Making a popup piston

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muddstopper

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I am thinking about making a popup up piston to put in a saw that I cut the base down .030. I am just wondering how much can you safely cut off the top of a stock flatop before ring lands start breaking off or the piston gets so thin it burns thru. Currently the saw runs like crap with no power in the cut. Its a playsaw I built out of junkyard scrap and I dont care if it blows up, so I am just experimenting to see what I can get away with and hopeing I can make it a real cutter..
Its a husky 55 rancher, if that makes a difference.
 
I read that number somewhere too. I havent ported the cyl yet and am thinking that makeing a popup out of the stock piston should bring port timing numbers back closer to stock. Then do the porting. Cutting the base .030 and removing the base gasket lowers the ports about 1mm. If I can make the popup about .030 the port numbers should be back or close to oem with a base gasket delete. I want to give that a try and then drag out the grinder.
 
It sounds like youve already cut the base? Just keep in mind if you cut a popup, youll have to cut the base more to account for that. Id measure to be sure you can do that without a bunch of freeporting on the exhaust. What kind of intake timing do you have now? Im not a pro by any means, but i dont see the advantage of a pop-up unless you already have excessive intake duration and dont want to lower the intake any further.
 
I havent checked port timing on this saw. I know I cant cut the base anymore, .030 is enough to elongate the pulse hole to where it just barely seals. Stock squish is usually a little over .040 on a stock 55.. The cheap chinese cyl I am using had .051 squish. It now has .0205 with the base gasket delete.. The base gasket is .008 so with the cutting of the base and losing the gasket thats .038. One mm is .039. To port the exhaust, I am guessing I would have to raise the port 1mm just to get back close to stock. Same for the intake. If I can cut .030 off the top of the piston and leave a .030 popup, then I would be close to stock squish and port timing should be close to stock. I will lose some compression but it should be more than stock. I havent checked compression as the saw currently sits. I got two compression testers and cant seem to find either one. i will check compression before I take the top back off. As hard as it is to pull the rope, it has got to be high. The saw does run as is, it just doesnt have any power and I am sure the low power is related to the port timing.
 
I will lose some compression but it should be more than stock.
Going from .020 to .050 will overcome any gains you might realize by “optimizing” the way you are talking about, imo. Why dont you just raise the exhaust?? You need to time it before you do anything else, also just my opinion.
 
Going from .020 to .050 will overcome any gains you might realize by “optimizing” the way you are talking about, imo. Why dont you just raise the exhaust?? You need to time it before you do anything else, also just my opinion.
At bdc the piston doest clear the bottom of the exhaust port. Its not much, I havent measured, but if it doesnt clear the exhaust port, then all the exhaust cant get out.. Raising the top of the exhaust port will make it bigger so more exhaust can get out, but I fear I have the same problem on the intake side. Now cutting the piston is a lot easier than porting and pistons are cheap if it dont work. Yes, I will lose some of the compression I gained from milling the base by cutting the piston. I can also probably just install a base gasket and the piston will reach the bottom of the ports.. I might do that first before doing any milling or grinding. I need to check timing with and without the gasket so I know at least where I am starting. Its all a learning process for me as this is the first saw I have tried any of this on.
 
I wouldnt worry at all what the piston is doing at bottom dead center in the exhaust port. Flow is basically nothing at that point anyway, so no reason to plan your port work around that. Idk what the number is but something like 90% of cylinder pressure is vented in the first ten degrees after opening.
 
what are you wanting this saw to do? Make as many rpms as possible? Or have lots of torque cutting firewood? Somewhere in between? I dont know how to definitively tell you make one or the other happen, but i can tell you that that cylinder will not perform better than it does now if all you do is increase squish from .020 to .050
 
Honestly, I am just playing. I am wanting to learn a little about porting and hotroding a saw. I had a bunch of leftover parts and decided to see what I could do with them. I just didnt want to mess up a good saw trying to learn. All I do with a saw is firewood. Building saws I always go back stock. I was hoping to get a liitle more out of a small saw without stepping up to a heavier saw. I am using 346xps now along with the 55s. I use a 272 for bucking the big stuff. I got rid of a 365 special because of the weight.
 
One other thing and correct me if I am wrong. Deleting a base gasket is done to get a smaller squish and increase compression. Milling a cyl base is done to decrease squish and to get better compression. When making a popup out of a stock piston you also have to mill the base to maintain squish and compression. It seems to me the fact that you mill a cyl a set amount and cut the piston into a popup the same amount, you would have the original squish around the squish band, but would have higher compression due to the dome of the popup going up farther into the cyl top and would result in more compression. Not as much compression as milling the base and keeping the piston stock, but more compression than you started with factory cyl and stock piston. Cutting the squish band and milling the base and keeping the piston stock would increase compression because you are decreasing the volume of the dome in the top of the cyl. All these different mods have one thing in common and they all increase compression. Now it also seems to me that when you start machining cyl and pistons, you are going to change timing numbers of the ports. Cutting a dome on the piston seems to me to be the easiest way to get those port numbers back to where they belong. I guess one would also have to find a way to deal with the dome pistons interference when exchanging fuel and exhaust as air moves thru the cyl. Widening the ports and changing port timing would be the easiest way to accomplish that
 
I havent checked port timing on this saw. I know I cant cut the base anymore, .030 is enough to elongate the pulse hole to where it just barely seals. Stock squish is usually a little over .040 on a stock 55.. The cheap chinese cyl I am using had .051 squish. It now has .0205 with the base gasket delete.. The base gasket is .008 so with the cutting of the base and losing the gasket thats .038. One mm is .039. To port the exhaust, I am guessing I would have to raise the port 1mm just to get back close to stock. Same for the intake. If I can cut .030 off the top of the piston and leave a .030 popup, then I would be close to stock squish and port timing should be close to stock. I will lose some compression but it should be more than stock. I havent checked compression as the saw currently sits. I got two compression testers and cant seem to find either one. i will check compression before I take the top back off. As hard as it is to pull the rope, it has got to be high. The saw does run as is, it just doesnt have any power and I am sure the low power is related to the port timing.

Working with these Chinese jugs can be really frustrating. The port timing is often compromised by the excessive squish and the plating usually wants to peel if you grind the ports.

If I understand right, you have already gotten squish close to 0.020 and you have dropped the jug almost 1mm, thus increasing your intake dramatically. Cutting a pop up now will reduce compression and get you very little. Run it as is or raise your exhaust roof to gain extra blowdown.

Cutting a pop up requires going deeper into the squish band than you already have for positive results. This is why getting timing numbers is critical because you risk over timing intake or freeporting exhaust if you take more base cut.

Try running your saw as is. You might be really happy with the results. If you want more RPMs try a minor increase in exhaust height, but beware the plating may not last if you grind.
 
One other thing and correct me if I am wrong. Deleting a base gasket is done to get a smaller squish and increase compression. Milling a cyl base is done to decrease squish and to get better compression. When making a popup out of a stock piston you also have to mill the base to maintain squish and compression. It seems to me the fact that you mill a cyl a set amount and cut the piston into a popup the same amount, you would have the original squish around the squish band, but would have higher compression due to the dome of the popup going up farther into the cyl top and would result in more compression. Not as much compression as milling the base and keeping the piston stock, but more compression than you started with factory cyl and stock piston. Cutting the squish band and milling the base and keeping the piston stock would increase compression because you are decreasing the volume of the dome in the top of the cyl. All these different mods have one thing in common and they all increase compression. Now it also seems to me that when you start machining cyl and pistons, you are going to change timing numbers of the ports. Cutting a dome on the piston seems to me to be the easiest way to get those port numbers back to where they belong. I guess one would also have to find a way to deal with the dome pistons interference when exchanging fuel and exhaust as air moves thru the cyl. Widening the ports and changing port timing would be the easiest way to accomplish that

You’re assuming your stock timing was good. In a Chinese jug of unknown stock performance. You need numbers from an OEM jug as a baseline. Even those can be improved for RPM or torque.
 
I've done a couple 55 ranchers.granted they had OEM top ends.but a gasket delete got me about 0.022 squish and did a muffler mod.saws ran much better then stock.but with the high squish which is normal in the Chinese cylinders.i would have cut the base leave the piston alone and get to 0.020 with a muffler mod.just my thoughts.

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Working with these Chinese jugs can be really frustrating. The port timing is often compromised by the excessive squish and the plating usually wants to peel if you grind the ports.

If I understand right, you have already gotten squish close to 0.020 and you have dropped the jug almost 1mm, thus increasing your intake dramatically. Cutting a pop up now will reduce compression and get you very little. Run it as is or raise your exhaust roof to gain extra blowdown.

Cutting a pop up requires going deeper into the squish band than you already have for positive results. This is why getting timing numbers is critical because you risk over timing intake or freeporting exhaust if you take more base cut.

Try running your saw as is. You might be really happy with the results. If you want more RPMs try a minor increase in exhaust height, but beware the plating may not last if you grind.
I have already ran the saw. Very low on power in the cut, but revs decent. The p/c out of the box had .051 squish without base gasket. I expected something like that as I have yet to find a china topend worth a crap. I have already cut the cyl .030 which is about as much as you can remove and still have a pulse hole left in the base. The only way I can see to get exhaust timing close is to raise the roof and I am guessing probably a full mm. That seems like a lot to me. Of course I say close and I dont even know what it currently is or what it needs to be. The saw is sitting in the corner as I have more important projects going on. I will find time sooner or later to tear back into it
 
If you dont have one, get a degree wheel, think i got mine for $12 or somethin. Really will help you quantify whats going on and the effect that some of these changes have. Keep in mind that the amount removed from the base does not equal the amount you would need to raise the ex port (i.e .030 off the base does not mean raising the ex .030) crank angle has a part to play. And you could do all the grinding on the ex port before you even have the piston chucked up in the lathe.
 
You cannot cut a popup on the piston and not drop the cylinder accordingly for the proper squish. You're not only negating any compression gains, but you're also killing squish velocity.

Forget BDC on the exhaust port. It has little to nothing to do with performance or port timing.

Don't do a thing to this saw until you get a degree wheel on it.
 
I have a degree wheel and know how to use it. I just havent had time to do anything. I checked squish, didnt like it and chucked the cyl in the lathe and then put it together and tried it out. Didnt like the out come and now just being a glutten for punishment. Like Randy Machoman Salvage, my plan is no plan. Adapt and Overcome. Anyways, I am listening to whats being said and before fireing up any more grinding and milling equipment, I'll pull out the tape and calipers and other measureing tools. Now to fix my leaking hotwater storagetank and dry out my flooded basement I woke to this morning.
 
I have a degree wheel and know how to use it. I just havent had time to do anything. I checked squish, didnt like it and chucked the cyl in the lathe and then put it together and tried it out. Didnt like the out come and now just being a glutten for punishment. Like Randy Machoman Salvage, my plan is no plan. Adapt and Overcome. Anyways, I am listening to whats being said and before fireing up any more grinding and milling equipment, I'll pull out the tape and calipers and other measureing tools. Now to fix my leaking hotwater storagetank and dry out my flooded basement I woke to this morning.
Can't like the ending to that post. Good luck.
 
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