trappermike
ArboristSite Guru
This easy piston swap with mods will really put some real fire into a 372 Husky,and cures what I consider to be a big design blunder in this saw. When I first pulled the cylinder off a new 372 to look inside,I was really impressed to see 4 large modern transfer ports,the first in a chainsaw,I thought "Wow this saw should really be able to make some power."
Then when I looked at the piston I couldn't believe what I was seeing,with the piston still on the rod at BDC,the full-skirt piston they use almost completely blocks any of the fuel air charge under the piston and in the crankcase from getting to those excellent transfer ports,what a design blunder! Look at one closely for yourself. So in 1998 I came up with a great piston swap the cures the problem completely,the new piston is about 1mm higher at the crown,so I machine the squish area on top of the piston leaving a high-compression dome which is even better,also this piston assembly is lighter than the stocker. The new piston as you will see installed easily allows all the fuel charge a direct unrestricted path to the transfer ports. It's made to fit with the stock cylinder gasket,so you can still juggle the squish gap if you wish. Now with the compression dome this piston is ideally suited for a ported motor with a raised exhaust port,you can run it with a stock ex. port but compression will be high. The skirt is slightly narrower than the stocker so you can't widen the intake port quite as far,no big deal.
For the 372 you need a Jonsered 670 piston,for a bigbore 372 you need to use a 272 Husky piston with the same machining specs.
Be sure when machining that you polish the piston crown after the machining is done,use some #400,and #600 grit emery cloth and finish up with any polishing compound on a cloth while the piston is still spinning in the lathe,it only takes a minute to finish the job nicely.
This mod will make great power even in a stock motor,but in a modified motor it will add a lot of power,and works even better with porting. I've built a lot of these over the past 2 decades,no failures.
Below is the diagram for machining:
Then when I looked at the piston I couldn't believe what I was seeing,with the piston still on the rod at BDC,the full-skirt piston they use almost completely blocks any of the fuel air charge under the piston and in the crankcase from getting to those excellent transfer ports,what a design blunder! Look at one closely for yourself. So in 1998 I came up with a great piston swap the cures the problem completely,the new piston is about 1mm higher at the crown,so I machine the squish area on top of the piston leaving a high-compression dome which is even better,also this piston assembly is lighter than the stocker. The new piston as you will see installed easily allows all the fuel charge a direct unrestricted path to the transfer ports. It's made to fit with the stock cylinder gasket,so you can still juggle the squish gap if you wish. Now with the compression dome this piston is ideally suited for a ported motor with a raised exhaust port,you can run it with a stock ex. port but compression will be high. The skirt is slightly narrower than the stocker so you can't widen the intake port quite as far,no big deal.
For the 372 you need a Jonsered 670 piston,for a bigbore 372 you need to use a 272 Husky piston with the same machining specs.
Be sure when machining that you polish the piston crown after the machining is done,use some #400,and #600 grit emery cloth and finish up with any polishing compound on a cloth while the piston is still spinning in the lathe,it only takes a minute to finish the job nicely.
This mod will make great power even in a stock motor,but in a modified motor it will add a lot of power,and works even better with porting. I've built a lot of these over the past 2 decades,no failures.
Below is the diagram for machining: