making stroker engines

[rmihalek]

I was wondering if the modified saw experts could describe some of the things that are required to make a stroker engine. How is the stroke increased? Is a new crank required? What other modifications are done on the stroker saws with things like ports, carb, timing, etc.?

Thanks!

 

[ehp]

On a 3120 stroker, the normal stroke is 42 mm, you take out that crank and send it to who ever is going to do your crank work , they take your pin thay connects to your con rod out and move it 2 mm out , so now you have a 46 mm stroke, 2mm more on top and 2 mm lower when the crank is at bottom dead centre, now if you guys are going to do this I would highly tell you guys to change the rod to a 084 stihl rod it is alot stronger than the stock 3120 rod

porting is pretty much the same as any big cc motor , it is done in degrees not the length of your stroke,


yes you need to change the coil to get rid of your rpm limiter , and you will need to retime your flywheel , just find the saw you are taking the coil out of and time it, then but the 3120 flywheel in the same place

one thing you need to do is open up the crankcase for the stroked crank because now your rod is out farther and it is going to hit the crankcase so you have to grind some off the case so the crank will not hit it,

a stroked crank 3120 saw is only good for racing because you will need to put a head on it , sure you could raise your jug the 2 mm so the piston does not hit the top of your jug but remember your piston also goes 2mm lower than stock so if you raise the jug 2mm ,that would mean the piston on the botton of the stroke would have 4 mm on the piston hanging out of the jug with no support and it will eat the intake skirt off of your piston

 

[rmihalek]

Thanks Ed. That's great info!

 

[rahtreelimbs]

Originally posted by rmihalek
Thanks Ed. That's great info!

Big, fat ditto on that one!!!

 

[sedanman]

When we stroke a v-8 we can't move the head away from the centerline of the crank so we must use shorter rods. This way all of the extra stroke takes place at the bottom of the bore. The longer crank can interfere with the camshaft and mandate the use of a small base circle cam. Another thing to remember is that as the engine gets bigger the compression ratio will go up if the combustion chamber volume is not increased.

 

[ehp]

yes that is true , but on a chainsaw the rod is so short that when you stroke a saw most times you have to cut some off the bottom of your piston skirt or if you get lucky you can find a rod with the same pin sizes just a little longer like the 084 rod it is 75.25 mm long the 3120 rod is 74 mm or like when i put a 372 rod on the 272 crank 272 is 62mm long the 372 is 65 mm long , it helps and the longer rod does help you make more torque

 

[rmihalek]

You lost me there, sedanman. Why would you want to move the head away from the crank? I'd think with a stroker engine you'd just have to accept the fact that you'll be running high octane fuel due to the increased compression. Then again, there's the issue of valve clearance that you have to deal with in a 4 stroke that doesn't apply to a 2 stroke. Might require machining out valve divots in the piston crown...

 

[sedanman]

If you have a 4" stroke the throw of the crank is only 2" long. The piston moves 2" up from the centerlline of the crank and down 2" for a total travel of 4". The deck height or distance from the centerline of the crank to the where the cylinder head mounts is not easily changed. If the stroke is increased by a 1/4" then the piston moves an additional 1/8" up and another 1/8'' down. You will have to make all the extra travel take place below the top of the engine block. If you dont use a shorter connecting rod the piston will poke out of the top of the block at t.d.c. and you won't be able to get the heads on. On a chainsaw you CAN change the deck height more easily by milling the top of the jug off then building a custom cylinder head with releif for the piston to fit into.

 

[ehp]

on a saw motor you have tons of room for your head to fit in to but what you have to be carefull of is running out of crome for the ring to run on at the top of your jug