Everything I wish someone had told me before my first NH35 build: what to buy, how the parts fit together, and the order to put them in. No jargon you don't need.
A modded watch is a handful of parts that were made to fit one another. Here's everything you need for a three-hand NH35 or NH36 build. Tick them off before you order so you're not waiting on one missing piece.
What goes where. Parts are spread out top to bottom in the order they sit in the watch.
The big parts
Movement. The NH35 (date) or NH36 (day and date). It's the engine, and every other choice follows from it.
Case. Usually comes with the case back, gaskets and the crown tube already fitted. Many include a crystal and bezel too, so read the listing before you buy those separately.
Dial. Most often 28.5 mm across, with small feet on the back that lock into the movement.
Hands. A set of three: hour, minute and seconds.
Bracelet or strap. Sized to the gap between the lugs. Some cases come on their own strap or bracelet.
Bezel and insert. The ring around the crystal, and the printed ring that sits in it.
Crystal. The glass. Sapphire is the usual upgrade.
The small parts people forget
Crown and stem. The crown screws onto the stem, and the stem goes into the movement. The NH35 and NH36 use the Seiko 351-200 stem, which you trim to fit your case.
Movement spacer ring. A plastic ring that holds the movement snug and centered in the case. Most cases include one. If yours doesn't, you'll need one made for the NH35.
Chapter ring. SKX-style cases use a separate ring with the minute track that sits around the dial. Some cases have it built in.
Gaskets. The rubber seals at the crystal, crown and case back. Fresh, lightly greased gaskets are what keep water out.
Spring bars. The little sprung pins that hold the strap. Bracelets and straps usually include them.
Tip: the easiest first build is an NH35 in a case with the crown at 3 and a stated 28.5 mm dial seat. Most 28.5 mm dials fit it, so you'll have the most choice.
Both are automatic movements from Seiko Instruments, and they're the same size: 27.40 mm across and 5.32 mm tall. Both hack (the seconds hand stops while you set the time) and both can be wound by hand. The difference is what they show.
NH35: date only. You'll need a dial with a date window at 3, or a no-date dial.
NH36: day and date. You'll need a dial with a wider day-date window at 3.
A dial cut for one won't look right on the other, so this is the first thing to decide.
What "3.8 crown" means
Cases put the crown either at 3 o'clock (a "3.0" case) or tucked down towards 4 o'clock (a "3.8" case, like the classic SKX007). The movement sits at a slightly different angle in each, so the dial feet and, on the NH36, the day wheel have to match.
NH35: the same movement works in both 3.0 and 3.8 cases. The date wheel is identical, so the date lines up either way.
NH36: the day wheel is printed for either 3.0 or 3.8. Check which one the listing ships, or plan to swap the day wheel.
Dials: many have two pairs of feet, one for 3.0 and one for 3.8. You snip off the pair you don't need.
Crown positions, seen from the dial side. The date window is at 3 in each.
Cases with the crown even lower, at about 4.1 (some Tuna-style cases), need a different date wheel and usually dial dots instead of feet. I'm leaving those out of the catalog for now to keep things simple.
My pick for a first build: an NH35 in a 3.0 case, with the crown and the date both at 3. That's what the catalog is built around right now, so every part you see is checked against it.
This is the order I pick parts in. Each part has to fit the ones before it, so go step by step and check one number at each. The builder follows the same order and does these checks for you.
My build order. The builder follows the same steps.
Start with the movement. NH35 for the date, NH36 for day and date. Every other choice follows from it.
Pick the case. The listing should say NH35 or NH36. Note its crown position (3.0 or 3.8), the dial size it takes and its lug width. Some cases come on their own bracelet or strap (Cases with Strap in the catalog).
Pick a dial that fits that case. Match the dial diameter the case is made for (usually 28.5 mm), the window (date or day-date), and feet for your crown position. If your case comes with a bracelet or strap, step 5 is already done.
Pick hands that fit the movement. NH hand holes are about 1.50 mm (hour), 0.90 mm (minute) and 0.20 mm (seconds). A set listed for NH35/NH36 will fit. Then check the minute hand reaches the minute track on a 28.5 mm dial.
Pick a strap or bracelet, only if the case doesn't include one. Match the lug width (22 mm on SKX007-style cases). Bracelets with curved ends must be made for your case shape; straight-end straps fit any case with the same lug width.
Optional extras. Each one can be skipped.
Crown. Most cases include one, but you can swap it. Crowns screw onto the case's crown tube, so buy one made for that case (for example "for SKX007 case").
Inner chapter ring. It has to fit the case. SKX007-style rings are about 27.5 mm inside and 30.5 mm outside. Some cases have one built in.
Outer bezel and insert. Only if your case has a bezel, and both must fit the case. Flat inserts pair with standard crystals; sloped inserts need a taller crystal so there's no step between them. SKX007-style inserts are usually 38.0 × 31.5 mm.
Crystal. They come in different styles, and the case decides the size (SKX007-style cases take 31.5 mm). Swapping one needs a crystal press to push out the old glass and press in the new one.
The NH35 and NH36 share these numbers. Dial and movement drawn to scale with each other.
The SKX007-style numbers, in one place
Other cases vary, especially the dial size (28.5, 29 or 30.5 mm) and lug width, so go by the listing for those. These are the numbers for the classic SKX007-style case.
Movement (NH35 / NH36)
27.40 mm, or 29.36 mm with the dial holding spacer; 5.32 mm tall
Hand holes
Hour 1.50 · minute 0.90 · seconds 0.20 mm
Stem
Seiko 351-200 (Tap 10, 0.90 mm), trimmed to the case
Dial
28.5 mm, Seiko NH feet
Chapter ring
About 27.5 mm inside, 30.5 mm outside
Crystal
31.5 mm (flat about 3.0 mm thick; domed versions are taller)
Bezel insert
38.0 × 31.5 mm is the common flat size
Lugs
22 mm wide, about 46 mm lug to lug
Lug width is measured across the lugs, not across the case.
If a listing doesn't give the number you need, ask the seller before you buy. A two-line message saves a return.
This is the order I build in, the same order I pick the parts in: movement, case, dial, hands, strap, with the optional extras fitted to the case before the movement goes in. Take your time; nothing here needs force. Wear finger cots from start to finish.
1. Start with the movement
Set the movement in its holder, dial side up. Everything that follows is built around it.
2. Prepare the case
This is where the optional extras go in, before the movement, so no pressure ever lands on the dial or hands. If you're fitting a new crystal, seat a new crystal gasket, place the crystal on top and press it in with the crystal press, using dies that touch only the edges. Keep it dead level. If your case uses a chapter ring, drop it in and line it up. If it has a bezel, press the bezel on evenly and fit the insert.
3. Fit the dial
If your dial has two pairs of feet, snip off the pair that doesn't match your crown position, flush and clean. Line the feet up with the holes in the movement's dial holding spacer and press the dial down gently until it sits flat. Peel off any protective film.
4. Fit the hands
Pull the crown to the second click and turn the hands forward until the date has just changed. Now you're at midnight.
Press the hour hand on pointing exactly at 12. Use the hand press and light, straight pressure.
Press the minute hand on, also pointing at 12.
With the crown still out, the seconds hand pinion is stopped. Press the seconds hand on pointing at 12.
Turn the hands through a full 24 hours. Check that they never touch each other or the dial, and that the date changes around midnight, not midday.
5. Case the movement and fit the crown
Take the stem out first: with the crown pushed fully in, press the small release lever on the movement and gently pull the stem out. Fit the spacer ring, then lower the movement, dial down, into the case from the back. Turn it so the stem hole lines up with the crown tube, then slide the stem back in until it clicks.
Screw the crown (the case's own, or your new one) onto the uncut stem and push it in. Measure the gap between the crown and the end of the tube. Take the stem out and cut a little less than that off. You can always file more, but you can't add it back. Keep test-fitting until the crown sits snug against the tube and screws down smoothly. A drop of thread locker inside the crown keeps it from working loose.
6. Close it up
Blow away any dust, then check the dial one last time. Make sure the case back gasket is seated and lightly greased, then screw the case back down.
7. Fit the strap or bracelet
Fit it with the spring bar tool. If your case came on its own bracelet or strap, it's already done.
8. Let it run
Wind it, set it and leave it overnight. Check the date changes around midnight and the time is holding. Then wear it.
A home build isn't pressure-tested. Treat it as splash-proof until a watchmaker has tested its water resistance.
Wrong crown position. A dial with only 3.0 feet won't sit right in a 3.8 case, and an NH36 with the wrong day wheel shows the day off-centre. Check both before you order.
Wrong window. A date-only dial on an NH36 hides the day, or shows half of it. Buy the dial for your movement.
The date changes at noon. The hour hand went on at 12 noon instead of midnight. Lift it off and refit it just after the date changes.
Hands that touch. A hand that isn't pressed on flat can catch another one and stop the watch. Look from the side at 12, 3, 6 and 9.
Dust under the crystal. It always shows up once the case back is on. Blow out the dial and crystal right before closing.
Stem cut too short. Cut a little less than you measured and file down to fit.
A step between crystal and insert. Sloped inserts need the taller crystal made for them. Flat inserts suit standard crystals.
Bracelet doesn't meet the case. Curved-end bracelets are shaped for one case. Match the lug width and the case they're made for.
Dry gaskets. Old or dry seals let moisture in. Use fresh ones with a thin film of silicone grease.