engine and drivetrain Damn! Scuffed Piston Skirts!

[thehackmechanic]

Pursuant to my other thread, I yanked the head and dropped the pan on my 72tii that ran fine, didn't burn oil, but had horrible leak-down numbers (all past the rings) and weak compression in one cylinder. I was expecting -- hoping -- to see stuck rings, and was planning to do a re-ring and dingle berry hone with the engine in the car, but instead I found that the rings weren't stuck at all. However, to my horror, what I found was that all four pistons are scuffed on the intake side, with two of them lightly scuffed on the exhaust side as well. 

 

This is the car that had a persistent lean running issue, finally addressed by rebuilding the KFish pump. It also had, when I bought it, a distributor that wasn't advancing, and a marginal radiator, all of which could've contributed to hot running, which could've created the scuffing.

 

The cylinder walls have barely visible scuff marks that I couldn't photograph.

 

Seeing this, I now think that I should've followed the advice that I never should've gone into the engine unless I was willing to face a total rebuild.

 

But here I am.

 

I'm going to try to get a machinist friend to come over to measure the bores for roundness.

 

Do the scuff marks automatically mean that the pistons are toast? There's no obvious detonation damage to the crowns. I haven't measured the ring lands yet.

 

Feeling physically ill...

 

--Rob

 

[Preyupy]

You said the car sat for years, there is a good possibility this was all caused by rust on the cylinder walls that was worn off once you got it started.  This would have acted like sand paper on the skirts for the first few revolutions.  This would have also messed up the sealing of the rings.  We call this Garage ROT! 

[thehackmechanic]

I like that answer. This is a good answer.

[FB73tii]

Do the scuff marks automatically mean that the pistons are toast? There's no obvious detonation damage to the crowns. I haven't measured the ring lands yet.

 

Rob--I am no piston expert, but it seems to me that any piston with a fair amount of use will show some scuffing. That said, the one in your photo does look pretty used up. Are they all like that? It seems a good "hack" fix would be to wet-sand out the scuff marks with some 400 grit, hone and re-ring and call it good. But I've never done that, so don't take my word for it. Will be interested to hear what others have to say that have BTDT. --Fred

Edited February 24, 2014 by FB73tii

[roadhog0]

Thats almost as much as my pistons were scuffed. My car also sat a for a while in a garage too... In all honesty, after I pulled my motor down I thought I could probably get by with a re-ring, hone, and put it back together. But I had already bought new pistons, built my head, etc so I said screw it... And FWIW, I'm up in the almost 4k range for parts and machine work. It adds up REALLY quick!

[jgerock]

[OriginalOwner]

hack,

 

the more fundamental question to me:  knowing what you now know:  would the engine have survived the 3,000 mile trip ??  I am clueless on matters such as this.

 

Cheers,

 

Carl

[thehackmechanic]

Carl, that is the big question. On the one hand I am kicking myself for having opened up this can of worms. On the other, that having been done, I hope to put it back together in a way that will give me a much greater peace of mind while avoiding Roadhog's $4k bill; I simply don't have that money to spend right now.

[OriginalOwner]

 hack,

 

my immediate thought:  the logical bigger picture to me is that you lucked-into the more-correct path:  you are now quite certain you won't be stuck 1,500 miles from home (and job !!) with at worst a broken engine, with attendant costs far in excess of $4k.

 

a lot of comfort when things "break" in one's own backyard.

 

Cheers,

 

Carl

Edited February 25, 2014 by OriginalOwner

[saaron]

Uneducated guy question - how would the pistons become scuffed?  Are they contacting the cylinder bores?  Or something ON the cylinder bores (like rust)? 

 

I wouldn't have thought it was possible for the sides of the pistons below the rings the physically touch the cylinder walls unless something was REALLY amiss. 

 

I agree w/the above perspective that now you at least know what you're dealing with.  This particular car of yours seems like it could be more of a keeper than some in the Hack Collection, so it's worth getting the motor squared away.

 

Scott

[Emyers]

_Rob,

  _{Pistons have specs as well, see if your machinist friend can measure the skirts and ring lands to see if they are still serviceable.}

_Earl

[jimk]

Normally scuffing occurs on the thrust side.  There is the unanswered question as to why are the pistons scuffed on the intake side as described in the original post?

[FB73tii]

Uneducated guy question - how would the pistons become scuffed?  Are they contacting the cylinder bores?  Or something ON the cylinder bores (like rust)? 

 

I wouldn't have thought it was possible for the sides of the pistons below the rings the physically touch the cylinder walls unless something was REALLY amiss.

 

The rod angle changes as the piston goes up and down, applying sideways (thrust) load to the piston. The piston rocks a bit in the bore, and takes more load on the thrust side (piston pushed down under combustion) than when it is coming back up. Eventual scuffing is normal, excessive is cause for concern. If you have the head off and one piston at the top of the stroke, you can rock it slightly about the wrist pin axis with minimal pressure. When building a race engine, where valve-to-piston clearances are very close, this rocking must be measured with a dial gauge and taken into account to avoid VP contact.  --Fred

Edited February 25, 2014 by FB73tii

[saaron]

Fred -- thanks for the explanation.  I think I understand.  So the question becomes how did the piston movement become excessive?  

 

The rings wore letting the piston rock about more than it should, or the cylinder walls wore, letting the piston move around than it should?  Or both?  And maybe other issues could be contributing - anything that would allow the piston to move around in the bore would contribute.  

 

Thanks again - I love learning about this stuff.

 

Scott

[eurotrash]

I have never understood how a piston that measures within spec above the rings is considered worn when showing scuffing below them.  I'm guessing that its the amount of metal loss on said piston skirt that allows the piston to rock more than a new piston?