How Robust Is The 4.0 Engine Block?

[Oyaji]

Per the thread title, I have a couple questions about the 4.0 I-6 engine block. I know that ultimately the solution will be to mic the cylinders to see what the condition of the block actually is, but would appreciate some general advice about what to expect before tearing engines down.

 

First, what is commonplace for wear? Does the 4.0 typically need to have the cylinders bored for an overhaul, or is it typical to be able to get away with simply honing and going back with standard pistons? (For that matter, how well do the pistons usually hold up - should one plan on replacing them too, or can a set of rings suffice?)

 

Secondly, can a simple re-ringing be accomplished with the engine in place by just pulling the head, dropping the oil pan, removing the rod caps, and pushing out the rods and pistons? Is there a difference in available clearance depending on 2wd versus 4wd? And do the main bearings generally hold up well enough to reuse them and only just replacing the rod bearings (provided that the rod bearings are still in the babbit)? 

 

I do realize that individual cases will vary, that ultimately there are no reliable shortcuts, and that blind faith is no substitute for diligence and proper method, but I am just looking for generality here. If it makes any difference whether HO or Renix engines, please note the differences in your experience. Any other tips would be greatly appreciated as well.

 

Thanks in advance.

[carnuck]

1 and 6 tend to wear the most due to uneven cooling. I would do a sonic check before boring. '91-95 blocks were "weakest" due to too much metal taken out for lightening. Thats why later years got a bolt in "ribcage" aka girdle to beef them up. I saw a '91 that cracked top to bottom and the only thing holding it together was the head and oilpan!

[cruiser54]

Yes, an in-frame overhaul can be done. The Renix blocks were high nickel content and wore the best.

 

I've never seen a 4.0 block cracked as described above. 

 

The fact is the girdle was ADDED to the same old block in order to reduce NVH, not strengthen the block due to lightening. Using the later model main bearing bolts/studs, you too can add the girdle to any 4.0. 

 

Anyway, it's all doable. 

[oldseddie]

My NASCAR engine builder buddy had me sonic check both the renix and 96 ho blocks I had to see which one had the most meat for boring due to core shift during manufacture. The ho block was the thickest. Now Cruiser says the renix has the most nickel content after Ben just bored it for the Diamond pistons.

 

[Eagle]

Minor year-to-year differences aside, the 4.0L and the AMC engines on which it's based are incredibly "robust." Not many people remember that one of the AMC predecessors was used for several years in Indy cars by a second-tier driver named Barney Navarro. I don't think there are many other street engines that can make that claim.

 

The conventional wisdom is that you can generally go to .060" over with no problems, and often to .090" over. Beyond that, the block should be checked for core shift to be sure you don't break through the cylinder walls. Whether or not a particular block will need to be bored as part of a rebuild depends on too many factors to have a valid answer. Primary, of course, is how many miles are on the engine. Considering that I have one that's currently at 287,000 miles and still runs well and doesn't use oil, my guess is that barring abuse anything with 150,000 or less can "probably" be rebuilt without boring. Maybe even up to 200,000.

 

The only way to know, obviously, will be to mike the cylinder bores and check for out-of-round.

[Alexia]

The 1997 block I disassembled with 220,000 miles for my stroker still had hash marks in the cylinders and the journal bearings could have been left alone if someone was struggling for money. http://imgur.com/a/e8vip#0

 

The engine in my 2004 Wrangler has been knocking for over 17,000 miles and just hit 157,300 miles.

[HOrnbrod]

The 1997 block I disassembled with 220,000 miles for my stroker still had hash marks in the cylinders and the journal bearings could have been left alone if someone was struggling for money. http://imgur.com/a/e8vip#0

 

The engine in my 2004 Wrangler has been knocking for over 17,000 miles and just hit 157,300 miles.

 

Same here for my stroker, except I used a 96 block (same 53020569 casting number as the 97-98 blocks) w. 150K on it . I only had them go .030 overbore though to make sure I had ample room left should a rebore ever be necessary. Hopefully it won't be......

[johnj92131]

Minor year-to-year differences aside, the 4.0L and the AMC engines on which it's based are incredibly "robust." Not many people remember that one of the AMC predecessors was used for several years in Indy cars by a second-tier driver named Barney Navarro. I don't think there are many other street engines that can make that claim.

 

The conventional wisdom is that you can generally go to .060" over with no problems, and often to .090" over. Beyond that, the block should be checked for core shift to be sure you don't break through the cylinder walls. Wheilther or not a particular block will need to be bored as part of a rebuild depends on too many factors to have a valid answer. Primary, of course, is how many miles are on the engine. Considering that I have one that's currently at 287,000 miles and still runs well and doesn't use oil, my guess is that barring abuse anything with 150,000 or less can "probably" be rebuilt without boring. Maybe even up to 200,000.

 

The only way to know, obviously, will be to mike the cylinder bores and check for out-of-round.

Barney Navarro was the engine builder of considerable reputation in the 1940's, 50's and 60's.  Not a second tier driver or a driver at all.  See this article for more information about Navarro:

 

http://www.hemmings.com/mus/stories/2007/12/01/hmn_feature16.html

[Eagle]

Barney Navarro was the engine builder of considerable reputation in the 1940's, 50's and 60's.  Not a second tier driver or a driver at all.  See this article for more information about Navarro:

 

http://www.hemmings.com/mus/stories/2007/12/01/hmn_feature16.html

 

Good catch. I remembered the name (and the engine, since I owned a Rambler American with the 199 engine), but I either forgot or didn't know that Navarro was the owner/builder, not the driver.

 

 

Navarro also chose an engine that had never before run in the Indianapolis 500--the Rambler overhead-valve 199-cu.in. six-cylinder, due both to its strong bottom end and thick cylinder walls. Navarro built both a fuel-injection setup for the engine and a variety of single- and double-turbocharger setups, eventually wringing in excess of 600hp from the little six-cylinder engine. According to Herman, Navarro tried to qualify at Indy with some form of that engine for three years, starting in 1968, but never actually qualified due to problems with drivers and with the suspensions of the cars.

[nitroxsteve]

My 88 I had to go .060" with 150k on it, but this truck was totally neglected I think the engine oil had 100k on it and of course it had no air filter.

[Oyaji]

Thanks for the responses.

 

In the 2+ weeks it took to get answers here, I went to the best machine shop in my area and asked them the same question. They said that for every 50 to 100 4.0 heads they run through the shop, they only see about 1 engine block come through for boring. Along with the factoid that the blocks were high-nickel-content cast iron, this was encouraging information.

 

On the downside is the guess that the Renix engines were more prone to overheating because of the (generally unfamiliar) closed cooling system and unknown operation and maintenance history of individual examples.

[carnuck]

Coupled with the craptastic Blackstone rads that had too small of cooling tubes.

[Eagle]

On the downside is the guess that the Renix engines were more prone to overheating because of the (generally unfamiliar) closed cooling system and unknown operation and maintenance history of individual examples.

 

The Renix-era engines are not prone to overheating. The problem is that owners of Renix-era XJs and MJs do not understand the "closed" system. They let the radiator get clogged, or they don't understand that the plastic bottle on the firewall is a pressure vessel and that, if it's leaking, the cooling system isn't working. The engines themselves are no more prone to overheating than any other generation of the AMC I-6.

[sloride]

In 25 years of owning my MJ, it's only over heated one time on me and that was due to a cracked reservoir bottle.

[Oyaji]

 

 

On the downside is the guess that the Renix engines were more prone to overheating because of the (generally unfamiliar) closed cooling system and unknown operation and maintenance history of individual examples.

The Renix-era engines are not prone to overheating. The problem is that owners of Renix-era XJs and MJs do not understand the "closed" system. They let the radiator get clogged, or they don't understand that the plastic bottle on the firewall is a pressure vessel and that, if it's leaking, the cooling system isn't working. The engines themselves are no more prone to overheating than any other generation of the AMC I-6.

Errrrm... that's exactly what I was guessing and tried to make clear.

 

The biggest problem is that all these trucks are over 20 years old now, and unless you are the original owner, you cannot vouch for how any individual example was operated and maintained. Considering how unusual the closed cooling system is to the general public, seems to me that unfamiliarity with it has led to a number of avoidable overheating events. That's just the sort of thing that takes the temper out of rings and collapses piston skirts... which loss of tolerance leads to excessive and premature cylinder wear. :(

[cruiser54]

I'll take an old Renix block that has been overheated before I take an HO with poorly machined bore and piston slap issues. 

 

There is no reason to believe that the Renix blocks have been subject to so much abuse. The cylinder head is what took the hit in an overheat condition anyway.

 

Plenty of HOs got hot also. There's no cure for stupid, no matter what folks are driving. 

[Pete M]

and there is a big difference between  "overheating" and "overheating enough to damage the block".   plenty of the prior for any old car/jeep, very very few of the later of our old 4.0s.  it's really not much of a concern when block-shoppin'. :thumbsup:

[Oyaji]

and there is a big difference between  "overheating" and "overheating enough to damage the block".   plenty of the prior for any old car/jeep, very very few of the later of our old 4.0s.  it's really not much of a concern when block-shoppin'. :thumbsup:

 

Indeed - but I wasn't referring to heat damage to the block, but rather heat damage to rings and pistons which then cause abnormal wear on the block. Whereas it takes a LOT of heat to affect the block directly, it doesn't take as much to de-temper the rings and collapse the piston skirts... which then start nibbling and scrubbing away at the cylinders.

 

You guys have no idea how pleased I was to hear that the Renix block is high-nickel cast iron, and accounts of sometimes still being able to see the original hone marks in the cylinders. That reminds me of old Datsun engines I rebuilt years back, whose high-nickel-content blocks showed little wear and often still-visible hone marks when torn down after 200,000 or even 300,000 miles.

[Eagle]

Indeed - but I wasn't referring to heat damage to the block, but rather heat damage to rings and pistons which then cause abnormal wear on the block. Whereas it takes a LOT of heat to affect the block directly, it doesn't take as much to de-temper the rings and collapse the piston skirts... which then start nibbling and scrubbing away at the cylinders.

 

You guys have no idea how pleased I was to hear that the Renix block is high-nickel cast iron, and accounts of sometimes still being able to see the original hone marks in the cylinders. That reminds me of old Datsun engines I rebuilt years back, whose high-nickel-content blocks showed little wear and often still-visible hone marks when torn down after 200,000 or even 300,000 miles.

 

Having at one time been the owner of a Javelin that had been overheated severely enough to take the temper out of the rings, IMHO once that happens you don't drive the vehicle enough to start damaging the block. With rings that have lost their temper, there is VERY low compression, so the engine kind of chugs along and doesn't make any power. It "runs," but not well enough to be useful.

 

In any case, it's easily checked with an inside micrometer to measure for out of round, and scoring is visible.

[Oyaji]

Thinking of posting a video (funny, but ultimately too much an invasion of privacy)  - 180,000 mile engine, no idea of its previous history beyond the fact that it wasn't registered for over 3 years (and I believe the story that it never ran since then). Gas tank has holes rusted through it, and the remaining metal is so thin I poked my fingers through it in the 2 places I tried. Dry compression numbers average - 80, 85, 83, 80, 78, 95; wet - 95, 105, 103, 108, 95, 125.  It will start and run... but only on 3-4 cylinders. *sigh*

 

I may fiddle with it and get it running before pulling it down (when I get time - as you might guess I have a lot of other things taking up my time just now). I have a suspicion that at least part of its low compression is from having stuck piston rings from having sat without running for so long... which still begs the question as to just what was the reason that it sat for so long.

[Eagle]

Obviously, your career must not have been with AMC. ;)

 

"Back in the day," at one time (around 1981 or '82) I stumbled into possession of a $50 1968 Javelin with the 232 c.i.d. version of the 4.0L's predecessor. I don't remember where I bought it, or why I bought it, but I did ... and it didn't run. I pulled the head, reamed the carbon ridge off the top of each cylinder, honed them with the engine in the car, and slapped the head back on. I went all out and even used a NEW head gasket.

 

Ran like a top.

 

Those engines are as close to indestructible as you'll ever find.

[COMJNUT]

my friend ownes a mechanic shop, both he and I have seen many 4.0's that are running on 2 or more failed cylinders for months on end just because the owner does not want to put money into their beat up cherokee.

[Eagle]

my friend ownes a mechanic shop, both he and I have seen many 4.0's that are running on 2 or more failed cylinders for months on end just because the owner does not want to put money into their beat up cherokee.

 

It can happen, but I believe Oyaji's concern is that doing so may damage the block to such extent as to leave it unusable as a base for a rebuild. The problem is, I suppose, if it's not your vehicle or your engine, how can you know whether or not a used engine will be suitable before you invest a bunch of cash into buying it and starting the work on it.