Mercury MerCruiser Quicksilver 18903T Rear Transmission Mounting Bracket — Hurth Inboard Applications

Mercury MerCruiser Quicksilver 18903T — Rear Transmission Mounting Bracket, Hurth Inboard

Overview

The Mercury MerCruiser Quicksilver 18903T is a genuine OEM rear transmission mounting bracket for selected MerCruiser inboard engines equipped with Hurth marine transmissions. It is positioned at the rear of the powertrain in the transmission and engine mounting assembly, where it supports the rear of the engine/transmission package and works with the front bracket, trunnions, mounting bases, and related hardware to maintain correct drivetrain alignment.

18903T is superseded to 18903T02, which is the current active Mercury OEM part number for this application. If your parts diagram shows 18903T, 18903T02 is the correct current replacement.

Important: Fitment depends on both the engine serial range and the installed transmission. MerCruiser inboard engines were available with Hurth and Borg Warner transmissions, and those configurations use different rear mounting brackets. Always confirm that the parts diagram for your specific engine model, serial number, and transmission calls for 18903T / 18903T02 before ordering.

Compatible Applications

• MerCruiser 5.7L / 350 CID MIE — Hurth transmission mounting (confirm serial range)
• MerCruiser 5.7L EFI MIE — Hurth transmission mounting (confirm serial range)
• MerCruiser 350 MAG MPI MIE — Hurth transmission mounting (confirm serial range)
• MerCruiser 350 MAG MPI Horizon MIE — Hurth transmission mounting (confirm serial range)
• MerCruiser 7.4L / 454 CID MIE — Hurth 630 transmission mounting (confirm serial range)
• MerCruiser 7.4L MPI MIE — Hurth transmission mounting (confirm serial range)
• MerCruiser 7.4L MAG MPI Horizon — Hurth transmission mounting (confirm serial range)
• MerCruiser 8.1L Inboard — Hurth transmission mounting (confirm serial range)
• MerCruiser 8.2L / 502 CID MIE — Hurth transmission mounting (confirm serial range)
• MerCruiser MX 6.2L MPI MIE / Horizon MIE — Hurth transmission mounting (confirm serial range)
• Always confirm by exact engine model, serial number, and transmission type — Hurth and Borg Warner-equipped versions of the same engine use different rear brackets

When to Replace

• Bracket is cracked, bent, or has visible structural damage
• Mounting holes are elongated, stripped, or damaged from wear or overtightening
• Heavy corrosion is compromising structural integrity or fastener contact surfaces
• Bracket no longer holds the transmission in the correct mounting position
• During major engine or transmission removal — inspect both rear brackets at this time
• When engine or transmission alignment cannot be correctly achieved after mount service

Installation Notes

• Typically two rear brackets are required per installation — confirm quantity from the Mercury parts diagram for your engine model and serial number
• Inspect the matching bracket on the opposite side, trunnions, mounting bases, and surrounding structure during installation
• Related hardware shown in Mercury Hurth 630 mounting diagrams: M12 × 45 screw 806195 and .500 lockwasher 38829 — replace fasteners if corroded or damaged
• The front bracket 66418T is also shown in Hurth 630 mounting assemblies — inspect it at the same time
• Check engine and transmission alignment after installation — incorrect mounting position from a worn or incorrect bracket affects driveline alignment and can cause vibration or coupling wear
• Torque all fasteners to the MerCruiser service specification for the applicable engine and mounting assembly

Technical FAQ

• What is the difference between 18903T and 18903T02? 18903T02 is the current superseding part number for 18903T. They serve the same function in the same mounting application. If your diagram shows 18903T, 18903T02 is the correct current OEM replacement. Stock carrying the original 18903T number functions identically.
• How do I know if my engine has a Hurth or Borg Warner transmission? Check the transmission model plate directly on the transmission housing, or confirm by engine model and serial number using the Mercury parts catalog. Hurth and Borg Warner-equipped versions of the same engine use different rear mounting brackets — the transmission type must be confirmed before ordering.
• Do I need to order two brackets? In most Hurth 630 mounting assemblies, two rear brackets are required (one per side). Confirm the exact quantity from the Mercury parts diagram for your engine and serial number before ordering.
• Why does the mounting bracket affect alignment? The rear bracket positions the transmission at a specific height and angle relative to the engine centerline. A bent, incorrect, or worn bracket changes that geometry, which shifts the output shaft position and affects the alignment of the drive shaft, coupling, and strut. Proper mounting bracket condition is the starting point for achieving correct driveline alignment.
• Should I replace both rear brackets at the same time? Yes, in most cases. If one bracket has failed or is significantly corroded, the other has likely been in the same environment and may be close to the same condition. Replacing both at the same service visit avoids a repeat job.

Parts Desk Insights

Best Use Case: This bracket is the right part for a MerCruiser inboard installation that specifically uses the Hurth transmission mounting system and requires rear bracket replacement or renewal during major service. Because the bracket positions the transmission at a specific height and angle, using the correct Mercury OEM bracket is important for maintaining driveline alignment after service — a fabricated or non-OEM substitute can place the transmission at a slightly different position and make shaft alignment difficult to achieve.

Common Issues: The most frequent problem is not confirming the transmission type before ordering — Hurth and Borg Warner configurations of the same engine use different brackets, and they are not interchangeable. The second common issue is ordering only one bracket when the assembly requires two. Check the Mercury parts diagram for your engine and serial number to confirm both the correct part number and the required quantity before ordering.