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Mercury / Mariner / Force

Bearings for the Small End of the Connecting Rod (2-Stroke)

The bearings for the small end of the connecting rod are the key components to transmit
the explosion force. To reduce the impact and noise level, the inner diameter of the
bearing cage will move against the piston pin that is called the “inner guide”. Because
the bearing for the small end of the cage is designed as a concave shape, it can reduce
weight and also ensure that rollers can easily obtain lubricant. The cage material of
these bearings is SCM 415 (JIS) alloy steel that has been subjected to the carbonitriding
and heat treatment. The overall bearing structure is high rigidity, allowing it to withstand
the impact of high-speed engine. The cage can be copper or silver-plated in order to
reduce the friction between the bearing and connecting rod during break-in and
increase the life of the bearings.
Bearings for the Big End of the Connecting Rod

Bearings for the big end of the connecting rod rotate around the crankshaft pin in a
eccentric motion. Because the centrifugal force is proportional to the square of the
rotational speed, the high speed engine application (over 8000 rpm) bearings must
have an especially strong cage structure.

Usually GLM’s needle bearing roller uses SCM 415/420 (JIS) alloy steel with
carbonitriding and heat treatment, same as cage material. It has better fatigue and
wear resistance. The cage should be electoplated (with copper or silver) when the
bearings for the big end of connection rod are used in engines with maximum speeds
of over 8,000 rpm. This plated surface ensures a smooth run-in between the cage
and the race. To avoid hydrogen embrittlement, we treat all electroplated bearing cages
with a de-hydrogen process after the plating. The cage material of these bearings are
STKM-13C or SPCC, and the deviation of the diameter of the roller is 3 micron.

What are the Main Causes of Bearing Failure?

• The cage cracks due to poor manufacturing and non qualified materials.
• The needle rollers fatigue caused by overloading.
• Wear on the rolling surfaces (Piston Pin, Crankshaft & Connecting Rod).
• Abnormal friction or insufficient lubrication causes overheating damage.
Troubleshooting Tips

For wear on the rolling surfaces inspect the following parts; Connecting Rod, Piston Pins
and Crankshaft. The big end of the connecting rod surface roughness should be below
0.15Ra. The roughness in the small end of the connecting rods should be below 0.2R.
The roundness and cylindricity of the big end of the connecting rods must be within
0.003mm. Parallelism between the big and small end of the conneting rods are 0.0008”/
4.0000” for High-speed engine and 0.0025”/4.0000” for normal usage engine.

The Piston Pins surface roughness below 0.15Ra and Crankshaft surface roughness
below 0.2Ra. Both parts roundness and cylindricity to be within 0.002mm.

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