Piston damage

[enjenjo]

I started checking the pistons out of this Ford 5.0. four of the 8 pistons show scuffing like this.  My initial thought was that these pistons were fitted too tight, but further inspection showed that the skirts on theses pistons were collapsed by about .015" on all four. The tops of these pistons are also black and shiny wet, rather than satin black like the other four. Now I am more inclined to think it was detonation, but why would it only do it on four cylinders? I know nothing about the top end of the engine, the bottom end was assembled poorly, probably the top end too. It was carbureted.

Any insight?

[wayne petty]

I started checking the pistons out of this Ford 5.0. four of the 8 pistons show scuffing like this.  My initial thought was that these pistons were fitted too tight, but further inspection showed that the skirts on theses pistons were collapsed by about .015" on all four. The tops of these pistons are also black and shiny wet, rather than satin black like the other four. Now I am more inclined to think it was detonation, but why would it only do it on four cylinders? I know nothing about the top end of the engine, the bottom end was assembled poorly, probably the top end too. It was carbureted.

Any insight?

pop the rings off carefully.. check the end gaps in the cylinder bores..

perhaps it got standard rings on oversize pistons..  or just boxed wrong..

any signs that the head gasket was installed backwards on one side and caused the engine to overheat on one side.. that happens a LOT..  that would have caused the rear 3 pistons to expand until they scuffed and then be distorted and collapsed as you describe..


on most Ford V8s.. one head gasket goes on right side up .. the other face down.. the coolant passages NEED TO GO AT the back of the block.. or coolant will flow straight up from the water pump and directly into the intake crossover.. the rear cylinders will over heat ..


since you don't have the top of this engine.. the rest of this are POSSIBLES..

any chance the carb was really screwed up???   where one side leaned out really bad.. i am taking that its a dual plane intake.. and that can happen..

could the PCV valve be REALLY OVERSIZED??? that has happened to a LOT of fords over the past 20 years.. one size fits all.. i researched and found that at idle..  block the pcv flow and the engine speed should drop 50 to 70 RPMs.. if the PCV is sized correctly..

years ago.. when i worked at a small engine rebuilder.. i ran the head department.  one engine kept coming back for excessive oil use.. 350 to 400 miles per quart..  it came back 3 times and we build 2 different complete engines and it still burned oil.. i got them to stop in with the truck.. early 70s ford with FE 360 motor..  the engine was clean enough to eat off of..  as was the rest of the truck..  i ask about the history.. he bought the truck.. it ran perfect. he did a major tune up and it started to burn oil..  he had it rebuilt at a different shop 3 times before he brought it to use..  i ask about that nice shiny PCV.. did that get changed..  yep.. but not since.. i told him to go to the ford dealer and buy one in a FORD or motorcraft package.. not something open across the counter.

there used to be available little PCV flow testers. you could put over the oil filler opening and it worked like a flow meter . telling you if the pcv calibration was enough or if you needed a larger calibration..

i have taken to using the compound fuel pressure vacuum gauges hooked to the oil dipstick tube and see what the crank case pressures or vacuum is doing.. as soon as a block off the breather port..    it should NOT build pressure at idle..  it will create vacuum at idle. but not a LOT really quickly..

the pinch off the PCV flow to check with a digital tach for 50 to 70 RPM drop is the most accurate for testing the calibration of the PCV valve for the engine/ carb..

PS.. if the PCV calibration is too small.. it will require more throttle opening and expose more of the idle transition slots and create excessive carbon on the piston tops and chambers..  

if the PCV calibration is TOO LARGE it will burn oil... excessive vapors into the intake..  too much idle mixture to compensate for the excessive vacuum leak/pcv too big.

over on another forum.. i was discussing on tunnel rams to use a needle valve to create an air leak into the plenum to allow the primary throttle blades to be closed far enough to not overexpose the idle transition slots causing an over rich idle mixture that fouls the spark plugs.


sorry for so much of an answer.. i wanted to cover the various issues for the forum membership to learn from..

[kb426]

Frank, my exp. has been excessive heat causes collapsed skirts. Wet usually means oil consumption for whatever reason. No compression, pinched rings, etc.

[enjenjo]

I got back to this today, My dial bore gauge just came. All the cylinders measure 4.033 to 4.034. None of them have more than .001 taper, so I think a hone job with new pistons and rings will be enough.

[Arnold]

My .02 cents I dunno..it lasted this long. Me personally..I would do just what you are doing.
  I guess that would depend on the patterns of the scuffing.
  Detonation will sure scuff some pistons in some engines as the piston rocks in the bores. Others it is much more subtle and just burns a hole in the piston
Depending on how much this botherred me and or depending on how much this engine is built..
 I would see that it is decked square,mains aligned,crank straight.
  Perhaps there is a core cast issue. lol..I have seen cores that were so bad it was a wonder they ran for long..and of course core issues where..ah ya..that is why it broke there dunh.Not worth thickness testing.
  Has it been bored? I have seen some not straight bores on stuff.Then again I am so old that I have seen old clamp on Kwik Way boring bars go in not straight decks I guess modern "in frame" boring. Do they still even do that?