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TKART magazine Under Review | TM OK and OKJ. A new beginning
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30 January 2016
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Like most kart engines, the new TM products are two-strokers. Four-stroker engines are still a small niche in karting

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125 cc

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

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

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OK and OKJ name the new clutch-less non-shifter engines that have replaced the KF and KFJ

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CIK-FIA certified at the first OK homologation session in history, valid for 9 years starting in 2016

Some manufacturers preferred to skip the homologation for the new OK/OK-J non-shifter engines, the new open bet in karting; but a leader like TM could not miss the great opportunity

The new OK engine concept aims to bring back the simplicity and compact nature of the old direct drive 100cc engines that so many kart lovers are nostalgic about. Compared to KF and KF-Junior engines, which had been coinceived and built in line with the TAG (Touch and Go) concept, the new OK and OK-J engines do away with the battery and related cables, the electric starter and relative wiring, and the clutch.

In addition, the water pump is again external, like it used to be in the 100’s, allowing for quick intervention and replacement if necessary - another important element of simplification. All things considered, then, the OK “revolution” seems to be driving a valid goal: return to simpler engines, that don’t need a huge number of expensive high-maintenance components to perform well, able to offer greater reliability and significant cost reduction. Even so, skepticism continues to run high on several grounds, beginning with the argument that it will take some time for these new engines to acquire the tried-and-tested “status” of today’s KFs. Another hot-button issue is the elimination of the electric starter. The polemic here is that having to push-start the kart makes things tougher and requires the more petite and inexperienced drivers to rely on help from a mechanic.
Both the OK (above) and the OK-Junior are more compact compared to the KF (below), thanks to the elimination of electric starter and clutch and the repositioning of the water pump (now external)
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The OK engine includes a simplified power valve (without lateral boost adjustment). The decompression valve (visible on the OK-J below) can be easily adjusted by pullling on it before starting the engine
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Finally, doubts have also been raised regarding the removal of the clutch: if the kart ends up off track, the engine will easily come to a halt, forcing the driver to either push-start his vehicle again or withdraw (in case of a competitive race).
All caveats considered, let’s see how a successful and important company like TM has interpreted the CIK-FIA guidelines for the new non-shifter classes.
For starters, TM’s OK and OK-Junior engines look very different compared to the KF and KF-J models, precisely because there is no starter and clutch and the water pump is external. Second, they are much lighter.

In fact, one of the major goals of the new class specifications was to enhance performance by cutting the regulatory weight limit (of kart plus driver): while for the KF minimum weight was set at 158 kg, for the OK it is set at 145 kg.

Now, the major factor of weight reduction is precisely structural simplification, which accounts for a loss of 3.5 kg (5 kg if we also consider the battery and its cables). The other “lightening” factor is the elimination of the front brakes, which brings the new OK in line with the old 100 cc class. Along with stark simplification, the distinguishing feature of the new engines is the decompression valve.

The introduction of the component has been discussed far and wide, but in this sense TM was facilitated, because it had been using it for years on other products, relying on external providers. In terms of details, technical regulations call for a small and precision-finished valve to ensure a perfectly tight hold (when shut) and prevent the formation of incandescent pockets in the combustion chamber that could lead to detonation and seizure.
OK specs make the power valve (above) mandatory and like the exhaust pipe (below) require it to be of the same type for all homologated engines
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The decompression valve is the revolutionary trait of both OK and OK-J engines. Exhaust-wise, instead the OK -J remains similar to the KFJ, since it still calls for a monotype exhaust and has no power valve
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The conduit that connects the carburator to the crankshaft is made with a brand new molding cast
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