So two weekends ago (the 11th and 12th) I drove up to the Sawtooth Mtns in Idaho, just north of Ketchum (gorgeous country) to go 4 wheeling and camping. Put about 800 miles on it in two days. It ran fine until on the way home I notice a slight knock at about 45-60 mph in 4th gear, at VERY slight throttle only.
When I got back home, the knocking seemed to get worse so I decided to drop the pan and check the rod bearings.

Before I tell you what I saw let me give you all the info I have in the motor;

• Has 40,000 mi on a rebuild, which I believe.
• I replcead all the gaskets, the crank bearings, and the cam and lifters before I put it in the truck about three weeks ago. The crank is turned 10/10.
• It has 60 PSI oil presure at speed, 40 at idle, hot, w/ 5w/30 oil.
• At this point it runs pretty darn good all the time. Good power(not great though), starts INSTANTLY, idles fine, except for this noise it makes at 45-60....

So after returning from Idaho, I pulled the pan and the rod caps they were all in supurb condition except for one which was NOT spun, but had seen some debris. I mic'ed the crank and then changed them all. Put the pan back on, filled it w/oil and fired it. Same thing. 45-60 mph LIGHT throttle. So then I thought it might be the oil pick up tube getting hit by the crank, so I dropped the pan again. NOPE. While in there I dropped a rod cap again and the bearing shell looked perfect -unused as a matter of fact, which it should have since it only had 30 miles on it. But that confirnmed that there is nothing instantly destroying the bearing shells.

Now here is the wierd part. It is not NEARLY as loud as a spun bearing generally is, and none of the bearngs WERE spun. Just one had had some debris go through it. It is a higher pitch (than a typical rod knock). It is NOT VALVE NOISE here folks; it sounds like a SMALL ball-peen hammer hitting a hardend steel anvil, w/every OTHER revolution. (in sync w/ the ehxuast pulse sound). I cannot hear it at idle at all nor can I hear it when I rev the engine lightly when parked. It may make the noise in gears other than 4th, but the T-18 is too loud in 1st, 2nd and 3rd for the noise that is too quite. I'm starting to think it is a wrist pin noise. But I'm pretty much out of ideas. I'm REALLY staring to think that I'd like to run it until it throws a rod (lol) and the replace it w/a SBC 400 like my car has Input please.

 

hey tom

what engine do you have? if you are only having problems with one rod bearing then I would concentrate on that cylinder. I would check to see that the oil passage in the crank isn't pluged up. also you said you mic'ed the crank, did you check the rod it self? maybe it is slightly out of round or oversized. You might also pull the plug one that cylinder and see if it is damaged(preignition or detonation) hope this helps. I hate to see a good jeep down. I like mine almost as much as my RS.

 

wrist pin or piston slap will sound similar to a bearing going out. have you checked the thrust clearance on the crank? if you measaured the crank and used the corect bearings for it that really should eliminate the rod bearings.

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ICON Motorsports
1st & 3rd
MM Black Diamond 538 F&AM

 

I lean toward wrist pin.

Maybe a cracked piston skirt.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R
and 2 POS commuters...

R.I.P. #3
"He can see air"

 

Slayer, I have a 401 V-8. The one bearing I had a problem with was cause by debris. But the damage was not significant enough to warrant a knock IMO. Since it doesn['t make noise at idle or when revved when parked, It is hard to determine which cylinder it is.

Ede, I did check the endplay on the crank and it was within spec. At this point all the bearings have been replaced, and it still makes a "ball-peen hitting a marble floor" sound.

Engineboy, none of the piston skirts are cracked I saw them all as I rotated the engine and replaced the rod bearing shells. I'm pretty much leaning towards wrist pin noise at this point, which brings me to another question; How long can I drive it like this? I am really not interested in pulling all 8 pistons and replacing them right now, since I don't know which piston it is.

 

How long can you drive it??.....ford 360/390s have been on the road for years with that noise, I think it was stock.

I think I would drive it till it either popped, or got to the point where you don't want to listen to it anymore.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R
and 2 POS commuters...

R.I.P. #3
"He can see air"

 

10-4 Engineboy. Thanks for confirming my "plan". lol. At this point the noise does not seem to be getting any louder; it was just as loud right off the bat as it is now, several weeks into operation. So I am going to run it -at least through the winter, and then probably do an Advanced Adapters SBC 400 Coversion. I think the 400 in my car runs WAY better, and is WAY, WAY tougher than the AMC 401. JMOH. Thanks!

 

Good luck, Let us know if there is any *bang* photos available..

Actually, without being able to "hear" the noise, since its not getting any worse, you will probably be OK.

Keep us informed.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R
and 2 POS commuters...

R.I.P. #3
"He can see air"

[This message has been edited by Engineboy (edited November 25, 2001).]