Bus overhauls


The classic view of the Aldenham overhaul process is of bus bodies being lifted and conveyed along the High Bay.  This is the High Bay in 1954 whilst still being converted from redundant rail depot to an overhaul works.  At the time, body overhauls were taking place in the adjacent Low Bay, with chassis still being sent to Chiswick.

Photo © Jim Andress



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 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.  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, with thanks for some of the detail to Brian Watkinson, who worked at Aldenham from 1965.



Aldenham railway depot was completed before the war in preparation for the extension of the Northern Line to Bushey Heath. It was used during the WWll for the manufacture of Halifax bombers.


LT retained it as a maintenance facility and from 1949 it carried out RT accident repairs and overhauls on TFs and lowbridge STLs; it also carried out the changes to bodies and chassis to create the short-lived SRT class.  At this time, its facilities were those of a train depot, not a bus overhaul works.  When the Northern line extension was ruled out and LT lost its planning application for a bus overhaul works at Ickenham, Aldenham was extended and adapted to enable the transfer of bus overhauls and the manufacture of body parts from Chiswick.


The overhaul of the post-war RT-type fleet started in 1949 and early 1950 with several pilot overhauls, and the main overhaul programme started in the second half of 1950.  Standard procedure then was body dismount and overhaul at Aldenham, with the chassis being sent to Chiswick for overhaul. It was only in late 1954 that complete overhauls at Aldenham started in bulk, and Chiswick was remodelled to undertake the overhaul of mechanical and electrical units and parts.


These early RTs were delicensed before entering the Works and were outshopped with the same body/ chassis/ stock number combination. The ability to change identities was wrapped up in the ability to mount almost any RT body on any RT chassis.  It appears from the overhaul records that chassis identification changes commenced in early 1955 for the RTLs, with the MCCW bodied RTLs being overhauled at that time leaving with different chassis but the bodies retained the same stock number. Some RTs also changed chassis from early 1955 but again the bodies kept their stock numbers. 


The different (and unpredictable) time 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 averages are provided by Brian Watkinson), 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.


The full "swapping" of identities thus commenced when, over a period of a few weeks in late 1955/early 1956, over 150 buses, RTs, RTLs and RFs, were delicensed and went for overhaul, becoming 'Works Floats'. The stock numbers of these buses did not re-emerge for some time, some as late as the final overhauls in 1970, although the physical buses re-emerged with different numbers; this allowed the whole business of tax disc and chassis number switching to begin in earnest. Over time, some of the original works float numbers re-emerged and others took their place as the Works Float was a system of production control and used to control the numbers of buses of each type or sub class in the works at any one time.


The vast majority of buses that entered the works were licensed buses.  These had the tax disc holder removed and taken to the Licensing Shop where it was fitted to the bus going out with that identity,  Those that were not licensed were usually going in to be part of the Works Float and were always delicensed prior to leaving the garage and driven on trade plates.


A 1950s BTF film of the Aldenham process is on YouTube here and here.  In the later years of a bus's life, a more limited process known as 'final overhaul' took place, when the bus was repaired with minimum structural replacement and stripping.  For the RT, this meant 'plonk-on' plates on the interior coving panels and exterior cant panels and many other repair schemes to wheel arches, staircases, battery risers etc, rather than full repair.


In practice, because the volume of RF overhauls was smaller (a smaller fleet than RTs), and with sub-classes within the fleet, most RFs body and chassis combinations did not change at overhaul  – for example, the body and chassis combination on RF486 is the same now as when built – although 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 (saloon heaters and different differentials) from the “standard RT” meant that they could not be allocated at random.  RLHs all regained the same body after overhaul, as did the GSs.



This refers to different pairings of bodies and chassis, but we have not yet mentioned numbering.  London Transport (probably uniquely) had a dispensation from the Ministry, it is believed (does anyone have any proof?), 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 was thus removed from the bus when it arrived at Aldenham, and was placed onto a bus which had been painted with the same fleet number and registration and emerged from overhaul and certification on or about that day.  Therefore the ‘new’ bus had no physical link with the ‘old’ one that was now to be 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 produced and fitted a new chassis plate to the newly overhauled bus with the same chassis number as the one removed from the original chassis.  The fleet number, the registration number and the manufacturer’s chassis number always stayed together, but not on the same physical 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.