Rotax engined 250cc machines were fast and furious, renowned for speed and performance but lacking in reliability. “Never slipstream a Rotax” became a mantra for many!
The following information confirms that the introduction of power valves to the Rotax cylinders was 1985. Therefore, GP Originals Rotax machines will need to be the pre-1984 non-power valve version.
Bob Clowes, Motiv Racing
I can confirm that the 1985 production engines were fitted with power valves.
Bob Clowes
For the 1984 season production engines had a new, lighter crankcase. Before 1984 all customer 256 motors had the heavier crankcase. Before 1985 the barrels were square in profile and had a larger water volume. More recent barrels were wasted to reduce water volume.
Niall Mackenzie
To whom it may concern. As an official rider for Armstrong Motorcycles from 1983 to 1986 I can confirm the following:
From the 1985 season onwards our machines were fitted with the Rotax 250cc Power valve spec engine supplied directly by the Austrian factory. Our team competed in the full 250cc Grand Prix Series and the 250cc British Championship.
Niall Mackenzie
Leif A Nielsen
The Rotax engine that Armstrong used, before they made their own engine, started with Power Valve in 1985, pretty sure of that. I have attached a Data Sheet from Rotax on the 1985 engine.
Leif A Nielsen
Rotax crank cases
The Rotax factory describes the revisions made in 1984 as a new engine. Up to and including, 1984 models of Rotax are welcome to race at GP Originals.
As mentioned above this means that the later power valve barrels (1985>) are not eligible as this is a significant change to the design. In addition, the carburettor flange on the 1985 model became a lighter, more flexible design to allow riders to use 38mm carbs without reworking. However, the new engines continued to be supplied with a 36mm Dellorto carburettor through to 1986.
We have assessed the revisions made to the crankcases and there were only very minor changes made from the 1984 version through to 1986, therefore we will allow up to 1986 cases only. The 1987 version allowed for an optional side load gearbox meaning this is another significant change – the 1987 cases onwards are not eligible.
Changes made to the later Rotax crank cases
Eligible 250cc Rotax engine
The following photos demonstrate a Rotax engine with Post-Classic era eligible barrels and cases.
Thank you to Simon Bartling for providing photos of the various incarnations of Rotax barrels and crank cases. BRP-Rotax GmbH & Co KG for providing historical data, Michelle and Steve Carthy for research and data collection. The riders and contributors shown in this article for sharing their valuable comments and experience.
