BMW K 1300 GT - Manual (Page 9)

BMW
Media
Information
10/2008
Page 9
In the process of upgrading the power unit of the K 1300 S, the engineers
at BMW Motorrad have given particular emphasis to supreme riding qualities.
Hence, the combustion chambers on the four-cylinder, the intake camshaft
angle and the valve timing as well as the stroke of the outlet valves have
been modifi ed accordingly, with the outlet valve springs being re-set from the
ground up.
Running in anti-friction bearings, the connecting rods are extra-light forged
components made of heat-treated steel. Measuring 119 millimetres or 4.68"
in length (K 1200 S: 120 mm/4.72"), they keep lateral forces acting on the
pistons to a minimum and guarantee superior engine refi nement despite the
longer stroke.
Together with their anti-friction bearings, the conrods weigh just 404 grams
(K 1200 S: 413 grams). In usual BMW style, the upper conrod opening comes
with a bearing bush conceived for a service life of more than 100,000 kilo-
metres or 61,000 miles.
The connecting rods are split horizontally in proven crack technology, with
the large opening in the connecting rod being “broken” along its centre level
by hydraulic force applied as an abrupt jolt. The fracture surface created in
this way allows extremely precise subsequent assembly without requiring any
further centring.
Newly developed lightweight box-type pistons measuring 80 millimetres or
3.15" in diameter, with a short piston apron and two slender piston rings
optimised for minimum friction as well as a slender oil scraper ring, serve to
increase cylinder stroke and, accordingly, the capacity of the engine.
The fl at upper surface of the combustion chambers, the new contour along
the bottom and the valve pockets all help to provide a stable thermodynamic
combustion process. Through its new contour, the fl oor of the pistons serves
additionally to optimise the weight of the entire unit: Together with the bolts
and rings, piston weight is a mere 287 grams (K 1200 S: 299 grams).
To remove heat from the piston fl oor under high thermal load, the pistons
are cooled at the bottom also on the K 1300 S by modifi ed oil spray jets in the
crankcase, helping to extend their service life.
The degree of crankshaft balance is adjusted to the different mass distribution
through the use of new pistons. To eliminate free second-order mass forces
inevitable on a straight-four power unit, the crankshaft, as on the former