1100 miles into my rebuilt motor (pre-emissions 350), it's knocking. Sounds like the bottom end in the back of the motor. Utmost care was taken when I bolted the machined parts together. All bearings were plasti-gaged, all torques on all bolts correct. Cam was degreed in and piston clearances and rings checked. Motor was pre lubed before starting (2000 rpm- 20 minutes).

Ran it real easy driving the first 500 miles, then changed oil and filter. Ran motor normal driving for another 500 and changed oil and filter again.

Now at 1100 miles, I can hear knocking from the block, I think? Sound increases with engine speed, load or not. Oil pressure is the same as when I first started it, 15-45 psi.

Here's what I have done so far:

Took a stethascope to the motor, sound coming from the bottom rear, hard to isolate.

Pulled 1 plug wire at a time (thinking a rod)- no changes.

Disconnected torque converter from flex plate (maybe tranny or TQ conv noise), ran motor- no changes

Removed all belts from accessories (just in case sound was transmitted down)- no changes

Removed fuel pump (weak spring in pump might cause rod to slap cam lobe)- no changes

Checked crank end play, no problem there.


HEEELLLPPP, I really don't wan't to take the motor back out, but I will if I have too, I figure it will cost about 100 bucks just for fluids and gaskets:mad:- not including my labor. I hope it turns out to be something stupid:zzz: and cheap:question: . Somehow, I doubt it though........

Any other ideas????????

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have you checked the cam?
this is a semi wild guess as I have only worked on Fords, but I had an engine blow many years ago because of a loose cam. The sound was being transmitted to the bottom end through the timing chain so it sounded like it was coming from the bottom end and we could never find what the problem was till it was too late.

i would go back and check the mains and rod bearings, if u spun one of those, then its toast, also check lifters(hope you put new ones in), but first i'd pull the oil pan and just look, gaskets are cheap and oil will be about 20-25 w/ filter and all

Yep, I was thinking that as well.... as a start.

BTW- New lifters, rocker arms, cam etc...

Originally posted by injectedEB
have you checked the cam?
this is a semi wild guess as I have only worked on Fords, but I had an engine blow many years ago because of a loose cam. The sound was being transmitted to the bottom end through the timing chain so it sounded like it was coming from the bottom end and we could never find what the problem was till it was too late.

Thanks dude:) that will happen if I wind up pulling the motor- I'll tear it down til the problem is located

I just had that same thing happen to a 350 with about 2500 miles on it after a rebuild. Spun the #5 rod bearing. Cranks toast so is the rod. Time for new stuff :( good luck!

Originally posted by mike
I just had that same thing happen to a 350 with about 2500 miles on it after a rebuild. Spun the #5 rod bearing. Cranks toast so is the rod. Time for new stuff :( good luck!

Thanks man:), Got the oil pan off checking rod and main bearings, as well as end play tonight. How the hell does a rod bearing spin? What fails?

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Whenever I start a new engine I first pull the distributer and insert a tool that I made , but can also be bought at most parts sellers. It is a 3/8" rod welded to an old chevy distributor oil pump driver shaft, without the distributer drive gear on it. I weld on the 3/8" dia. extension so it's long enough to extend out past the intake manifold, this is so I can chuck it up in a hand drill. Then you just crank the oil pump with the drill so you can get oil to all the bearings and cam lifters before you start the engine. You will feel the difference in pressure once you've cranked the oil pump with the drill motor fof a short time. Of course if you did this then it could be some other problem.

it sounds like you have done about everything you can do besides droping the pan. i hate to say it, but thats next. mike

How well did you clean the block prior to the rebuild? Did all the oil passages get cleaned out? If you did spin a bearing, it probably wasn't oiling well. 15psi seems a bit on the low side for a new motor.

Originally posted by FWP


Thanks man:), Got the oil pan off checking rod and main bearings, as well as end play tonight. How the hell does a rod bearing spin? What fails?

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Oiling, or the caps are torqued wrong or the wrong sized bearing... I have yet to figure out what the shop that built it did wrong. Doesnt matter much now though its outa warranty and needs a ful rebuild. Personally Im just happy it wasnt a main so all I need is a new crank and rod. :D Looks like this though <br>http://ajax.extremejeep.com/mike/Pictures/newengine/DSC00051.jpg

Thanks for the ideas guys. Good picture there Mike....Something is really screwy with the main bearings[crazy]or crank. I get wear marks (shiney- not the usual grey colored wear) on one half of a bearing and none on the other half. For example- The bottom half front bearing has one wear area towards the rear of the motor, and no wear on the front, the top half is shiny all around except for 3/8 inch on the sides. Almost like the bearings are the wrong size, or the crank was ground wrong (It was supposed to be 10/10)

The mains plastigaged to spec (1.5-3) when I first put it together......It's really bizarre.......I've never seen anything like it......Anyway, I'm taking the bearings to my machinist for further evaluation.

Well, it is normal for the bottom bearing to wear more on the mains. Notice that the top bearing has a groove for oiling the rods, but the lower bearing does not. That is because the bottom bearing carries the most load, and grooving it for oiling purposes drastically decrease the load carrying capability of it. As for why the rear half of the lower bearing is worn, the block could be warped, or the crank may not be straight. And as far as spinning the bearings, there could be many reasons. Were the rods resized? If so, they could have been mismachined. The tabs on the bearings are not for keeping them from spinning. Bearing crush holds the bearing in place. New, the bearing OD is just slightly larger than the bore diameter, causing them to crush into place. Also, the backs of the bearings, and the insides of the rods, must be absolutely clean and dry when the bearings are installed. They could have assembled it dirty also.

Best way to tell if the block is (for the garage kinda mechanic) to place the crank in the block with the caps torqued, no timing chain installed, or rods/pistons either. The crank should turn easy, with little effort. Also, you can place the front and rear main bearings in the block, oil them up good, and place the crank in the block. Dont install the caps. Use a dial indicator at the center main and turn the crank. If you have main clearances less than .003", then you should have no more than .002" runout on the crank.

If you only had 15psi of pressure, then I would think that a standard set of bearings might have been used with the reground crank. That motor should have at least 30psi.

Something to think about too, if the crank is not straight, then the plastigauge will not be accurate. Also, the plastigauge must be placed at 90* from the parting lines. The bearings are thinner at the parting lines (eccentricity) for oiling purposes. If they were clearanced checked there, then the bearings may have been set up too tight, but I would think the oil PSI would be a lot higher if that was the case......

Possibly, the rods had too much clearance. When you use the plastigauge, you have too make sure the crank does not turn, the rods do not twist when you tighten the nuts, and you place the plastigauge in the bearing shell in the rod, not the cap. If you put it in the cap, then when you tap the bolts to loosen the cap to get it off, it can screw up the reading, making it seem tighter than it really is. Put two rods on the same crank journal, and put a feeler gauge on the sides of the rods to prevent the side clearance from allowing them to move.

Another I'd check is to see that the main caps/block were marked, and the rods/rod caps. Make sure they were installed in the correct orientation.

Let us know what the machine shop says!