eyolf
Addicted to ArboristSite
Somebody asked a question or two regarding how to "stroke" a crankshaft; There a couple of ways, but none are cheap or easy.
Way back when, when rod caps came off, even on two-strokes, the rod journal could be built up with weld, and reground to diameter at the new swing point. Since the heat-treat was no longer any good, they wouldn't last. Later, some electrodeposition and metal-spraying companies could put a hard-surfaceing deposit on the rod journal that partly alleviated this trouble.
later cranks came apart in 3 or more pieces, and those were easy: press apart, rebore the crankpin holes to the new location, at a larger diameter, install bushings to reduce back to the original crankpin size, and reassemble.
EHP talks about 2 piece Stihl cranks: these can be dealt with somewhat like a one-piece set-up.
BUT
You still must deal with rod length problems and/or/ piston head/deck height situation. I have built some Honda 4-stroke strokers for modest increases in stroke the steel rod can be shortened by heating and hydraulically comprssing the rod, allowing the use of factory pistons.
Longer strokes call for a custom machined rod, or if you're really lucky, maybe something else will work: Honda 185 four stroke engine can be stroked abouit 2 mm with a CR125 rod, but will require bushings in piston.
Another option can be a custom piston with a short wrist-pin to deck height. keep in mind that you've radically changed the stroke-rod-length ration, and will also effect port timing radically.
My uncle raced snowmobiles, I helped load and unload, other gopher work, and did some wrenching. We worked a lot with an engineer at the snomobile company that sponsored us There were many many engines that responded very poorly to stroking... actuallly lost power.
My opinion, unless you have time and money to burn, it probably isn't worth it for a chain saw. 4-strokes can often use stroking to extend a narrow powerband on an over-cammed engine...I've assembled several Honda XL-350's to around 460cc, that would make a Yamaha TT500 with cam, carb and pipe look ill.
Way back when, when rod caps came off, even on two-strokes, the rod journal could be built up with weld, and reground to diameter at the new swing point. Since the heat-treat was no longer any good, they wouldn't last. Later, some electrodeposition and metal-spraying companies could put a hard-surfaceing deposit on the rod journal that partly alleviated this trouble.
later cranks came apart in 3 or more pieces, and those were easy: press apart, rebore the crankpin holes to the new location, at a larger diameter, install bushings to reduce back to the original crankpin size, and reassemble.
EHP talks about 2 piece Stihl cranks: these can be dealt with somewhat like a one-piece set-up.
BUT
You still must deal with rod length problems and/or/ piston head/deck height situation. I have built some Honda 4-stroke strokers for modest increases in stroke the steel rod can be shortened by heating and hydraulically comprssing the rod, allowing the use of factory pistons.
Longer strokes call for a custom machined rod, or if you're really lucky, maybe something else will work: Honda 185 four stroke engine can be stroked abouit 2 mm with a CR125 rod, but will require bushings in piston.
Another option can be a custom piston with a short wrist-pin to deck height. keep in mind that you've radically changed the stroke-rod-length ration, and will also effect port timing radically.
My uncle raced snowmobiles, I helped load and unload, other gopher work, and did some wrenching. We worked a lot with an engineer at the snomobile company that sponsored us There were many many engines that responded very poorly to stroking... actuallly lost power.
My opinion, unless you have time and money to burn, it probably isn't worth it for a chain saw. 4-strokes can often use stroking to extend a narrow powerband on an over-cammed engine...I've assembled several Honda XL-350's to around 460cc, that would make a Yamaha TT500 with cam, carb and pipe look ill.
