Simples!
Buy the M5 (5mm) 8mm x 0.8mm as well as the M5 (5mm) 10mm x 1mm.
Then I can report back here![]()
BOTH sizes will fit. The most important thing is M5 (5mm) inside diameter.
The reason why 8mm x 0.8mm is the correct size.
1. Most modern DE razors (95%) have a head thickness of 8mm to accommodate a 4.48mm x 0.8mm thread. Most Older DE razors have 8mm head thickness to accommodate a 4.31mm x 0.79mm thread. (10-32 thread).
Due to the head thickness and to have a washer that sits flush without overhang, the 8mm will fit athestically better (no overhang) than the 10mm size.
2. The 0.8mm size when threaded between the bottom plate and head of the handle will look invisible to the eye in most applications. The 1mm in most applications will be visible. This is even more evidient in some razors that have a 8mm x 0.2mm slight protusion into the top cap for a snug fit.
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