Can someone explain just how you increase the stroke length? (Stroker--? )

cvarcher

Member
Say you have an Sl70 72cc engine and you buy that 88cc larger bore kit. That doesnt change the stroke at all but just increases the bore and combustion volume-correct? For stroking dont you have to buy a slightly longer piston rod and have a deeper head? Years ago I seem to remember that when you mill the head that brings it closer down to the piston head and yields more compression . And porting was done to the valves somewhere (I forgot).
 

darrel gunderson

Active Member
Trail bikes has a 51mm stroker crank for the 3 or 4 speed type flywheels. Also the matching piston for either the stock head or their race head. Makes it a 108cc engine by changing to this setup. You have, with this being an SL70 a 4 speed mechanical advancing type flywheel. HTH
 

Spizz

Active Member
Stroke is the location of the lower connecting rod pin relative to the crank centerline. An increase in stroke would mean that pin is moved further away from the centerline.
 

Adam-NLV

Well-Known Member
So a stroker crank is longer thus the pin moved further away from the crank's center line. That would increase the compression of the 88cc kit to a 108cc.
I have the 88 kit and was thinking if you have to split cases anyway, might as well add the longer crank for $169 from Tb & bump it up.
 

cvarcher

Member
Thankyou Kirrbby. I read all of that .So again that 88cc kit does not include anything to lengthen the stroke ,but simply enlarges displacement capacity by bore size and a larger carburetor,and valves. I didnt see anything on stroker parts in TB.
 

kirrbby

Well-Known Member
Your sl70? Is the same basic motor as a CT70 H. The H is important because the crank is different than the 3 speed version. The H model is a 4 speed like your SL70. So search it that way, pretty sure tb still has a stroker crank for you. DRATV has them too.
 

4-gear

Member
A big bore alone doesn't give you same feel as when you add a stroker crank. I have never had one of the modern offerings, but like how Powroll does it. Moving the pin farther out on the weights and shrinking the rod. So cam chains and stock gaskets work just fine as opposed to spacer plates. The extra volume comes from the piston going lower in the cylinder. I have a few of their cranks and bore kits now and they rip, and have for many years.
 
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