LT chassis and body changes

London Transport's overhaul system, which involved identity swapping, is rather hard to explain, and was for many years poorly understood.  We have here tried to summarise the key features and effects of the practice.

The practice of swapping bodies and chassis at overhaul goes back a long time, at least to the B-type, London’s first standardised bus dating to before the First World War.   The 1930s STL fleet showed it most obviously after the war, with different body types appearing with fleet numbers that they never bore when new.  These notes relate primarily to the RT and RF families.

The system of overhauling London buses derived initially from a requirement of the Metropolitan Police for the first motor buses.  This involved taking the body off the chassis and disassembling and refurbishing both.  At least with the opening of LT's Aldenham Works after the war, if not before, the variable times it took to rebuild the separate body and chassis (7-8 days on average for a full RT body overhaul, 6-7 days for a chassis; these times are provided by a former Aldenham employee, but the elapsed time for an overhaul was often longer) would have meant that there would be a stockpile of one or other at the end of the line, whereas (provided there were spares in the system), and at least in theory, the body and chassis that happened to become available at the same time could be married and sent out – one of the benefits of a highly-standardised fleet.  The chassis overhaul did not involve the overhaul of running units (engine and gearbox) – these were left for garages to change and had a separate overhaul cycle at Chiswick.

In practice, because the volume of RF overhauls was smaller (a smaller fleet than RTs), most RFs regained their own body at each overhaul – RF486 did – but this was not universally true.   When RFs were built, the body was fixed to the chassis outriggers by means of rivets.  A body lift required the rivets to be drilled out; when remounted, the body was secured by bolts.  This led to a school of thought that the body and chassis were not usually physically separated at overhaul, but this has now been disproved.  With RTs, it was relatively unusual for a body and chassis to be reunited, except in cases such as Green Line RTs, where the slight differences in both body and chassis from standard meant that they could not be allocated at random.  RLHs all regained the same body after overhaul.

This refers to different pairings of bodies and chassis, but we have not yet mentioned numbering.  London Transport, uniquely, had a dispensation from the Ministry that allowed them to swap bus identities.  It is clear that a bus being out of service for overhaul whilst still taxed would represent a wasted cost.  LT therefore adopted the practice of removing a bus’s identity when it went in for overhaul and allocated that identity to a newly overhauled bus, usually the same day.  The tax disc for MXX463 was thus removed from the bus when it arrived at Aldenham, and was placed onto a bus which had been painted as RF486 with the registration MXX463 and emerged from overhaul and test on or about that day.  Therefore the ‘new’ RF486 (and the fleet number was always linked to the registration number) had no physical link with the ‘old’ RF486 that was now being overhauled.  This system caused some puzzlement amongst crews - Stuart Perry reports that drivers at Muswell Hill had a particular liking for RF500 in the early 1960s, but recalls one elderly driver saying that Aldenham had 'ruined it' when a different bus returned after overhaul with the same identity.

So when (for example) RF475 MXX452, which was delivered to LT with chassis number 9821LT828 and body number 7993, arrived at its first overhaul on 13 Sep 57, a new RF475 emerged the same day with a different body and a different chassis.  However (and this is the confusing bit), the logbook for MXX452 shows the chassis number 9821LT828.  LT therefore removed the chassis plate from the original chassis and screwed it on to the chassis of the newly overhauled bus.  The fleet number, the registration number and the manufacturer’s chassis number always stayed together, but not on the same chassis.

Without some extra system, this would have caused a real problem in tracking the physical chassis.  The body had its own number, which it retained for life (a system originally adopted by the General).  The chassis however had now lost its manufacturer’s number, and was therefore allocated a ‘Chassis Unit number’ (CU) by LT.  This number, only allocated on first overhaul (except for the RT chassis that were fitted with ex-SRT bodies when new, which had them fitted at that time), stayed with the chassis for life and was borne on a plate riveted (rather than screwed) to the chassis frame (above the engine on an RF).  The CU numbering system was introduced in 1951, during the first RT overhaul cycle, and applied to all subsequent types where the body and chassis were separated.

So when our example RF475 arrived at Aldenham on 13 Sep 57, body 7993 and (newly allocated) CU number 9502 were separated and overhauled, then married together again at the end of the process.  On 30 Sep 57, the newly overhauled bus was given the identity of RF500, replacing another RF500 which arrived that day, and was sent back to Muswell Hill from where RF475 had arrived.  The body and chassis from the ‘old’ RF500 then went through the process and emerged as RF504 three weeks later.  And so on.

RF500 next went in for overhaul on 18 Sep 61, with another ‘new’ RF500 appearing two days later.  Body 7993 and CU 9502 emerged from overhaul as RF471.  With the overhaul period now extended to 5 years, RF471 went in again on 25 May 66 and 7993/9502 emerged as RF486.  It will be noted that all three overhauls resulted in the chassis and body pairing being retained.

To return for a moment to the manufacturer’s chassis number, the original RF486 MXX463 was delivered with chassis 9821LT847.  This number was made up of the AEC model number 9821LT (as distinct from the Regal IV model 9821E supplied to other operators) and the unique number 847.  LT's 700 RFs reached number 1369 (RF700); the missing ones were 9821Es and other variants.  As noted above, this number swapped chassis but stayed with registration and fleet number, so is still carried by RF486.

We can recommend Alan Bond’s LT Vehicle History series (published by Transport Interests) for more detail.