The most recent article in the Gazette about rebuilding a twin cam Blackburne engine appeared in edition number 74, as long ago as December 1984, from the erudite pen of Roger Buxton. This article was a source of much useful information, and served as a working document when rebuilding my own engine. During both the relatively quick stripping process and the rather longer rebuilding process, many photographs were taken, which may be of interest to those contemplating intimate personal contact with a Blackburne twin-cam engine. The engine in question turned up in 2004 through Robin Hildyard, who acquired the Torode "special" which had evolved in the early post war era, and included a Blackburne engine. The story is told in Trenchard Torode's article in Gazette number 136. The engine, number DA 102, has seen service in several cars, originally powering BMC 446. A strip and rebuild was judged necessary because the engine had not been run for 50 years despite being overhauled at that time. During much of the intervening time it appeared to have resided, with one spark plug removed, in a rather damp shed.

The engine was complete, but had been subjected to certain modifications. A Bosch distributor was arranged to drive directly from the front end of the exhaust camshaft, the cam cover having been machined to accommodate it. The water outlet connection on the head, fitted with post-war pressurised filler cap, was also non-standard. These modifications were due to the use of a Bond Minicar body shell, and therefore a non-Frazer Nash radiator, on the Torode special as constructed in the late 1940s/early 1950s. Stripping down was fairly straight-forward and indicated that most parts were in quite good condition. One exception to this was the condition of the needle-roller cam followers, which were all shot. This was to prove one of the more ticklish problems to resolve.

Stripping the engine also revealed quite a degree of design sophistication. The arrangement of downward facing head studs, while un-orthodox, and demanding a certain amount of abuse to an innocent ring spanner in order to make a tool able to untighten or tighten the nuts (Roger Buxton's article explains this quite clearly), nevertheless allows the complete head with camshafts and rocker gear to be removed as a unit without disturbing timing or valve clearances. This feature was to prove useful later, as will be recounted. The top timing chain sprocket which drives the exhaust cam (the inlet and exhaust cams are geared together) can be parked on a little ledge formed by two pins fitted to the steel housing for the timing chain, so unless this is also being removed, timing is not lost. The timing chain support structure, which also houses a Weller spring-blade type chain tensioner and the aforementioned two pins which support the top sprocket when it is removed from the camshaft, is simply attached by two studs to the crankcase, accessible after lifting the cylinder block. To remove the assembly it is also necessary to remove the intermediate gear wheel which meshes with the crankshaft drive gear, and carries the lower timing chain drive sprocket. Removing the large nut at the back of the crankcase allows the spindle to be withdrawn, freeing the intermediate gear, lower timing chain sprocket and the timing chain assembly.

The demountable half of the rear main bearing housing also contains the oil pump, which is gear driven from the pinion fixed to the rear of the crankshaft. Undoing the two main bearing nuts and the oil pipe inlet and outlet connections allows the entire assembly to be lifted off. If the oil pump needs to be inspected or worked on, the assembly is easily separated to reveal the gears inside. Slightly less reassuring aspects of the design are the oil pressure supply feed to the upstairs camshafts which passes upwards by means of drilled oilways which cross the mating passages in the alloy crankcase/cast iron cylinder block joint with only a paper gasket to prevent leakage of high pressure oil lubricating the valve train, and also, if anything more alarming, the flow of cooling water from a passage cast internally into the light alloy crankcase upwards into the cylinder block via a transfer port in the lower face of the block. Again, only the condition of the two mating surfaces (which tend mutually to corrode) and the thin paper gasket between the two components prevents the undesired dilution of lubricating oil in the sump with cooling water. Not wishing to dwell too much on such less than ideal arrangements, dismantling proceeded to completion, the last act being to lift the crankshaft, complete with roller and ball bearings located at each end, clear of the crankcase. This was no trivial feat, as the crank with front and rear main bearings in place weighs about 80lb.

It was decided not to re-use the original light alloy connecting rods, owing to their light-seeking reputation. Even if the probability of this occurring turned out to be low, the rarity of this type of engine justified the use of unbreakable rods of known provenance. New rods were ordered from Farndon Racing, a subsidiary of Robson Engineering on the outskirts of Coventry. These were manufactured from forged steel blanks. The execution was superb, and included the necessary off-set in the little end bush to allow for the spacing of the bores in the cylinder block, which do not coincide exactly with the crankpin spacing. The original alloy rods were used as patterns from which to reverse-engineer the new rods. Since the extra machining operation to fit shell bearings would cost effectively nothing if carried out during manufacture, but would save about £50 per rod in white-metalling costs, the decision was taken to use shell bearings. The crankshaft was measured; the crank pins proved to be both round and all the same size to within 5 "tenths", ie about half a thousandth of an inch (1.8560/5 inches). Fortunately the size of the crankpins permitted a perfect choice of bearing shells to give exactly the right clearance, without grinding the crank. The shells chosen were from the Ford Escort 1.4 LX, engine type FUH (the CVH engine). Standard crankpins for this engine are 47.9mm or 1.886 inches in diameter. Big end bearings 0.75 mm undersize, to suit crankpins 47.15mm or 1.8560/5 inches in diameter were ordered (two sets) to provide six pairs of shells. These were given to the connecting rod manufacturer to ensure a perfect fit in the rods after machining.

The crankshaft was taken for some TLC, which included crack testing (uncracked, to my great relief) and having all the plugs removed so that quantities of carbon could be removed from the oil ways. Apparently most of the oil ways were badly clogged. This work was carried out by Messrs. Baynton Jones in Shaftesbury. A change was also made to the direction of oil flow from the pump to the filter. Originally the pressurised cavity cast into the front nearside of the crankcase housed a wire mesh oil filter. It was desired to use modern replaceable paper cartridge filters to fit into this housing to improve filtration to a level compatible with shell big end bearings. This required the oil flow direction to be reversed, as originally it flowed radially outwards from a central feed. This was judged likely unfortunately to blow the paper elements to pieces. As a precaution, fittings were made and pipework modified to reverse the direction of the flow. Messrs. Baynton Jones made the necessary fittings, and the pipework was altered on the work bench in the garage as part of the many activities to prepare the crankcase for reassembly. The paper filter fits a significant number of Mercedes engines from the 1970s or 1980s, and is surprisingly low in cost when purchased from Europarts. The part number I have for this component is Hengst, E117H D07.

Double overhead camshafts

The light alloy crankcase and sump were laboriously hand scraped and fettled to remove the accumulated grime and oxidation of over 50 years. The basic condition of these two components was good, with gasket surfaces nicely undamaged. A number of studs had to be removed from the crankcase during the dismantling process in order to get the lengthy sump to release without damage. These were not replaced at this stage, because of the expected multiple sump fitting and removal operations during the rebuild process. The crank was fitted to the crankcase on the bench, by which time the assembled mass was about 100lbs or so. This was manageable, but would not be for much longer as more and more components were fitted, and the weight increased. It was decided to put the crankcase with crankshaft on to the (cleaned) garage floor. From this stage on, engine assembly continued using the floor-mounted approach.

Because of the use of separate crankcase and block assemblies and the narrow cylinder bore of the engine, it was not possible to fit the pistons into the bores with the block fitted to the crankcase. Pistons must be separately inserted into the bores one at a time, following which the block is lowered on to the crankcase. With the crank fitted to the crankcase, the simplest way to achieve a stable and level upper crankcase surface to receive the block is to fit the sump and then stand the engine on this. However, after fitting the block complete with pistons, the rods must be drawn down on to the crank pins one at a time, and the big-end bearing caps fitted. This latter activity naturally requires that the sump be off, so it was clear that the sump would be on-and-off several times during the assembly process. Having a number of the sump studs removed during this stage made life much easier as it was fairly simple to fit or remove the sump casting.


The new connecting rods were removed carefully from their cardboard box and the pistons fitted using the "Aga" method. Those fortunate enough to have an Aga or other similar stove, will recognise it as a good friend to the old car fettler, as it allows components which need to expand for ease of assembly to be placed in a suitable position on the top, choosing the right heat to give the appropriate expansion. Once up to temperature (a minute or two with aluminium pistons), the gudgeon pins slide easily into the little end bushes with no force being needed. This is much preferred to the use of hot water, as it tends to go cold quickly, and it makes everything wet. This latter aspect is not very satisfactory when trying to keep everything clean, while using some light oil to assist assembly. The dry hot stove method is much preferred. A refinement to this general approach is to put selected components into the freezer, which in my case conveniently resides in the garage. With inner parts chilled and outer parts warmed, assembly of shafts/bearings/housings etc is usually very simple. This is direct evidence that not all domesticity is the enemy of the old car enthusiast.

With gudgeon pin circlips fitted, pistons were placed in the bores, sliding up from below with the aid of a piston ring clamp and lots of lubricant. Care was taken to prevent the rings popping out of the top, as this presents a tricky retrieval situation. The rings are reluctant to go back in at the top of the bore (no lead-in or taper as at the bottom), but they have to because the big end will not pass up the bore, and the piston cannot be removed from the rod in-situ. Shells were fitted to the rods before fitting pistons to the bores, in preparation for lowering the block on to the crank. Pistons were pushed to the top of the bores, to reduce the risk of big end bolts damaging any of the crankpins when lowering the block on to the crankcase. The crank was turned so that all crankpins were at as low a position as possible relative to the top deck of the crankcase, again to minimise risk of damage to the crankpins when fitting the block. Fitting the block was a reasonably stressful part of the reassembly, and was one stage which definitely required another pair of hands to guide the lower ends of the connecting rods inside the crankcase to avoid damage to the crankpins. However, this only took a minute and then the block could be lowered into place over the studs. With the block in place, a few of the holding down studs were attached and done up loosely to stop the block moving.

The pistons fittted into the block