BMW 121ci Specs: 89 x 80 mm

BMW 121ci specs: 89 x 80 mm (3.5039 x 3.1496 in) bore and stroke, shared by all 2 versions. Displacement, compression and factory ratings, plus rod ratio.

At a glance

Displacement	121 ci
Cylinders	4
Bore	3.504"
Stroke	3.150"
Compression	8.1:1
Advertised power	100–130 hp
Years	1968–1979

Engines in this family

Engine	Displacement	Bore × Stroke	CR	HP
M10	121 ci	3.5039" × 3.1496"	—	100–130
M10B20	121 ci	3.5039" × 3.1496"	8.1:1	110

Factory cars with a BMW 121ci

• BMW 2002: 1968, M10, 100 hp; 1969, M10, 100 hp; 1970, M10, 100 hp; 1971, M10, 100 hp; 1972, M10, 100 hp; 1972, M10, 130 hp; 1973, M10, 100 hp; 1973, M10, 130 hp; 1974, M10, 100 hp; 1974, M10, 130 hp; 1975, M10, 100 hp; 1976, M10, 100 hp
• BMW 3 Series / M3: 1977, 110 hp; 1978, 110 hp; 1979, 110 hp

What varies across the family

All are oversquare — bore larger than stroke, which favours revs over low-end torque.

Each engine page carries rod ratio, mean piston speed, bore/stroke character and cylinder head specs where published, and links into the calculators with its dimensions already filled in.

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