258 knock when cold

[jamesweed]

hello, My stock AMC 258 i6 / 4.2l produces a very noticeable knock for about 3-5 seconds upon first start of the day when cold. After that and when warmed up its not so noticeable. I'm getting ready to do a 4.0 head install with TBI and want to be sure the bottom end is healthy before doing so. Its doesnt burn oil and Ive done compression check with good even AMC 150 psi results.

That being said I dont believe the engine has ever been rebuilt. its prob. got about 150000 miles on it. Just looking for opinions on if I should rebuild the bottom end now or run it as is.

Thanks.

 

[coldair]

What kind of oil pressure is it running?

 

[IOPort51]

I would be thinking "piston slap". the piston skirt has enough play to hit the cylinder wall until it's warm. It is kind a byproduct of the use of unleaded fuel , from what I have read. There are ways to check for it and it will be a lot easier to check for play at TDC and BDC of all six pistons with the head off. From what I have read it is not catastrophic like a rod knock and you can replace the piston or live with it. Subaru has a major problem with it in some of their cars and from what I understand they have declared it a non issue. Do a Google search on Subaru/piston slap and you may find the knowledge you seek.
Most of this information is from my own memory and should be confirmed before it is considered anything more than the ravings of a senile old man.

 

[CJim7]

I would be thinking "piston slap". the piston skirt has enough play to hit the cylinder wall until it's warm. It is kind a byproduct of the use of unleaded fuel , from what I have read. There are ways to check for it and it will be a lot easier to check for play at TDC and BDC of all six pistons with the head off. From what I have read it is not catastrophic like a rod knock and you can replace the piston or live with it. Subaru has a major problem with it in some of their cars and from what I understand they have declared it a non issue. Do a Google search on Subaru/piston slap and you may find the knowledge you seek.
Most of this information is from my own memory and should be confirmed before it is considered anything more than the ravings of a senile old man.

IO, piston slap is a big problem on our jeep motors I had to tear down my 401 and have the #8 cylinder sleeved by a shop because I developed piston slap when a wristpin froze. Anytime the piston skirt contacts the cylinder wall...it's bad news.

Now, a froze wristpin causes the damage very quickly, but occasional piston slap will eventually cause the same damage down the road.

Here's the factor to consider:
on a short stroke engine built for RPM's (like a Subaru, Datsun..most engines really), the damage can possibly be under the working range of the piston rings. That will not be an issue. On AMC 258 i6 / 4.2l 's, and AMC V8's the longer stroke exposes the damaged area to the range of the rings. When I tore down my 401, the damage was actually above the lead edge of the piston with #8 piston all the way down in the cylinder

So a short-stroke engine doesnt suffer the same consequences as a longer stroke engine when piston slap is present.

 

[IOPort51]

I had a slap in the AMC 258 i6 / 4.2l in my J-10. Major annoying. motor already 60 over and no plans to rebuild. Had about 75k on it so I let it go, went about 50 k that way. the J-10 is now in semi retirement but still capable of running.
I am with you 100% and with the J-10 spending 99% of its time at 70+ MPH the dynamics are much different from the way I would be driving my CJ. I would fix it ASAP but it is always good to have a good idea what the problem could be before just checking rod bearings and scratching your head when they mike out OK. As always, we report, you decide.

as another point to consider I think Subaru s boxer engine may have had something to do with their problem as well.

 

[jamesweed]

What kind of oil pressure is it running?

Its got about 20 psi at idle. I can definately still here it at idle, but isnt noticeable at higher engine speeds. I'm leaning towards rebuilding the bottom end now, maybe not totally necessary, but if I don't it'll probably start smoking oil about a week after I put the new head on. Like I said not sure this things ever had a rebuild, Ive ony had it for 9 months, the PO wasnt very big on maintenance from what ive seen on other areas of the jeep.

 

[coldair]

Sounds like the bearings on the crank and cam are good.

 

[coldair]

If it was me i would pull the pistions out mic them and the do the cylinders too make sure they arent oblonged. You might get away with just a set of pistion and rings.