Prop Shaft Assembly and Crown Wheel and Pinion Replacement
Following replacing the universal joint (UJ) on my 1924 Cowley (see Mag 321 page 20), a following test run emitted a regular crunching sound from under the floorboards! Oh no, I had reassembled it incorrectly! However, as the crunch only was heard once every revolution of the rear wheels, I decided it was not the UJ, which revolved considerable faster, but from the rear axle, the sound being transmitted up the torque tube.
Removing the differential cover, investigation resulted in finding a half broken tooth on the Crown Wheel (photo 1). Nothing for it but to remove the whole assembly, (photo 2), fairly easily done after removing the drive half-shafts – only then to find 3 broken teeth on the Pinion (photo 3). Minute shards of broken teeth had to be very carefully flushed out of the diff casing, as they were probably the cause of the subsequent major tooth failure!
The prop shaft was removed by inserting a stout lever in the rear upper chamber in the torque tube (photo 4), and then unwinding the barrel bolt anti-clockwise until the whole assembly could be withdrawn – this takes ages and must have been at least 40 full turns of the bolt!
Several things were immediately apparent; the Barrel Bolt in the rear assembly was rattling between the Thrust Bearings, because the thrust faces of these Bearings were the wrong way around, (a failure of a previous owner, photo 5) and the edges of the Barrel Bolt were therefor so worn (photos 6 & 7) that the inner tube spacer was now far too long to give the thrust bearings a tight clamped support (photo 8). Furthermore, the pinion was so loose on the taper at the end of the prop shaft that it had disintegrated the woodruff key slot and taper (photo 9).
Luckily, spares officer Clive from the Bully Club had a new Crown Wheel and Pinion set (but note that this club Pinion only fits the narrow taper on a prop shaft), and I duly ordered and had it delivered very promptly. As the tapered end of the existing prop shaft was past repair, a second-hand one was sourced from a local member. This was when I realised there were two types of shaft tapers – wide (the full diameter of the shaft, and narrow (taper starting below the shaft diameter, photo 10). My newly acquired shaft had to be turned down from a wide to a narrow one, (photo 11). (See Barry Blights article in Mag 194, page 32, regarding the various types of shaft).
A radical re-building of the entire rear end shaft assembly was required. A new Main Bearing and two Thrust Bearings were purchased, (off –the-shelf, as they are all metric), the edges of the Barrel Bolt milled true and clean, and the internal spacer turned shorter so that the Thrust Bearings could be clamped tight to the Barrel Bolt. The assembly (photo 12) was put together in the correct fashion, Thrust Bearings facing inwards, the whole unit tightened from the Collar (photo 13), and a new hole drilled into the shaft to accept the locking stud and retaining wire.
It was wise at this stage to investigate the differential. To my alarm, all the bolts holding this to the Crown Wheel were loose! Internal pinions and stub axles were checked (photo 14), the main bearings found to be sound, but in any event could not be replaced as manufacturers do not now supply them with the extended inner races that act as spacers for the bearing locking nuts (photo 15). Loose half-shaft keyed ends to the wheel hubs were renewed with Club replacements (photo 16), and the hubs then driven onto the shafts to provide a much tight fit.
The prop shaft was fiddled into the torque tube to meet the splines of the UJ, (not easy, but do-able), and the Barrel Bolt wound (endlessly) into the torque tube – difficult to get the small threads inside the torque tube to mate with the Barrel Bolt threads which had been slightly damaged in the milling process – until the pinion was just showing in the diff casing (photo 17). The diff was reassembled with the main bearings’ inner race extended flanges pointing outwards, and then the half shafts re-installed.
Now came the worry of getting the Pinion to mesh correctly to the Crown Wheel. The Barrel Bolt was wound to and fro until the far end of the Pinion was exactly flush with the inner diameter of the Crown Wheel teeth – only viewable with a dentist mirror and a powerful torch beam! The mesh distance was adjusted to about 5 thou. by adjusting the locking nuts and washers either side of the main bearings to move the diff assembly left and/or right (photos 18 & 19). In practice this was done by wiggling the Crown Wheel until there was virtually no free play.
I was also alarmed to find the locking bolts which peg the castellated washers tight against the main bearing’s retaining nuts were also loose! A local Club member had found he could not move forwards, only in reverse, because one of these nuts had come undone and jammed the whole assembly! The diff cover was replaced with a new Club gasket (photo 20), the axle filled up with gear oil, a quarter pint poured into the torque tube chamber (essential) and then taken on a test drive – no whining on ‘drive’ or ‘overdrive’. I was lucky to get the correct mesh first time!.
So why had this all happened? I surmise that the incorrectly installed Thrust Bearings had not helped, but I believe it was also because the rear Thrust Assembly had run dry. I recommend a quarter pint of oil is poured into the torque tube lug cavity every now and then, particularly if planning a long journey. The rattling of my old UJ, and the wobbling of the loose Barrel Bolt in its housing, had also stressed the Pinion on its taper – which in turn caused a very bad mesh with the Crown Wheel…and had thus broken the teeth on both gears.
After these replacements, and within a week, the car subsequently drove over 450 miles to, during and from the 2017 Spring Rally at Tiverton with no problems, and a noticeably smooth and quiet transmission.
Service Information Sheets CO/9 and CO/15, and Geoff Bull’s article in Mag 128 page 17 are very useful references for removing and tuning the Crown Wheel and Pinion. John Hine June 2017.
Double click picture to enlarge.