BRM had some previous experience of stressed skin construction with the BRM P25, so Rudd was in a good position to be able to exploit the new technology to the full. This previous experience meant that Rudd's use of the monocoque was somewhat different from the pioneering Lotus's frame. Where the 25 had been a channel-section frame with an open top, within which the driver sat, the P261 chassis was a slim tubular-section, into which a hole was cut to allow the driver to gain access. To replace the P61's subframe the side pontoons of the P261 chassis were extended behind the driver's seat, and the engine was mounted between them. Within the pontoons, rubber cells were used to retain fuel. This caused complications early in the P261's life, as BRM's new, high-exhaust version of the "P56" V8 engine was not ready for the start of the 1964 season, and holes had to be cut in the pontoons to allow the exhaust pipes of the older, low-exhaust version to pass through them. The centre exhaust engine appeared at the 1964 Italian Grand Prix in Graham Hill's new chassis "2616" and this and "2617", which was Jackie Stewart's regular car in 1965, were the only two P261s which did not have the exhaust slots. These were plated over on the earlier cars which remained in service, but could be opened up if necessary to fit outside exhaust engines, as happened in the 1967 Tasman Championship.