BST40 Slide Guide Wear Indicators

BST40 slide guide, new, depressions without wear

Mikuni BST40 slide guide

Notice the ~0.020" (0.51 mm) deep depressions in the bottom corners/concentric to the bore (circled in green), as well as the four circular ~0.023" (.58 mm) depressions.

Contrast this with the same areas of a slide guide from a KTM 640LC4 with under 20K miles (32,187 km) (claimed)(note: wear was accelerated by slide lift hole drilling). The depressions in the bottom corners/concentric to the bore (normally found in the area bounded by the red circles) as well as the four circular depressions are worn completely away. The plastic is worn completely through adjacent to the bore at 10 and 2 o'clock (when viewed in the installed position). Maximum wear measures 0.085" (2.16 mm), which works out to .0042"/1K miles (.07mm/1K km) (!).

Emulsion tube wear sets in when the circled depressions are somewhere around .010" (.25 mm) or less deep, and causes overly rich mixtures in the lower portion of the rpm range. Measure with the tail of a caliper. Zero as shown, then find and measure the lowest spot in the immediate surrounding area. Repeat for the other side.

BST40 slide guide from KTM 640 LC4, <20,000 miles claimed, drilled slide lift hole, all depressions worn away
BST40 slide guide, new, alternate view
BST40 slide guide from KTM 640 LC4, <20,000 miles claimed, all depressions worn away, alternate view
 BST40 slide guide, zeroing caliper inside depression