Found a 401 with a bad slug

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JeepManiac
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Joined: Sun Aug 12, 2018 6:42 pm

Found a 401 with a bad slug

Postby JeepManiac » Tue Sep 11, 2018 1:57 pm

There is a 401 for sale near me (technically a boat ride away) and it came from a 70's J-truck. I have an '87 J20 with a 360.

It apparently has a hole in a piston. Low miles (70K) and looks good according to the seller. When he turns it over there is no damage to the cylinder wall but there is play like a rod knock in the piston. I'm wondering what might be wrong with the rod and if it just needs new bearings.

I'd love to get away with a basic refresh with new bearings, seals, hone and rings. If I were to just replace the damaged piston (if there is not damage to the rod) is there any need for a rebalance? I'm just not sure how the external balance works, and if I can replace the piston with a stock replacement piston and be good to go? I made sure it comes with the flex plate.

I'm looking around at a lot of different swaps to make it a decent tow rig. I would love to keep it AMC and toss a 401 in it.

440sixpack
Posts: 332
Joined: Sun Jul 31, 2016 6:31 am

Re: Found a 401 with a bad slug

Postby 440sixpack » Tue Sep 11, 2018 3:18 pm

If the piston has a hole in it changes are it's collapsed which will make a noise when it changes directions. or it could be the wrist pin.

If you're luckier than me you can find a matching piston weight check it and drop it in. and if you're really lucky a light fresh up will do it. it's possible but not probable.


I would plan on it being a rebuildable core and if you get off easier than that it's a bonus. you won't really know until you tear it down and check it out.

FSJ Guy
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Re: Found a 401 with a bad slug

Postby FSJ Guy » Wed Sep 12, 2018 1:17 am

There's no such thing as a "light refresh" on an engine rebuild. LOL!

Once you have it apart, you will want to do all sorts of things and before you know it, you're bored 0.030 over, fancy new pistons, "RV" cam and rebuilt heads to match. And.... $4K later, you have a nice fresh 401.
:- )
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fulsizjeep
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Re: Found a 401 with a bad slug

Postby fulsizjeep » Wed Sep 12, 2018 7:11 am

"It apparently has a hole in a piston."

What does this mean? Really?

How much do they want? Has it been opened up? How do they know there is a hole in a piston? Is the crank OK?

A 70s FSJ odometer does not show how many increments of 100,000 miles are on it. Too many unknowns right now to understand if it is scrap or a rebuilder.

Buyer beware.
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440sixpack
Posts: 332
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Re: Found a 401 with a bad slug

Postby 440sixpack » Wed Sep 12, 2018 8:05 am

There is a light refresh , if you're a cheap skate or if you don't plan on keeping it. rings, bearings , seals , lifters and so on. not what I'd do but it's done all the time.

JeepManiac
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Re: Found a 401 with a bad slug

Postby JeepManiac » Wed Sep 12, 2018 12:33 pm

The fella sent some pics. It might have had a rod knock, but no hole in the piston. He said "it's been a while since I pulled it out and I think I thought the damage was worse until I got the heads off." There is definitely play in one of the pistons/rods

Honestly I get it. I've done stuff like that with a tranny I pulled and found again 6 years later. He was going to swap it into a FSJ, but the FSJ he ended up buying came with a machined 360 block ready to be built. Sometimes a lot of time can pass during these projects.

The initial description made me think the previous owner was spraying nitrous to be honest.

Casting number on the block checks out.

Also I understand what a "correct" rebuild is and a refresh. I'm trying to understand when a balance is needed on these. I built a very high strung 4.7L stroker for my XJ. It was bored/honed and decked with a new "built" aluminum head. The one thing I don't fully understand though is when rods need or have to be resized. I used new rods so I didn't have to worry about that. I mean if you torque everything up and plastigauge it and are within spec.......then you're within spec. I plan to just thoroughly inspect this engine if I get it and see if the cylinders are serviceable with a hone and so on. I would likely have it hot tanked and cam bearings replaced.

440sixpack
Posts: 332
Joined: Sun Jul 31, 2016 6:31 am

Re: Found a 401 with a bad slug

Postby 440sixpack » Wed Sep 12, 2018 3:56 pm

If it has a rod knock you need to be more worried about the crank than the rod. rods are cheap and easy to get, 401 cranks not so much.

The easiest you're going to get off in this case is a crank grind and a rod check with new bearings. not a big deal. it's downhill from there if you have a problem.


You won't pick up whether rods need resized or not with a simple plastigauge check. resizing is more about roundness .. just plan on having them checked. looking at the bearings will tell you a lot.


It sounds like it worth checking out. it may all be fixable in a standard rebuild.


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