The air filtration system differs significantly between the standard TD, the TD Mark II, and the TF, reflecting the progression from the smaller H2 carburettors through to the TF's individual filter arrangement. The crankcase breather system that connects the engine ventilation to the air intake also differs between the TD and TF, making this a section where the model and carburettor type must be established before ordering.
TD and TD Mark II Air Filter
The standard TD uses a single air filter assembly positioned above the twin carburettors. The assembly mounts on a stud with a wing nut at its top, and an air manifold bridges across from the filter to feed both carburettors simultaneously. Bolts secure the air manifold to the carburettors, with a gasket between the manifold and each carburettor body. The air manifold-to-carburettor gasket differs between the standard TD (for the smaller H2 carburettor mouth) and the TD Mark II (for the larger H4 carburettor mouth), the Mark II retains the same single-assembly layout but uses the larger gasket specification.
A hose connects the air filter housing to the rocker cover breather boss, so that crankcase gases are drawn into the air filter and then into the induction path.
TF Oil-Wetted Air Filters
The TF uses a pair of individual oil-wetted air filter assemblies, a front assembly and a rear assembly, mounted directly onto each H4 carburettor. Each assembly contains a replaceable wire-mesh filter element supported on felt pads inside the housing. The filter-to-carburettor gasket on the TF uses the same H4 mouth size as the TD Mark II, and the assemblies are secured to the carburettor bodies by bolts. Oil-wetted filtration traps airborne particles on the oil-coated wire-mesh element, with the element serviceable as a wear item rather than the whole assembly needing replacement.
TF Crankcase Breather
The TF's crankcase ventilation follows a different path from the TD. The TF rocker cover does not have a breather boss, instead, a separate breather pipe is fitted to route crankcase gases via hoses to the air filter system. Supergrip hose clips are supplied in the correct specification.
This is a practical difference for owners: a TD rocker cover cannot simply be swapped onto a TF engine, and vice versa, because of the breather arrangement.
Performance Air Filters
For owners seeking improved engine breathing and a different engine-bay appearance, performance air filter options are available in both 1¼-inch bore (for H2 carburettors) and 1½-inch bore (for H4 carburettors). Chrome accessory filters provide a period-appropriate appearance with modest air flow improvement. SU-style chrome foam filters, carrying the SU logo, offer a similar visual effect with the original manufacturer's badging. K&N pancake filters offer the highest flow rate, up to 40 per cent more air flow than conventional filters, with high levels of filtration, and are supplied pre-oiled and ready for installation, with service packs for ongoing cleaning and re-oiling.
Stub stacks are a separate accessory fitted between the carburettor mouth and a K&N or SU-style pancake filter, further improving air flow into the carburettor. When fitting any performance air filter, richer carburettor needles may be necessary to maintain correct mixture strength with the increased air supply.
Ordering Considerations
The original-equipment air filter specification is determined by both the model and the carburettor type. The TD uses the single manifold-mounted assembly with the H2-specification gasket. The TD Mark II uses the same single-assembly layout but with the H4-specification manifold gasket to suit its H4 carburettors. The TF uses the individual oil-wetted filter pair mounted directly on its H4 carburettors, with a separate crankcase breather pipe.
For performance filters, the choice is based purely on carburettor bore size, 1¼-inch for H2 or 1½-inch for H4, and on the appearance and flow-rate preference.