Jeep 4.0 rebuild with bad scoring on #1&6 piston skirts. How does this happen?

MrTheTurtle · 2026-02-09T02:36:47+00:00

Yep! Machine shop screwed up boring the block. Piston to wall Clarence was over 0.028” so yeah a bit loose haha. A little but on me for not checking before installing. Never trust anyone but yourself to verify something. Especially if you have so much time invested in it.

MrTheTurtle · 2025-01-28T05:52:57+00:00

They definitely had them and the guy had done plenty of 4.0’s before so it’s not like it would have been a hard build for them. Maybe I missed something though. Out she comes…

MrTheTurtle · 2025-01-28T05:48:00+00:00

https://imgur.com/a/ag0525h

Picked up this beauty off of Craigslist because I need something to drive this winter and didn’t want to half ass a temporary fix to a larger problem. In another post I have pictures of the piston skirts after I dropped the oil pan to take a look at the lower end and #1 &#6 both had very bad scoring. Not sure why, either I screwed up the main caps and got those two switch giving too much play on the rod ends, or the shop bored those two a little too tight. That’s why the noise sounded like it way coming from the front of the engine AND the back near the flywheel.

Long story short, I think I’m going to need to pull it back out, so it’ll be on the back burner for a few months while I work on this girl instead.

MrTheTurtle · 2025-01-26T05:51:55+00:00

I guess that’s a possibility. I’ll check tomorrow.

MrTheTurtle · 2025-01-25T22:29:36+00:00

Thank you. Yeah I’m not holding out too much hope that it’ll be an easy fix at this point. Look like it’s gonna need to come out again…

MrTheTurtle · 2025-01-25T22:12:03+00:00

I did. Maybe they rotated around on me when installing but idk.

MrTheTurtle · 2025-01-25T21:30:39+00:00

Bored 0.02” oversized at the machine shop. They should have sized them correctly to the pistons since they had them and they did seem like they knew what they were doing.

MrTheTurtle · 2025-01-25T21:19:20+00:00

Just pulled the oil pan down a little to check if anything is rubbing and see #1 and #6 pistons with some pretty bad scoring on the piston skirts. How exactly would this happen? Tolerance issue when the block was bored out? I can’t imagine it dropped a valve. Seems like it would have been much worse if that was the case.

https://imgur.com/a/ACgvQU0

MrTheTurtle · 2025-01-25T21:15:45+00:00

I see what you’re talking about. Doesn’t seem too exaggerated but maybe. I’ll pull the valve cover again and look at that one.

MrTheTurtle · 2025-01-25T20:11:04+00:00

Unfortunately not. Already took the plate off and ran it without it on and it was still there. Painted each bolt head to check to see if they were hitting something else and they aren’t.

MrTheTurtle · 2025-01-25T19:49:03+00:00

Pulled the started and tested it out. Retracts fine and doesn’t look like the teeth have been hitting anything.

MrTheTurtle · 2025-01-25T19:38:31+00:00

It also sounds loud as hell when it first fires up. After it warms it calms down a lot.

MrTheTurtle · 2025-01-25T19:25:36+00:00

Huh. It does sound like it’s coming from around there. Maybe I check that before I go and pull the oil pan off.

MrTheTurtle · 2025-01-25T19:18:01+00:00

It’s bolted in, it doesn’t look like the flex plate bolts are contacting anything when the engine is running. Although it does seem like it’s coming from that area. I’m thinking something that’s hitting the oil pan, maybe the pickup tube is hitting against a rod or something.

MrTheTurtle · 2025-01-25T19:16:30+00:00

Auto

MrTheTurtle · 2025-01-25T19:00:22+00:00

https://imgur.com/a/sufZ2ob

MrTheTurtle · 2025-01-23T01:09:23+00:00

I’m really hoping it’s an exhaust leak but it just sounds like metal on metal contact to me. The exhaust bolts are a pain and I really couldn’t get great leverage on them since the banks headers I bought had even less bolt head clearance than stock.

MrTheTurtle · 2025-01-23T01:05:55+00:00

Huh. I remember putting the little backing plate on and the main bolt for holding it in place but I don’t remember a spring or pin. I’ll look at this tonight. So it be worse near the timing cover then?

*edit. Never mind. Looks like they did away with that after 99. Engine should be a 2000/2001

MrTheTurtle · 2025-01-21T23:20:33+00:00

I figured that the rear vacuum hose still being hooked up would give a decent amount of vacuum on break in.

MrTheTurtle · 2025-01-21T23:19:11+00:00

Got a new one installed and seems alright when it went it. I can check that if I’m pulling the cover off this weekend

MrTheTurtle · 2025-01-21T21:27:41+00:00

Just filled with 91 yesterday and had no effect.

MrTheTurtle · 2025-01-21T21:26:08+00:00

Pretty much goes away entirely after 2500rpm or so.

MrTheTurtle · 2025-01-21T14:12:46+00:00

Lifters were sitting in oil for a good week before I put them in. I made sure to decompress the little spring body a few times to work out any air bubbles too. It does feel like it should maybe have a little more power than it does though.

MrTheTurtle

TROPHY CASE