The new 401 ran good and sounded great for a day or two and about 300 miles then it began to knock badly. I took it to the machine shop where the motor was done and he agreed to pull the pan and check things out. He found nothing until he started cancelling spark from each cylinder. He found that with no spark to the #7 the knock was gone. He said that it has a collapsed piston in #7. What is could be the cause for this and what should I look for when I pull it out to investigate the cause?
You need to take the pan off and remove the #7 bearing cap. Chances are that the bearing is spun. I the bearing is good, then look for a bad wrist pin in the piston (may have to pull the head and remove it). Those are the only 2 things that would cause a knock that would disappear without spark.
Bob
tufcj
1969 AMX
1967 Rambler Rogue
If you need a tool and don't buy it...
you'll eventually pay for it...
and not have it.
Henry Ford
The machine shop has already had the pan off and checked all the bearings. They said that the bearings are all fine. I do plan on pulling the head and pulling the piston. I am just holding my breath and hoping that the block was not damaged at all.
While the piston is out mike the crank and check the bearings. We got a rebuilt a few years ago that had the crank cut .030, but had .010 bearings in it.
A friend got a Ford factory rebuilt 351 that was bored .030 and had standard pistons in it.
Machine shops aren't perfect. Double check everything.
Bob
tufcj
1969 AMX
1967 Rambler Rogue
If you need a tool and don't buy it...
you'll eventually pay for it...
and not have it.
Henry Ford
Got the No. 7 piston out. The skirt on the top side is badly scored. Don't have the tools to accurately measure the crank or cylinder, but the cylinder wall doesn't "feel" too bad as far as scoring. Can't "feel" any slop in the wrist pin.
Compression just prior to tear down was:
#7 150
#5 150
#3 143
#1 157
Wow something went wrong with that puppy. Is the piston pin hole offset with those pistons? Looks like it was installed wrong, or the piston pin bore was offset to the wrong side forcing all the thrust to the galled side. I've never seen that before. BTW no such thing as a collapsed piston, just rings also called a stuck ring. I may be wrong but I don't think it caused the scuffing.
Jeff
1979 Jeep CJ-7 401 AMC, GMC 6-71 Blower 10% overdrive, Bugcatcher with electronic fuel injection and a little N2O for fun, all self built. (this supercharger stuff is easy)
Wow.. I would be asking for someone to measure the cyl walls /bore and the piston diameter..that is some serious skirt damage.. the only time I have seen wear like that was on a teardown that had 300,000 miles on a Small Block Chev.. the bore was badly "egg shapped" letting the piston thrust against the Cyl wall.
"A man's got to know his limitation's"
Dirty Harry.
Yes, we're going to be talking to the machine shop and Ross tomorrow. Funny thing is now I can't find anything on Ross Racing Pistons on fastengineparts.com (maddog) where we got these. Although it says Ross pistons are available under the Master Engine Kit listing for AMC 401 Performance II, when you click on Pistons, then the Ross link, it just says
"Found (0) Makes.
We are sorry, Your search did not turn up any makes under this category. Please try another search."
When I get on Ross's web page, I'm not finding any AMC pistons listed. We just got these a couple months ago.
Yeah I got the same search results for AMC Ross Pistons on the FastEngineParts web page. They do have them listed on eBay though within thier eBay Store. Guess they don't keep thier standalone web page upated.
The Pistons are lubrictaed by Oil coming from a squirt hole in the connecting rod from the oposite bank. If the scored piston is #7, then it would be lubricated by the #8 Connecting Rod squirt hole. The squirt hole is in the seam between the Cap and the Rod. Was the #8 Connecting Rod installed turned the right way ? If it's turned over the wrong way, then the squirt hole is not facing in the direction of the #7 Piston.
Just a guess.
Also, if the rods are reconditioned, it's possible that the squirt hole was eliminated because the mating surfaces of the Rod and Cap are machined to egg shape the rod journal hole so it can then be made round again. If I remeber correctly, this is why there is a hole in the rod bearings at the mating edges so oil can escape and feed the squirt holes.
Just another guess.
I'll have to look at all that tonite. I did notice that the top half of the No. 7 rod bearing seems to be wearing more right in the middle rather than evenly over the entire length.
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