Suzuki burgman's tech is also shared.Locally Honda India shared unicorn's engine specs with HeroHonda's Hunk and CBZ extreme ( read on internet)

I cannot help but agree with what Abhimanyu says here and can only add a bit to it. The engines cannot be seen just in isolation but need to be viewed as a component of the complete motorcycle. The block dimensions also define the physical architecture of the engine and as a consequence its placement, frontal area, weight and a few other variables when it is put to use on the bike frame. These bore and stroke dimensions have resulted in probably the best optimization in terms of performance, scope for further improvement in the stock performance, the physical engine dimensions, the dynamic loads on reciprocating components at achievable high revs and attainable reliability with cost effective materials during manufacture. You'll see this happening a lot in Mother Nature's creations too

Like Abhimanyu says, none would like to lose even a single possible 'cc' of swept volume from the allowed limit. 599 is as close you get to 600 and even though there are a number of possible combinations to get that figure, the reasons elaborated above probably make these dimensions the best of the lot. And with the kind of specific power being demanded in this category, playing around with bore and stroke could upset the relationship between performance and reliability.

But all said and done, this is a strange 'co-incidence', at least in a fiercely competitive product category and whatever we state stays firmly entrenched in the world of conjecture.

Yes Satyen, my bad, the Intruder engine is a product of the corporation between the two manufactures. There are also scooter and small displacement engines that have been developed by them.

However, sharing of engines for the flagship bike line like GSXR and ZX, I don't think thats going to happen.

abhimanyu31 - That's the closest explanation one can give & I believe its true as the regulation comply with FIM; perfect; fair enough.

That lead me to believe that FIM probably should control the litre class motorcycles too isn't it? I tried hard enough to find them, but couldn't; meanwhile, here're some of the spec of top contenders...

2012 Honda Powersports CBR1000RR Specifications - 999CC; 76.0mm x 55.1mm; 12.3:1
2012 Yamaha YZF-R1 Specifications - 998CC; 78.0mm X 52.2mm; 12.7:1
- 998CC; 76.0mm x 55.0mm; 13.0:1
2012 Suzuki Cycles GSX-R1000 - 999CC; 74.5mm x 57.3mm; 12.9:1
BMW Motorrad UK. - 999CC; 80.0mm x 49.7mm; 13.0:1
Aprilia - 999.6CC; 78.0mm x 52.3mm; 13.0:1

and, I see quite a different combos than the 600 class; that makes me think...
1. The FIM regulations are quite different for litre class
2. Engineering allows to produce better results & still comply with the FIM regulation

Can you help find 2012 FIM spec for litre class pls?

PS - I think 2012 allows upto 1200cc isn't it?

^^^ the complete rule book is yet to be officially released, however, they have released a supplement for the changes the link is http://www.worldsbk.com/images/stori...%20October.pdf

#1 If we analyze the facts closely, we'll find a good reason. Before we had 67x42.5 combination, all other 599cc combinations used before were smaller bore & longer stroke (like 65.5x44.5).
Additionally, all these 4 bikes have been keeping the same Bore x Stroke since 2004 onwards, except for YZF R6 which adopted it in 2006.

And amazingly, this particular R6 claimed to be the highest revving bike with redline around 17.5 krpm. (Although later it was found that the tacho used to read slightly optimistically).

The point is that any more short stroking and big boring will result in higher redlines. And in such a situation, the entire machine has to cope up with the additional revving... The valves, the cams, the crankshaft, everything! And today's tuning potential is enough to extract maximum from an engine, even without modifying its Bore-Stroke.


#2: GSXR 750 dates back to the 70's and 80's so probably they thought they should retain it. And one thing: odd man out capacity bikes have made very good sales like 675, 750, 848, etc.
There are multiple known and unknown reasons for it. I'm talking about the road. GSXR 750, in the past has sold better than its smaller 600 and bigger 1000 twin.

Why the competition ignores this segment could be due to lack of proper racing in this segment, who knows?

And about your query about Litre class rules, well, IIRC, it was a maximum allowed 81mm bore. Also, under 1000cc inline 4/ V4's and under 1200cc V2/ L2 are considered race-able. But this is old so it might have changed.