Recently a competitor set a lap record and then was excluded because the squish clearance was found to be too small at the post race check. When the head was removed, it became clear that a castor based oil had been used on practice days and so carbon had built up resulting in an illegal squish measurement. Unfortunately, the competitor had to be excluded because, in fairness to others, he had raced with an unfair advantage of the higher compression.
Castor based oils have been popular for decades in motor sport and particularly on air-cooled two strokes where success is enjoyed but with gumming up of rings and build-up of carbon. Pistons and rings are frequently changed on air cooled two strokes so the down side of castor based oils was not much of a problem here.
Modern water cooled motors, using nikasil cylinders and a modern synthetic oil perform well for many times longer than the old-school engines and will not need honing and new pistons on a frequent basis. Having said that running synthetic oil at 50:1 also results in minimal carbon build-up whereas running a rich mixture of castor based oil will quickly build carbon on top of the piston and very soon your engine will be faster but illegal!
In Micro and Mini Max engines, where the compression ratio has been lowered to limit the power output, engine builders all set the squish close to the minimum of 2,40mm so if you run a castor based oil it won’t be long and you will be in trouble. Have your engines squish checked from time to time just to be sure – your engines squish will be measured as raced and you will not be allowed to remove carbon before measuring the squish at a strip.
Questions and answers:
Q: Will I be allowed to remove carbon before the squish is checked at a race meeting?
A: No the squish is measured “as raced” and if the measurement is too small you will be excluded.
Q: Which oils contain castor oil?
A: There are many oils on the market that are castor based ranging from Shell Sport M, Castrol R40, etc. – pure castor oil is available from pharmacies. All of these will build carbon quickly so be warned – there is no legal advantage to be had.
Q: Can I build up carbon and then get my tuner to just remove the carbon in the areas where the squish is measured?
A: This will provide you with an unfair performance advantage but it will also be illegal and result in your exclusion. In all Rotax classes modifications are forbidden and "Modified" is defined in rule 4.1 as: any change in form, content or function that represents a condition of difference from that originally designed. This is to include the addition and/or omission of parts and/or material from the engine package assembly unless specifically allowed within these rules. Rule 4.2.Internal additions: No additional material may be added except in the case of engine repairs and shall only restore the engine or components to original specifications. The use of thermal barrier coatings/ceramic coatings on or in the engine and on or in the exhaust system is prohibited.
The answer is a clear NO – so remove all the carbon (or replace the piston) and in future run only a high quality fully synthetic oil mixed at 50:1 as prescribed by the manufacturer and required in the regulations. If for any reason you do not have XPS on a practice day then please note that Motul, Shell, Castrol and many more have suitable synthetic oils so there is absolutely no reason to use castor based oil.