Manual - Chapter 1 Engine

20. Examination and renovation: cylinder block

  1. The usual indication of badly worn cylinder bores and pistons is excessive smoking from the exhausts. This usually takes the form of blue haze tending to develop into a white haze as the wear becomes more pronounced. However this must not be confused with the smoke cloud which these machines all seem to produce on starting, especially when hot or when the machine has been resting on its side stand. All K-series models do this in varying degrees and assuming that oil consumption is negligible, there is nothing that owners can do about it except remember to use the centre stand.
  2. The other indication is piston slap, a form of metallic rattle which occurs when there is little load on the engine. if the cylinder head end of each bore is examined carefully, it will be found that there is a ridge on the thrust side, the depth of which will vary according to the rate of wear which has taken place. This marks the limit of travel of the top piston ring
  3. Measure the bore diameter just below the ridge both along the gudgeon pin axis and at right angles to it using an internal micrometer or a bore gauge. Take similar measurements at the middle and bottom of each bore. Although no service limits are specified for taper and ovality, these dimensions should not differ significantly from each other. Take the largest of these measurements to be the bore inside diameter.
  4. Subtract the piston skirt diameter (see Section 21) from the bore diameter to calculate the piston/cylinder clearance; if this exceeds the specified service limit. either the pistons or the cylinder block must be renewed. Only very careful measurement can reveal whether this is necessary; in view of the expense of this solution owners are strongly advised to have their findings confirmed by an authorised BMW dealer before taking further action.
  5. If sufficiently accurate measuring equipment is not available, an approximate idea of the degree of bore wear can be obtained by inserting a new piston (less rings) so that it is appropriately 3/4 inch from the top of the bore. Use feeler gauges to measure the clearance between the piston skirt and the bore at the point of greatest wear. It must be stressed that this is only approximate, to be regarded purely as a quick check before taking the cylinder block and pistons to an expert for accurate measurement.

The history of BMW

1928 - Cylinder head R63