Kohler K91 Service Manual page 112

takes on a shiny or bright appearance . Scratches
on rings and piston are caused by abrasive mate­
rial such as carbon , dirt , or pieces of hard metal .
Detonation damage occurs when a portion of the
fuel charge ignites spontaneously from heat and
pressure shortly after ignition . This creates two
flame fronts which meet and explode to create
extreme hammering pressures on a specific area
of the piston . Detonation generally occurs from
using f uels with too low of an octane rating .
Pre-ignition or ignition of the fuel charge before
the timed spark can cause damage similar to
detonation . Pre-ignition damage is often more se­
vere than detonation damage - often . a hole is
q uickly burned right through the piston dome . Pre­
ignition is caused by a hot spot in the combustion
chamber from sources such as : glowing carbon
Stuck, Broken Rings
Abrasive Scratched Rings
Figu re 1 1 -1 0. Common Types Of Piston And Ring Damage .
deposits , blocked fins , improperly seated valves or
wrong spark park plug .
See Figure 1 1 - 1 0 for some common types of pis­
ton and ring damage.
S ervice
K-Series service replacement pistons are available
in STD bore size , and in . 003 " , . 0 1 0 "
. 030 " oversizes. Replacement pistons include new
piston ring sets and new piston pins .
Service replacement piston ring sets are also
available separately for STDI . 003 " (same ring set
for both sizes) . . 0 1 0 " 020 " and . 03 0 " oversized
• .
pistons. Always use new piston rings when install­
ing pistons . Never reuse old rings .
The cylinder bore must be deglazed before service
ring sets are used .
Abrasive Worn R i ng s
Scored Piston a n d Rings
020 " . and
• .
1 1 . 9