When the Merco 49 was developed to meet a need for what was then the top power requirement for R/C it was proportioned with the possibility of a larger version to follow (just as "29's" gave rise to "35's"). The Merco 61, therefore, has virtually iden tical overall dimensions to the "49" and uses the same crankcase. It is, however, very much more than a bored out "49" as both bore and stroke have been increased in raising the capacity and there are other subtle differences in transfer and exhaust port timing, etc., plus the addition of transfer ports in the skirt of the piston to improve transfer and promote bearing cooling. The "61", as it has come to be called, has been developed as a specific 10 c.c. design and has had the unusual distinction - for a British production engine, at least - that some half a dozen prototypes were extensively flight tested under competition conditions throughout 1963 by leading R/C flyers
Although a big (and noisy) engine, the Merco is extremely easy to handle. Starting is excellent, following a generous prime - or with just finger choking when warm. An identical carburettor unit to the '49" is fitted, giving fully flexible throttle response and a normal safe "low" setting of approximately 3,000 r.p.m. The air bleed control enables the low speed setting to be established at an optimum - neither too rich or too lean - although exact adjustment is a little difficult to establish until a fair amount of running-in time has been achieved. At least an hour's running in time is probably necessary to ensure complete consistency of throttle action, although this may be less (or more) with individual engines. Although employing a ringed piston and a ball race crankshaft which should minimise running in requirements, Merco's are usually set up tight enough to be a little on the "sticky" side when absolutely new. But we are pretty sure that, as a consequence, they will outlast many a lighter and freer (as manufactured) glow engine.
"61" is not. a high revving. engine and seems quite happy starting and running on either straight or moderately nitrated fuel, .with virtually:no vices or peculiarities. It will also start readily at reduced throttle settings. Maximum power appears to be developed in the region of 11,500 to 12,000 r.p.m. with a fairly flat peak and outstanding lower speed torque. Best propeller size would probably be one giving 10,000 to 10,500 r.p.m. static, which means a generous diameter size. A 12 x 6 would probably be right for most R/C applications, although a 13 x 5 or a 14 x 4 might be a proposition on a slower flying or very heavy model. Anything under a 11 x 6 is likely to lead to over-revving in the air, with no advantage. In any case, the Merco is not all that happy trying to push the r.p.m. up past 12,000 with straight fuel.
Structurally, and asregards material specification, the "61" is virtually identical to the "49", with a number of detail changes..The crankcase unit is a beautifully smooth pressure die casting in L.33 alloy with a sand blast finish and embodies the lower cylinder housing and stub exhaust, and stub intake pipe. The liner is of leaded steel (EN.1A) casehardened, ground and honed to finished size. Overall diameter is 1.0625 in. which, with a bore of 0.940 in., gives quite thin cylinder walls for a robust engine of this size, with an intricate pattern of ports cut through the walls. Exhaust ports consist of two pairs each approx. 3/16 in. square, with four similar sized but equi-spaced transfer ports diametrically opposite and slightly lower giving some 60 per cent overlap. In addition, two circular holes 1/4 in. dia. below the transfer ports connect with corresponding holes in the piston to provide skirt transfer. An orthodox transfer passage is retained in the crankcase unit outside the cylinder, thus transfer is effected in the normal way but with; an additional flow path through the piston into the transfer passage. . - - -
The piston itself is turned from a low expansion light alloy and is fitted with a pair of cast iron rings (Weliworthy), conventionally arranged. The crown of the -'piston is flat with a well faired in rectangular deflector. The piston walls below the gudgeon pin boss are turned away to approximately 1/32 in thickness. The hollow hardened steel gudgeon pin of 1/4 in. dia; is fully floating without retalnmg rings or end pads. The connecting rod is a light alloy forging in RR 56 with reamed plain beirings and each end generously slotted for lubrication.
The cylinder is a loose plug fit into the crankcase unit and seals on a gasket. It is encircled by a turned dural jacket and the whole assembly is secured via three short and three long 1/8 in. whit. screws through the head - three extending into the crankcase and three bolting the head to the jacket. The head is machined from dural and generous in bulk, although the top is well finned. The plug-in portion is shaped to give a hemiphericil combustion chamber and the glow plug is mounted in the centre. No gasket is used between the head and the top of the liner on which:it sits, spacing between head and jacket being provided by a narrow flange on top of the liner.
The 1/2 in. dia. crankshaft is of EN 1A steel, case hardened and ground between centres to finished size. Crank web is extremely generous in thickness, but machined away for counterbalance. The shaft is carried on a 1/2 in. ballrace in the main crankcase unit and an 8 mm. ballrace at the front. Outside the front housing the shaft steps down to 3/4 in. dia. with a conventional threaded length for the prop. retaining nut and washer. The prop. driver is turned from dural and mounts on a split taper collet.
The carburettor unit is pure "Merco 49", with the body machined from dural housing a dural barrel valve and brass spraybar assembly. Barrel and spraybar are separate and mounted from opposite sides so that the throttle movement does not rotate the fuel entry pipe. The barrel movement is linked to an exhaust flap in the conventional glow-throttle manner, the exhaust flap closing completely some little time before the throttle is completely closed. A vertically mounted screw on the carburettor body provides adjustment for barrel throttle stop in the closed position. Open position stop is fixed and independent of this adjustment. A further (horizontally mounted) screw on the right hand side of the carburettor body controls bleed air entering the carburettor under the barrel for fine adjustment of mixture when the barrel is closed to the slow running position. This particular control is very near the propeller disc and rather easier to reach with a screwdriver than the fingers for precise adjustment.
Apart from the fact that the Merco "61" really looks an outstanding example of precision model engineering production - everything about it is quality plus. Workmanship throughout is outstanding, for example, and everything about it appears built to last, although it is quite compact for a 10 c.c. power unit. There is little doubt that for sheer B.H.P. output there will be other engines of similar size to beat it - but few, if any, will be able to beat it in the speed range for which it was developed - up to 12,000 r.p.m. This, after all, is the speed range which really counts for R/C flying.
Frankly for British R/C modellers who want the "top" size engine, there are few reasons for not automatically settling for a Merco "61".
One must remember that it is not a mass produced engine, so demand could well outstrip supply for some time to come - especially, as we are sure, there will be a pretty hefty export demand. To our mind, too, the "61" puts the hitherto attraction of the "49" in the shade. Here for virtually the same overall dimensions and only a negligible fraction of an ounce increase in weight is over 20 per cent more power. Any model which will take a "49" will equally well take the "61", with the benefit of swinging a slightly larger diameter propeller and that extra power available when wanted. You do not have to use maximum power all the time, and the throttle is one of those really effective types where the full speed range of the engine is available on command.
The Merco has also received Design Centre recognition, which to the best of our knowledge is the first time any model engine has received such distinction. Not that it needs any such label to promote it-and in any case we doubt that the Design Centre knows anything about model engines, anyway. The Merco "61" we venture to prophesy, will become one of those engine "classics" in the manner of the well loved McCoy "60" spark ignition of the late 1940's. It costs more money than other engines of similar size, but it is well worth saving up for. Our own order is going in at once.
My thanks to Terry McDonald for supplying copies of this material.