Mastermind
Work Saw Specialist
I've seen the same increase in usable RPM when going from a windowed, to a non-windowed piston in a 372 and a 7900.
hard time finding the right piston husky55
- Thread starter samhop
- Start date
Help Support Arborist Forum:
stihlbro
ArboristSite Operative
Y'all getting all technical and stuff on me.
.....there maybe a balance between what was removed/added. The crankcase is tunneled to feed from the lower. The original cylinder wall shrouded the crankcase tunnel, I just matched the cylinder to crankcase. The full skirted piston is certainly utilizing the cc tunnel feeding the lower transfers now. So really, a lot has changed. I'm sticking to a 51 piston can be used as a replacement if original Oem can't be found. 
Hmmm...thereby "stuffing" the entire underside of the piston. I'll have to think on that one.
Window or not, I'll definitely be going back in mine to unshroud the transfer ducts. I really appreciate the tips! I've enjoyed this conversation.
Makes me want to buy another 51 piston and test before and after the piston swap.
Mastermind
Work Saw Specialist
Exactly.......but the lighter windowed piston would probably make for a better cookie cutter. Reasoning on that, the piston would be lighter, and the lesser case compression would make for a freer revving engine. Maybe.
These sort of discussions are very enjoyable. Hey Joey, thanks for showing up my friend.
stihlbro
ArboristSite Operative
I've actually lost some sleepover this whole windowed vs. non windowed piston in a 45mm cylinder. There are some differences in the cylinders from the factory. Huskys seem to leave the bottom of the cylinder alone. Evidently some of the jonsereds can be either way. Using the full skirt piston reminds me of the 365/372. There ain't but one way to sort all this out and that is to modify both, do everything the same.........decisions.......decisions?????
I know this, the 44mm one I done, it was based on a work saw, very little porting. Didn't take much and the carb would not supply enough fuel on the top end.
I know this, the 44mm one I done, it was based on a work saw, very little porting. Didn't take much and the carb would not supply enough fuel on the top end.
pops21
Addicted to ArboristSite
Was the stock saw in your video a closed port cylinder and CP piston?
pops21
Addicted to ArboristSite
Ya know the dolmar 5105 also went to a non windowed piston too. The 5100 piston is windowed. I'm really digging this thread. My 55 is the air injection closed port saw too. I haven't used it in over 2 years. I might just get it back out this weekend.
stihlbro
ArboristSite Operative
Yes it was. Two identical saws. Same bars, chain, mufflers, fuel.
Mastermind
Work Saw Specialist
I've been talking to a well known AM parts supplier about having the CP 55 piston repopped. I'm heading to the top of the page to start a new thread......
thanks for bringing me up to rpm
my 2 cents
i dont buy the idea that opining a lower transfer to the base would effectively reduce base pressure, because the increased volume is filed or surpassed by the non-windowd piston.
stock windowed piston = 51g non windowed piston = 60g so whats that 3.33cc difference.
and the inverted cup idea is a fail jest because of that hole math thing that i cant do, and the way gasses work, you know volume - pressure - energy. gas pushes a wave but everything pushing (or in the cup) is compressed. think explosives, mock+speeds or plasma temps before gas starts doing weird stuff .
but WTFDIK
my 2 cents
i dont buy the idea that opining a lower transfer to the base would effectively reduce base pressure, because the increased volume is filed or surpassed by the non-windowd piston.
stock windowed piston = 51g non windowed piston = 60g so whats that 3.33cc difference.
and the inverted cup idea is a fail jest because of that hole math thing that i cant do, and the way gasses work, you know volume - pressure - energy. gas pushes a wave but everything pushing (or in the cup) is compressed. think explosives, mock+speeds or plasma temps before gas starts doing weird stuff .
but WTFDIK
not running yet but got the cylinder ported.
i cut out 5 grams out of the lower so thats about 1.85cc of volume i added to the base, but the piston took up an extra 3.3cc so the base volume is smaller by 1.48cc "proxmitly"
jest tried to copy what Stihlbro did thanks foe sharin the info
than i did a light muff mod.
still need to get some fuel line / filter and and an bar oil tank cap b4 i can runer.
base sealint will be dry in the morning and i will presser test it when im weihting for the ods and ends i need to finnish this freebe up.
thanks for the help!
i cut out 5 grams out of the lower so thats about 1.85cc of volume i added to the base, but the piston took up an extra 3.3cc so the base volume is smaller by 1.48cc "proxmitly"
jest tried to copy what Stihlbro did thanks foe sharin the info
than i did a light muff mod.
still need to get some fuel line / filter and and an bar oil tank cap b4 i can runer.
base sealint will be dry in the morning and i will presser test it when im weihting for the ods and ends i need to finnish this freebe up.
thanks for the help!
I went back in mine this evening and case matched the cylinder base. WOT RPMs didn't change at all. Should hold better RPMs in the wood. Throttle response is still very good. I also posted pics of my windowed piston.
http://www.arboristsite.com/communi...on-rebuild-porting.249660/page-4#post-4644058
http://www.arboristsite.com/communi...on-rebuild-porting.249660/page-4#post-4644058
Last edited:
Must be a reason some of the 590 topends were from the factory like this. 
From what I have heard alittle stronger when other cylinders in the jonsered line up was done like this too.
From what I have heard alittle stronger when other cylinders in the jonsered line up was done like this too.
nice work Brad.
am i right in thinking that's no change in rpm running windowed piston.
any thoughts on the effects of windowed piston in a standard closed port cylinder? i ask because i windowed a piston in a 372xp BB kit i did last year and im wondering if it was a good idea or not - any thoughts
Last edited:
To show folks they dont know what they are talking about on making a closed port into a open port going backwards.
My race MVP 166 bridge was removed to make it open port and my MVP copied cylinder on my partner 5000 had bridge removed to make it open port too. They ran pretty dang good.
My race MVP 166 bridge was removed to make it open port and my MVP copied cylinder on my partner 5000 had bridge removed to make it open port too.
They ran pretty dang good. 
Big_Wood
westcoast dweller
To show folks they dont know what they are talking about on making a closed port into a open port going backwards.
My race MVP 166 bridge was removed to make it open port and my MVP copied cylinder on my partner 5000 had bridge removed to make it open port too.
So what are you saying? Are you saying that nobody knows what they are talking about cause they say open port cylinders are no good? If so i will say that i have never heard anyone say they are no good, just that the closed ports are better. I find it annoying to hear people talk about other people not knowing a thing cause they say open ports are not as good as closed. These people are assuming that they are saying open ports are junk when really they're not. They are saying not as good not that they are complete ****. Everyone knows open ports run pretty damn good, just not as good. I know that closed ports are better because of various saws with both options i have run. If this is not what you meant in your post then please disregard. I'm mostly just posting this so the particular people see it and realize how most guys are really thinking instead of just assuming and blowing it way out of proportion. There's even a video where a guy refers to the "snobs" who say open port are no good and says they run pretty damn good. Well all i have to say to him is we know they run pretty damn good! No beef with you kev. Mostly just a stab at a few particulars who have been picking scabs for a while.
