G-Mark .12 Opposed Twin

Name G-Mark .12 Opposed Twin Designer unknown 
Bore 11.2 mm Stroke 10.15 mm
Type Glow-plug Ignition Capacity 0.122 cuin
 (2 cc)
Production run unknown Country of Origin Japan
Photo by Ron C Year of manufacture 1979



When he reviewed this engine in Model Airplane News of March, 1980, Peter Chinn began by saying that his initial impression was that the engine was "a neatly made novelty, ingenious in certain respects, obviously a collector's item, but unlikely to attract attention as a practical alternative to single-cylinder engines of similar displacement." [1]. He went on to say that after full evaluation, the engine turned out to be far better than anticipated.

A factor in his initial impression may have been experience with past opposed twin two-strokes, especially small ones. As noted elsewhere in these pages, elaborate Scotch-yoke schemes aside, this type of engine must be simultaneous-firing in order to use the crankcase as a pump in the normal way. Designers face two problems: attaching the conrods to the two-throw crankshaft, and delivering equal weight of charge to both cylinders during transfer and scavenging.

Amateur builders may opt for a permanent assembly, but commercial engines require a higher degree of serviceability, so normally some provision must be made to provide removable straps or caps on the big-end of the conrods. This leads to both complexity and bulk in the crankcase. And the smaller the engine, the more delicate the arrangement.

Induction is also a challange. The double throw crankshaft results in the cylinder axis being offset (unless the conrods are offset from the centerline like the Ross boxer engines). A single rotary valve, front or rear, will lead to some degree of starvation at the cylinder furthest from the point where the mix enters the crankcase. Solutions to this problem add complexity. The Davies-Charlton Tornado used both front and rear rotary valves fed from a central carby. Some German designers have used the central crankshaft web as a "drum valve" in an attempt to equalize the feed. This works, but requires German precision. Others opt for a centrally located reed valve and accept that this makes induction somewhat symmetric and the engine prone to starting backwards. As usual, there are no easy answers and no such thing as a free lunch.

A Novel Solution

G-Mark's solution to the big-end problem is novel and surprisingly effective. As seen in the manufacturer's exploded illustration, the conrods are quite conventional with a normal big-end and a ball-joint little-end, a'la Cox. Obviously the rear one can just slip on, but as the shaft appears to be solid, how is the front rod attached? A close look at the drawing reveals a grub-screw in the front crank-web that provides a clue.

In fact there are two grub-screws. They lock in place an insert disk that sits in a recess in the unusually deep front crank web. The two crank throws are machined integral with the central web. This latter is drilled out to provide some degree of flow between the throws. One of the crankpins is longer than the other and slips into a hole drilled in the front web so that it partially overlaps the central cavity. A crescent relief on the pin allows the disk to both align and lock the central section in place after the front rod has been placed over the crankpin.

Now think closely about how you'd setup to machine something like this. It must assemble with the crankpins centered exactly on the diameter of the front crankweb, equally displaced about the shaft axis. Tricky, especially as it must assemble like this "in the field" as well as in factory jigs. This worried me sufficiently that I did not attempt full disassembly for photographs. To be sure of alignment, I'd have to rotate the shaft in a collet with a DTI on the center web. Easier to leave it alone and use the MAN photo to show the setup. It is interesting to note that on my example of this engine, the locking disk is solid, not a donut.

The shaft assembly may appear "delicate" but remember that with both pots firing, or not firing, all stresses on the crankpins will be balanced at all points of rotation. Things may get a bit ugly if one cylinder stops--say when throttled down. But you can't argue with success, and experience has validated the practicality of this unusual design.

The rest of the engine, seen disassembled here, is just as well made as the shaft. The main crankcase is a neat die-casting with integral back-plate mount. The offset cylinder mounts are threaded for the blacked steel cylinders. A hole in the top of the case locates the reed assembly, centralizing it over the central crankweb. The cylinders, like the piston/rod assemblies are rather Cox-like with opposed exhaust ports and opposed, internally fluted transfer ports. Chinn measured the exhaust duration at 134 degrees, with a transfer period of 112 degrees. This is very conventional. The cylinders retain the exhaust collector rings, pressing them onto "O" rings that sit in cast-in grooves. The collectors are identical, located and locked against rotation by a raised arc cast into the rim. The offset hole for the cylinder in the collectors has the neat effect of aligning the two exhausts. Very clever. Unlike the Cox design, the turned cylinder heads attach to the cylinder with four slot-head screws and take conventional glow-plugs.

The crankshaft is stepped, 9mm at the rear and 7mm at the front. It is carried in two ball races held in a turned aluminium housing. The front bearing only is of the sealed type. The assembly attaches to the case by four socket-head screws. The propdriver locks to the shaft with another grub-screw that bears against a flat on the 7mm section. An unusual feature is the stamped, five-finger spring washer that sits in a recess on the rear face of the driver. The fingers are raised to press against the central portion of the front race. The cad-plated prop nut is a commercial item that is forged with an integral washer.

The reed valve is mylar, housed in a cylindrical turning that plugs into the crankcase. It is sealed by another "O" ring and secured by a 2mm cross-point screw whose head is recessed into the rear of the mounting flange. The reed design and its retention spring are again, distinctly Cox (of the "old" pattern). The R/C throttle plugs into the center of this assembly with yet another "O" ring and two cross-point screws. The throttle is quite conventional, being of the "non-compensating" type. It has a fixed air bleed hole and a screw sets the idle limit. Fuel inlet is provided by a neat nylon banjo. Not so neat is the throttle arm. As moulded, this would have interfered with the crankcase, so the tip has been circumcised with side-cutters. This, and the prop nut, are totally out of character with the quality exhibited by rest of the engine, but completely original.


The engine was supplied in an injection moulded plastic box, embossed with the manufacturer's logo, and fitted with a foam insert for the engine. The instruction leaflet shows the three possible arrangements for lighting up the plugs:

  1. Separate cells (1.2 to 1.5 volt each) for each plug (ie, two batteries, four leads; treating each cylinder as a "separate" engine).
  2. A single 1.5v, 6A cell applied to the two plugs in parallel (ie, one side attached to the engine body; the other to both plug tips. Three wires in all).
  3. A single 3v, 6A cell applied to the tips of the two plugs. This effectively lights the plugs in series. It requires only two wires and has the advantage that if one plug blows, the other will loose current so there is no chance of starting one pot only.

In the USA, the engine was distributed by Cannon R/C systems (I built and flew several of their propo radio kits, which were well designed and performed well). instructions suggest 20-25% nitro in the fuel. They are dated November 15, 1979. At this time, a full, factory recondition of the engine could be had at a cost not to exceed $76.50. The speed range is quoted as 2,800 to 14,000 rpm with a 7x4, or 8x3 prop. The MAN review confirmed these, and managed to record a screaming maximum of 21,000 rpm on a 6x3 wood prop. The peak power was developed at about 18,500 rpm. [2].


I really should give the little G-Mark a run. MECA rules still allow you to call an engine "NIB" if it's had a test run or two, but given its pristine condition (and what I paid for it!), I'm reluctant to see Peter Chinn's praise confirmed. From an innovation perspective, the design should be an inspiration to designers of small, opposed twin two-strokes. Yes, a high degree of precision would be required to mount the central section while machining the crescent that both aligns and locks the thing together, but no more than a careful worker who is prepared to spend time making jigs is capable of. The real eye-opener is how light the crankshaft for such an engine can be and yet deliver reliable, outstanding performance.


[1] Chinn, Peter: Engine Review: G-Mark .12 Twin, Model Airplane News, Volume 100, Number 3, March 1980, p42, Air Age Inc, Darien Conn. USA.
[2] ibid. p82.




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