The cylinder head is one of the most critical assemblies on the XPAG and XPEG, and one where production-change detail is essential to parts identification. The basic architecture is shared across all TD and TF engines, cast iron overhead valve with pushrod-operated rockers, but the head gasket, valve sizes, rocker shaft length, rocker bush specification, and rocker adjusting screw length all changed during production, creating multiple specifications that must be identified before ordering.
Head Gasket Identification
Three distinct head gaskets exist, determined by engine number. The block was revised to round water passage holes at engine XPAG/TD2/17969 in June 1952, with the head following at XPAG/TD2/22735 in November 1952. TDs to engine 22734 use an oval coolant hole gasket; from 22735 through all TF 1250 production the gasket has a round hole; the TF 1500 uses a unique gasket for its 72mm bore. Spark plugs also changed at 22735, from the longer-reach Champion L87Y to the shorter-reach Champion N9Y.
Cylinder Head Depth and Compression Ratio
The standard TD head depth was 76.75mm for 7.25:1 compression. The Mark II head was machined to 74.37mm for 8.6:1; the TF 1250 used 75.16mm for 8.1:1; the TF 1500 retained 76.75mm but achieved 8.3:1 through its larger 72mm bore. The minimum head depth for any 1250cc XPAG is 73.575mm, corresponding to 9.3:1 at Stage 2 tuning specification, the point at which further machining is not advised.
Cylinder Head Reconditioning
All TD and TF cylinder heads are reconditioned to lead-free specification with hardened exhaust valve seat inserts. Four distinct specifications are maintained on an exchange basis, each requiring the customer's own unit for reconditioning: TD to engine number 22734 (oval coolant hole), TD from 22735 (round hole), TF 1250 (round hole), and TF 1500 (larger bore). The engine number and target compression ratio must both be specified when ordering.
Rocker Assembly Changes
The rocker shaft exists in two lengths: 14-inch (TD to engine 9007) and 14?-inch (TD from engine 9008 through all TFs). These are not interchangeable. At engine number XPAG/TD/9008 in June 1951, the exhaust rockers received longer bosses and bushes (inlet rockers unchanged), and the bush specification changed from four short bushes to eight long bushes. The adjusting screws were then revised between engines 17297 and 17298 in June 1952, when shorter pushrods with longer adjusting screws were introduced.
These breakpoints mean a rocker assembly cannot be transferred between an early and late TD without checking every component against the engine number. Later production brought three further changes. The exhaust valve material specification was improved at engine number XPAG/TD2/18291 in July 1952. The height of the valve spring faces was reduced at engine number XPAG/TD2/27867 in May 1953, with the valve guides slightly more protruding.
Locking wire holes were drilled in the heads of the rocker shaft support bolts from engine number XPAG/TD2/28167 in May 1953.
Valve Specification
Inlet and exhaust valve sizes differ between the standard TD and the competition or TF specification. The standard TD uses a 1 5/16-inch inlet and 1 3/16-inch exhaust valve; the TD Mark II and all TFs use larger 1 7/16-inch inlet and 1 3/8-inch exhaust valves, carried forward from the Mark II to the TF. Exhaust valves are available in standard material or hard-wearing Stellite specification, the latter being more durable for engines in regular use. Valve guides are available in original cast iron or an improved silicon bronze alternative offering lower friction, reduced wear, and better heat dissipation, recommended for engines in regular use as the improved heat transfer helps protect the exhaust valves from burning.
Rocker Covers
The rocker cover is model-specific: the TD cover has a breather boss, the TF does not. Both are available in original pressed steel or cast aluminium, the TD alloy cover is a smooth cast pattern with a breather boss, the TF alloy cover a ribbed pattern without. The rocker cover gasket is common across the entire T-series from TC to TF, and the studs and thumb nuts are universal across both models.
Ordering Considerations
The cylinder head contains more production-change breakpoints than any other section of the TD and TF engine. Essential references when ordering are: the engine number (for head gasket, rocker shaft length, rocker specification, bush type, pushrod and adjusting screw length, and spark plug), the engine prefix (for bore and compression ratio), and whether reconditioning is on an exchange basis or replacement components are needed individually.