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as well,” says Berli. “The cylinders are cast by a supplier of ours, and we machine them to our required tolerances before being sent to another partner for the Nikasil coating process.” The use of bolt-on cylinder heads (rather than dead-end cylinders) adds a small amount of weight but makes the machining and honing processes slightly quicker and easier. It also gives flexibility to accommodate changes to the cylinder heads, for example in the cooling fins, combustion chamber or compression ratio. “We discussed the pros and cons of dead-end versus open-deck; in the end though we just had too many different customer requests not to opt for the more flexible of the two approaches,” Kehe recalls. “With an open-deck engine, we can design for double ignition, increased cooling fin requirements, and other changes.” Two bearings hold the crankshaft to the crankcase: a ball roller bearing at the front near the PTO, and a needle roller bearing at the rear (next to the generator). As Kehe explains, “The rotational mass and force of the propeller could be a real problem if they’re allowed to be transferred to the inside of the crankcase, so we needed a strong bearing on the front end to withstand and absorb the axial and radial forces from the prop. “A ball roller bearing works better than a needle bearing for that. A needle bearing is better on the back end, because that whole area needs to run more freely than the front and has a greater length that needs to be covered.” The con rods are fabricated as single (non-split) components from hardened high-strength steel billet and coated entirely in copper (except inside the bearing inserts). The crankshaft is therefore manufactured as three parts – a forward crank and pin, a rear crank and pin, and a crank web for joining the two. “It’s an established design approach that makes sense for many reasons, one of the biggest for us being to avoid the split bearings that come with using split con rods,” Kehe says. “This way we can use non-split needle bearings in the con rods’ big and small ends. “Split bearings can cause real mechanical trouble down the line, and split rods need additional nuts and bolts to hold them together. That increases the rotating mass, which complicates the engine management because you have to compensate for the extra offset and vibration with every crank rotation and piston stroke.” Considerable design work went into the piston. As Kehe notes, it is potentially the most vulnerable part of any two-stroke as it is subject to the heat of combustion with every turn of the crankshaft, with no time to cool down and relieve the high- temperature stresses on it. “Four-strokes have an easier time of it: they can be designed to shoot oil Suter Industries TOA 288 | Dossier