Wear on piston skirt

[morbius18]

This is a husqvarna 365 special cylinder that was cleaned up and reused with a meteor piston. About 1 tank of fuel through it.

Is this normal on the skirt wear? Ignore the scratches, they came from me test fitting the piston in thr saw while porting.

 

[Andyshine77]

Totally normal.

 

[cus_deluxe]

Hard to tell from pic. Looks like a pretty flat floor of intake? Thats a little more wear than i would expect from just one tank, maybe give that lower intake lip a bit more bevel if anything.

 

[Andyshine77]

Hard to tell from pic. Looks like a pretty flat floor of intake? Thats a little more wear than i would expect from just one tank, maybe give that lower intake lip a bit more bevel if anything.

I don't know why, but that's how every new saw I see looks after just a few tanks. More chamfer wouldn't hurt though.

 

[morbius18]

I haven't touched the bottom or top of the port, this is all OEM shape and bevel. I'll be dropping it slightly from 76* to about 82*, so some of ths will disappear.

 

[rd35]

Well, there's one thing for sure....I will definitely be pulling the carb after the first tank of gas on the next new piston I install. It would seem from what I am seeing that the piston is undersized for the bore.....but I do realize that a certain amount of "undersize" is necessary for clearance during a cold start WOT full power warm-up to prevent piston seizure. Very Interesting!!! (which is why I love this site....learn something new every day!)

 

[Chainsaw Jim]

That's just the way most pistons wear and it usually shows up the moment you start cranking on the pull rope. I've noticed it happens quickly, but never gets any worse. If you look you'll see it isn't the intake port because the rub mark reaches across the entire skirt evenly and matches to the black lines on the cylinder, just above the port.
I've never discussed the subject with more knowledgeable folks than me, but if I had to come up with an idea why it happens I'd say it's because the pistons are tapered wider at the bottom. That's why the black lines on the cylinder are heaviest at the top of the stroke and fade out and disappear before reaching bottom. I'm assuming combustion pressure off the front of the piston prevents the exhaust side of the skirt from wear.

 

[blsnelling]

+1 Totally normal.

 

[morbius18]

+1 Totally normal.

Good to know, thanks.

Does only havin 1 ring cause any piston instability vs 2? This is my first foray out of Stihl saws

 

[ericm979]

The rings do not keep the piston straight in the bore. They only seal combustion gasses in.

 

[blsnelling]

I only run a single ring 99% of the time.

 

[67L36Driver]

+1 Totally normal.

This is not.

Nor this.

 

[morbius18]

The rings do not keep the piston straight in the bore. They only seal combustion gasses in.

I understand what rings 'do', but it's a little more in depth of what they actually do. From an engineering perspective, the more points of contact, the more stable an object is. When a piston is traveling vertically in a bore, it's motion stabilizes it. At the limits of its motion, ie both sides of tdc and bdc, it is subject to horizontal thrust, ie Piston slap.

More contact points would stabilize this motion. But it's a compromise. More rings equals more parasitic drag and more friction. Less power and more heat are the result. The hotter the engine, the more it wears and the less efficiently it runs. Multiple rings also seals compression better, and on 4 stokes reduces emmissions.

So, I guess it's all a compromise; longevity, less emissions, less power vs stability, more power, cooler running.

And longevity wise, I doubt it make a difference that would be noticable.

 

[ericm979]

Rings aren't a contact point as far as keeping the piston centered in the bore is concerned. They're pressed against the bore by spring tension or combustion pressure and the piston floats within them, just pushing them up and down.

The general rule of thumb in motorcycle two strokes is that two rings are a benefit below about 9000 rpm. Above that the reduced friction of the single ring is worth more than the lost compression.

 

[drf255]

It “can” be normal, but your intake floor looks rough.

Hit the port with a large 10mm and then small fine 4mm diamond ball. The sand by hand and if possible scotch brite drum after.

It can be normal, but I’d say that’s rough after one tank. I’d also check piston to bore, it should be under .003”.

I don’t think that’s normal in this case. Just my opinion. I wouldn’t let an intake port like that leave here.

 

[pioneerguy600]

It “can” be normal, but your intake floor looks rough.

Hit the port with a large 10mm and then small fine 4mm diamond ball. The sand by hand and if possible scotch brite drum after.

It can be normal, but I’d say that’s rough after one tank. I’d also check piston to bore, it should be under .003”.

I don’t think that’s normal in this case. Just my opinion. I wouldn’t let an intake port like that leave here.

I always finish my bevels with a piece of machine paper,320 grit or finer, to take off any ragged edges left from a diamond ball bit on the port edges. Just a hand operation, no special tool required and all edges inside a cylinder needs a slight chamfer.

 

[morbius18]

It “can” be normal, but your intake floor looks rough.

Hit the port with a large 10mm and then small fine 4mm diamond ball. The sand by hand and if possible scotch brite drum after.

It can be normal, but I’d say that’s rough after one tank. I’d also check piston to bore, it should be under .003”.

I don’t think that’s normal in this case. Just my opinion. I wouldn’t let an intake port like that leave here.

This one is a stock port bottom and top opening. I was going to lower the port from 76 degrees to about 80-81 degrees, I'll be able to fix the rough edge and remove a little of the wear area on the cylinder wall. I just haven't gotten a chance to do it yet.

 

[drf255]

I doesnt look stock OEM, but pics can be deceiving.

Even the tunnel looks like someone played there after the factory cast it.

 

[morbius18]

I doesnt look stock OEM, but pics can be deceiving.

Even the tunnel looks like someone played there after the factory cast it.

I widened the ports to about 65% of bore but left the top and bottom alone. The intake was opened by me. This was prior to getting a degree wheel.

 

[drf255]

Check piston to bore. Even with a perfect port, if the clearance is too much it will shave down the piston skirt.

I know what you are saying, but that intake roof and floor is choppy. Perhaps it’s not it’s first rodeo. I’m sure you see the giant plating crack as well.