I don't remember exactly. As what I know. From 199X to 2010, O.S. .21 engine use 16.6mm/16mm bore stroke ratio. While NR REX was square engine. Then MES was invented. NR use 16.26mm/16.8mm bore stroke ratio. The O.S. way make MES is almost no advantage in buggy engine. The O.S. VZ(16.6mm/16mm bore stroke) did not use MES, but a rib in the centre of the exhaust port. Until recent years, 201X. O.S. launch the XZ, 16.27mm/16.8mm bore stroke ratio, the first smaller than 1 design. XZ has MES. Longer stroke make MES in buggy engine possible/efficient. The NR bore stroke ratio continue to decrease. The latest NR .21 engine use 15.88mm/17.6mm bore stroke ratio.
We can see the bore stroke ratio is decreasing. I believe the designer want to maximise exhaust and scavenging efficiency. They are Contradictions, cannot gain both.
http://www.rcuniverse.com/magazine/article_display.cfm?article_id=770
Model Name: O.S. 21VZ-B V-Spec
Displacement: 0.211ci (3.46cc)
Bore: 0.654" (16.6mm)
Stroke: 0.630" (16.0mm)
Practical RPM: 3000-40,000rpm
Output: 2.48hp @ 33,000rpm
Weight: 12.7 oz. (360g)
Carburetor: 21C Slide-Valve
Glow Plug: O.S. P3 Turbo
http://www.osengines.com/engines-boat/osmg1621/index.html
O.S. 21XZ-M Inboard Marine Engine
Stock Number: OSMG1621
Displacement: 0.21 cu in (3.5 cc)
Bore: 0.640 in (16.27 mm)
Stroke: 0.661 in (16.8 mm)
Practical rpm: 4,000-45,000
Output: 2.71 hp @ 33,000 rpm
http://www.osengines.com/engines-car/osmg2043/index.html
O.S. Speed R2101 Engine
Stock Number: OSMG2043
Displacement: 0.213 cu in (3.49 cc)
Bore: 0.633 in (16.08 mm)
Stroke: 0.677 in (17.2 mm)
Practical RPM range: 4,000-45,000
Output: 2.7 hp @ 33,000 rpm
http://www.novarossi.it/2012/index.php/en/products/off-road-engine-nova/3-5-cc/keep-off-21-4.html
NR KEEP-OFF.21-4
Displacement: 3,49 cc
R.P.M. (max power): 36.000 RPM
Practical range: 6.000-38.000 R.P.M.
Bore x stroke: 16,26x16,80 mm
Sleeve: 4 ports
Rear ball bearings: ceramic
Front ball bearings: steel
Crankshaft: 14 mm - turbo - tuned
Carburetor: plastic - 9 mm - reverse
Glowplug: Turbo (C6TGC)
Exhaust position: rear
Starting: -
Weight: 360 g
Fuel type: 25% nitro
http://www.novarossi.it/2012/index.php/en/products/off-road-engine-nova/3-5-cc/keep-off-21-7xl.html
NR KEEP-OFF.21-7XL
Displacement: 3,49 cc
R.P.M. (max power): 36.800 RPM
Practical range: 6.000-40.000 R.P.M.
Bore x stroke: 15,88x17,60 mm
Sleeve: 7 ports
Rear ball bearings: ceramic
Front ball bearings: steel
Crankshaft: 14,5 mm - turbo
Carburetor: plastic - 9 mm - slide
Glowplug: Turbo (C6TGC)
Exhaust position: rear
Starting: -
Weight: 370 g
Fuel type: 25% nitro
Best regards,
Leo