Marathon GPM 34mm

Watch Mod: Recasing

Marathon makes a sharp mil-spec watch, the 34mm GPM (General Purpose Mechanical). The dial and hands are clean and beautiful—and lit by radioactive tritium that glows on its own.*

The stock case isn’t for me—small (34mm), angular, skinny 16mm lug width, and only 30m water resistance. The good news: it runs the NH35 movement. If the hand stack and dial fit, you can drop it into a bunch of aftermarket NH35 cases—the mod is dead simple.

I didn’t want to risk buying a new GPM just to test it, so I found a second-hand one for a great price—way below typical used prices.

The case I used wasn’t my first choice: The hand stack (about 3mm tall) didn’t fit Lucius Atelier’s 36mm Ultra-thin Explorer case, but it dropped right into cases made for the taller NH34 (GMT) movement. I hope to swap the flat crystal on the LA case with a domed one.

The dial’s a little smaller than the standard 28.5mm, but any case with a built-in rehaut or chapter ring will cover it fine.

I removed the logo from the photos to avoid confusion with official Marathon products.

*The tritium has a half-life of 12.5 years; it’s sealed in tiny glass tubes coated with phosphor. The tritium gives off low-energy electrons that make the phosphor glow. The radiation is fully contained and undetectable.

Case Material	Stainless Steel with Black DLC
Width	36 mm
Thickness	14.0 mm
Lug to Lug	41.5 mm
Strap Width	18 mm
Movement	NH35
Lume	Tritium Tubes
Crystal	Domed Sapphire with Blue AR
Crown	Screw-Down
Lowest Water Resistance	?? m

Gallery

Part	Description	Cost	Supplier	Notes	Alternate Part
Case	36mm Barrel Shape Titanium Case 20ATM AR Sapphire Glass Big Crown	$32	Tandorio	Includes bezel, crystal, chapter ring, caseback, crown, and spring bars. Very high quality case. Some tool marks on crown. Excellent blue AR coating on crystal. Water resistance untested. Not compatible with SKX casebacks.	Stainless steel version available
Crown	36mm Pilot Diver Watch Case Titanium or 316L	$35	Tandorio	Only the crown was used from this case. You can contact the Tandorio store on AliExpress and they will sell you just the crowns for their cases.	Use the onion crown that comes with the titanium case, above.
Dial, Hands, Movement	Marathon GPM 34mm (used/secondhand)	$180	Marathon Watch	I purchased mine secondhand on r/watchexchange for well-below the going rate for used. MSRP for new is $450. Long Island Watch has the best price, or wait for Marathon's black friday sale.	Marathon GPM 34mm at Long Island Watch
Strap	Hemsut Horween Chromexcel Leather Watch Band Quick Release 18mm (Brown)	$21	Hemsut	Excellent leather
Strap	18/20/22mm French Troops Parachute Elastic Nylon Watch Band	$3	Generic		Amazon seller
		$271

Notes

Links are provided as helpful suggestions only; I have no affiliate relationship with any vendor or supplier.
Gaskets, springs, adhesives, stems, and other "consumables" are included with each item unless noted otherwise.
Prices are in US dollars.
Listed prices do not factor in tax, shipping, and discounts.
Updated April, 2025

Build Notes

The watch as it arrived. The original owner removed the permanent lugs so they could use a custom strap, a 16mm silicone band from Barton.

Removing the snap caseback revealed a custom rotor (with a few handling marks).

I'll have the radiation and safety information engraved on the new caseback.

The hands appear to be attached to the pinions by spacer tubes. This could be for a couple of reasons:

A single set of hands could be used with different movements.
The tubes allow them to add space for the tritium tubes.

I felt it was too risky to make any changes to the hand stack.

The brightness of the tritium tubes was enhanced in the photo above. In actual experience, the brightness is less than regular luminous paint, but it’s constant and usable throughout the night.