Revisions to Adjustable Idler Gear Hub and Mounting Cap screws
M-215-003
(June 2007)

Valid for
All models built before April 2007 with MP7 engine

Case description
Effective April 18, 2007, a revised adjustable idler gear hub (part no. 21012985) and mounting capscrews (part no.
21015266) were implemented into production on MP7 engines, replacing the previous hub (part no. 20520535) and
capscrews (part No.20477171). The revised idler gear hub can easily be identified by the counterbore at the lower
mounting holes, and the revised mounting capscrews can be identified by the part number stamped on the head of the
screw.
Note: Does not apply to Mack Trucks Australia.

Parts
Timing Gear Train — MP7 Engines

Key

Qty.

Part No.

Description

Replaces

1

1

21012985

Hub, adjustable idler gear

20520535

2

5

21015266

Capscrews, adjustable
idler gear hub mounting

20477171

Repair
The adjustable idler gear hub mounts against the gear thrust plate and timing gear plate by five flange-head mounting
capscrews that pass through the plates and into the threaded holes in the cylinder head (upper three mounting

capscrews) and the cylinder block (lower two mounting capscrews). The counterbore at the lower mounting screw
holes in the hub and the improvements made to the mounting capscrews provide improved thread engagement and
clamp load retention for the lower mounting capscrews in the cylinder block.
On engines manufactured prior to the April 18, 2007 implementation date, instances of stripped or partially stripped
lower mounting bolt holes in the cylinder block have been reported. When replacement of the adjustable idler gear
hub on an MP7 engine manufactured prior to April 18, 2007, is necessary, the revised hub and mounting capscrews
should be used. Additionally, if stripped or partially stripped lower mounting bolt holes in the cylinder block are
encountered, the threads can be repaired by installing Heli-Coil® thread inserts as outlined below.

Heli-Coil® Thread Insert Installation Procedure
Note: Heli-Coil® thread inserts, insert kits and all applicable tools are available at most industrial suppliers, or can be
ordered directly from the manufacturer by visiting the Emhart® Teknologies web site at www.emhart.com. A tool kit
(Emhart® Teknologies part no. 5403-12), which contains the proper size drill bit, high-speed Heli-Coil® tap, a tang
removal tool and quantities of three different length inserts, is available. For this repair, use the 12 mm length insert
only.
Note: To avoid having debris from the drilling and tapping operations enter the engine, tape a sheet of paper or plastic
over the timing gear plate, leaving a small opening at the two mounting bolt holes which are to be repaired.
This repair is performed without removing the timing gear plate.
1. Place a mark on a 12.5 mm drill bit, 28 mm (1.102″) from the tip of the bit to indicate the maximum drilling
depth. This measurement is the depth of the mounting bolt hole measured from the outside surface of the
timing gear plate to the bottom of the hole.
2. Carefully drill both holes to the maximum depth. Use cutting oil during the drilling operation, and stop drilling
when the mark on the drill bit reaches the outside surface of the timing gear plate.

Caution
Under no circumstances can the 28 mm (1.102″) maximum hole depth be exceeded.

Note: DO NOT deburr or chamfer the hole larger than the major thread diameter.
3. Place a mark on the Heli-Coil® bottom tap (Heli-Coil® part no. 2093-12, M12 x 1.75–12), 18 mm (0.709″) from
the tip of the tap, which indicates the minimum depth to which the hole must be tapped. This dimension is the
thread depth from the outside surface of the timing gear plate.
4. Tap both mounting bolt holes to the minimum depth as indicated by the mark on the tap. Use cutting oil for
lubrication during the tapping operations.
Drill and Tap Damaged Mounting Bolt Hole (Cylinder Block and Head Shown with
Timing Gear Plate Removed for Clarity)

5. Assemble the M12 x 1.75–12 thread insert (Heli-Coil® part no. 1084-12CN120) onto the Heli-Coil® insertion
tool until the heel of the tool contacts the drive tang of the thread insert.
6. Install the insert into the drilled and tapped hole until it is a maximum of 1/2 thread (0.9 mm [1/32″]) below the
cylinder block surface. When the insert is completely installed, remove the insertion tool.
Note: The specified insert (part no. 1084-12CN120) is 12 mm long. Longer inserts must NOT be used.
The insert must be installed completely into the cylinder block threads, and no part of the insert can remain in
the timing gear plate. This will prevent pulling the insert out, ruining the insert and possibly damaging the
cylinder block threads should removal of the timing gear plate be necessary at a later time.
Install Heli-Coil® Insert

7. Using the appropriate tang removal tool, break off and remove the tang from the insert.

Note: If the appropriate tang removal tool is not available, use a small flat-blade screwdriver to break the tang
and a small needle-nose pliers to remove the tang from the hole.
8. Remove the plastic or paper sheet from the timing gear plate, using care to prevent any of the drilling and
tapping debris from entering the engine.
When reassembling the adjustable timing gear, use the new hub (part no. 21012985) and five new mounting
capscrews (part no. 21015266). Tighten the capscrews to 100 N•m (74 lb-ft).
Note: The adjustable timing gear hub mounting bolts are five-time use bolts, which includes the initial installation.
Punch-mark the head of the bolts upon installation to indicate the number of times the bolts have been used.

Issued by
Technical Service
Mack Trucks, Inc. engages in a continuous program of testing and evaluating to provide the best possible product.
Mack Trucks, Inc., however, is not committed to, or liable for updating existing chassis.