The MG TD and TF were powered throughout their production by variants of the XPAG and XPEG four-cylinder overhead valve engine, a robust, well-proven unit whose origins trace back to the TB Midget of 1939. Understanding which engine variant is fitted, and specifically which engine number prefix it carries, is the essential starting point for all TD and TF engine parts selection.
XPAG Engine Variants in the TD
The TD used the 1,250cc XPAG engine throughout, producing 54 bhp at 5,200 rpm with a 7.25:1 compression ratio and twin SU H2 carburettors. From engine number XPAG/TD2/9408 in July 1951, the TD2 specification introduced a revised block, sump, larger bell housing, new flywheel, and stronger clutch fork shaft for the 8-inch clutch replacing the original 7¼-inch unit. The block was further revised with round water passage holes at engine number XPAG/TD2/17969 in June 1952, and the cylinder head received round coolant holes at engine number XPAG/TD2/22735 in November 1952, at which point the spark plug specification changed from the longer-reach Champion L87Y to the shorter-reach Champion N9Y. A revised camshaft with new valve timing, subsequently used on the TF, was introduced at engine number XPAG/TD2/24116 in January 1953, changing the tappet clearance from 0.019 to 0.012 inches and improving torque at lower revs.
The TD Mark II and TF Engines
The TD Mark II (TD/C) was MG's factory competition variant, tuned to Special Tuning Stage 1/1A specification and producing 57 to 61 bhp with an 8.6:1 compression ratio. The cylinder head was machined from the standard 76.75mm depth to 74.37mm, inlet valves were enlarged from 33mm to 36mm and exhaust valves from 31mm to 34mm, stronger valve springs were fitted, and the 1¼-inch SU H2 carburettors were replaced by 1½-inch SU H4s on a larger intake manifold with a bigger air cleaner. All Mark IIs were fitted with twin fuel pumps, a manual ignition advance/retard control, and Andrex adjustable friction shock absorbers front and rear in addition to the standard hydraulic dampers.
The final drive ratio was raised from 5.125:1 to 4.875:1. From engine number XPAG/TD3/17029 in June 1952, Mark II engines received the XPAG/TD3 prefix. Approximately 1,710 Mark II cars were built. The TF 1250 (XPAG/TF) produced 57.5 bhp at 8.1:1 compression with a head depth of 75.16mm.
The TF 1500 (XPEG/TF), from chassis TF/6501 in July 1954, enlarged the bore from 66.5mm to 72mm while retaining the 90mm stroke, for 1,466cc and 63 bhp at 8.3:1 compression, a 17 per cent increase over the XPAG. TF 1250 and TF 1500 production ran in parallel from July to September 1954, with the last TF 1250 being chassis TF/6950.
Engine Identification
The engine number was stamped on a small octagonal plate, originally fixed to the left-hand side of the flywheel bell housing and later moved to the right-hand side of the crankcase, and was repeated on the guarantee plate. The prefix determines the specification: XPAG/TD for early TDs (engines from 501 to 9407), XPAG/TD2 from July 1951 (engines from 9408), XPAG/TD3 for Mark II engines from June 1952 (engines from 17029), XPAG/TF for the TF 1250 (engines from 30301), and XPEG/TF for the TF 1500 (engines from 501). Because the TD and TF are now over seventy years old, engine swaps are not uncommon, the engine number must always be checked against the chassis specification before ordering parts.
The Crankshaft
The XPAG and XPEG crankshaft is the single most critical component in any engine rebuild, a three-bearing forged steel design that is far less rigid than the five-bearing crankshaft in the later B-series engine. Replacement crankshafts are extremely difficult to source. Any rebuild should begin with a specialist crack test and journal measurement. The crankshaft material specification was improved from engine number XPAG/TD2/27551 in April 1953.
Ordering Considerations
The engine number prefix is the single most important reference when selecting engine parts. Establish whether the engine is XPAG/TD (early, 7¼-inch clutch), XPAG/TD2 (from July 1951, 8-inch clutch), XPAG/TD3 (Mark II), XPAG/TF (TF 1250), or XPEG/TF (TF 1500). The cylinder head gasket specification, oval or round coolant hole, is determined by engine number at the 22735 breakpoint. Gasket sets, pistons, cylinder liners, connecting rods, and clutch components all vary across these specifications.