Series /6 Technical Notes
Enthusiasts interested in rebuilding or modifying /6 engines should note a change in machining that occurred during the /6's three-year production run. Engine blocks for motorcycles built through August 1975 (model years 1974 and 1975) have the same size of cylinder spigots as earlier /5 engines. Engine blocks for /6 motorcycles built during or after September 1975 (i.e., 1976 models) have larger spigots. Base sleeve outside diameter on the cylinders increased in corresponding fashion, from 97mm to 99mm. Similarly, pushrod tube diameter also increased, from 15.8mm to a 17.8mm. It is also important to consider that BMW were notorious for using part overruns from the previous model year to outfit new bikes, even when such changes occurred, so it is possible that some 1976 /6 models made in the autumn of 1975 may have the smaller diameter engine parts left over from the '75s.
The down tube frame gussets on /6 are slightly enlarged compared to their /5 predecessors, and this visible change (along with other minute details and brazed-on fixtures like the brake line clip) can be useful in identifying and typing a frame from a picture. (/7s have even larger down tube gussets and a welded horizontal cross-brace for even more rigidity, also clearly identifiable from cursory visual inspection.)
Compared to the /5 short-wheelbase motorcycles that preceded them, /6 bikes utilize a beefed-up subframe. Starting in the 1974 model year, BMW equipped their cycles with 13.25" (337mm eyelet to eyelet) shock absorbers, slightly longer (7-10mm) than the previous OEM suspension units. The top eyelet of the shocks was machined taller, and the heftier shock mounting plates on the improved subframe were drilled higher to accommodate the change. As a result, most /5 shocks (usually recognizable by their long shrouds) will not mount with proper clearance on later Airheads, although /6 and /7 units can be retrofit to /5s.