The XPAG is the engine at the heart of the post-war T-Series, the four-cylinder unit that powered the MG TB, TC and TD, and in its enlarged XPEG form the TF 1500. It is also one of the most tuned and raced small engines Britain ever built. This page sets out where it came from, what it is, where it was used, how to identify one, why tuners love it, and the weak spots to watch. It draws on the widely-published record; corrections and additions from owners are welcome.
Origins: the Morris X-series
The XPAG was not designed from a clean sheet. It grew out of the Morris “X” series of overhead-valve engines laid out by Claud Bailey in 1938, itself a thorough redesign of the earlier Morris side-valve units, with a stiffer crankcase, modern shell bearings and a six-port cylinder head that breathed well and left room to develop. The first of the family was the 1,140 cc XPJM, a lively 10 hp unit for the Morris Series M saloon. MG took that design, enlarged and strengthened it, and the result, first fitted to the TB Midget in August 1939, was the XPAG. War intervened almost at once, and the engine's real career began in 1945 with the TC.
The engine in detail
The XPAG is a 1,250 cc overhead-valve four, with a bore of 66.5 mm and a stroke of 90 mm, breathing on MG applications through twin SU carburettors. Compared with what it grew from it was substantially strengthened: a stiffer block and a crankshaft running in three main bearings, short connecting rods, light-alloy Aerolite pistons, thin-wall shell bearings, larger valves, and a large-capacity sump with a full-flow oil filter. In the various T-Types it produced somewhere between about 54 and 57 bhp depending on model and state of tune, rising as compression and valve sizes crept up through the TD and into the TF. Modest figures on paper, but the engine was smooth, willing, and, as the world would discover, astonishingly receptive to tuning.
Where it was used
The XPAG powered almost every MG of its era: the TB in 1939, then the TC, TD and TF Midgets, and the Y-type saloons, the YA, YB and YT, from 1947. A single-carburettor, milder version, the XPAW, powered the Wolseley 4/44, which kept the engine in production until 1956, after the last TF had been built. For roughly fifteen years the same basic unit ran under a remarkable spread of cars.
The XPEG: the TF 1500's engine
The last and largest development came in 1954. For the TF 1500 the TD Mark II block was re-cored so that it could be bored out to 72 mm, lifting capacity to 1,466 cc and creating the XPEG, with larger valves and 63 bhp. It was the final flowering of the design. For how the change was phased in through TF production, see the TF production changes page; for the TF 1500 in detail, the MG TF 1500 page.
Identifying an XPAG or XPEG
A few things help place an engine. The number is stamped on the block, on an octagonal plate low on the right-hand front, on the guarantee plate on the bulkhead, and on the block plinth, carrying a Nuffield-system prefix such as XPAG/TD or XPAG/TF, or simply XPEG for the 1500. There is also a well-known casting change: the earliest engines, the TB, TC and early TD, have banana-shaped water passages on the block deck and head, while later TDs and all TFs have round water holes, the so-called banana block and round block. A casting date code near the dipstick can also be decoded. For reading and matching the numbers on a TF in particular, see Identifying a genuine TF 1500.
For where every number lives on the car and what the prefixes mean across the range, see the chassis and engine number decoder.
A tuner's engine
If the XPAG is remembered fondly out of all proportion to its size, it is because it tunes so well. From the late 1940s a whole industry grew up around extracting power from it: staged tuning kits, gas-flowed heads, high-compression work, twin and triple carburettors, and supercharging. Goldie Gardner's record-breaking EX135 ran an XPAG-based unit to set international class speed records, and a young Colin Chapman chose the XPAG for the Lotus Mk VI, where a well-developed example gave around 85 bhp and helped make Lotus's early name. Specialists such as Laystall offered light-alloy cylinder heads. The engine's stiff bottom end and good breathing meant it would take the work and keep running, and it is still why a T-Type can be made genuinely brisk without losing its character.
Common problems and weak spots
For all its toughness the XPAG has a handful of known weaknesses, most of them age and fuel related rather than design faults.
- Unleaded fuel: the original cast-iron exhaust valve guides and unhardened seats relied on leaded petrol, so on modern fuel it is usual to fit phosphor-bronze guides and hardened seats and to consider the valves themselves.
- Oil leaks: the rear main was never fitted with a modern lip seal, so a degree of seepage is normal, and there are well-known ways to reduce it without butchering the engine.
- The timing-chain tensioner: the earliest engines, up to XPAG 883, had none, a tensioner arriving from engine 884, so a rattly chain on an early unit is worth investigating.
- Cooling: the XPAG can run warm in modern traffic and hot weather, so correct coolant, a sound water pump, the right thermostat and a clean system all matter, and uprated cooling is a common and sensible fix.
- Bearings and internals: on hard-worked or high-mileage engines, main and big-end wear, classically at the third throw, and water-passage corrosion are the things a rebuild addresses.
Read next
The models that use the engine: MG TB, MG TC, MG TD and MG TF. See also the MG TF 1500, its production changes and originality guide. For work on the engine, see the Tuning Guide and Servicing sections. Corrections and additions from owners are welcome.