The four stroke engine's FIRST AND FOREMOST factor, is camshaft timing. After that, the oil pump, coolant pump, alternator, and magneto (if it has one) needs to be addressed. Distributors are oftentimes okay, but sometimes reluctor wheels (electronic) or breaker point pivot position will cause them to operate slightly different than in the other (like... wear faster, or be less-stable).
Counter-rotation in propeller driven airframes and boats is done for dynamics and efficiency reasons... and it's easy to describe:
I have a 17 foot runabout with an inline six that is FAR from stock. It swings a big propeller, with lots of pitch, and basically no rake... it generates lots of thrust at very high speed, which happens to be coincident with lots of planing lift- there's very little hull in the water... mostly the drive unit, the propeller, and about a square foot or so of hull for the whole 1500lb boat.
The propeller is 'screwing' it's way through the water to generate this thrust, just like an electric drill-driver pushes a deck screw through a plank.
For every action, there is a reaction, and just like the drill twists in opposite direction of the screw, my boat is trying to 'lean' in the direction opposite of prop twist.
To counteract the force of prop torque, there's a 'trim tab' mounted on the cavitation plate behind the propeller that directs a small portion of the prop wash to one side, which helps at slower speed, but at extremely high speed, I basically have to 'steer' the boat slightly sideways to keep it going in a straight line.
And of course, when I grab the throttle and snap it back to idle, the boat reacts by 'snapping back' to straight orientation, and usually a fair lean in the 'opposite' direction.
Some of this can be attributed to the engine's spinning mass slowing rapidly, but most of it, is the fact that the propeller's bottom half (in clean, deep water) is pulling one way, while the top of the propeller (at high speed, is ventilated... running in aereated water) is not. the 'sideways' walking is downright aggressive with a very high pitch prop.
When docking at idle, the propeller's twisting effect 'walks' the boat to one side in forward, and the other in reverse. We actually use this circumstance to aid in docking, especially on inboard boats (outboard and Inboard-Outboard drives pivot the propeller angle, so they can be 'steered' fairly well in reverse-thrust, so it's not as necessary).
Aircraft have exactly the same scenario.
When multi-engine setups are used, it is most common that the propellers are set up for counter-rotation- one turns to the left, other to the right, so that the propulsion's natural side thrust of one, will be counteracted by same from the other side. It results in higher efficiency, and improved handling.
On many marine drives, the engine is actually mounted 'backwards' in the hull... flywheel forward, and transmission drives off the 'front' of the crankshaft. This is done because the flywheel diameter needs clearance from the bottom of the boat... they tilt the engine, and drive it from the nose, so that the entire engine system can be closer to the bottom of the boat, and more in-line with the propeller shaft. When they do this, they build new oil-pans, relocate oil pump drives, change the camshaft, distributor drive, water pump, etc., so setting them up for opposite rotation is no big deal.
Also note that in some marine applications, the engine drives towards the rear of the boat (directly out to the propshaft), while others drive forward to a gearbox, which then drives rearward to the propeller (a "V"-drive). In V-drive systems, it's not unusual for the engine to be oriented with flywheel towards the aft, but driven off the front of the engine. Just depends on the boat.
Another option, is to set up the transmission so that forward is reverse, and reverse is forward. On transmissions that use a 1:1 ratio, it's as simple as changing the bellcrank shift lever connection position. On planetary transmissions, the gear ratio for reverse and forward are NOT the same, so it's a more complex task.
Many industrial engines (Waukesha, for example) have the option of rotation as a 'natural' part of the design. This is easy to figure out when you're holding the camshaft. If the camshaft bearing surfaces are all the same, AND there's cam-gear mounting on BOTH ends of the camshaft, it is symmetrical... so a reversable engine. To reverse the rotation, just remove the camshaft, flip it end-for-end, reinstall it, reset ignition timing (distributor!) and I THINK... flip the oil pump housing... and you have opposite rotation.
As noted, two stroke ported engines will run either direction. They're usually DESIGNED to favor for one direction... the most obvious being piston-thrust-angle. If you place a rod down the exact centerline of the bore, you'll find that the centerline of the crank does NOT intersect it... the piston will be offset to one side. This is to keep the rod thrust angle closer to 90 degrees for LONGER, hence, generating more useable torque through the power stroke. It means that the rod angle is steeper during compression, which has some impact on compression curve. When running this backwards, it certainly will NOT run as well in the opposite direction.
Now, there's been a few two-stroke gassers designed to run in BOTH directions... and some were used to run in reverse unintentionally...
Those designed to run backwards... as noted by others... golf-cart two strokes frequently are set up to start in reverse, which allows the cart the ability to back up without having to have a gearbox.
Really large ships running 'oil engines' (bunker oil, so substantially thicker than diesel, but still compression-ignition) have no gearbox... they start in-gear, and they have direct reverse, but HOPE to never have to use it.
The Mercury Mark 78 'dockbuster' was an inline six two-stroke that had no neutral, and no shifting. When you pulled the throttle to neutral, it killed the ignition. When you moved the throttle to forward and hit the button, it started in the forward direction... when you moved it to the reverse position, a cam reversed the ignition (to the opposite side of TDC), and activated a second (reverse-operating) starter. It worked... uh... but not great. On heavy boats, coming into the dock, frequently there was too much drag on the propeller for the starter to actually START the motor, and as a result, the pilot would not be able to control his arrival, the boat would strike the dock.
Here's a demonstration of the Mark 78... move to about 0:56 to see it:
Now, if you wanted to drag race an overconfident Mustang driver... the best thing to do, is place a pink-slip bet with the following: "I'll drag race you, I get to pick the course, and the rules." When they're foolish enough to agree, you have them meet you at the starting line, and the rules are... six blocks, in reverse.
You show up in a Saab model 95... a three-cylinder two-stroke. As you coast up to the start line, push in the clutch, shift to reverse, cut the ignition, flip it back on, and bump the clutch... now the engine is running in reverse. Pop to neutral, wheels on the start line, put it in first. When the flag drops, hammer it, shift through the forward gears, while steering backwards. He'll beat you for the first 20 feet, but after that, he'll be out of gears, and you'll keep accellerating.
On a somewhat unrelated note... OMC made a two-stroke 4.5l V8 outboard motor that was rated at 300hp... but due to many factors, was capable of much more, for those willing to invest time and effort... but here's a guy who 'de-marinized' and horizontallized one, mounted it in a car chassis:
And if you like silly screaming power, take a laugh at this test drive:
Edited by DaveKamp - 16 Nov 2019 at 1:10pm
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True confestions 
I coast start my old D6 Cats if I shut them off at lunch time if there is a hill in the right 
But there is no mistaking what you have done. Oil came blowing out of the wet air cleaner, the second time was dry air cleaner. But what a cloud of dust. So you tend to grab the throttle and shuter down 
john(MI) wrote:
