I asked around for a copy of the relevant article to get chapter and verse on this. I can only assume that AEC built a few specials with a compressor one side and an exhauster the other side of the timing case, because the timing gears for a 505 will not mesh with a similar gear for a 470. My guess is either that these BRS lorries had an engine driven compressor which had a through drive to an exhauster and then a through drive to the fuel injection pump; more easy to achieve with the much smaller rotary DPA pump but not impossible with an in line. Alternatively the simple solution would be to have the exhauster driven from the transmission.
The article is from the newsletter of “Friends of Classic London Buses of the Fifties”
Fitting an AEC 505 engine into a vacuum-braked 470 Reliance. The impossible we can do, miracles…
By Brian Catchpole
The Problem:
The AEC “MU” Reliance was introduced in the mid fifties with the newly developed 410 cu inch (6.75 litres) lightweight wet-liner engine lying horizontally under the floor. As operators demanded more power, the engine was stretched to 470 cu inch (7.68 litres) by increasing the bore size from 105mm to 112mm. In this form it developed an unhealthy reputation for overheating and head-gasket failure. This generally occurred between the cylinder and one of the cooling system ports leading to loss of coolant, failure of the saloon heaters and, eventually, a sump full of oil/water emulsion. When left to cool down, the remaining coolant could drain back into the cylinder causing hydraulic lock when attempting to start the engine. Bent con rods were not unknown as a result. The heads then have to come off and, at the very minimum, the gaskets replaced. I have been round this circuit approx 6 times in 14 years ownership of 1958 Maidstone and District Reliance 470, 390 DKK. Heads have been skimmed, liners replaced, different heads used and expensive handmade copper gaskets tried out, all to no avail.
The Cause ?
Theories abound: the cooling system capacity was inadequate; the radiator was too small, liner height variation and the thinner liners of the stretched engine being commonly mentioned. My own view is that the design was flawed from day one. The engine has only one head stud in the land between the cylinders. Instead of the conventional 6 studs surrounding each cylinder, the “lightweight” 410/470 makes do with 5. A glance at the illustration of the heads shows just how easy it is for the gasket to blow from 2 adjacent cylinders to the rather large waterway. Any imperfection in the flatness of the block or heads or difference in liner heights simply accelerates the problem.
What did AEC do about it?
Initially, they decided that fitting high-tensile head studs would allow the head to be held down tighter. Torque settings were increased from 90 to 110 lb ft. Subsequently a revised design of head gasket was specified, eliminating the little rubber O-rings which were supposed to seal the waterways. None the less the problem did not really go away until well into the 1960s when the dry-liner 505 (8.80 litres) engine was developed, complete with 6 head studs per cylinder. Most, if not all, 505 engines were fitted with air compressors and DPA rotary diesel injector pumps. By this time vacuum brakes were a thing of the past and in-line pumps were also rapidly becoming extinct. A very few vacuum-braked 470 engines were fitted with DPA pumps but finding the adaptor which allows the 505 DPA pump to fit to the end of the 470 exhauster is close to mission impossible. Likewise finding an exhauster that was originally fitted to a 505 also proved impossible, if indeed such a combination ever existed.
What did operators do?
Once the 505 was available many operators decided to change their 470 engines for the newly- developed engine. Whether by design or by good fortune, AEC designed the 505 to fit directly into the existing MU Reliance chassis using the same engine mounting points so the changeover was, on the face of it, quite straightforward. For those with air-braked vehicles fitted with DPA rotary fuel injector pumps this was indeed the case. Our own, air-braked, ex-Western Welsh Harrington Grenadier ABO 145B was fitted with a 505 years ago and the heads have only been off once in 27 years. For older vacuum- braked chassis with in-line pumps the cost and trouble of re-engineering the ancillaries was probably prohibitive. Most operators pensioned off their vacuum-braked 470 Reliances as soon as they decently could. M & D certainly replaced most of theirs, but 390 DKK survived mechanically unmodified, well into the 1970s, probably because of the expense incurred in modifying the body-work from touring coach to DP spec in the mid 1960s. M & D wanted a return on their investment.
So, can a 505 be fitted into a vacuum braked chassis?
The answer is Yes, but not without solving a few problems along the way. Having reluctantly decided that 390 DKK would have no future without an engine change to a 505, I canvassed opinion among fellow owners and various AEC experts. The responses varied from “it’s a doddle” to “it’s impossible”. Undaunted, the search was on for a suitable engine. An engine coupled to a 5 speed synchromesh gearbox was located in the South Wales valleys. This had come out of another air-braked Western Welsh vehicle which had originally been fitted with a 470. Hence it was fitted with a rotary pump and air compressor. The decision was made to install the complete engine/gearbox but to use the vacuum exhauster and in-line injector pump from the existing 470.
What’s involved?
Removing the complete engine/gearbox unit is straightforward. At least it is if you have a set of 4-post lifts, a pile of pallets, a pallet truck and a few competent helpers. We started after lunch and had it all out by about 4.30 pm one November afternoon. The process entails lifting the bus up, undoing everything leaving the engine mountings finger tight, and draining all the fluids. Lower the bus down gently with a pile of pallets underneath until the weight of the engine/gearbox unit is firmly on the pallets. Undo the engine mountings and raise the bus up. Finally, pull the complete unit clear using a pallet truck. It is really that simple. (See photo on previous page.)
Engine removal revealed the previously inaccessible and filthy centre section of the chassis. There then followed 3 cold winter months of chassis scraping, removal of corroded chassis lube pipes, rust treatment, undercoating and silver-painting so that the replacement engine could be fitted into a spotless engine bay.
In parallel, the 505 engine was cleaned and the compressor/diesel pump assembly removed. The injectors were removed and sent for servicing and to be set up to run with an in line pump. Although the mounting arrangements for the compressor and vacuum exhauster onto the engines are apparently identical, the lubrication arrangements are different. A core plug has to be fitted into the block where the compressor drains oil into the crankcase and the pipe work for the oil feed/drain transferred from the 470 to the 505.
The vacuum exhauster then has to be fitted. At this point, it becomes apparent why some experts believe that this conversion is impossible. The main timing drive gears at the front of a 505 are quite different from those on a 470. The teeth are cut at a different angle. Hence, a vacuum exhauster from a 470 simply will not mesh with the drive gear on a 505. The picture shows the difference. Given that finding an exhauster that was originally fitted to a 505 was impossible, the only solution is to take the driven gear from a 505 compressor and have it re engineered to fit the vacuum exhauster. Unfortunately this is complicated by the fact that the compressor driven gear has a parallel centre with key way, whereas the exhauster needs a taper fit centre. There is nothing, however, that cannot be solved by a combination of a good engineering shop and a lump of cash.
There remains the problem of connecting the in-line injector pump to the 505 injectors. This arises because the in-line pump uses the larger size nuts and the injectors the smaller size associated with DPA pumps. Thus, the injector pipes from either of the donor engines are useless. The only solution is to have a new set of pipes made up with different size nuts at each end. Then bend gently to shape making sure they will clear the chassis as space is very tight.
Having sorted out these problems over the winter, we undertook the refitting in April and it was nearly as straightforward as the removal process. The exhaust down-pipe fits straight on with 3 bolts, and all the drive-shaft mounting points and cooling connections are identical. Some new pipe work had to be created to connect the diesel dribble return, and the air intake trunking from the air cleaner has to be modified as the 505 manifold inlet is larger than the 470. Finally the injector pump timing has to be reset as the in-line pump setting is very different from the DPA.
It was then a question of filling with oil and water, bleeding the diesel pump and pressing the starter. It ran but was rather lumpy and lacked power. Some experimentation with the pump timing on the road has resolved much of this and the engine now works well, albeit some way off the timing spec given in the manual. Perhaps we fitted the driven gear one cog out?
Was it worth it?
Nine months on and with several trips behind us, we now have a vehicle which feels like it will not let us down. It is no faster than it ever was and doesn’t have the get up and go of a DPA pumped 505 like ABO. It is noticeably noisier than the 470 but at least the cab and saloon heaters work for the first time in years. What more can we ask?
Thank you:
My thanks go to my son Mark and fellow members of the North Kent Vehicle Preservation Group who assisted at all stages. Also to Goddens of East Malling who re-engineered the exhauster driven gear, Cooks Autodiesel of Faversham who serviced the injectors/made new pipes and to John Evans of Bargoed who supplied the 505.