bag. Full kits may appear later as inventory shows Available; until then, sized drivers plus a holder are the practical path.
FAQ
Which watch screwdriver size do I need for a caseback?
Match tip width to the screw slot. Prefer a flat tip that fills the slot; start small and step up. There is no single universal “caseback size.”
Are caseback screws usually flat or Phillips?
Often flat/slotted. Confirm by looking at the head before you pick a tip.
What happens if I use a tip that is too small?
It cams out and strips the slot. Re-seat with a tip that fills the slot instead of adding force.
Do magnetic tips help?
They can hold a screw on the blade for placement. Still use a parts dish and do not rely on magnetism alone.
Should I open a waterproof watch to “just check”?
No — not unless you can reseal and verify water resistance. Wrong size tips make that job worse; the larger risk is the seal itself.
What should I buy to hold different sizes?
A W&S screwdriver carousel base for the desk (tools separate) and/or a W&S screwdriver case for travel and storage.
Shop
- W&S screwdriver carousel base (base only — tools sold separately)
- W&S screwdriver case
- All W&S tools on Amazon