FAW Sub-Box Input Gear Assembly: An Expert Analysis of 5 Core Components

Welcome to this expert technical analysis of the FAW Sub-Box Input Gear Assembly, the complete “input” group for the auxiliary “sub-box” of the manual transmission in the FAW Jiefang CA4251P66K24T1A3E5 6×4 diesel semi-trailer tractor.

The FAW Sub-Box Input Gear Assembly is the critical component that receives 100% of the power from the transmission’s main box and transfers it into the auxiliary box (the range/splitter section). This mechanism is what turns a 6-speed main box into a 12-speed transmission, providing the high and low ranges necessary for a heavy-duty tractor.

This guide is an essential resource for professional transmission rebuilders and parts specialists. We will deconstruct the entire FAW Sub-Box Input Gear Assembly into its 5 serviceable components, including the main ‘Input Gear’ (1701431-A8P), its critical ‘Deep groove ball bearing’ (6218 NR/YA7), and the associated locating plates and circlips.

Understanding the “Sub-Box” Logic

To understand the FAW Sub-Box Input Gear Assembly, one must first understand the “sub-box,” also known as the auxiliary transmission or range box. Heavy-duty transmissions like the one in this FAW tractor are often “6+1” or “8+1” speed main boxes. To achieve the 12, 16, or 18 speeds needed for heavy hauling, this main box is bolted to a 2-speed “sub-box.”

This sub-box has two settings: “Low Range” (a reduction gear) and “High Range” (a direct drive). The driver shifts through the main box pattern (e.g., 1-6) while in “Low,” then flips a switch. This engages the sub-box synchronizer, shifting it from Low to High. The driver then shifts through the *same* 1-6 pattern again, which now function as gears 7-12.

The FAW Sub-Box Input Gear Assembly (1701431-A8P) is the “front door” to this sub-box. It is in constant mesh with the output gear of the main transmission, and it is the component that all the synchronizers and gears inside the sub-box receive their power from. Therefore, this single gear and shaft assembly handles the full, multiplied torque of the engine in *every single gear*. Its durability is absolutely paramount, as a failure here renders the entire transmission inoperable. The FAW Sub-Box Input Gear Assembly is a zero-failure-tolerance component.

Core Component: The Input Gear (1701431-A8P)

The heart of the FAW Sub-Box Input Gear Assembly is the ‘Input gear-sub box’ (1701431-A8P). This is a massive, helically-cut gear that is permanently integrated with its shaft. This component is forged from a single billet of high-manganese, case-hardened steel (like 18CrNiMo7-6) to provide an incredibly tough inner core that resists shock-load and an glass-hard outer surface that resists wear.

The helical (angled) cut of the gear teeth is a critical design choice for the FAW Sub-Box Input Gear Assembly. This design allows the gear teeth to engage gradually, which is significantly quieter than “spur” (straight-cut) gears. More importantly, the helical cut allows multiple teeth to be in mesh at the same time, spreading the immense torque load over a much larger surface area.

A chipped tooth on this single gear, often from a bad shift or driveline shock, is a catastrophic failure. It would send hardened steel fragments throughout the entire auxiliary transmission, destroying all other gears and bearings. This makes the FAW Sub-Box Input Gear Assembly‘s gear one of the most durable components in the driveline.

The Main Bearing: Deep Groove Ball Bearing (6218 NR/YA7)

The ‘Input gear-sub box’ shaft is supported by the ‘Deep groove ball bearing’ (6218 NR/YA7). This is the single most important component for the alignment and low-friction rotation of the entire FAW Sub-Box Input Gear Assembly.

The “6218” designation indicates this is a large, heavy-duty bearing with a 90mm (3.54 in) bore. A ball bearing is used here because it is exceptionally good at handling both **Radial Loads** (the separating force from the helical gears) and **Axial Loads** (the “thrust” or push/pull force) in both directions.

The “NR/YA7” suffix is the most critical part:

• **N:** Indicates a “snap ring groove” is machined into the *outer* race of the bearing.
• **R:** Indicates that the “snap ring” (or circlip) *is included* with the bearing.

This snap ring is the “logic” of the mounting system. The bearing is pressed onto the FAW Sub-Box Input Gear Assembly shaft. The entire assembly is then inserted into its precision bore in the transmission case. The snap ring (1701358-A7G) then fits into this “N” groove, locking the *outer* race of the bearing to the case. This single ring prevents the entire FAW Sub-Box Input Gear Assembly from moving fore or aft.

A failure of this bearing is a primary cause of transmission failure. It is often first heard as a high-pitched “whine” or “rumble” that is present in all gears and is tied to vehicle speed. If this bearing seizes or collapses, the FAW Sub-Box Input Gear Assembly will be destroyed.

The Alignment System: Circlips and Locating Plate

The final three components of the FAW Sub-Box Input Gear Assembly are dedicated to precision alignment and “shimming” (setting the correct end-play).

**1. ‘Circlip-input gear bearing’ (1701357-A7G):** This is a large snap ring, or circlip. Its function is to lock the *inner* race of the 6218 bearing onto the ‘Input gear-sub box’ shaft. This prevents the shaft from sliding *out* of the bearing.

**2. ‘Locating ring-circlip’ (1701358-A7G):** This is the snap ring that fits into the “NR” groove on the *outer* race of the 6218 bearing. It locks the *outer* race of the bearing into the transmission case. These two circlips, one on the inner race and one on the outer, create a “locked” assembly that is fixed to both the shaft and the case.

**3. ‘Locating plate-input gear bearing’ (1701432-A9K):** This is a “baffle” or “cap” that is often bolted over the bearing on one side. It serves two functions: it acts as a final, redundant “stop” to prevent the bearing from moving, and it acts as an oil “slinger” or shield, protecting the bearing’s seal from direct splash and helping to channel lubrication into the bearing.

Together, these 5 components form the complete, serviceable FAW Sub-Box Input Gear Assembly. A transmission rebuilder must replace these circlips, as re-using a “sprung” or “fatigued” circlip is a major risk. If one fails and falls into the rotating gearset, it will destroy the entire transmission. The FAW Sub-Box Input Gear Assembly relies on these small parts.

FAW Sub-Box Input Gear Assembly Parts List

The following table provides the complete, detailed breakdown of all 5 components identified in the ‘Sub-Box Input Gear Assembly’ diagram for the FAW CA4251P66K24T1A3E5 tractor. A full rebuild of the FAW Sub-Box Input Gear Assembly requires these precision-matched parts.

Technical Specifications and Assembly Context

The specifications for the FAW Sub-Box Input Gear Assembly are defined by its function as the primary, constant-mesh input drive for the transmission’s auxiliary (range) section. The FAW Sub-Box Input Gear Assembly is a precision-matched set.