John Deere stx38 - Manual (Page 75)

4/23/97
3 - 43
ENGINE REPAIR
This detonation shockwave (“hammering”) of the piston
inside the cylinder causes damage to piston crown (C),
top ring and groove (F), piston side skirts (E), and pin
locks (D).
The following is a list of possible causes for detonation:
1. Lean fuel mixture.
2. Low octane fuel.
3. Advanced ignition timing.
4. Engine lugging.
5. Buildup of carbon deposits on piston or cylinder
head, causing excessive compression.
6. Wrong cylinder head or excessive milling of head
increasing compression ratio.
PRE-IGNITION—
Pre-ignition is the igniting of the fuel-air mixture prior to
proper ignition spark. Pre-ignition causes internal
shock, resulting in pings, vibration, and power loss.
Severe damage is caused to the piston crown (A),
rings, and valves results from pre-ignition.
Check the following for causes of pre-ignition:
1. Hot internal carbon deposits.
2. Incorrect spark plug (high heat range).
3. Broken ceramic insulation of spark plug.
4. Hot sharp edges on valves.
IMPROPER ROD AND PISTON ALIGNMENT—
Check rod and piston alignment when piston shows a
diagonal wear pattern (A) extending across the skirt of
the piston. Contact with the cylinder wall shows on
bottom of skirt at left and ring lands on the right (B).
CYLINDER BORED IMPROPERLY—
A cylinder bored at an angle to the crankshaft may
result in improper ring contact with the cylinder which
may cause the following:
1. Rapid and uneven piston ring wear.
2. Rapid and uneven piston wear.
3. Loose or broken piston pin retaining rings.
4. Excessive oil consumption.
A broken piston pin retaining ring caused the damage
(A) shown. Piston pin retaining rings loosen or break
due to:
1. Rod misalignment.
2. Excessive crankshaft end play.
3. Crankshaft journal taper.
4. Weak piston pin retaining rings.
5. Incorrectly installed piston pin retaining rings.
Inertia can also cause a broken piston pin retaining ring
to beat out the piston and cylinder, causing extensive
damage.
M29947
D
C
F
E
M30039
A
A
M29948
B
M29949
A