Crystal Mods SKX & 5KX – The Seiko SKX and 5KX are both shipped with the Seiko Hardlex Crystal. The Seiko Hardlex can, and does, scratch pretty easily. If you are an engineer, work in the building trade or live an active lifestyle, you probably know how easy it is to damage the Hardlex crystal. Crystal modding is one of the most popular mods to perform. Not only are you upgrading a stock item for something of a better quality, you have the option to change the appearance of the watch.

Sapphire Crystals

Sapphire is the second strongest jewel on the Mohs mineral hardness scale, second to diamond. A sapphire watch crystal is a watch glass that is usually made using a sapphire-quartz crystal combination. These are often used in high end watches due to their resistance to damage as well as their aesthetic appearance.

Mineral Crystals

Although there are many replacement mineral crystals which you can use to replace a damaged or scratched Hardlex crystal these, unlike sapphire crystals, will mark relatively easily. They are at a lower cost and do offer top hat, domed, raised domed etc. The likelihood that you will scratch a replacement mineral crystal is increased should you opt for a domed design. This is because it protrudes from the watch making it a little thicker. This will make it the likely point of impact when you glance by something, like a door handle.

Hardlex (OEM) Crystals

The stock crystal that is shipped with the SKX and 5KX has a beveled edge. This means the crystal tapers off at the edge. This is what creates the gap between the inside edge of the bezel insert and the high point (top) of the crystal. The crystals on the market without a beveled edge will remove this gap and give a solid surface from the bezel insert across the crystal.

Different Styles of Crystal

There are now so many crystal options on the market domed, double dome (DD), top hats, raised flat, raised DD, AR colours, no AR. Where do you start? It would be hard to mention them all here. All the leading suppliers have numerous photos of their products so you can gauge what you are getting. It seems that the beveled edge is what confuses people. There is more detail on this contained in the “Useful Information” at the end of this crystal guide.

Anti-Reflective Coating

Another thing to note when choosing crystals is the AR coating – “Anti-Reflective Coating”. There are a number of colours, blue, red, purple, also clear. There is also the option for no coating. So it is whatever suits your mod. Blue gives a cool contemporary look, while a red is more suited to vintage style mods.

Crystal Suppliers

There are lots of great suppliers out there. Crystals are one of the most important components of your watch. Saving money on this item may not be in your best interest for a number of reasons. They both fall back to quality control. The first is obviously the water resistance of the watch. If the crystal has any imperfections this can lead to water ingress. Not what you want on your dive watch! The second is again imperfections in the finishing process which will leave sharp edges that can make installing them difficult. You get what you pay for!

Guide to Removing the Crystal – Crystal Mods SKX & 5KX

To remove the crystal, the watch will need to have previously had the movement, dial, and hands removed. You may need to remove the bezel and/or the insert so that the crystal will come out, this will depend on the crystal and insert installed on the modified Seiko. Once all the necessary parts have been removed and stored away you can begin to remove the crystal.

Tools and Consumables

  • Finger Cots
  • Crystal Press
  • Watchmakers paper – optional
  • Microfibre cloth – optional

Removal Process

The SKX or 5KX case which is being modded should be looking pretty bare at this point. The only components remaining in/on the case are the crystal, its gasket, the bezel, and maybe the chapter ring. The simple process of pressing the crystal out from the watch will allow us to remove all of the final components.

You will need to have a crystal press to hand, something cheap and cheerful will do the job. The press will be supplied with various sized connections known as dyes. 40mm should be suitable for the base of the press, while 30mm or 32mm fitted to the top will work to press on a smaller dye, which you will place on top of the watch crystal in order to remove it. This is only necessary if you have chosen a budget press. Professional presses have fittings to raise the watch higher to remove the crystal without the free floating dye.

To remove the crystal, the watch case will need to be face down on the 42mm dye at the base of the press. Place a 26mm dye on the crystal from the inside of the case (where the movement goes). This should clear the chapter ring and rest against the crystal. Now, slowly plunge the dye’s together and the crystal will pop out with a satisfying thump.

Dyes are attached with a thread.
Set up for removing crystals.
Removed crystal left in press.

That’s really all there is to it. If you wanted to add a little protection to the crystal, you could use a small piece of watchmakers paper that comes with the new crystal or a microfibre cloth. This will stop grease or muck transferring to the crystal from the dyes.

Be sure to store all your parts away safely in your containers. Don’t loose the crystal gasket, they can sometimes want to stay inside the stainless steel case, but other times they will come away with the crystal.

Seiko SRPD81 case - Disasembled into component parts -
Crystal Mods SKX & 5KX

The watch will now be completely disassembled. If your chapter ring remains in place please see the chapter ring section of the guide for details on how to remove this. If your watch is a little dirty it may be worth treating it to a nice bath to remove the dirt and muck built up over the years. You may want to consider an ultrasonic cleaner if this is something that you feel you will be doing more of.

Guide to Fitting a Crystal in your SKX & 5KX

If you’re all set to start putting your Seiko mod back together, you will need to have read about chapter rings, and have successfully fitted your original or modified chapter ring. Once this process is complete you can start to fit your crystal.

Tools and Consumables

  • Finger Cots
  • Crystal Press
  • Microfibre cloth
  • Air blower/Puffer
  • Degreaser/Isopropanol
  • Small paintbrush – optional
  • Calipers – optional
  • Watchmaker’s paper – optional

Fitting Process

Use finger cots as a minimum when handling a crystal, gloves may be better for butterfingers! Ensure your crystal is clean. It is recommended you inspect all your mod parts when your receive them as any problems can be raised with the supplier at this point. This primary inspection doesn’t mean that you can forgo further inspections of parts during installation. The crystal being one of the most important, certainly the inside of the crystal. This will be hard to clean when installed. This may require you to go back to the removing the crystal again.

If your crystal needs cleaning, there are various products on the market that will degrease, but isopropanol will do the trick if it is not coated, a lint free cloth will help with this and a little paintbrush. Acetone is likely to remove any anti-reflective coating. Check, check and check again before you are satisfied that the crystal is clean and ready for installation.

Newbie Tip #1: Fit the case back, this will help steady the watch in the dye on the base of the press while you are pressing in the crystal.

Install the gasket in the case. This is easily done by placing it on with some tweezers and working it in with your fingers or some pegwood. Using anything steel and/or with sharp edges may damage the gasket. Avoid where you can and stick to blunt soft objects to manipulate the seal into its final resting position. Use gloves or finger cots if you intend on fingering it. Finally, at we suggest best practice is new crystal, new gasket. But you can do your Seiko mod however you like, this is only a guide and best practice often means more money.

Newbie Tip #2: When you inspect a crystal gasket, you will see that both the top and bottom differ. The bottom is square with 90 degree edges, while the top has a tapered, leading edge to allow the crystal to slip into place.

You can now place the crystal flat on top of the seal. You will need to view this from all angles until you are satisfied that this is flat. Starting with your crystal wonky can leave you with a wonky finish.

Placing the crystal gasket.
Ensuring it is firmly in place.
Align the crystal.

The press will need to be fitted with a 30mm, 31mm or 32mm dye on the top, and the watch case placed on the 38-40mm dye at the base.

At this point, if you have set your seal correctly, and your crystal is sitting nice and even on the top of this, you can begin to press the crystal.

A firm pressure will press the crystal into its gasket bed. If you are using a budget press it will be necessary to press lightly and rotate the watch to ensure the crystal is being evenly pressed down into the gasket. A little bit, then rotate 90 degrees, followed by a little bit more, rotate 45 degrees, press then 90 and so on.

When you are satisfied that your crystal is in its bed, use your eyes to inspect the crystal’s height and angle. If you notice anything, this can be checked with verniers or another measuring device you may have available.

Ready to set crystal.
Engaging the press.
Inspection – Wonky crystal.

If you have not successfully pressed the crystal in flat, you can try to press it again with the press. If this is not successful then it will be necessary to remove the crystal and start over. If a second installation is required, inspect the crystal gasket as this may have been deformed during the previous installation. When installing a new crystal, it is best to use a fresh new gasket. If this is the case, don’t despair, just order a new one – or five.

The gasket may need inspecting.
A correctly fitted crystal.

The crystal may have picked up some grease and muck while installing. It may be a good idea to give this a wipe with an anti-static cloth. Finally you will be able to inspect the side of the crystal for dust.

Wipe the crystal after installation.

Further Information About Aftermarket Crystals for the SKX & 5KX.

Listed below are the crystals that are suitable for each insert type. We have separated these to flat and sloped. Please don’t take this as gospel, we are talking about modding Seiko’s, so it is each to their own decisions.

Crystals for Flat Bezel Inserts

Flat bezel inserts are like the stock inserts that come shipped as standard with and SKX or 5KX. They sit at the same height as the recess in the bezel insert, and the same height as a standard Hardlex crystal. (Side note: there is an approximate 0.3mm difference in thickness between the stock crystals and the aftermarket replacement sapphires.) This leads to the the stock bezel insert on these models sloping down ever so slightly to the crystal. Flat aftermarket inserts are produced to work in unison with the AM sapphire crystals for flat bezels.

  • Flat Sapphire 3.3mm thick with beveled edge: This is the standard size aftermarket crystal which can replicate the original crystal look. Retaining the same thickness but offering superior scratch and damage protection. This is available without a beveled edge. These are slightly thicker to work with than the aftermarket bezel inserts, which do not slope down like the OEM inserts.
  • Low Double Dome: This will raise the crystal above the height of the flat bezel insert towards the centre of the crystal. These are available with or without beveled edge
  • Top Hat: This works well with vintage mods – more on this in the sloped insert options.

Crystals for Sloped Bezel Inserts

  • Raised Crystals typically around 5mm thick: these are suitable for sloping bezels. Again, there are various options, most of which all can retain the iconic Seiko beveled edge. Be sure to select the beveled or non-beveled to suit your mod.
  • Raised Flat Crystal: These will have the appearance of a Sub with a sloped bezel which finishes at the same height of the crystal.
  • Raised Double Domed: The dome of the crystal continues on the same angle as the sloped insert.
  • Top Hat: This works well with vintage mods as it can give the appearance of a vintage acrylic crystal that sits proud above the watch. They can also make a watch look boutique, and sometimes futuristic.

Date Magnifier

A date magnifier also known as a cycl… We stopped there as we can’t use that word as modders because it has been trademarked. A magnifier can be fitted to any crystal with a flat top. Not suitable for a dome crystal for obvious reasons. Although we will explain that the magnifier on a domed crystal is possible, but it is quite a hassle to maintain a high quality finish. Magnifiers are available pre-installed to the crystal or this is something you can do yourself. Both date and day/date can be sourced to suit your Seiko mods movement calendar complication.

Single Dome – Double Dome – Difference?

From looking at the exterior of the finished mod there is nothing to discern any physical differences between these two types of crystals. The difference should be clear from the name, one has one dome (on the exterior of the watch) the other has two (on the underside of the crystal also)! The difference is how the dial can be viewed through the crystal. The single dome will appear to warp the appearance of the dial when the watch is viewed from sharp angles. Through a double dome crystal the dial appears the same whatever angle you view it from.

Double Dome – Clear view from all angles.
Single Dome – Distorts the view of the dial from wide angles.

If you’ve found this guide helpful, be sure to explore our collection of other informative guides at We offer a wealth of resources to assist you in your modding journey. We’re here to support your passion for watch modification.

Useful Information:

Crystal Size

  • 31.5mm x 2.97mm

OEM Hardlex Part Number

  • 315P15HN02

OEM Gasket Part Number

  • 8660-0630

Procuring a Seiko 5KX

Seiko have lots of options in the 5KX range a link to the collection can be found here.