4.0 kW from 66.8cc in a 12.7 lb powerhead — more power than an MS 391, 1.1 lb lighter, and the reason crews that cut hardwood all day keep choosing it over the 362. The headline is the magnesium piston, the first in a production chainsaw: magnesium alloy expands less and melts higher than aluminium, so Stihl could tighten clearances and shave 25% off the piston's mass, which is why the saw revs the way it does. The honest caveat is the one owners raise themselves: M-Tronic on this platform is well proven, the piston is not — nobody has fifteen years on one. Early owners a few seasons in report no trouble, and the saw's reputation among fallers has steadily improved from scepticism to preference. At $1,250 it sits between the MS 362 C-M and the MS 462 C-M and takes 16- to 25-inch bars.

Best for: Fallers and firewood producers who want 70cc-class cutting speed at 60cc-class weight and will keep it on a 20- to 25-inch bar.

Specifications

Engine class: 60cc class (by displacement)
Engine: 66.8 cc
Rated power: 4 kW
Bar length: 18 in
Bar range: 16-25 in
Single-pass cut: about 16 in (bar minus 2)
Chain: 3/8" pitch / .050" gauge (33 RS3)
Weight: 12.7 lb, powerhead only, without fuel (15.7 lb with the 18" bar and chain)
Power type: Gas
Use case: Professional
Warranty: 1-year consumer / 3-month professional (Stihl policy effective Jan 1, 2026)
Model: MS 400 C-M

Manufacturer figures except the engine class and single-pass cut, which are derived here. What each spec means

4.0 kW at 12.7 lb
First magnesium piston in a chainsaw
HD2 filter, pre-separation air

What owners like

4.0 kW from 66.8cc at 12.7 lb — out-powers the heavier MS 391 and out-pulls the MS 362 C-M
Magnesium piston is 25% lighter than aluminium, which is what lets it accelerate like a smaller saw
M-Tronic on this platform is the version owners consider sorted
HD2 air filter plus pre-separation stretches cleaning intervals through full days in dusty felling
Rated for 16" to 25" bars, so one saw covers felling and bucking

Common complaints

$1,250 — $410 more than an MS 391 of similar displacement
Long-term durability of the magnesium piston is genuinely unproven; owners say so openly
M-Tronic faults still need a dealer diagnostic tool, not a screwdriver
3-month warranty if you earn money with it under Stihl's 2026 policy