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Scratches on the pistons

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petterfilip

Registered User
Östersund, Sweden
I have scratches on the pistons of my Supercub engine that I do not understand what they are caused by. They are found on all four pistons, but most on cylinder 1 and 3. They are at exactly the same place at the top of the piston when it is mounted. It is a Lycoming O320 A2B. Corresponding scratches is in the cylinders, but very superficial.
I will be very grateful if someone can give me an explanation for this.
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Think Cruiser nailed that for you... what oil are you using??? One of the reasons I use 20W50 Phillips vs 100W here in Canada and preheat religiously for late fall/winter flying.
 
In the two stroke world we call those scratches a "scuff" usually caused by a "seize". Just part of the fun! Did your engine ever lose a little rpm for a bit and then recover to the original rpm? Piston overheats and expands more than the cylinder inner diameter and metal starts rubbing off. This creates additional heat and the cycle continues until power is reduced or the engine locks up and stops.
 
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We see a fair amount of this in our flight school aircraft, not usually a pre-heat issue for us but more from screaming RPM starts. Effect the same from both, no oil in the barrell initially. We get good results from multi-vis oil to get oil moving faster and Camguard to help a bit of lube left for the start.
 
Those scratches if only on the top side of the pistons when they are installed are caused from dry starts. When you let the engine sit to long all the oil will drain off the tops and when you start her up you have no lubrication. If it was cold starts or to high of RPM at startup it would also be on the bottom piston skirt. This is very common on engines that sit for months on end between flights.
Dave
 
Yep. In the ultra light world running a standard pattern and diving in with partial power from 900' agl, propeller forcing the engine to high RPM and not enough fuel so the mixture goes lean EGT's soar and then touch and go at full power. It's called a "cold seize". EGT's above 1200. I think aluminum melts at 1300?. Strangest thing to cut back on power and watch EGT climb and CHT drop. I didn't think 4 strokes really suffered much from that. Is it possible that a lot of these could be traced to touch and goes?
 
Yep. In the ultra light world running a standard pattern and diving in with partial power from 900' agl, propeller forcing the engine to high RPM and not enough fuel so the mixture goes lean EGT's soar and then touch and go at full power. It's called a "cold seize". EGT's above 1200. I think aluminum melts at 1300?. Strangest thing to cut back on power and watch EGT climb.
 
Most everyone is postulating an oil problem and that of course could be the driving influence but notice how the scuffs are on the "bigger" sided of the piston. I'm thinking piston cylinder wall clearance disappearing (piston expanding faster than the steel liner) and no room for the oil. If the cross hatch from the original hone goes away (walls get smooth) then there are no "channels" to hold the oil.
 
I pre-oil (GND mags, run starter, sparkplugs removed) until 30PSI when re-commisioning an engine after storage.

I started using 20W-50 today too.
 
Dave, what you say about "dry starts" sounds wise. That might be the cause of the scuffing.
I had another thought... As I use the super cub on floats and skis I have been thinking that it might be caused by water or snow sucked into the engine through the carburetor. Any ideas about that? Is there anyone who has seen the same thing on pistons from an engine that has not been used on floats or subjected to snow? If so, I can exclude that theory.
/Tomas
 
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Just curious, what type of fuel are you using in Sweden? Do you have large lead deposits in the upper cylinders and on top of the pistons?
 
1&3 are in the back, and get the hotest... Could it be from getting to hot on floats? Looks a bunch like my sled engine after it got hot grrrrrr.
 
George.. you got up too early today... ONE of those is at the front.. :smile:
 
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