Prognossis not to good

[Merc69]

The motor os out of PsyOp and the oil pan off. The number 8 piston has scoring on the skirt and there was metal in the oil pan and some larger slivers of metal actually on the pickup screen.

Tomorrow the whole thing comes apart.

I hope the cylinder walls are not screwed up as it has aleardy been bored 60 over. Has anyone had this happen to them on the number 8 cyl ?

[Wesdog]

Take a look at these photos and tell me if yours looks the same:

http://www.fullsizejeeps.com/jeeps/files/Misc-Pictures/401_piston_damage2.jpg
http://www.fullsizejeeps.com/jeeps/files/Misc-Pictures/401_piston_damage3.jpg

[ August 24, 2003, 07:00 PM: Message edited by: Wesdog ]

[Merc69]

It won't let me look at it

[Wood's Wagon]

GOOD GRIEF???? somthing is majorly wrong there. That piston is TOAST. about the only thing you could do if its allready bored is hone the crap out of it and get it as smooth as possible. I had a minor scoring on one of my cylinders, but that is just terrible. i am suprised it didnt seize on you.....good luck

[Merc69]

All I can see is the skirt right now but that is the general look from what I can see. What happened ? Any ideas ?

[NomadJack]

I had that on my whole left bank when I tore mine down, with a broken piston skirt on #8. Fo course, since I had a "virgin" block, it wasn't a problem to get the cylinders bored 30 over. I would imagine that your only option would be to get the cylinders sleeved, though I don't know much about this process.

[Wesdog]

The cast piston damage above was the result of a lean condition and a piston to cylinder clearance on the low side of the factory spec. The piston is from a .030 over 401 with under 10k miles on the engine. There were actually 3 pistons showing damage, the one shown was the worst. All 3 were common to one plane of the 2 plane Performer intake. The lean condition was caused by an electric fuel pump that was going south but still supplying enough fuel to appear to be good when tested. Under heavy engine load the pump couldn't keep up and the level in the fuel bowl farthest from the carb fuel inlet was low which affected the mixture. It was that side of the intake plane that the damaged pistons were on. 2 on one side of the block and one on the other. No other explaination could be found for the damage and it was looked at closely. The cooling system had been upgraded and was working correctly. The engine did not overheat to my knowledge but that really depends on where the temp measurements are made.
The bottom line is the piston overheated, changed shape and contacted the cylinder wall. The same sort of thing can happen if a steam pocket gets trapped in the cooling system in the block/head and liquid coolant can't get to critical areas to cool them. #8 has been a problem area in the AMC engines I have dealt with.
In the above case the pistons were replaced with TRW forged pistons and the cylinders bored slightly to obtain to correct clearances for forged pistons. The engine now has 4K miles on it and is good so far.

[ August 24, 2003, 07:31 PM: Message edited by: Wesdog ]

[Wesdog]

I went through this situation twice unfortunately. The 1st time the cause was identified as a lean condition but the actual cause of the lean condition was not identified. It was thought to be a vacuum leak in one of the accessories or intake runners. Also the 1st time the damage was more severe because the cooling system didn't have the same capacity as it did the 2nd time. I didn't even know the damage had happened the 2nd time because there were no engine noises to go with the damage. It was only discovered because the engine was disassemblied to install Indy Aluminum heads.
If you have read any of my previous posts you will notice I caution others regularly about running lean and tuning their engines properly. Keeping the cooling system up to snuff is another of my pet peeves.

NomadJack, If you had a whole bank of pistons showing this sort of damage the 1st thing that comes to mind is cooling system problems since that would be common to the same bank/head.

Merc68, the cause of your problem may be totally different especially since you found metal in the pan. How many miles on the engine? Any other symptoms?

[ August 24, 2003, 08:41 PM: Message edited by: Wesdog ]

[NomadJack]

Actually, I think the damage was a combination of cooling in the block (the block has been hot-tanked), and a lean condition due to warpage in the intake manifold (intake was just resurfaced last week), so the combination lean condition and improper cooling would certainly do some severe damage.

[Merc69]

I have less than 1,000 miles on the rebuild. I was pulling a trailer to the dump to get rid of a bunch of stuff. Less than 2,000 lbs of junk though. I was running down the highway at 65 mph and started to notice the temp gauge started to get warmer than I had ever seen. Normally the gauge read below the center mark. This time it started to get above the middle mark. Then I noticed a banging noice from the motor. Pulled off and the noice has been there ever since. I limped home after things cooled off not towing the trailer. The temp gauge never got above the middle of the gauge again.

I am running the original temp gauge...I have no idea if it is accurate. I did notice that the heater hoses got extreemly hot during the diagnostic portion while at home. The port at the rear of the intake manifold did leak some fluid during one of the times I was running it trying to figure things out. This was the port with the christmas tree vacume switch (that I have nothing connected to). I removed the switch and blocked off the port the switch went to and no more leak. The hoses still ran very hot though.

[Wesdog]

Bruce, sounds like a real careful inspection is needed by someone who really knows what to look for. If you know how to do that great. Otherwise, you might want some help before moving forward again. You don't want to go through this a 2nd time.

[Merc69]

Between myself (very basic knowlegde of mechanical things), and a friend that has been building motors for over 20 years, the machince shop who did the work and others I hope we can get this thing straightened out. Thanks for the input.

Forensic Mechanic work...Maybe it's time for a new hobby :D

[imported_horsepuller]

WESDOG: Will using forged pistons reduce the chance of such an occurance in the future?

[Chero77]

You probably don't want to use forged pistons for a daily driver. Forged pistons expand and contract quite a bit compared to cast pistons, thus you have to run loose piston to cylinder bore tolerences. Might get some piston slap and smoking at startup. Racers put up with this stuff, but your typical daily driver would probably find it annoying.

The damage in the picture could be caused by too tight of a piston to wall clearance, but there are other possibilities as well, the most likely in my mind is a rod that is slightly bent or rod or crank journal that is tapered. I don't think a lean condition by itself would do that. Super lean running engines usually cause detonation which may in severe cases lead to damaged parts but not usually of the type shown in that picture. The pic looks like a machining problem. I'd guess the rod and or crank journal was tapered forcing the piston to one side.

[Crazy_Jeepman]

yup from what I can see of the top of the piston Lean does not appear to be the problem at all. To tight!!

[NomadJack]

The tight condition could point back to a cooling issue as the piston and block are perhaps expanding at different rates causing the piston to expand into the cylinder. If I remember correctly, scuffs on four corners indicates a cooling issue while uneven scuffing or scuffing all over indicates more mechanical issues.

[Wesdog]

Originally posted by horsepuller:
WESDOG: Will using forged pistons reduce the chance of such an occurance in the future?Forged pistons expand more than cast pistons but they also expand more evenly than cast pistons due to their construction as I understand it. Combined with a larger clearance between the piston and cylinder wall this tends to make the sort of problem shown in the photo above less likely. They are also more expensive than cast pistons. I don't find the engine startup noises associated with using forged pistons to be a problem. I wanted to insure the previous problems didn't return and forged pistons were the best option for me at the time. There are some problems getting them now last I heard unless you have a custom set made.

[ August 25, 2003, 08:17 AM: Message edited by: Wesdog ]

[Wesdog]

As far as the cause of the problem shown in the photos above all the machining issues were checked and rechecked. The rods etc. were all carefully examined and there were no problems with any of the parts or the machining identified. A torque plate was used to bore the cylinders. All these questions were asked and asked again at the time. The guy doing the work knows his stuff and has been building both stock and high performance engines for over 30 years. The only suspect parts in my mind were the pistons themselves but unfortunately I don't have them anymore. I agree that there are other potential causes for this type of damage but in my case the cause was as stated.
Merc69's engine is a 360 and hopefully his problem will be more clearly identifiable. The 360s have thicker cylinder walls to start with so unless his has been bored significantly over it shouldn't have the thin wall problems associated with the 401's like mine. I would recommend going on the wide side of the piston to cylinder wall clearance spec for cast pistons in the AMC 360 & 401 V8's but that's just my personal opinion. The 1st time I had the problem the bore had been done on the tight side of the spec because of the 401 wall thickness. The 2nd time the bore was opened up to the wide side but there was still some damaged discovered. There are still some questions in my mind about the construction of the cast pistons used but they are not available for inspection anymore.

[ August 25, 2003, 08:12 AM: Message edited by: Wesdog ]

[gsmikie]

yep...... hypereutectic 390 alloy pistons are dual process heat-treated (T6), making them 30 percent stronger than ordinary untreated hypereutectic pistons. The lightweight, rigid rib skirt design stabilizes the piston in the bore. The 100 percent CNC-machined piston crowns, high top compression ring location, gas accumulator groove, drilled oil returns, and spiral lock retainer grooves make this an outstanding performance piston. All this adds up to a piston that fits tighter than forged pistons, providing improved oil control, reduced blowby, increased ring life, and no cold start-up knock--and they won't cost you an arm and a leg! These pistons are available in standard, .020, .030, .040, and .060 oversize.

[Wesdog]

The tranny god has spoken! I'll see if I can get the model and manufacturer of the cast pistons I had the problems with. I think they were Badger's.

[Merc69]

The block, pistons, heads and associated parts are at the machine shop. Forensic wrenching indicates there was a intake leak that leanded out the #8 piston sufficently that it superheated the piston. The result was the piston heated enough to expand and score the cylinder wall.

Possible causes
1 - intake manifold leak
2 - vacume leak
3 - intake cylinder leak

There were two additional cylinders that indicated a possible leak and leaning out.

Solutions: Sleave the cylinder that is scored. hone one other cylinder. Head is being checked at the machine shop. After reassembeling check the intake for vacume leaks and other possible vacume leaks.

Pray to the Jeeps Gods that everything is good to go smile.gif

[Merc69]

Checked with the machine shop today. They said three pistons needed to be replaced and then informed me that the shop that did the head work needed to be contacted.

It appears that the head shop had installed bronze valve guides that were way to sloppy and there was oil leaking into the cylinders past the guides.

The guy at the head shop claimed they were not that way when the heads left his shop. Come on now...less than 1,000 miles. Oh well they are off my list after this. Live and learn.

Pistons and other work should be done around Wednesday next week :D

[Wesdog]

Bruce,

You might want to read through this thread:

http://pub186.ezboard.com/famcforummodifications.showMessageRange?topicID=12 60.topic&start=1&stop=20

Wes

[bigun]

it just goes to show you have to check everything or have a machine shop that you trust to do it.