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372 Pston swap/mod

Discussion in 'Saw Building 101' started by trappermike, Jul 20, 2018.

  1. trappermike

    trappermike ArboristSite Guru

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    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: 372 Piston Mod.jpg
     
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  2. trappermike

    trappermike ArboristSite Guru

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    I can understand Husky wanting to use a full-skirt piston in the 372 for better longevity but why did they put no windows in it? The optional 2100 "Jungle" type piston was also a full skirt design too,but it had 2 long windows on each side,I don't know why they didn't use something like that..
    Using the new type machined piston I would recommend a simple ex. port mod- Raise the port 2mm,and widen 1mm each side,then put a new and generous chamfer on the port. Of course you can modify the ex. port any way you want,but this simple mod will help take advantage of the new piston,if you're not sure what to do.
    Also "windowed" pistons always have the fuel/air charge flowing thru them which cools the piston substantially,polishing the piston crown also reduces the heat in the piston.
    You can install the piston without machining the top,but will have to use a couple extra cyl. gaskets and check the squish gap.
     
  3. Leafy

    Leafy ArboristSite Operative

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    I'm beginning to think that manufactures block the transfer ports at bdc because no new air flows into them at bdc. All the air going into the combustion chamber was already in the ports by the time the motor got to that part of the stroke. That's why you see on all the 2 stroke software making the transfer ports longer makes more power.
     
  4. trappermike

    trappermike ArboristSite Guru

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    I don't think Husky intentionally blocked the fuel charge in the crankcase and under the piston from getting directly to the transfer ports,I've never seen another 2-stroke in 45 years that did that,I think maybe there was a disconnect between the piston and cylinder designers.
    Another thing you can cheaply do when getting the piston machined is to have the wrist pin taper-bored at the same time,both mods are simple and inexpensive.
    The next thing I would do without a second thought is to take the carb and entire intake from a 385 or 390,and finally get a better size carb that these motors really respond well to,they are ideal for this saw. I would look for a used assembly,but if I couldn't find used I would still spend the money for new parts and end up with what I really want. Now you can really pour a lot of fuel-air charge into those great transfer ports.
    After you have installed the piston I would re-install the cyl.,then with a sharp narrow scribe I would put the piston at TDC and then scribe the outline of the ex. port on the piston,then go to BDC and scribe the outline of the intake port on the piston. Then you get a better idea of how much you may widen,raise or lower the ports.
    I've seen lots of photos here of people "Knife-edging" the divider in the 372's transfer ports,they're usually not as nice as I'd like to see,first trim away the excess metal with your dremel,then finish the job with a small round file and triangle file,then when you are done you have a pretty nice,smooth,sharp, knife edge job.
    I would not hog out the transfer passages bigger...leave them alone ,they are good as is and you need some velocity thru them to deliver the fuel charge in thousandths of a second.
     
  5. roy wilson

    roy wilson ArboristSite Lurker

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    i took off the 385xp carb to see the difference and its a bit bigger bodywise the but the bore is the same siize as the 372 one , but could not get the caliper in to measure the venturi , so what is the size difference on them ?nd it is the walbro wj 118 -1 carb 2003 model year , so maybe a later one is bigger doesnt seem to be much really so chose not to fit it as it does get used on bigger wood sometimes , but did use the windowed 272xp 52 mm piston and machined 40 thou off it to 8mm across the crown to help the transfer timing a bit and raised the xhaust port to a centre hump as it s a bit flat and widened it 40thou each side and the intake the same and lowered it a bit and knife edged the dividers and removed the material from side transfers for less strangulation to be level with the base for better flow , compression is brutal lol had to put in a decomp to help with startlol it runs well and set it rich at 12500 to bed it in safely but not cut wood yet
     
  6. Leafy

    Leafy ArboristSite Operative

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    Why'd you arch the exhaust? Flatter is better for power and the factory seemed to think it wouldn't wear out the ring too fast being that flat. Every oem jug I have that was made in the last 20 so years has a flat exhaust roof. Rings are better now than they were in the 80s.
     
  7. roy wilson

    roy wilson ArboristSite Lurker

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    it was very slight , about 20thou was a bit excessive mentioning a hump bit of an error , i didnt mention it was a AM kit and with a light in the plug hole it actually had a dip in it , so when the piston was lowering , the light appeared at the sides , not in the middle , so raised the middle a tad higher than the side, almost nothing , dont want to have a bad blow down timing, installed a caber 52mmx 1.5 mm single ring
     
  8. old guy

    old guy Addicted to ArboristSite

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    What is taper boring the wrist pin & why?
     
  9. roy wilson

    roy wilson ArboristSite Lurker

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    probably to save weight and strain on connecting rod and bearings , , any less reciprocating weight less weight will help acceleration and load on componnts , my idea i suppose
     
  10. wcorey

    wcorey ArboristSite Guru

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    Obviously the 2100 needs a windowed piston because it has side fed transfers.
    With the 372 having bottom fed transfers, wouldn't you need to raise the cylinder wall at the lowers to take advantage of the piston windows?

    That said, seems like either way/if anything, a 7910 type piston design with the really dished in sides would be the way to go there.
     
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  11. trappermike

    trappermike ArboristSite Guru

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    I normally don't use taper-bored wrist pins on work saw sbut you can,it's so easy to have the machinist do it when doing the piston,it's a quick simple mod for a machinist. I've seem some factory saws using them in recent times.
    The windowed piston gives the most open and straight shot to the transfers,plus keeps the piston cooler.
     
  12. wcorey

    wcorey ArboristSite Guru

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    Yes on a side fed transfer but still I don't understand how this would work on a bottom fed transfer like the 372 where the cylinder wall goes most of the way to the bottom of the piston, blocking the windows.
     
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  13. trappermike

    trappermike ArboristSite Guru

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    You have to look at both pistons installed on a 372. The fuel/air charge in the crankcase and under the piston need the most direct route to the transfer ports possible.
     
  14. trappermike

    trappermike ArboristSite Guru

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    The 385/390 carbs are definitely bigger and flow more air,using an inside caliper I measure the venturi of saw carbs, the WJ can flow more air than the HS,but either one is a good step above the stocker.
     
  15. CJ Brown

    CJ Brown "I learned this from a hockey card..."

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    Here is a 372xp cylinder. The transfers are not fed via any flow around the piston - they are fed from the bottom/sides of the cranckcase. A windowed piston might be effective for cooling, but won't feed any more charge to the transfers as far as I can see.

    372xp Cylinder.JPG
     
  16. Leafy

    Leafy ArboristSite Operative

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    Look at the crank case. The side feeds for the transfers are in the case and then look at the piston at bdc compared to those.
     
  17. CJ Brown

    CJ Brown "I learned this from a hockey card..."

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    I have looked at that. The side skirts on the 372 piston are much higher than the front/rear skirts - are you saying the side skirts actually dip below the bottom of the cylinder at bdc?
     
  18. CJ Brown

    CJ Brown "I learned this from a hockey card..."

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    Pic for reference.
    372xp Piton Cylinder.jpg
     
  19. trappermike

    trappermike ArboristSite Guru

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    You are looking in the wrong place, install the piston on the rod,then push it down to the bottom of the stroke with the cyl. off. Now visualize for a moment that most of the fresh fuel/air charge drawn in is now mostly contained under the piston and in the crankcase. How is all that trapped charge gonna get to the transfer ports in a fraction of a second,there's no windows in the side of the piston to release the charge under it,and the full piston skirt sits down close to the crankcase and crank,very little mixture can easily exit the crankcase here too.
    The piston is blocking most fuel transfer at BDC. It goes against every other 2-stroke I've ever seen.

    Push the piston to BDC on a 066 or 395(Or any other saw) and you will see a glaring difference,you can see straight into the crankcase thru the piston and there is no restriction for flow.
     
  20. CJ Brown

    CJ Brown "I learned this from a hockey card..."

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    Are you saying the piston side skirts are sealing against the crankshaft counterweights, effectively sealing off both transfer tunnels?
     

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