372 Pston swap/mod

[trappermike]

The full skirt piston is blocking the fuel charge in the crankcase from it's traditional paths out of the crankcase,look at one and see...
If they had used a full skirt piston like the 2100 "Jungle" type piston with long tall side windows transfer flow would be ok out of the crankcase.
The transfer ports are open for only a very short time,impeding flow at all will cost power.

 

[cus_deluxe]

At bottom dead center the transfers have already been open for close to 60 degrees? I may be missing something here as well...

 

[Leafy]

Bdc is when the piston speed is the highest which might be the highest transfer flow depending on primary compression.

 

[trappermike]

Keep in mind the whole transfer process(duration) has to happen in about a thousandth of a second at high speed. One can't afford to impede flow.

 

[wcorey]

Who cares if the lowers are partly blocked at bdc, doesn't seem like there'd be much of any positive pressure left in the crankcase to be pushing the charge up by then.
I thought that the majority of the charge is supposedly in the cylinder in the first 10 percent or whatever of the uppers opening anyway...

And then you can still always raise the cylinder walls at the lowers for additional clearance.

 

[trappermike]

Where did you hear that? I only suggest the piston swap,maybe it's not for you...run whatever piston you prefer.

 

[Yotaismygame]

From what i've read on here the 268 piston is the desired piston to use for a pop up. It's considerably lighter than the OEM piston.

This thread describes it well

https://www.arboristsite.com/commun...372xp-with-the-cylinder-skirt-removed.309504/

 

[trappermike]

Yes!

 

[Fordhook]

Arr yes. I get it now Mike. Ok this changes every thing and my job in now on hold till I get this piston. Thanks for your PM. It's not too late to change in direction. This wont change my ports heights or anything actually and I'll recover some of my lost swept compression. It may even contribute to a better swirl in the combustion chamber with the popped piston.

Lucky I come back and re read this post!

 

[trappermike]

If anyone wonders why I recommend the 670 Jons piston instead of the 268 piston(Yes I know they are the same) is so some one doesn't make a mistake and get of of the several earlier Husky pistons,which would be far from ideal. I also always insist on genuine factory rings on an aftermarket piston if porting is done.

 

[Yotaismygame]

Whats wrong with the earlier 268 piston? I thought the non windowed piston was the one to get. A oem 266xp piston would be okay also right?

 

[trappermike]

You absolutely want the windowed piston in the 372,it has large modern transfer ports but the stock full-skirt non-windowed piston completely blocks off the fuel charge in the crankcase and under the piston from getting to those excellent ports at BDC,at high rpms the transfer ports are only open for about 1 thousandths of a second so you want nothing impeding the flow of those gases. A large pocket of the fuel/air charge is trapped under the piston,a windowed piston allows it straight out into the transfer ports unimpeded.

 

[Yotaismygame]

Ill have to do some more reading now, again. I swear I've read the opposite of what you just said minus all the fuel charge info. I remember one thread by MM where he put a 272 piston in a 372 BB and the results were not good.

 

[trappermike]

The 272 piston is the ideal one I use on BB 372. However if you put it on a cyl. with an already widened ex. port you can get into trouble,the 272 intake skirt is narrower,so port widening can only be done after installing thee 272 piston and then determining how wide you can safely widen the ports(ex. and intake),also I have no idea what else did to the motor. It has never failed for me. But...use whatever piston you want.

 

[Leafy]

Mastermind doesn't like the 2 series piston in a 3 series because he builds around high case compression and the 2 series piston really drops case compression because it takes up so much less case volume.

 

[Yotaismygame]

Guess I was remembering incorrectly, is it indeed the windowed piston. Mike do you have a lathe? I'm gearing to build a 372 and i'm either going to use a 268 piston or the Dominant saw piston. Yes the Dominant saw piston is spendy, but to do the 268 piston ill need to do some measuring, source out a lathe, and then hope my measuring was right. Or I can spend a little more money and just bolt it on.

EDIT: This thread is so old I had forgot what was already talked about

 

[Yotaismygame]

The other day I tore apart a 576xp and noticed it comes with case stuffers. Sadly I put it back together before I thought about seeing if the case stuffers would fit a 372 crank. The cranks looked very similar between the two saws but I don't have a clue if they are.

 

[trappermike]

At the beginning of this thread I have a diagram and specs for machining the piston,it is a very simple job for any machinist and is not costly,just read my instructions. If the Dominant piston is a full skirt piston without windows ,use the 670 or 268 windowed piston,they are also far lighter too,forged pistons are heavier still,and require more piston clearance because they expand more. In saws other than the 372 and 385/390 design a forged piston can be an asset in a race saw.

 

[Yotaismygame]

https://dominantsaw.com/products/wiseco-dominant-372xp-50mm-high-performance-piston

Im not sure if you've seen it or not. I'd like to hear your opinion.

So if I take your measurements to a machinist approximately what squish will I end up with while using a base gasket?

 

[Yotaismygame]

Hyway makes a 268 and 272 windowed popup. Do you see any advantage to starting with this piston? They just added a dome to the piston for some extra comp