Peak 2T - Aprillia RSA125

Not exactly karting, but this category is kinda for all things 2T since there’s not too many places left for it. This came up in a 2T facebook group today. Details, pics and specs of what’s often considered to be the peak of two stroke development, the Aprillia RSA125.

What’s the most interesting part of this to you?
For me, the balance shaft and oil pump caught my eye. Not to mention the head volume of 8.x cc. Front rotary we’ve seen in karting of course. Not sure what the oil pump is for as it was a premix engine at 5%

Starting with the people behind the top level of two stroke GP racing:

Tech Specs


Cylinder and Porting








Piston and Transfer ports


Carb and powervalve





Balance Shaft, Oil Pump, Detonation Sensor

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Oh my that’s a work of art…you were asking why the oil pump since it’s premix? In that version, the pump picks up oil from the gearbox and pushes it into the gear&pinion that transmits rotation from balance shaft to the shaft that moves the rotary valve, as the two shafts are at 90 degree angle. Oil is needed to keep the gears lubed

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What was the purpose of the rotary valve? To operate the power valve or was it some other function controlling mixture through the transfer ports? Also, I’ve seen windowed pistons before but this design takes it to a different level :heart_eyes:

absolutely beautiful, thanks for sharing, super interesting. damn near 60hp out of 125cc is so insane.

The rotary valve does the same thing as a reed pack does, stops the mixture from escaping as the piston comes down and pushes mixture from case to transfer ports, allowing durations that a piston port would not allow you to do. The difference is a reed pack self-regulates so it optimizes charge across the rpm range, while a rotary, although more stable at higher RPMs, it provides a fixed setup.

Power is impressive, I wonder how a modern KZ with a 42mm carb and modern ignition would fare against it. The other part I would be curious to learn more about, is that in karting we stopped developing the rotary valve designs years before GP motorcycles, which continued on that path, paired with a power valve on the exhaust while we went straight to reeds and no exhaust valve

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Reeds, tiny carburetors, and fixed exhaust timing are only there because they’re required to be. The TM KV95, the last of the big-carb-rotary-valve engines, is still impressive to drive and very fast.

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What aren’t we racing these? Too complex?

Like these aprilia engines? This particular one you are staring if memory serves me right was not available for sale to users, you could only lease the entire bike for a race program with Aprilia. My guess on the price tag of this engine, just south of $50K. Again fuzzy on memory but you could buy the bike as a collector for about $100K, about half of that if used after a race season. You can find RSW models (the privateer versions, with a less refined engine) for about $20K. These are not standard, mass produced units

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Well that answers that. I was looking at all the bits and thinking, this thing must be a bear of a bottom end rebuild. It looks expensive.

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Maxter made a TaG KZ motor with exhaust PV. Not in production now, though, I think. Seems that on a gearbox motor, the valve would just be in full open mode in karting.

I remember when Aprilia was dominant in 2 stroke racing. 60 HP = lots of rebuild intervals. That much peak power must have made the motor very peaky.

You are right, Aprilia was dominant also from a technical standpoint, they’ve engineered amazing stuff, and they were not alone. In the '70s all the way through the '90s it was 2 stroke heaven in Italy. I don’t even think I can recall how many manufacturers we had, coming up with all all sorts of new tech for 50cc and 125cc platform (plus aprilia with the 250cc V2) with either their own engine or in cooperation with big engine makers like rotax etc. Some of them branched to karting like TM, some others had their engines used in karts (formula Europe was a stock moto with a Cagiva engine…same exact trajectory we had in US with Honda 20 years later). As a teenager I remember we had dozens of magazines talking about all the new stuff coming out. I recall Aprilia also had a 50cc and 125cc, 2 stroke, harley davidson looking bike…complete nonsense, but good times nonetheless. The usual trajectory would be getting a 50cc at age 14, 125cc age 16. In parallel, you’d have a kart, a motocross, trial, enduro or whatever racing thing you wanted to have fun on. Depending on your budget, you could get new stuff or old stuff, but you could afford something for sure. Big numbers in the markets translated to big numbers in racing, with huge R&D spending budgets that the Companies could afford.