Truedough

I bought a '97 Grand Cherokee for $500 a few years back. Everything was good except a junk 4.0. he must have never changed the oil because it had a grenaded piston and the oil looked like tar. Recently I finally got enough money to rebuild the engine. i bought a rebuild kit online and sent the block and head to a machine shop for some basic machine work. I only had 120,000 on it so i was hoping to just have it hot tanked, wrist pins and cam bearings installed. They convinced me to go .030 over. I got a good deal under the table. All i did with the head was lap the valves. I had to send the pistons and rings back for the .030 ones. Anyway i rebuilt it and in less than 50 miles it was making a noise like a collapsed lifter. It wasn't. i pulled the head and found all pistons with rub marks on the cylinder walls but #5 was the one making the noise. It is so bad that I will be lucky to bore it again to salvage it. I researched the P/N's for the pistons and rings and it turns out they sent the 93-95 ones. i did more research to find they are slightly different yet they are the same bore. Is that my problem or just coincedense? i am not an engine builder and could have screwed something up without knowing. Does the company that gave me the wrong parts owe me a long block or am i screwed?

dave1123

I don't believe the earlier pistons are the problem, although I could be wrong. I HAVE built many Chevy engine in my life and to me, it looks like they fitted the pistons too tight. Did they fit the pistons or did they just bore it? Check the piston clearance specs for the earlier pistons as opposed to the specs for the 97s. Each piston should be feeler-guaged on assembly and the cylinder howned if it's too tight. That's the way to do it right. That's the way we did it on race engines because of the high heat and stress involved. The earlier pistons may be heavier which will require more clearance because of expansion. It also may throw off your crank's balance if the difference is substantial. In engine rebuilding, it's the little things that will come back to bite you.

That's another thing with going to 30 over. If the cylinder walls are scored, the block is a throw-away. Maybe not. Check with the shop that did the work and see what they suggest.

I have a friend with a classic 51 Pontiac that had a granaded piston. We had just had that one cylinder rebored 30 over and it runs perfectly. The oversized piston is the same weight as the standard one. We went 30 over because that's all we could get. These pistons are fragile cast iron.

Bustedback

Yeah, I think the bores were too tight. The aluminum piston will expand faster than the cylinder walls and cause the piston to get stuck or break the bottom half out of it.

Truedough

Thanks for the input. i will check clearances on the others and get with the machine shop. Here is a picture of the #2 piston. I haven't pulled the rest but this one's cylinder walls are consistant with 1,3,4 and 6.

Is it possible that it is lack of lubrication? I had a problem with the oil pressure gauge falling on its face at idle when warm. I am 98% certain it is the gauge as I tried 2 senders, both used, as well as a manual gauge. The manual gauge read 9lbs but i questions its accuracy. i know that is low but i believe the book only calls for 13. it went up to 30+ when I revved it but never went much above 1000 due to all the clacking the engine was making.

dave1123

No, from the pics, I'd say there is a clearance problem. The section that is scuffed is the first part to get hot and expand. Looking at those pistons, I see the skirts are very heavy when compared to Chevy V8 pistons. This means to me, they expand more.

The lube question has me wondering. On GM V8s, there is an oil squirt hole in the rods that lubricates the cylinder walls whenever the hole indexes with the bearing supply hole in the crank journal. If yours has the same feature, do the holes in the bearing shells align with the hole in the rods?

Truedough

i inspected the bearings and i did them correct. i was pretty confident but for some reason my memory could not give me a conclusive answer. The bearings on the rods only go one way, both top and bottom. they both have a half circle cut out oppisite of the tang. it lines up with a little notch by the stud. Seems a bit hokey to me but that is obviously the design. i guess i will call the machine shop in the morning.

dave1123

No, that's right. That little notch is the cylinder oiler hole. After I told you about the oil squirters on the Chevys, I remember that notch on the rod cap lubricates the opposite cylinder of the pair. The old Hemis had full floated wrist pins so they needed an oil passage up thru the rods. It was little things like that plus 5/16" thick cylinder barrels that caused it to weigh 750 lbs! You could bore that sucker 1/8" and still supercharge it! That's the old 392 from '58. Keith Black actually bought the casting molds from Chrysler so he could continue producing them for racing.

I guess you didn't need to know that. I get carried away sometimes. Sorry.

Truedough

Well i talked to the machinist today. He doesn't think it was his work that caused it but he is willing to look at it and if salvageable, fix it for free. My company does a lot of business with his and he understands my money situation.

As I stated before, I am no engine builder and am still new to it. he thinks it may have been the ring gap. I was unaware that I needed to check it. Does that look like something that might be caused by rings that should have been filed down? he has not seen pictures yet and only going off what I have told him. he also asked if it over heated. i can assure it has not.

dave1123

It's possible, but I doubt it. From the localized scuff, I still think it's piston skirt clearance. Usually, piston ring end gap problems cause the rings to break or scuff the walls around the entire diameter, no just in one spot. They might on V8s because gravity gets involved with the pistons no running vertical. I know V8 bores tend to egg-shape because of this, or wear off center. When you cut the wear-lip at the top of the cylinders on a V8, they will be heavier on the bottom.

Checking the ring gap is one of those little time-consuming jobs that will come back to bite you if not done. One thing you have to remember is pistons are not round, they are designed so that they become round when heated.

It sounds like the machinist is willing to work with you to resolve the problem. That's good.

Truedough

So, i brought the block back to the machine shop. he thinks it overheated. Before I brought it I looked at it again. When i took it apart i was in a hurry but on further inspection i noticed the cam bearings were spitting out. See the picture. 3 of the 4 look this way. he said they just wore on the bottom and did not spin. The cam is fine. anyway, they say i got an air pocket and never got it burpped out. he even mentioned a bullitin he found stating the problem and that there is a special procedure, well more than just filling the radiator and running and burping the hoses. Something about filling the heater hoses and what not due to the air pocket. is he on to something or just blowing smoke?

Bustedback

That cam bearing does not look right to me.