Full circle crank.

[rolloman]

Is a full circle crank always a good thing?

 

[Jacob J.]

It depends on what your target performance characteristics are. A full circle crank in theory "stuffs" the mixing area of the crankcase, therefore forcing more fuel/air mixture up through the transfers into the combustion chamber. However, it's been my experience that I get better bottom end out of saws that previously had the full-circle crank when I install a standard crank in them. The Husky 357 is one example. I run a 28" bar on mine for thinning in softwoods and it does better with the non full-circle crank.

Most hotsaws use a full circle crank because the target performance is "high and fast." High RPM and fast chain speed.

 

[trappermike]

Full circle cranks have been used in factory built 2-strokes for many decades,notably on Japanese motorcycles and snowmobiles. Most had 2 large holes punched through the flywheels(Counterweights) on each side of the con rod big end,to lighten that side of the crank to make up for the extra weight of the con-rod and piston assembly. It used to be a practice to "Stuff" these balancing holes with cork and epoxy and similar things to increase the crankcase compression ratio. The Kawasaki Mach 111 motorcycle was a rare example where these balancing holes were plugged by the factory with aluminum plugs.Some skilled hot saw builders have built custom(Steel) full circle cranks for competition saws.They worked well,but were much heavier than stock cranks.

A few (ok,many) years ago I wanted to improve on this,so I designed and had built full circle cranks(With 2mm longer stroke) from titanium,with the balancing holes plugged with magnesium.They featured polished counterweights and knife-edged con-rod.
These cranks were actually a few grams lighter than a stock crank. I built and sold 3 of these,and it was a lot of work!