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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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    Your squish should end up close in the low 20's,but because of different engine production variants it will differ but it's usually pretty close to ideal,if you're fanatical about final squish gap you can simply alter the base gasket thickness. The Dominant piston is the same style as stock(I just saw),so I'm not interested in it,but the Hyway pistons sound ok IF they are OEM quality,I'm wondering if they are just stock pistons with the mod done,if so they would be great,maybe save you a few bucks on machining.What is the dome height,I specify .040"(1mm) on mine. Although some may think it petty,I like my piston domes polished,they run cooler and of course resist carbon build up,if it's not done you can do it yourself at home with a little elbow grease.
    I would not recommend modifying a Meteor or or aftermarket replacement piston,they use an inferior aluminum alloy that does not come close to OEM in hotstrength or % of silicon,nor will I use aftermarket rings in a ported engine.
     
  2. Yotaismygame

    Yotaismygame saw me up

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    I have a 372xp with a Hyway BB with their 372 popup piston in it now. It's just a stock piston with a dome added to it. It's been reliable so far. What I'm wondering about their 268 popup is with the added dome on it already I would assume all the machining would still need to be done but the end result would net a higher popup. Do you think that's a positive thing or not necessary? I'm definitely going to get a 268 piston and have it machined with your measurements but which piston should I use? Hyway popup, Meteor, or try and find a OEM piston?
     
  3. old guy

    old guy Addicted to ArboristSite

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    Read carefully when you buy those Hyway pistons, I ordered the 268 piston with a popup thinking it was a 268 cut down to work in a 372, what came 2 weeks later straight from china in a Hyway box was a 268 piston with a popup added.
    Very nice piston, casting inside looks better the Meteor 268 I have, also it is lighter than the cut down unwindowed meteor piston, but I can't use it unless I have the popup cut off and a new one machined.
     
  4. Yotaismygame

    Yotaismygame saw me up

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    I never had any confusion with there products. They also make a 372 popup...

    Imagine if they did make 268 piston cut down to work in a 372. That would be great.
     
  5. trappermike

    trappermike ArboristSite Guru

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    Without seeing these popup pistons and knowing who makes them etc. I can't really comment or recommend them. If it's for a saw of your own that you will cherish and own a long time I would only use OEM piston and perform the mod,it may cost more but I know I've got the best quality and it's done right,and it won't fail,I won't use cheap or inferior parts in a performance engine,that's begging for trouble. You won't have saved any money if the piston fails and wrecks the cylinder too. "Ya pays yer money and ya takes yer chances"
    I've used aftermarket pistons in stock engines in many applications but not for performance engines, and they were fine.
    Also you don't want to take a domed piston and machine it more,piston crown can get too thin,ring land can break away,melt, etc.
    Each to his own choice,for yourself you can choose quality or what your wallet likes more. Because I was building engines for other people I couldn't risk an embarrassing failure,or paying for all the extra damage a failure may incur by experimenting with cheaper parts.
     
  6. Yotaismygame

    Yotaismygame saw me up

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    All of that is understood. But you didn't answer my question. If the hyway 268 piston is just a stock piston with a added popup then it should be able to be machined down for a 372 netting a bigger popup. Do you think thats beneficial or not?

    Also I'm not seeing anywhere I can buy a oem 268 piston for a reasonable price.
     
  7. trappermike

    trappermike ArboristSite Guru

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    I don't know if the Hyway piston was cast with the extra dome and the piston crown is still as thick as stock or if it is not. If it is already domed and made to fit a 268 then I'm guessing you would still have to machine the the top down another 1mm to make it fit in a 372 with stock cyl. gasket etc. This would result in a very high dome (080",2mm?)and compression would be FAR higher than I would ever use,even if you raised the ex. port a lot. The 1mm dome from my mod already makes the compression too high for my liking with a stock ex. port,that's why I say it's for modified(raised) ex. ports. A 2mm dome is far too much,compression will be way too high even if you mod the ex. port a lot. I've never made a dome that high even in a race motor with maximum ex. duration,it doesn't take much extra dome to increase compression a lot.
    A 2mm dome would be far too much for any motor with a normal squish gap,except maybe a diesel engine...
     
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  8. Yotaismygame

    Yotaismygame saw me up

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    Thank you. I appreciate the input. This is at least the third time Ive looked into doing this and I'm seeing both the windowed and the non windowed piston being used. I already have a few oem 266 non windowed pistons in my possession and other than using a meteor windowed piston I dont see any other options unless I want to spend over $100 for a new oem 268 piston.

    What if I cut my own windows in a 266 piston? This saw is just for myself and something to tinker with when I get bored.
     
  9. trappermike

    trappermike ArboristSite Guru

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    Personally I wouldn't care if the OEM piston is twice as much,after all the money and long hard hours you put into building the engine I want it to be bulletproof and last,I wouln't risk my project blowing up because I took a shortcut and saved a few bucks somewhere,it's not the same as throwing a cheap piston in the 'tiller rusting out in the garden. I'm as cheap as anyone else (Trust me on that!)and try to save a buck anywhere I can. But if you can find a good piston and ring that will do the job that's great,I just personally don't have a lot of experience with today's aftermarket saw pistons to recommend any.
     
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  10. trappermike

    trappermike ArboristSite Guru

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    If the Hyway domed 268 piston has as much metal in the piston crown as stock I suppose you could machine the squish an extra 1mm to fit the 372,THEN machine the dome down too, back to 1mm,that could work,in theory the squish and compression should be ok. But you can gamble on the quality,it may prove fine.
    There's the answer you are looking for I think.
     
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  11. Yotaismygame

    Yotaismygame saw me up

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    What kind of compression numbers do you generally get with the 268 popup?
     
  12. trappermike

    trappermike ArboristSite Guru

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    With the mod I do compression with a stock ex. port is around 200 psi(Far too much for my liking),with the ex. port raised 2mm the comp. is about 170,which is perfect for good midrange power and top end power from a 72cc saw with more ex. duration.
     
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  13. trappermike

    trappermike ArboristSite Guru

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    You can experiment cutting windows in a non-windowed piston,I've done it before and you will be rewarded with more power,it won't be quite as good as a factory windowed piston but it will work,only you must take care to leave the "webs" connecting the skirts to the wrist pin boss under the windows a little beefier than a stock windowed piston or one of the skirts can break off,and well you know the rest...
    What about the 670,268,272 MWP pistons I see advertised on Baily's website,the cost is nice,are they no good quality? I've never seen one for real. The pictures look like stock windowed pistons.
     
  14. Yotaismygame

    Yotaismygame saw me up

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    I haven’t seen a nwp piston either. But based off all the other things they sell it seems like they just rebrand parts. Same goes for hyway, except I do believe hway makes there own top ends, or at least does the finish work on them. Hyway is the only AM company making pop up pistons. General consensus for best AM pistons is meteor which are Italian made.
     
  15. Yotaismygame

    Yotaismygame saw me up

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    Just checked my 266 projects and both of them have windowed pistons. But I’m probably going to go with a meteor.
     
  16. old guy

    old guy Addicted to ArboristSite

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    I have one unwindowed 268 meteor piston that the windows are plainly seen on the inside, they just don't come all the way thru, would be easy to open em up.
     
  17. trappermike

    trappermike ArboristSite Guru

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    If you already have free piston to work with try it,just leave the webs a little beefier for extra strength,AM piston aluminum alloy will not be quite as strong as factory.
     
  18. mdavlee

    mdavlee Tree Freak

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    I didn’t care for the windowed pistons. I felt like I lost torque. You could push on the saw more before the windows. I have used a few of the Dominant ones. They make good compression with jus a gasket delete. With a tiny bit off the base and chamber they’ll push 190-200
     
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  19. mixxer

    mixxer Lord of Combustion

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    Very much agree with your desired compression range for a strong work saw without special octane needs above pump premium ...

    Question about the windowed pistons ...

    I haven't had my 372 down yet so I haven't seen...

    Typically I have known 2 stroke pistons to have skirts cut away on the lateral sides below the wrist pin axis... Such that when the piston is at BDC, the lateral cut aways line up with the cylinder base beginnings of the transfer ports..

    Are you saying that when the piston is at BDC on the 372 , that the lateral area between the wrist pin and dome is far enough down in the cylinder that it would block the transfer entrance, requiring windows above the wrist pin to unshroud this area??
     
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  20. trappermike

    trappermike ArboristSite Guru

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    You will see and understand as soon as you take your 372 apart.
    I like to run premium in all 2-strokes,even if comp. ratio doesn't require it,piston runs cooler and today's gas blends are very questionable. It's not a lot of extra cost on a couple gallons of gas for your saw,not like filling your vehicle,that is a huge extra cost if you don't need it. I like to pamper my saws. Here in Canada we don't have lotsa methanol in the fuel either,which can cause trouble in a stock 2-stroke, methanol will NOT mix with mineral 2-stroke oil(try it,pour some methanol and mineral mix oil in a jar and shake it up,it will not mix) and may cause mysterious seizures,synthetic oil is safer because it will mix with methanol in gas.
     
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