Making a popup piston

[muddstopper]

Been doing a lot of reading about making popup pistons for chainsaw. From what I have read, everyone just takes a stock piston and machines some of the squish area away and leaving a raised area in the middle. Simple enough I guess until you have to mille the base of the cyl to get your compression back. Then you have to grind the transfers and the intake/exhaust ports to get your timing back. Tell me why you cant just weld up the top of the piston, leaving the squish area intact, and then mill the welded area to the level that you need it. This way, you wouldnt have to do a lot of milling to the cyl or any grinding to all the port areas since timing should remain unchanged. Reading other sites, it seems that using 4043-6% silicon content, or 4047-12% silicon aluminum tig rods can be used for welding on racing pistons, so why not a chainsaw piston.

 

[blsnelling]

Exhaust and transfer port timing do not change. You're lowering both the crown on the piston and the cylinder, do they go together. Your are increasing intake duration. That's a good thing.

 

[Nitroman]

Been doing a lot of reading about making popup pistons for chainsaw. From what I have read, everyone just takes a stock piston and machines some of the squish area away and leaving a raised area in the middle. Simple enough I guess until you have to mille the base of the cyl to get your compression back. Then you have to grind the transfers and the intake/exhaust ports to get your timing back. Tell me why you cant just weld up the top of the piston, leaving the squish area intact, and then mill the welded area to the level that you need it. This way, you wouldnt have to do a lot of milling to the cyl or any grinding to all the port areas since timing should remain unchanged. Reading other sites, it seems that using 4043-6% silicon content, or 4047-12% silicon aluminum tig rods can be used for welding on racing pistons, so why not a chainsaw piston.

Nothing wrong with it at all. This is the best way as far as I am concerned.

 

[fordf150]

Problem with welding a pop-up is the added weight. Several manufacturers have been forced to lighten pistons or reduce rpm for longevity.... Welding a pop-up takes you in the direction that you may experience early failure of the bottom end

 

[Poleman]

Nate, was that any part of Dolmar changing piston weight/design in the 7900 and 7910. Isn't there like 20 grams different in pistons from the older windowed to the newer slab sided ones?? Or was this a performance thing or both?

The Dolmars bottom end seems pretty well built and I've only seen one that needed new bearings.

 

[pioneerguy600]

Cut the squish band and shave the cylinder base, run a stock piston, best of both worlds.

 

[sunfish]

I had a 365 "special" with a welded pop-up. You could feel the extra vibration from the added weight. I didn't like that.

I've done a couple pop-ups here on the lathe and they made strong runners.

 

[Nitroman]

Problem with welding a pop-up is the added weight. Several manufacturers have been forced to lighten pistons or reduce rpm for longevity.... Welding a pop-up takes you in the direction that you may experience early failure of the bottom end

And then you lighten the piston.

 

[fordf150]

Nate, was that any part of Dolmar changing piston weight/design in the 7900 and 7910. Isn't there like 20 grams different in pistons from the older windowed to the newer slab sided ones?? Or was this a performance thing or both?

The Dolmars bottom end seems pretty well built and I've only seen one that needed new bearings.

havent researched it but that is my understanding. They had some issues with crank failures so they reduced the weight of the piston and lowered the rpm to eliminate that issue. Not sure how true that all is but it is the widely available rumor spread on internet forums.

 

[Poleman]

Lol....damb rumor mills......
I've yet to have any bottom end issues and and mine all run hard. Mix of both type pistons.

Thanks.....sorry to side track.....

 

[blsnelling]

One thing to consider. Cutting the squishband requires significant rework of the transfers just to get back where you started. Same with the exhaust unless you're after a lower exhaust. I find that using a pop-up piston is a much more effective approach for most models. I also believe it's a good thing not having to make the transfers so huge when raising them back up.

 

[Poleman]

Good food for thought Brad.....never looked at it in those terms.....makes sence!!

 

[muddstopper]

Exhaust and transfer port timing do not change. You're lowering both the crown on the piston and the cylinder, do they go together. Your are increasing intake duration. That's a good thing.

Since I have just started down this path of modifying saws, I have a lot to learn. From what I have read, everyone is trying to get squish to around .020. My 55 husky has a current squish of around .040. I am thinking to get a popup I would have to cut the squish band about .020 and then mill the base .040 to get a squish of .020 and have a popup of .020. I am still studying timing, but considering decreaseing squish by milling the base should change when the piston is dropping pass the ports by the same .020. In other words, cutting the base would raise the height of the piston in the cyl so the piston would have to travel the extra .020 before passing the exahust port, and would go above the intake port the same .020 sooner. I havent even tried checking timing with a degree wheel, but am thinking a .020 change in opening and closing of the ports would create a change of 2 or 3 degrees of actual timing. To the good or bad, I dont have a clue.

 

[blsnelling]

Almost You're right on almost all of that! Squishband typically refers to the head of the cylinder. To make a popup you're cutting the crown of the piston. Your math is correct though. Your exhaust is going to open about 2° later. Same with the transfers. By this token, blowdown stays the same. That's the amount of time, in degrees of duration, that the exhaust is open before the transfers open. Your intake will be affected twice as much. While you lowered the cylinder .040", the bottom of the intake skirt did not move at all. This is where intake port timing is measured, at the bottom of the intake port when it first opens as the piston moves upward. This increase in duration is almost always a good thing, IMHO.

The bottom line, you wouldn't have to change the ports at all to see a benefit from the machine work you suggest. You're good to go and sound like you know what you're getting into

 

[muddstopper]

I aint sure I have a clue as to what I am getting into. I have a couple of old pistons I am going to play with. I dont have enough spare parts to do much playing around with, but enough to practice cutting on. Wont have a runner, but I wont be messing up good parts either. I'll probably put a old case and top end together and check all the degree numbers, do a little cutting and milling and see how the numbers compare. Then if I am satisfied with the results, i'll look for a blown runner and just see how it goes from there.

 

[blsnelling]

A good rule of thumb is to leave the crown no thinner than .100". You can turn up to a .035" on many pistons, some more, some less.