Strap for Horological Machine N°4 HM4 Thunderbolt Red Gold

The lug width of the MB&F Horological Machines 40.RSL.R is 20mm. This dimension is integral to the 54mm case design and is critical to maintaining the original proportions of the timepiece. To verify the measurement accurately, place a digital calliper at the centre of the spring bar slot, measuring the distance between the inner edges of the lugs.

For the MB&F Horological Machines 40.RSL.R, we recommend an alligator strap with vegetable tanning, which amplifies the sculptural character of the titanium and rose gold case. Alternatively, shell cordovan offers an almost metallic depth of colour, whilst vegetable-tanned calf with a tight grain allows for controlled patina development over time. For intensive daily wear, a cotton sateen NATO strap or FKM rubber provides practicality without aesthetic compromise.

To change the strap on a 20mm lug width watch, you will require a dedicated spring bar removal tool. Position the watch face-down, insert the thin blade of the tool into the spring bar slot, push upward to compress the internal spring, and simultaneously withdraw the bar. Once both bars are extracted, the old strap detaches easily. For installation of the new strap, repeat the procedure in reverse, ensuring the bars are fully seated and the internal spring is secure.

All straps designed for 20mm lug width watches are compatible with the MB&F Horological Machines 40.RSL.R, provided the lugs accommodate standard spring bars. Milano Straps offers a specialised collection of 20mm straps in leather, NATO, rubber and other materials, selected for proportion and construction quality. Always verify that the strap ends present consistent thickness (approximately 5–6mm) to guarantee perfect contact with the case.

The lifespan of a leather strap depends on the tanning method and usage patterns. An alligator or shell cordovan strap with vegetable tanning, if maintained with care (conditioning with specialist creams every 3–4 months), can remain in excellent condition for 8–12 years. A vegetable-tanned calf strap typically withstands 5–8 years of daily wear. The patina that develops over time constitutes an aesthetic asset. Factors such as humidity, prolonged sun exposure and perspiration accelerate ageing: given the visual prominence of the 40.RSL.R case, the strap deserves consistent maintenance.