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The AERA Technical Committee offers the following information regarding the piston oil clearance on 1992-2004 Mitsubishi 4D34 and 6D34 diesel engines. Engine builders have reported occasional piston to cylinder wall scuffing shortly after rebuilding these engines. The engines have failed pistons in multiple cylinders at the skirt’s largest diameter.

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AERA is aware of at least three revisions to the recommended piston-to-cylinder wall clearance for these engines. The current, recommended standard is .0045?-.0060? (.114-.152 mm) for all engines, with a note to err toward the high-end specification. The significant thing about this revised piston clearance is the minimum specification was the former maximum clearance required. Failure to adhere to this additional clearance may allow piston scuffing to occur.

At the time of this publication, AERA was unable to locate an aftermarket piston source for these engines. It’s been reported that there is no instructional piston fitting information accompanying new pistons from Mitsubishi.


I’ve heard a lot about the benefits of fractured rods in automotive engines. Are there any fractured rods available for Caterpillar diesel engines that you’ve heard?

There is a new fractured rod available for 1989-2005 Caterpillar C7, 3114, 3116 and 3126 engines from Caterpillar dealers as a replacement part.

Caterpillar claims the new fractured connecting rod is a product improvement and offers many advantages which include:

  • Being able to accept higher loads directly because of the clearly matched joint surfaces.

  • The fractured surface of the connecting rod and cap allows easier assembly of the rod and cap.

  • Eliminates the need for a locating dowel.

  • Eliminates fretting on the joint surfaces.

  • Improves the quality of the machining process because of reduced machining steps and a controlled machining process.

  • The machining process is further enhanced by the fracture machine, which incorporates automatic insertion of the connecting rod bolts and fastening equipment for the connecting rod bolts. This eliminates mismatch of connecting rod and cap components.


This fractured rod, p/n 213-3193, can be used as a replacement for the engines shown in Figure 1, above, and can be intermixed with earlier style rods within an engine. There is one important difference when using the new rod: it requires a different tightening procedure and torque value than the previous rod (p/n 2W-9128).

For information on receiving all of AERA’s regular monthly technical bulletins and other association services and benefits, call toll free 888-326-2372 or send an e-mail to [email protected]

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