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Tkart magazine

Under Review | S2: the evolution of TM Racing’s OKJ and OK engines

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12 April 2020


Like most kart engines on the market. 4-stroke engines are still a small niche

125 cc

125 cc has been the standard for some time now, while in the past it used to be 100 cc


Push-mode made easy thanks to a decompression valve on the cylinder head that helps the piston


The following are the “families” of kart engines: gears and single speed. The S2s are single speed clutchless engines

CIK-FIA 2019

The CIK-FIA one for OK and OKJ in force since 2019


TM Racing has updated its S-Junior and S-Senior engines for the OKJ and OK classes, for which 2019 marked the beginning of the second cycle of homologations. The arrival of the S2 model brings with it less noticeable changes compared to the previous version, which was introduced when the KF class and the related electric starter, the free exhaust valve and the clutch departed the scene. Therefore, on its new models, what the Italian make has done is introduce improvements to its engines that have already produced excellent results.

Indeed, the first generation of TM Racing engines to come out with the return to direct drive can boast titles such as the 2017 FIA ​​Karting World Championship, won by Dexter Patterson, and the 2018 WSK Super Master Series, by Gabriele Minì: an impressive curriculum vitae that removes any doubt on the performance of the engines. Perhaps that is also the reason why the thermal management part of the new models approved for the 2019 - 2021 period has remained almost unchanged from the two predecessors. From the outside, the cylinder of the S2 Junior and Senior is indistinguishable from the S, both as regards its structure and the cooling channels. The same goes for the cylinder head, which houses the decompression valve to facilitate push starting. The only update involves the ports, always the subject of continuous research, which have been optimised to facilitate filling the combustion chamber.

The more substantial changes can be found in the crankcase. First of all, we need to mention air intake: the duct has been raised by 12 millimetres and now has a slightly different internal profile, mostly due to its new position. The difference between the Junior and Senior, in this case, lies in the angle of the reed valve pack with respect to the plane of the cylinder, respectively equal to 27° and 22°: the 5 degrees less fit better with an engine, like the one for the OK category, designed for 16000 rpm, compared to the OKJ’s 14000 rpm. Obviously, this was followed by an analysis concerning the surfaces of the lateral port inlets, machined using numerical control machines to ensure the utmost precision.

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