The lower engine encompasses the crankshaft, connecting rods, pistons, bearings, timing chain and gears, camshaft, oil pump, and sump. The 1275cc A-Series and 1500cc Triumph engines share no lower engine components, and the internal differences between them are extensive, affecting the crankshaft, timing drive design, oil pump arrangement, sump, rear seal, and flywheel fixing.
1275cc Crankshaft and Bearings
The 1275cc crankshaft runs in three main bearings and drives four connecting rods. Reground crankshafts are available, and when ordering, the required bearing undersize must be stated. Main bearing and big-end bearing shells are supplied as complete engine sets matched to the crankshaft size. Thrust washers control crankshaft end float and are available in standard and oversize thicknesses.
The spigot bush at the rear of the crankshaft supports the gearbox input shaft and is available in two external diameters to suit different gearbox input shafts, the correct size must be confirmed before ordering.
1275cc Timing Gear and Camshaft
The timing drive uses a duplex (double-row) chain running between the crankshaft sprocket and the camshaft sprocket, with no separate tensioner, the duplex chain is designed to run untensioned. The camshaft runs in three bearings pressed into the block and is located by a thrust plate. The oil pump is driven from the camshaft via a drive flange, and the distributor drive is taken from the same shaft assembly. The crankshaft pulley at the front of the engine doubles as the fan belt pulley and carries a rubber-bonded vibration damper.
1275cc Sump
The 1275cc sump is a pressed steel unit secured to the block's lower flange. Early 1275cc engines had a thin lower block flange while later engines used a thicker flange, and the sump gasket must match accordingly. The oil pressure relief valve is housed in the block and is accessible without removing the sump.
1275cc Pistons and Rings
Pistons for the 1275cc are available in standard and oversizes (typically 0.010", 0.020", 0.030", and 0.040" over standard). Two piston ring configurations were used during 1275cc production: an earlier four-ring set and a later three-ring set. The correct ring set must be matched to the pistons fitted.
1275cc Rear Crankshaft Seal
The original 1275cc rear crankshaft sealing arrangement is a scroll-type design, a helical groove machined into the crankshaft that flings oil back toward the sump as the shaft rotates. While effective when new, this seal type becomes progressively less effective as the components wear, and is the most common source of oil leakage at the rear of the engine on cars with high mileages. A rear oil seal conversion kit is available that replaces the scroll arrangement with a modern lip seal, recommended during any rebuild where the rear of the crankshaft is accessible.
1500 Crankshaft and Bearings
The 1500cc crankshaft also runs in three main bearings but is a completely different forging from the A-Series item. Bearing shells, thrust washers, and seals are all specific to the Triumph engine and not interchangeable with any 1275cc components.
1500 Timing Gear and Camshaft
The 1500 timing drive uses a single-row chain with a separate spring-loaded chain tensioner, a fundamentally different arrangement from the duplex untensioned chain on the 1275cc. Chain stretch is a known wear characteristic on the 1500: as the chain elongates with use, the valve timing retards progressively, causing reduced power and rough idle. The tensioner compensates for moderate stretch, but once it reaches the limit of its travel, both the chain and tensioner must be renewed. The camshaft is secured by a lockplate rather than the thrust plate used on the 1275cc, and the distributor is driven via a separate pedestal arrangement.
1500 Oil Pump and Sump
The 1500 oil pump is a different design, bolted to the block with three bolts rather than driven from the camshaft. The sump and sump gasket are specific to the 1500. Pistons are available in standard and oversizes (typically 0.020", 0.030", 0.040", and 0.060" over standard).
1500 Rear Oil Seal and Flywheel
A key difference at the rear of the 1500 engine is the crankshaft rear oil seal arrangement. The 1500 uses a conventional lip seal carried in a separate housing bolted to the block, a more positive sealing method than the A-Series scroll arrangement. The 1500 flywheel is secured by four bolts rather than the six used on the 1275cc, and the spigot bush diameter differs between the two engines.
Gasket Sets
A conversion gasket set covers the lower engine gaskets (sump, timing cover, oil pump, distributor drive housing, and associated joints) and is used in conjunction with the head gasket set when carrying out a complete engine rebuild.