An extensive development program involving the use of Burt-McCollum single-sleeve valves was undertaken by Bristol in 1926. With this sleeve valve concept, the usual poppet valve gear (cam, cam followers, push rods, rocker arms, valves, springs, etc.) was replaced by a train of spur gears that drive small sleeve-valve operating cranks at one-half crankshaft speed. Their crankpins protrude inside the crankcase where each engages a spherical sliding coupling on its associated valve sleeve. The sleeve receives a combined reciprocating and rotating motion that causes any point on the sleeve to describe an elliptical path wrapped around the circumference of the sleeve, with a closed circuit being completed every two revolutions of the crankshaft. Four specially-shaped ports around the circumference near the top of each sleeve pass similar ports cut in the cylinder wall. This action opens and closes passages to the intake and exhaust manifolds. During the compression and power stroke, the sleeve is at the top of its travel and its ports rise above the level of internal sealing rings in the cylinder head, which in the case of sleeve valves, are called “junk heads.” Two spark plugs near the center of the junk head provide ignition.