Case Backs for Seiko Mod Builds
Find replacement case backs for your Seiko mod builds in a wide range of options at Watch&Style. The collection covers sapphire (clear) case backs that let you display the NH35/NH36 movement and sterile (solid) case backs that keep the build closed and minimalist — available in both Slim and Thick profiles to suit your case stacking preferences. Whether you're showing off a decorated NH movement, switching from a stock case back to a modded look, or matching the case back finish to your case build, this is where you find the case back that closes out your build.
Sapphire vs. Sterile Case Backs
Sapphire case backs feature a clear sapphire crystal window that lets you see the NH35 or NH36 movement working underneath — popular for builds that want to display the rotor, jewels, and balance wheel. Sterile case backs are solid metal with no window and no engraving — the cleanest, most minimal closure for builds that prioritize a low-key look. Both types are CNC-machined from 316L stainless steel and tested to water-resistance standards appropriate for daily wear.
Slim vs. Thick Profile Options
The collection offers two case back profiles. Slim profile case backs reduce overall case thickness when paired with a slim sapphire crystal — useful for bringing a build closer to a dress-watch wearing profile. Thick profile case backs maintain the standard case back depth — the safer choice for builds that want stock-equivalent water resistance and case feel. Match your case back profile to your overall case stack: slim case + slim crystal + slim case back for the thinnest possible build, or thick variants for a more traditional wearing profile.
Finishes & Style References
Finish options span polished steel (the dominant choice), brushed steel, sandblasted black, matte black, gold, rose gold, and Chameleon (color-shifting) variants — so the case back can match (or deliberately contrast) the case finish on your build. Most case backs in this collection are sized for the standard SKX007 case family, and most aftermarket conversion cases on the site are designed to accept the same case back dimensions.
Fitment & Movement Compatibility
Case backs in this collection are sized for most replacement and conversion cases on the site — confirm the case back compatibility note on your specific case product page before ordering. The case backs are designed for the NH35 and NH36 movement footprint with the standard gray spacer (note: some case backs require the specific spacer included with your movement; check the product description for spacer compatibility). Pair your case back with matching cases, crystals, bezels, dials, and bracelets from adjacent collections to complete your build.
Frequently Asked Questions
Will the case back available on your website fit my watch?
First, we need to identify your watch, as our case backs fit specific Seiko models: SKX007, Seiko 5KX models, SSK GMT models (except SSK023), Seiko SNZF17 (Sea Urchin), and SKX013.
There are 2 variants of case back on your website — for 7s/4r movement and NH movement. Which one should I use?
This depends on the movement you'll use. If you're using the original Seiko movement (7s36, 4r35, 4r36, 4r34) that uses the thick black spacer, select the case back for 7s/4r movement. Get the case cover for NH movement if you'll use an NH movement with the gray spacer.
What is the difference between 7s/4r and NH case backs?
The 7s/4r case back has a thinner thread to match the movement spacer, while the NH case back has a higher thread. The difference in thread height helps keep the movement secured and prevents it from moving — that's why getting the correct case back is important.
I ordered an NH case back for my watch, but I am using the original Seiko movement. Is there anything I can do to make it fit?
Yes, you have two immediate options. In our experience, sometimes the NH case cover will also fit the Seiko movement with the black spacer — so try installing it first. If it doesn't fit, you can: (1) trim the case back thread until it fits using sandpaper or a handheld rotary bit, or (2) replace your movement spacer with a gray spacer (from an NH movement) if you have one available.
What is the thickness of the slim case back and the thick case back that you have?
Case backs measured for NH movement:
| Thick Case Back | Slim Case Back | Slim Sapphire | Thick Sapphire |
|---|---|---|---|
| 4.7mm with thread, 2.5mm without thread | 4mm with thread, 2mm without thread | 4.6mm with thread, 2.5mm without thread | 5.54mm including thread, 3.2mm without thread |
What is the diameter difference of the 2 sapphire case backs available?
| Thick Case Back | Slim Case Back |
|---|---|
| 20mm | 27mm |
Does it come with a case back gasket?
No, the case back gasket is sold separately. Our cases use the same gasket and case back, so there are no issues interchanging them.
What type of crystal is used in the exhibition case back?
We use sapphire.
What is the water-resistance rating of your case back?
The regular thick case back has a 200m guaranteed water-resistance rating, while the slim case back has a 100m rating — assuming the case back is properly installed.
I bought a replacement case from your web store and I want to change the case back. What case back should I buy?
Our case covers fit all replacement cases we offer, including the 36mm, 38mm and 40mm cases. You can use our sapphire case back on any of our cases.
Should I choose a slim or thick profile case back?
Slim case backs are thinner than the standard case back, reducing overall case thickness when paired with a slim crystal — bringing the build closer to a dress-watch profile. Thick case backs maintain the standard depth for a stock-equivalent feel and a higher water-resistance rating (200m thick vs 100m slim). Match the profile to your overall case stack.
What's the difference between a sapphire case back and a sterile case back?
A sapphire case back has a clear sapphire window in the center, letting you see the movement underneath — popular for displaying the NH rotor and jewels. A sterile case back is solid metal with no window, for the cleanest, most minimal closure. Both are 316L stainless steel; the choice is aesthetic.
Browse the full case back collection above to find the case back that closes out your next Seiko mod build.