1) Checking Pistons - Rotax MMH-912 Series Maintenance Manual

3.13.1) Checking pistons

Effectivity 912/914 Series
Edition 1 / Rev. 0
BRP-Rotax
Maintenance Manual
See Figs. 72-86 and 72-87.
On engines of the 912/914 Series, light alloy full skirt cast pistons are
used. The piston axis is offset by 1 mm (0.039 in.).
Remove piston rings using a pair of piston ring pliers. Make absolutely
sure to re-fit rings in their initial position. Remove any oil carbon residue
from piston rings and ring grooves. The best way to clean the groove
root is by using a piece of a broken piston ring. Increased deposit is to
be expected when using AVGAS 100LL. Remove carbon deposits from
the piston crown.
Inspect and measure piston. Measure the cylinder and determine the
permissible installation clearance in accordance with 00-00-00 sec. 9.
If the determined installation clearance is greater than the permissible
installation clearance, the piston and / or the cylinder must be replaced.
See 72-00-00 sec. 4 and 00-00-00 sec. 9.
Check the groove (2) for the piston pin circlip. If any burrs are detected,
carefully remove them. If the groove is excessively worn (> 0.3 mm =
0.012 in. than the retaining ring), replace piston.
There are 2 tolerance groups, red and green (piston marked with a red
or greencolored dot). The difference between them is 0.01mm (0.0004
in.). The "red" piston is the smaller one. The nominal piston size is
stamped on the piston crown. Oversize pistons are not available. The
piston is supplied only complete with 3 rings.
Determine the diameter of the piston pin.
See 72-00-00 sec. 4.
With the rings (3) fitted on the piston, measure the flank clearance with
a feeler gauge (4). See Fig. 72-87. Increased deposits in the tubular
spring (see Fig. 72-89 (3)) of the oil scraper ring indicate that AVGAS
100LL has been used.
72-00-00
page 77
May 01/2007