Revisions to this document are noted
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SUBJECT:

SIL 02-WT-24, Rev. A
May 20, 2024
Page 1 of 3

New 3-Piece C3 and C4 Cast Piston Assembly
3000 SeriesTM

MODELS AFFECTED:

Introduction:
Bonded pistons for the 3000 Series C3 and C4 clutches are changing to a 3-piece cast piston design. The new
piston assembly includes a cast piston, an inner lip seal and an outer square cut seal as shown in Figure 1.
Indianapolis production has intermittently introduced the 3-piece cast piston design to transmissions built in
January-April 2024. Therefore, a portion of units built in early 2024 contain the new cast piston design, and the
balance of transmissions contain the bonded pistons. This approach will continue until stock of the former design is
depleted. Transition of Hungary production is targeted in Q3 2024.
For the foreseeable future (and until further notice), new transmission production will continue to transition between
C3 and C4 clutch, bonded and cast piston designs. Effective immediately, service parts and related service kits
from Parts Distribution Centers (PDC) will begin shipping only the cast piston and related seals. The bonded piston
P/N 29561880 has been exhausted from Indianapolis PDC.

FORMER
BONDED PISTON
P/N 29561880

CURRENT
CAST PISTON
P/N 29571670

CURRENT
INNER SEAL
P/N 29558027

CURRENT
OUTER SEAL
P/N 29567711
6676837

Figure 1. Detail Parts of the New 3-Piece Cast Piston Design

JT / SL9157EN

6741487

Copyright © 2024 Allison Transmission, Inc. All Rights Reserved.

SIL 02-WT-24, Rev. A
May 20, 2024
Page 2 of 3
The new cast piston is backward compatible in 4th Generation transmission (and later), a design that was
introduced in CY2006. The new 3-piece design is not backward compatible with WTEC III (pre-4th Gen)
transmission applications.
NOTE: Following rebuild, repair or overhaul of an Allison transmission, it is important to follow Allison
procedures, including dyno testing, resetting fast adaptive settings and test driving the vehicle. Failure
to follow these instructions and to properly cycle the clutches may lead to poor, initial shift quality and
customer complaints.
Running changes to the C3/C4 Piston Clutch Assembly will be made in production. Since the clutch assembly
function is unchanged with use of either piston design, the part number for the C3/C4 Piston Clutch Assembly
(P/N 29542751) is not changing.

Figure 2. C3/C4 Piston Clutch Assembly Comparison
NOTE: There is no TCM calibration impact when switching a 4th Generation (or later) transmission from
the bonded piston to the 3-piece cast piston assembly or vice versa.

Table 1. Former and Current Part Numbers and Quantities per Transmission
Former C3/C4

Current C3/C4

Description

Bonded Piston

Qty/Trans

3-Pc Cast Piston

Qty/Trans

Piston

29561880

2

29571670

2

Inner Seal

—

0

29558027

2

Outer Seal

—

0

29567711

2

Copyright © 2024 Allison Transmission, Inc. All Rights Reserved.

SIL 02-WT-24, Rev. A
May 20, 2024
Page 3 of 3

Stock Availability:
Effective immediately, only current service stock is available from global Parts Distribution Centers. Limited
quantities of the former, 4th Generation service kits containing bonded pistons are available from global Parts
Distribution Centers.

Order Placement:
Since the former design is being replaced with three individual parts, orders for the former design will not be
automatically switched to the new design. It will be up to the Parts Manager to reference the Parts Catalog and
order the correct part numbers and quantities for the transmission being repaired (refer to Table 1).

Service Strategy:
Either design is acceptable for use in 4th Generation (or later) transmissions. Allison recommends replacing both
C3 and C4 bonded pistons with new cast piston assemblies if either bonded pistons has failed.

Service Kits:
Service kit P/N 29550017 that contained two of the former 4th Generation bonded pistons will start including two
cast pistons, two inner and two outer seals in their place. The kit part number will not change and there will
be a running change to the kit content.
New service kit P/N 29572089 has been released for MY24 transmissions that already contain the cast pistons.
The new kit content is the same as kit P/N 29550017 but will not include the cast pistons since they are considered
"reusable" if undamaged. Additionally, the new kit includes two of the new inner piston seals (P/N 29558027) and
two of the new outer piston seals (P/N 29567711).

Copyright © 2024 Allison Transmission, Inc. All Rights Reserved.