They use 2 countershafts - hence 3 stage speed reduction - from "Honda GX(?)" motor to California stamps. Using flat-belts.
If they do that, with likely huge collective and individual experience, and ability to improvise, it looks highly likely that is the best achievable non-megabucks solution.
Rich S
------------------------------------------ In the USA the inexpensive alternative to a new Honda GX(?) is this:
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Supposedly they stand up well to HP upgrades for go kart racing.
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The one on my sawmill has given very little trouble when run mostly at full power. Only the plastic air filter latch broke and I made a sheet metal replacement. The mounting and shaft dimensions are inch-sized industry standard so it was a direct replacement for the 5.5HP Tecumseh engine I had borrowed from my log splitter.
The L095 Lovejoy jaw coupler that had been adequate driving the log splitter's low inertia hydraulic pump tore up its rubber spider on the sawmill, likely from lugging at low speed against the inertia of the motorcycle wheels when the belt tension clutch didn't release, a new pulley needed to wear in smoother. A larger L100 coupler has held up well. The horsepower and RPM to shaft torque calculation needed to be adjusted for the uneven power flow of the larger single cylinder gasoline engine. Shaft diameters also need to be large enough to survive the peak torque.
This is a power level where surplus and hardware store components for small electric motor drives become inadequate. I've broken a cord in a vee belt driving the tractor's added hydraulic pump, under load the belt's remaining cords rolled it a quarter turn to the outside and it slipped on the pulleys. I don't know enough of conditions to estimate the stress when it failed.
My machine tools allowed me to adapt replacement parts when my original guesses based on whatever was available were wrong. A common problem has been pulleys held by setscrews slipping and needing either a flat filed on the shaft or a milled keyway added. With inexpensive found parts the shaft and pulley key sizes may not match and require a key sized for the wider slot and milled to fit the narrower, or an inch/metric adapter.
These cut keyways in pulleys but you need a slotted plug to guide them which you may have to make if non-standard.
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Shaft key slots can be cut on a lathe with a milling attachment. I'm definitely not skilled enough to cut one accurately with a chisel. The alternative of pulleys plus tapered hub adapters is quite expensive.