Pinching piston to correct piston slap

[davidwyby]

Figured there wasn’t much to lose with a worn piston.

Old trick I came across online. I pinched it with calipers on it a little bit more each time. Got up to .020” increase in dia. with pressure and was afraid it was gonna break. When I released the pressure, it would pretty much go back to the same size. Heated it a little and cooled it with pressure applied. Got .002-.003” increase in width and slop down from .005”ish to .002”ish. Motoseal is drying now, hope to see tomorrow how it does.

@huskihl
@afleetcommand

 

[Yukon Stihl]

Having access to a bunch of machine shop stuff...
You could try knurling the piston like they did years ago

 

[davidwyby]

I started out looking for flaring, then knurling, then ended up squishing. Reasons are in this link. Basically no knurling machine. I might have tried punching or peening, but this seemed best.
https://www.practicalmachinist.com/forum/threads/knurling-pistons.384013/

 

[srcarr52]

You might be able to restore the proper clearance at the center of the skirt but you are going increase the clearance at the edges of the skirt (more oval shaped). I suspect you'll have problems with the intake skirt dragging on the intake port because the skirt edges are not there to keep the piston from tipping into the intake port slightly.

 

[davidwyby]

Good point I didn’t think of. I spose it will mess with the port timing a little bit because it will “leak” or open and close more gradually.

 

[davidwyby]

Maybe actual flaring of the whole skirt with some sort of round tapered slug like a lathe center would be better.

 

[huskihl]

Good point I didn’t think of. I spose it will mess with the port timing a little bit because it will “leak” or open and close more gradually.

It’ll really alter the timing if it snags the intake and the bottom half of the skirt blows through the case and takes out the cylinder

Be different if it was a dumpster find and no parts readily available

 

[davidwyby]

It’ll really alter the timing if it snags the intake and the bottom half of the skirt blows through the case and takes out the cylinder

Be different if it was a dumpster find and no parts readily available

Well how wide did you grind the port anyway?! Jeez!

 

[Woodpecker]

Well how wide did you grind the port anyway?! Jeez!

 

[jacob j.]

It’ll really alter the timing if it snags the intake and the bottom half of the skirt blows through the case and takes out the cylinder

Be different if it was a dumpster find and no parts readily available

I have to wonder if the wrist pin bore will still be spot on as well.

 

[davidwyby]

I have to wonder if the wrist pin bore will still be spot on as well.

Another good point but the pin still moved freely. Pic doesn’t show it well, but all the pinching is below that.

 

[Nutball]

I started out looking for flaring, then knurling, then ended up squishing. Reasons are in this link. Basically no knurling machine. I might have tried punching or peening, but this seemed best.
https://www.practicalmachinist.com/forum/threads/knurling-pistons.384013/

I wonder if straight line cuts would work too? I run a box cutter blade into nylon mandrels to make some ridges to make the final fit tight in the cylinder.

 

[davidwyby]

I wonder if straight line cuts would work too? I run a box cutter blade into nylon mandrels to make some ridges to make the final fit tight in the cylinder.

I think it would have to be a chisel tapped in to push material up. A cut would just remove material. This is how rolled threads are made vs. cut threads. (Think U bolts) We get ACME threads rolled into 2” shaft sometimes…pretty impressive. Way faster cheaper easier than cut threads, but the machine is huge and huge $.

Plastic tends to bulge up when cut.

 

[Nutball]

I didn't know if aluminum would bulge any when "cut". My "cut" in the nylin wasn't like a lathe tool cut, but just a razor blade edge pressing in, which makes the material bulge out on both sides of the blade as you said.

I figured it was a similar effect to knurling.

 

[davidwyby]

Yeah, I know what you mean. It’s like plastic has tension in it that makes it expand around a cut. Aluminum not so much.

 

[huskihl]

Well how wide did you grind the port anyway?! Jeez!

All of it

 

[Cat 525]

I started out looking for flaring, then knurling, then ended up squishing. Reasons are in this link. Basically no knurling machine. I might have tried punching or peening, but this seemed best.
https://www.practicalmachinist.com/forum/threads/knurling-pistons.384013/

 

[Cat 525]

Maybe just get a new saw. You don't have to fret then

 

[Stump Shot]

I started out looking for flaring, then knurling, then ended up squishing. Reasons are in this link. Basically no knurling machine. I might have tried punching or peening, but this seemed best.
https://www.practicalmachinist.com/forum/threads/knurling-pistons.384013/

Trust me, if you have a lathe already all that is needed is the knurling tool and some common sense. Put some masking tape at the top of piston to soften jaw placement and knurl skirt by hand back and forth while advancing tool. Hand fit to bore until it just floats down bore slowly with spark hole blocked.
I look at this as a real job in two ways. One, you have an old saw that no piston of any kind can be found to replace it. Or, two, racing, when looking to reduce friction and drag.
And yes, oil retention to the skirts is increased, reducing future wear, giving the knurls a reasonable work/ing life.

 

[Stump Shot]

Maybe just get a new saw. You don't have to fret then

...or just half a saw, the half that doesn't have the piston.