First off, let's find out what makes the RCV 58 CD tick. The crankcase splits horizontally about its thrust line, with the upper and lower cases being held together by six Allen bolts. Removal of the bolts and gently separating the cases reveals the intricate innovative workings of the RCV. Our next job is to gently lift out the crankshaft along with bearings, thrust washer/seals, prop driver, piston and conrod assembly; all can be removed in one go. Now up to the top rear of the engine to remove the induction manifold and carb, which is held in place by two Allen bolts. The carb is retained on the manifold by a pinch bolt and nyloc nut. Finally, the last piece of the casing to remove is the valve cover, retained by four Allen bolts and sealed with a fibre gasket. Once removed the rotating cylinder can be pushed out from the top downwards; the cylinder is a very tight fit in the bottom bearing so a lot of pressure has to be used.
The crankcase is made of three pieces; all are very high quality pressure die-cast alloy, with machining to the same high standard. The top cover is the simplest part with just four shallow cooling fins running from front to rear and four countersunk holes for fastening to the upper case.
The bottom case separates from the top and contains half of the engine mounting lugs and horizontal stiffening webs that run from the maincase to the front bearing housing, with a further web running centrally at the bottom. It’s bored for the six assembly bolts and has a chromed steel breather nipple fitted to the front crankshaft housing. Turning it over and looking at it from the inside, there are two grooves at the front for the thrust washer / seals that lie either side of the front bearing and two stiffening webs that run axially across the main portion of the crankcase.
The upper maincase contains the other half of the horizontally split engine mountings, stiffening webs and internal thrust washer / seal grooves, along with the six tappings for the assembly bolts. Looking inside one sees a large, very smooth-running lower cylinder bearing, and right at the top the plain bearing / timing bush with its three ports. From the outside the case has a further stiffening web running from just below the heavily-finned cylinder to the front bearing housing. The manufacturers name ‘RCV’ is in raised machine finished letters on the plug side of the case, with the size and type on the exhaust side.
Just below the top of the case at the rear is the induction port with two tappings for a short, pressure die-cast induction manifold, which is angled down and sealed by a fibre gasket. Moving around the ‘head’ in a clockwise direction the first point of interest is the port that’s tapped for a standard long reach four-stroke plug (O.S. 4T recommended by RCV). Moving on we reach the opposite side, which has been tapped for the steel exhaust manifold, two lock nuts and a very quiet alloy silencer that’s fitted with a chromed pressure nipple. Rounding up the case right at the very top is the valve / bearing and a very small oil drain hole that exits just below the top of the rotating cylinder.
Keith Laws’ ingenious device, the rotating cylinder with its built-in valve, is next in line for description. This is machined from a steel billet, case hardened and finished by very fine internal and external grinding. At the bottom is the large helically-cut bevel drive gear, heat-shrunk on to the main cylinder and chamfered internally for flywheel clearance.
Just above the gear is the main bearing seat, which has a machined-in oil drain groove. Finally, at the very top of the cylinder is the plain bearing area, which is cut with the single square valve port and has a vertically-machined groove directly opposite for oiling the bearings as the cylinder rotates; this area acts as the combustion chamber.
PISTON, CONROD AND CRANKSHAFT ASSEMBLY
The very short die-cast alloy piston has a machine finish and is fitted with a fully floating, hardened steel gudgeon pin that inserts into the piston from the rear, retained by a small PTFE pad. A 1 x 10mm diameter raised portion on its crown forms a squish area at TDC when the engine’s running. The lower skirt has been machined at both sides for flywheel clearance. Looking inside, two large webs support the gudgeon pin and a small, machined recess in the crown caters for small-end clearance. External machining just below the piston crown provides a home for the un-pegged iron piston ring.
The conrod is CNC-machined high tensile alloy, with bronze bushing for both big- and small-end bearings. Both bearings have oil holes, the small-end bored centrally at the top whilst the big-end has two holes - one central at the bottom, the other bored through from the top. Actually, there are two further lubricating grooves machined horizontally across the front and rear faces of the big-end.
Note, I’ve not dismantled the crankshaft assembly as the bearing seals, thrust washers and alloy prop driver are firmly pressed together with precision.
The crankshaft is from a single steel billet with a pressed-in, hardened big-end crankpin.
The conrod is retained to the crankshaft by a steel thrust washer and circlip that locate in a machined-in groove at the rear of the pin. In front of the fully counterbalanced flywheel lies the hardened bevel drive gear for the rotating cylinder. Forward of this is the first of the two high quality, smooth main bearings followed by a seal that’s recessed to align with the crankcase breather vent. Moving along is the rear steel thrust washer / seal, immediately in front of which is the second main bearing and final thrust washer. The last part of the shaft contains the Woodruff-keyed alloy prop driver and output shaft. The prop shaft has been cut with a 1/4” UNF thread for the steel prop washer, main prop and split locking nuts.
Of twin-needle design, the carburettor has a pressure die-cast alloy main body that houses a steel throttle barrel with a finely ground surface finish and a bore of 7mm. The latter is fitted with an adjustable chromed steel throttle lever and is bored centrally for the ‘O’ ring sealed brass idle needle.
The main needle valve assembly is from chemically-blackened steel, and is fitted with a very positive steel ratchet clip. Last parts of the carb fitments are the fibre washer, sealed barrel retaining screw and chromed fuel nipple. In common with all our engines, the unit is sealed to the induction manifold by an ‘O’ ring and retained by a steel pinch bolt. Operation of the carb was very smooth from the start, proving very easy to adjust and holding its settings throughout the test.
ON THE BENCH
With an O.S. type F plug fitted, initial starting was done with the aid of an electric starter. Once turning the engine started very easily and settled down to a fast tick-over. Opening the throttle to dial in the top end mixture, I kept a rich setting to prevent over-stressing the internals. This engine took a lot of careful running in (as the fits are very tight), using the recommended 11 x 6” prop over a period of 2 hours using the ‘5 minute run and allow to cool’ method at various throttle settings. After this period the engine would start readily by hand, taking no more than a couple of flicks.
Throttle response is excellent from the very low idle to full throttle without any glitches, even after idling for prolonged periods.
In terms of fuel, consumption is miserly, though RCV recommend to only use fuels containing a 15% synthetic / castor mix, of which not more than 6% is castor; pure castor oil fuels aren’t recommended as these will gum up the works. RCV also recommend a nitro content of 10% but the engine was found to run quite happily on 5%.
Rotating cylinder valve engines are unique and a great step forward over conventional four-strokes with no valves, camshafts, pushrods, belts or gear trains to time, break or gum up (in the case of valves). Wear of the 58CD cylinder is minimal, due to the cylinder rotating as the piston goes about its business.
The 58CD is the first of the new generation of crank-driven models and has true versatility when it comes to usable prop diameters and pitch. All its predecessors take the prop drive from the engines’ rotating cylinder that turns at half engine speed, giving a very limited range of large diameter, high pitch props.
This is a very compact, slim engine and, judging from the response it’s been getting, this is the one modellers have been waiting for. The 58CD draws lots of interest and admiration wherever it’s taken and it definitely has a ‘wow’ factor that’s missing from lots of the other badge-engineered four-stroke engines that we see around our flying fields these days.