For a high degree of internal streamlining the largest practical piston valves are necessary. The largest valve diameter in common use was 14 inches (355.6 mm) on American locomotives with large cylinders, and this figure is used for the 5AT, rounded down to 350 mm on account of the 5AT's much smaller cylinders. To reduce cylinder clearance volume, which would otherwise be excessive with such small cylinders, a single 350 mm diameter valve is substituted by two 175 mm diameter valves per cylinder, giving the same port openings as the larger single valve. This arrangement has the general advantage that whilst port openings are proportional to the valve diameter, the steam pressure forces on the valve heads and stem are proportional to the square of the diameter, therefore such forces per valve are reduced by a factor of 4 when the valve diameter is halved, allowing twin valves to be correspondingly lighter.
The design is based on that for the Chinese Railways modified QJ Class, which was a development of that successfully used on the SAR 26 Class No. 3450, which gave good thermal performance and very low wear, and which was in turn based on principles laid down by Porta in his (unpublished) 1975 paper "The Mechanical Design of Piston Valves". The principal features of the piston valves and spindles are as follows:
- Extreme lightness for minimum bearing pressure and hence low wear, and minimum inertia load on the valve gear.
- Guarantee of near-to-perfect steam tightness by elimination of the usual valve tail rod, thereby allowing the valve ring retainers to bear on the valve liners and cover the ring gaps (the nominal ring retainer - liner bearing pressure is considerably lower than that between the tail rod and its bush, e.g. ~ 50% lower in the case of the modified QJ). Even bearing of the valve on the liner is made possible by having the valve spindles pin-jointed at both ends.
- Multiple narrow rings, minimising the sealing duty of any individual ring, reducing ring-liner friction and wear, and enabling steam tightness to be maintained for extended periods of service without inspections or maintenance.
- Ring-controlled events, with thin tapered inlet edge lands for optimum inlet flow coefficient.
- Use of (some) bronze valve rings which deposit bronze on the liner surfaces to reduce wear.
- Ring retainers are bolted on, for ease of renewal.
- Small diameter stem joining valve heads, which minimises loss of steam chest volume (connecting tube on SAR loco. no. 3450 reduces steam chest volume by ~ 15%).
- Long steam lap and provision of exhaust lap. The resultant length of the ring retainers favours the fitting of multiple rings whilst maintaining lands of adequate width for low stress and low valve - liner bearing pressure.
- Exhaust diffusers are fitted to allow exhaust steam to sweep and cool the ring retainers. This function is aided by all features which allow short cut-off to be used, giving high expansion ratio in the cylinders and consequently moderate exhaust steam temperature.
- Insulated valve heads, to inhibit heat transfer between live and exhaust steam across the heads.
In contrast, the BR 5MT piston valves are heavier, do not seal 100%, have too few rings, have a poor inlet flow coefficient at small port openings resulting in large indicator diagram triangular losses and poor breathing at low cut-offs. They do not have the whole of the ring retainer cooled by exhaust steam; they have excessive heat transfer across the valve heads, and they have insufficient lap.