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View Full Version : Keep Losing Pistons!!??**##



70ssmuscle
05-15-2008, 10:23 AM
Thanks for taking the time to read this one. Hope you like a challenge.

Brief background: 69 AMX 390 automatice car. Vehicle involved in underhood carberator fire. Minor cosmetics and charring under the hood. Nothing major.

In lieu of the engine fire the entire cavity was torn down and engine sent to our normal machine shop.

The engine was returned fully assembled from the rebuilder and installed. The claim was that the engine was bored .030 over and had all of the necessary components replaced. New pistons etc.

The vehicle was driven a short period of time and soon after developed an engine noise that was believed to be lower end. We did the tear down and found several of the pistons and walls were scored heavily appearing to have been overheated from a lack of lubrication. The engine was presented to the machine shop for repair with the promise they would take care of it.

Again the machine shop assembled the motor and we re-installed. The noise returned without ever driving the car down the road. This time we called in the engine rebuilder and made him refund our money and sought the help of another highly recommended shop to do the machine work. This time we found that the last machine shop tried to quick fix the engine and never replaced all of the pistons that were bad.

This time it was sent to the new machine shop and had to be bored to .040 over due to the extensive damage. Engine received all new pistons and we did the reassembly/re-install checking all clearances ourselves. While we were re-assembling the motor we discovered that the new Clevite crank bearings did not have the small notches cut into them to allow the external connecting rod hole to get the much needed oil to “spurt” onto the piston wall thus lubricating the wall and piston. We chamfered the bearings to accommodate the oil passage.

Oil was primed, pressure was checked, engine was fired and she ran for a good 100 miles without incident. Much to our disbelief, the noise returned again. This time it was #7 & 8 pistons with the same scoring as before collapsed and scored from heat. The other cylinder walls were showing signs of bad things to come as well.

We are at a loss at this point. My engine man is an ASE Advanced Performance tech that has been building engines for years and has also been part of an alcohol drag team. I have all the confidence in the world that he is on point. The new machine shop is also baffled.

I am looking for any advice/ recommendations or further contacts you may have to assist with this problem. Looks like I’m also going to need a reliable source for more replacement pistons and parts. Possibly forged but I don't see this as being necessary for a stock driver.

I could just be dealing with defective pistons. I can accept once but twice from 2 different sources? I'm starting to look elsewhere.

Any ideas from the AMC experts are welcome.

70ssmuscle

Big Bad AMX
05-16-2008, 02:07 PM
Thanks for taking the time to read this one. Hope you like a challenge.

Brief background: 69 AMX 390 automatice car. Vehicle involved in underhood carberator fire. Minor cosmetics and charring under the hood. Nothing major.

In lieu of the engine fire the entire cavity was torn down and engine sent to our normal machine shop.

The engine was returned fully assembled from the rebuilder and installed. The claim was that the engine was bored .030 over and had all of the necessary components replaced. New pistons etc.

The vehicle was driven a short period of time and soon after developed an engine noise that was believed to be lower end. We did the tear down and found several of the pistons and walls were scored heavily appearing to have been overheated from a lack of lubrication. The engine was presented to the machine shop for repair with the promise they would take care of it.

Again the machine shop assembled the motor and we re-installed. The noise returned without ever driving the car down the road. This time we called in the engine rebuilder and made him refund our money and sought the help of another highly recommended shop to do the machine work. This time we found that the last machine shop tried to quick fix the engine and never replaced all of the pistons that were bad.

This time it was sent to the new machine shop and had to be bored to .040 over due to the extensive damage. Engine received all new pistons and we did the reassembly/re-install checking all clearances ourselves. While we were re-assembling the motor we discovered that the new Clevite crank bearings did not have the small notches cut into them to allow the external connecting rod hole to get the much needed oil to “spurt” onto the piston wall thus lubricating the wall and piston. We chamfered the bearings to accommodate the oil passage.

Oil was primed, pressure was checked, engine was fired and she ran for a good 100 miles without incident. Much to our disbelief, the noise returned again. This time it was #7 & 8 pistons with the same scoring as before collapsed and scored from heat. The other cylinder walls were showing signs of bad things to come as well.

We are at a loss at this point. My engine man is an ASE Advanced Performance tech that has been building engines for years and has also been part of an alcohol drag team. I have all the confidence in the world that he is on point. The new machine shop is also baffled.

I am looking for any advice/ recommendations or further contacts you may have to assist with this problem. Looks like I’m also going to need a reliable source for more replacement pistons and parts. Possibly forged but I don't see this as being necessary for a stock driver.

I could just be dealing with defective pistons. I can accept once but twice from 2 different sources? I'm starting to look elsewhere.

Any ideas from the AMC experts are welcome.

70ssmuscleI'll get into more specifics with more info but if the engine builder isn't familiar with AMC engines (the first one obviously wasn't) then assembly mistakes are quite common as they don't go together like a Big 3 V-8. So, just a few questions for you just to cover some basics and eliminate a few things: you mention you checked the clearances, what were they and especially what were the bearing tolerences you used? Was a new timing chain cover and oil pump installed? What was the oil pressure range at idle and speed (2500+ RPM)? Was it cylinders 7 and 8 each time or?

Also, the water passages could very well be blocked and creating hot spots around the cylinders. Pretty hard to find too but in 40 year old cast iron blocks it's a distinct possibilty.

And, you don't need forged pistons - cast are fine unless you'll be running the car hard. And since you're now at 40 over I'd recommend a 4-core radiator and high-volume water pump or you're likely to have over heating problems.

If you haven't already take a look at these threads for rebuild and assembly tips, ideas and other issues:
http://www.amx-perience.com/AmericanMotorsForum/showthread.php?t=316

http://www.amx-perience.com/AmericanMotorsForum/showthread.php?t=790

'69 and '70 390 clearances are in this one:
http://www.amx-perience.com/AmericanMotorsForum/showthread.php?t=98&page=2&pp=10


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