Mack 12.0L Rocker Shafts - Engine Builder Magazine
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Mack 12.0L Rocker Shafts

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What is the correct procedure for disassembling Mack
12.0L rocker shafts equipped with an engine brake?

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The AERA Technical Committee offers the following information regarding a
caution when disassembling the rocker shaft for 2000-2007 Mack 12.0L MP7 and
MP8 engines.


This information applies to engines equipped with the PowerLeash™ engine brake.
In this system the exhaust rocker arm incorporates an integral engine brake
valve and piston. When removal of the rocker shaft is necessary, the pistons
must be retained to keep them fully retracted in the bores. Suitable tie wraps
or mechanic’s wire can be used to secure the pistons in place as shown in
Figure 1.

Failure to secure the engine brake piston before removing the rocker shaft
assembly will allow the piston to drop from the bore as the shaft is removed.
Should this occur, it may not be noticed, or it may be difficult to push the
piston fully back into the bore.

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Additionally, plungers are a match-fit to the rocker arm, and inadvertent
mix-up of components must be avoided. Assembling the rocker shaft to the engine
or operating an engine with the engine brake pistons not fully retracted will
result in breakage of valve train components and significant engine damage.

Note: The tie wraps or mechanic’s wire must be removed only after the rocker
shaft has been reinstalled on the engine.

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