Gbox / Trans / PTO / Clutch

CHapter 8 - Gerabox, PTO, Clutch

Gearbox

Before dismantling, an oil sample was taken from the gearbox as part of the condition assessment. Some wear material was identified across the samples, although the history of the lubricants and corrosion found elsewhere in the vehicle meant the results could not be considered in isolation. The gearbox oil was subsequently drained and replaced to establish a known baseline for future monitoring.

Access to the gearbox also allowed closer inspection of the input shaft, bearings and PTO drive gear. These components showed no obvious signs of abnormal wear or damage and were considered suitable for continued service.

One issue identified during the work was a broken locating tab on the gearbox oil pump retainer. Rather than leave a known defect buried inside the assembly, the damaged retainer was removed and replaced.

The replacement retainer and surrounding components were thoroughly cleaned before installation. A new O-ring and key were fitted, with particular attention given to confirming correct key engagement and alignment before final assembly.

Clutch Inspection

Removal of the PTO housing provided the access required to remove the bell housing and properly inspect the clutch.

The condition of the clutch assembly was considerably better than might be expected from a vehicle of this age. The pressure plate and mating surfaces showed no significant scoring, pitting or recessed wear, while the flywheel appeared to have been machined previously.

The clutch friction disc measured approximately 3.5 mm of friction material on each side, consistent with an assembly showing very little wear. The clutch shaft and associated bearings were also inspected and found to be in excellent condition.

The release bearing was removed, cleaned and inspected. With no significant wear identified, it was retained, regreased and refitted along with the cleaned and lubricated release arm.

There was little justification for replacing good components simply because the assembly was apart. Inspection and measurement provided enough confidence to retain the existing clutch rather than replacing it.

Bell Housing & PTO Interface

Before reassembly, the clutch and PTO housings were wire brushed, sanded and chemically cleaned. Threaded holes were cleaned with taps and thread-cleaning tools, and the clutch housing was dry fitted before final installation.

Particular attention was given to the relationship between the gearbox input shaft, PTO drive gear and clutch housing. Measurements were taken between the housing faces and bearing locations to confirm that adequate clearance would remain once assembled.

With an estimated gasket thickness of approximately 1 mm, the assembled clearance was expected to be within tolerance

This check was important because the position of the bearing and housing directly influences the available clearance. Confirming the dimensions before final assembly removed the risk of assembling the driveline with the PTO drive gear contacting the clutch housing.

The clutch was then installed and the bell housing refitted with the mounting fasteners torqued to specification.

PTO Inspection

A workshop lifting tool was fabricated to support and remove the PTO housing from the side of the chassis. The size, weight and location of the housing made controlled handling important, particularly when aligning the assembly during removal and installation.

Once removed, the PTO gears, shafts and bearings were inspected. Despite the age of the vehicle, the internal components were in very good serviceable condition with no significant wear identified that justified replacement.

The decision was therefore made to retain the original gears and bearings wherever their condition allowed.

Externally, the PTO housing and associated components required considerably more attention. Corrosion was removed, surfaces were cleaned and treated, and threaded holes and mounting surfaces were prepared before reassembly.

PTO Output Bearing

Not every part of the reassembly went according to plan.

During installation of the PTO housing, the output shaft bearing was inadvertently loaded through the bearing cage while attempting to align the assembly. The bearing was damaged in the process and was immediately rejected

A replacement bearing was sourced, and before installation both the shaft and new bearing were measured to confirm correct fit.

The new bearing was then installed into the PTO housing. The housing bolts were initially used to maintain alignment and secure the gasket during assembly.

The housing subsequently had to come back off again after discovering that the retaining circlip had been overlooked. While frustrating, correcting it immediately was preferable to accepting an assembly known to be incomplete.

PTO Sealing Arrangement

The PTO output shaft uses a two-seal arrangement which was retained during the rebuild.

The inner Viton seal acts as the fording seal, while the outer seal retains the lubricating oil within the assembly. Both seals were replaced rather than reusing the existing components.

During assembly, the sealing surfaces and yoke position were checked carefully. The inner seal was lubricated before installation and the yoke temporarily fitted to confirm the seal position before proceeding with the outer oil seal.

The output yoke was also derusted, cleaned and refinished before installation. A new O-ring was fitted and the assembly completed, with the yoke retaining bolt torqued to 100 Nm.

This was one of those relatively small areas of the build where taking the time to understand the original sealing arrangement was more important than simply replacing parts.

Engine-to-Gearbox Drive Shaft

With the surrounding driveline dismantled, the engine-to-gearbox drive shaft was also removed and inspected.

The existing coating was removed, corrosion treated and the shaft refinished before being returned to service. As with many components on the truck, there was no mechanical reason to replace it; it simply required inspection and refurbishment while access was available.