engine and drivetrain Damn! Scuffed Piston Skirts!

[FB73tii]

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.

 

One of my favorite sources, Engine Builder Magazine (www.enginebuildermag.com), just happens to have an article on this (linked below). I also find a lot of useful engine stuff in Circle Track magazine (www.circletrack.com). Rob--you may find this article interesting as well:

 

Pistons & Rings: Details every engine builder should know

http://www.enginebuildermag.com/2007/02/pistons-rings-details-every-engine-builder-should-know/

"Scuffing is often the result of overheating, but loss of lubrication, detonation and preignition can also be contributing factors. When an engine runs hot, the pistons swell. This reduces the clearance between the piston and cylinder walls. The cylinder bore can also distort adding to the problem. If the piston scuffs, it will wipe metal off the side of the piston.

Where the scuffing occurs will give you a clue as to what might have caused it. When overheating is involved, the scuffing will be primarily on the upper ring lands and on the sides near the wrist pins. There may also be oil carbon and lacquer burned onto the underside of the piston indicating it got too hot. Scuff marks on the lower skirt area often indicates a lack of lubrication (check the oil pump and pickup screen). Scuff marks on the edges or corners of the thrust sides of the piston may be the result of bore distortion. Scuffing on both thrust sides would indicate binding in the wrist pin.

Normal wear also takes its toll on pistons. The constant pressure and reciprocating motion in the cylinder bore causes wear on the piston skirt as well as the wrist pin bosses and ring lands. Elevated temperatures and high loads cause microwelding between the rings and lands, resulting in rapid land wear.

Sometimes a wrist pin will work loose and chew into the cylinder with each stroke of the piston. The underlying cause here may have been improper installation of the retaining lock rings on a full floating wrist pin, improper fit or installation of a pressed-in wrist pin, a twisted or bent connecting rod, excessive thrust end play in the crankshaft or taper wear or misalignment in the crankshaft rod journal."

Here's another article. It's about small engines, but still applicable.

Reading Pistons

http://www.theultralightplace.com/pistons.htm

Edited February 25, 2014 by FB73tii

[Original_Customs]

Scuffing is a normal part of engine wear (as mentioned).  Make sure the scuffing hasn't increased the size of the piston slightly due to the edges of the scratches curling up.  If so, the wet sanding option mentioned could be used to bring the size back down.  New rings and a ball/dingleberry hone would be an acceptable stop-gap process.  Obviously it's not really a rebuild, but you'll achieve your goals.  Compression goes back to normal and a few more miles before the full rebuild.  The most likely worst-case-scenario is that you end up having the compression drop off again, which means you wasted some time and a little money.  Best case scenario, you get another couple years out of it or more.  Seems like a worthwhile gamble to me.

[thehackmechanic]

Thanks guys. I actually had found that first article on piston scuffing during an on-line search.

[Harv]

If it were me, and no one was looking, I'd measure the ring land and if that checks out, I'd go the wet sand and hone and re-ring route.

 

John

[thehackmechanic]

As I'm preparing to do this, I thought I'd order up a set of Deves 1901 rings. Found a good price, ordered 'em... then got an e-mail saying out of stock. I've since called six vendors. They're coming up as NLA everywhere. Anyone buy these recently?

[thehackmechanic]

I called Deves directly, They had a set already made up. Not cheap at $119, but I jumped on them.

[roadhog0]

FWIW, my machine shop doesnt like Deves rings due to the old style oil scrapers. Hasting have an improper ring gap, so TotalSeal (conventional C4124) are the ones I am using. They cost like 90 bucks. Yes they are special order, you have to call em and tell em you want em then they'll make em. 

 

Your other option is buy a 500 dollar known motor and put it in place of the current motor, rebuild the Tii motor over time, once rebuilt swap em, sell 500 dollar motor for 500 bucks. 

 

Good luck Rob!

[TobyB]

 

If it were me, and no one was looking, I'd measure the ring land and if that checks out, I'd go the wet sand and hone and re-ring route.

I've done that.  It works.  And ran Deves rings in it, too.

 

I have wanted to try glass- beading a set of pistons that looked that ugly but had good lands... haven't

had a chance- most I have found that are that scuffed are also way out of spec on the top land, too...

 

t

[thehackmechanic]

I took the pistons to the machinist who has my Lotus engine. He said they pass the thumbnail test. I asked "so I should clean up the scuff marks with emery paper or something, right?" He said "NO! Don't do anything that would remove metal. It'll just increase piston slap. Leave 'em as is." That's what I'm doing.

 

While I'm waiting for the rings to arrive, I did a quick check of the tightness of the rings. On #1 piston, the top ring is looser than all the others. I'm trying to figure out, by matching up the scuff marks, if I somehow mislabled #2 as #1, since #2 is the cylinder that had lower compression. I'm still trying to find the smoking gun for that. I did not have the presence of mind to check the orientation of the ring gaps as I popped the pistons out (ie, maybe the ring gaps on #2 were lined up). Missed opportunity there. As I said above, the leakdown test showed that ALL of the 40%-ish leakdown was coming out the crankcase, on ALL cylinders.

[mike472]

Besides measuring the piston and bore clearance You should check the side clearance in each ring groove with a new ring using a feeler gauge. This and checking the gap size in the ring after it's pushed squarely down the bore. I've rebuilt engines with Deves rings and made the mistake of putting a new set of rings on pistons that measured good on bore clearance, but had way too much side clearance in the ring groove. Smoked like crazy after I put it together and found these things on the post mortem after my smarter mechanic friend told me to check them. Ended up buying new 9.5 cr pistons and boring to the next oversize. Good luck.

[eurotrash]

I had to use Deves rings in my recent rebuild as well.  Interestingly, my builder was a little weirded out by the oil ring design, but said that the word he got from his inquiries was that they control oil very well and thus are more of a power drain than the Hastings. But over time, the control they offer will last longer.  So, final word was that they are good for DD or street applications, not real desirable for race engines, which are the bulk of his business.

[thehackmechanic]

I searched this forum thoroughly and selected Deves on the basis of positive comments from guys like Toby, then was surprised by the difficult availability and the high cost. At $119, they were twice the price of the Goetzes, but there were quibbles about the Goetze's one-piece oil control ring.

[jgerock]

I used the Goetze "standard" rings on my refurb'd engine.   +1 on measuring the end gap while installed the new rings in the existing bores (use a piston without rings to get the ring square in the bore.  I got lucky and didn't have to file the gaps (even bought a little hand crank ring gap filer from Amazon.com).

 

I did borrow a ring spreader tool from Bill Riblett to R&R the rings.  Very handy and I didn't tear up my fingers.

 

 

[MrForce]

What about having the pistons coated? I bought a hurt LS engine and teardown showed scuffed pistons. Sent them back to the mfr and they suggested a clean up and coating.

 

Rob

 

Edited February 26, 2014 by MrForce

[TobyB]

 

 

 On #1 piston, the top ring is looser than all the others. 

Oh, cripes, then stick a feeler gauge in there.  If they are outside spec (I think it's only like .003 or so) then don't reuse the pistons.

 

What eventually happens is that the gap gets big enough to let the ring twist in the groove, and it then fractures and finally punches

through the top land and leaves via the exhaust valve .  And makes a mess on the way.

 

2 things cause this- wear, over time, and also overheated piston crowns.  That top land tends to concentrate heat,

soften, and then either move or wear much faster.  The result is the same either way- your #1 ring comes apart.

 

Sorry to bear bad news, but I learned this one the hard way with a 'really good block that just overheated once or twice'.

It then ate the head as the #1 piston spewed the top ring...  and no, the pistons 'looked' so good that

I didn't bother to measure carefully.  

 

Pistons are expensive, but then you really do get a new motor, and another 100k out of it.

 

And Goeteze wasn't making/ selling rings when I first stuck a $60 set of Deves rings into a 'refresh' motor- it was a trial

balloon on my part.  It seemed to work, so I've been using them since.  But obviously not recently, if I still think they're 'cheap!!!'

 

hth

 

t

Edited February 26, 2014 by TobyB