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AMC 401 Rebuild Path Forward Sanity Check

Printed From: TheAMCForum.com
Category: The Garage
Forum Name: AMC V8 Engine Repair and Modifications
Forum Description: AMC-made V8 engine mechanical, ignition and fuel from basic repair to high-perf modifications
URL: https://theamcforum.com/forum/forum_posts.asp?TID=97656
Printed Date: Mar/28/2024 at 11:52am
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Topic: AMC 401 Rebuild Path Forward Sanity Check
Posted By: LowryCJ
Subject: AMC 401 Rebuild Path Forward Sanity Check
Date Posted: Dec/12/2018 at 10:29pm
Hello Everyone,
This is my first post to TheAMCForum and I am looking for a bit of a sanity check before I proceed forward with my current project.  So my grandfather and I rebuilt an AMC 401 back in 1996 and it was bored 30 over at the time (I was about 12 so grandfather rebuilt and I watched and got tools).  This engine was for a 1972 CJ5 crawler with a torque flight 999 automatic transmission, Dana 300 transfer case and Dana 44 Axle's with a 4.27:1 gear ratio.  Long story short he passed away in 2001 and the jeeps frame was broken in 2002 (Thanks Uncle!).  The jeep then sat under a tarp that was replaced every few years until it was offered to me earlier this year as we cleaned out my grandmothers property.  I have now completely disassembled the jeep to do an aftermarket TDK frame swap, and have pulled the 401 engine and done a complete tear down.  So here is where I am at, there was some water in cylinder 7 which had rust and pitting.  So I have three options as I see it right now...  

The 1st option would be to sonic test the block and provided the cylinder walls are not too thin, to bore at either 0.06" over or 0.085" over (BullTear.com Custom Pistons).  Given the 4.27:1 gear ratio, I don't need the larger bore for power, but think I would be forced to going to the 0.085" over to clean up the pitting.

The 2nd option would be to sleave the cylinders and bring them back to the stock 4.165", or the 30 over 4.195".  If I sleaved and bored to 0.03" over, I would consider pistons from the current engine that still looked like they are in good shape (the engine has under 5000 miles since it was rebuilt, but also 22 years).

The 3rd option would be to buy a block that could be rebuilt at a 0.02" or 0.03" over and hold off on using the old 401 engine core.  Currently I may have found a lead on an AMC 401 core on this forum at a reasonable price (and very gracious seller who may be able to facilitate a closer meeting point).  

Thanks for sticking with me...  so here is my question.  What would people on this forum suggest for my path forward?  For some extra horsepower I am thinking I will upgrade rockers, and go with roller lifters with new push rods.  So is it worth investing the money to go with option 1 (0.085" Over bore and new pistons), or option 2 (cylinder sleeves), or should I be looking at a better condition core?

Thanks for any insights!           



     

  




Replies:
Posted By: 69 ambassador 390
Date Posted: Dec/12/2018 at 11:28pm
.030 is not that large of an overbore.  You should be safe up to .060.  I bet your block would clean up at .045, so you could use off the shelf Wiseco Pro Tru pistons.  They are cheap and of good quality.  I have installed a couple of sets this week.  Don't sleeve all 8 because it will cost a large amount and gain you nothing.  You would be surprised at how much rust will clean up with a little bit of material removed.  I would not be afraid of .060 over either.  More than that, sonic check.

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Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: LakesideRamblin
Date Posted: Dec/13/2018 at 12:41am
Like lawyers, you will get different opinions from folks here.  Not bad, just different.  You have good options.  Personally,  I would start by determining if your current block would bore out to no more than .060.  If so, keep it and build it.  If too far gone, then buy the reasonably priced core but only if it has been magnafluxed.  If it has and checks out good, then you are on your way to your rebuild and a reasonable expectation of a long lasting engine with better build options.  My 2 cents.  Good luck with your decision.

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LakesideRamblin
69 Rambler 360
73 Javelin 360
"If you could kick the person in the pants responsible for most of your trouble, you wouldn't sit for a month." T. Roosevelt


Posted By: FSJunkie
Date Posted: Dec/13/2018 at 2:11am
I have a Buick 231 bored .060". It's a 3.86" bore on a 4.24" bore center spacing, so there is .38" of material between adjacent cylinder walls. Look how close together those cylinders are and how tight the water jackets are and tell me the walls are not thin. Hasn't blown up yet.



Meanwhile a 401 bored .060" has .525" between adjacent cylinder walls. Quite a bit thicker. And not a piece of junk...unlike Buick.




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1955 Packard
1966 Marlin
1972 Wagoneer
1973 Ambassador
1977 Hornet
1982 Concord D/L
1984 Eagle Limited


Posted By: jcisworthy
Date Posted: Dec/13/2018 at 4:10am
They get much thinner where the water circulates between the bores

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Specializing in dyno services, engine building, and cylinder head porting

rbjracing.com
Phone Number 518-915-3203


Posted By: farna
Date Posted: Dec/13/2018 at 5:59am
The main issue is the cylinder wall thickness. Have the block sonic checked before boring or 0.030". AMC only recommended a 0.020" overbore on the 401, but they were looking at maintaining max reliability over a long service life. Many have bored them 0.030 - 0.040, but more is questionable. While the engine may run fine for drag racing, it will generally overheat much easier with thin cylinder walls, so regular street use and rock crawling could be a problem.

With only 5K on the previous build I'd have the cylinders checked for trueness and wear. If it checks out ok just put in new rings and go, maybe a light hone to get a good cross hatch pattern so the rings break in quickly.

If not that, either find a block that will work with a 0.030" overbore or sleeve the current block back to stock bore. You don't gain enough from an 0.030" overbore to be concerned about it, and then you should be good for a second rebuild. Won't be cheap to sleeve 8 cylinder, $65 or so each last time I checked (a few years ago). Depends on machine work costs in your area, could be $100 each. Also depends on what you can find a good rebuildable core for.  For long term reliability these are probably the best routes, but as noted, just my opinion -- there will be others!


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Frank Swygert


Posted By: Trader
Date Posted: Dec/13/2018 at 7:45am
Sleeving and going stock bore has one big drawback - expensive stock forged pistons. Depending on what you are planning to do with the engine.
As Frank stated I'd check that 0.030" over as is now and go with the Wiseco pistons or the pistons you have.
If you need to bore, Wiseco offers the same 0.045" over pistons that should be good if the block is tested.
Overheating is a problem when wall thickness is less then 0.200". AMC had some castings that can go to 0.060 over without problems (some have gone 0.090"). Need to have you block checked if going past the 0.030" bore it already has.
I would put sleeving as the last option or if just one or 2 cylinders have core shift and need to be sleeved to utilize the block. #7 seems to be the worst cylinder for core shift from prior posts.


Posted By: White70JavelinSST
Date Posted: Dec/13/2018 at 8:36am
My thoughts are, since the engine was proven to run and run reliably for 5k miles, the only reported damage is cylinder 7, sonic check cylinder 7 for sure, if possible sleeve cylinder 7, bore it back to .030 over, hone for crosshatch pattern in all cylinders. Make sure you run a long 5/16" drill bit into the oil passages to verify each passage was fully drilled at the factory to the full spec size of 5/16. Thoroughly clean everything, check clearances, re-use pistons, new rings, new bearings, cam and valve-train as you were thinking.  Good to go.


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70 Javelin SST, second owner, purchased 1972


Posted By: Shawn_Watson
Date Posted: Dec/13/2018 at 9:02am
What about honing the bores true and using Line2Line's abradable piston coating on the existing pistons to take up the gap as opposed to boring out to the next piston size?


Shawn


Posted By: DragRacingSpirit
Date Posted: Dec/13/2018 at 12:11pm
Sleeve the bore of the offending cylinder. Hone and reassemble with same pistons and new rings.


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Best 1/4 mile 8.99, 1/8 mile 5.71, 60 foot 1.27, no power adders


Posted By: shootist
Date Posted: Dec/13/2018 at 1:48pm
My vote would be with White70 and DRS, Sleeve 7 as a reliable and cost effective approach.

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Posted By: Trader
Date Posted: Dec/13/2018 at 2:21pm
Lowry didn't say #7 needed a sleeve. I stated IF there is core shift, #7 seems to be the usual problem when over boring is required, and a sleeves are viable options.
Have the block inspected, there may be plenty of material in all cylinders.


Posted By: PROSTOCKTOM
Date Posted: Dec/13/2018 at 5:56pm
Personally I would sleeve the bad cylinder and go with it.

Having recently had a discussion with Kevin Fontenot's aka: (Stickshifter) about 401 wall thickness here's what he had to say about the matter.

"The max bore on a standard iron block is 4.250 and I would never do that.  The max I would do is 4.200 and even then I would check wall thickness with a sonic checker before starting.  Being a virgin block makes no difference.  And you need to check all cylinders in many difference location, especially around the head bolt areas near the top."  



With that said, I would trust Kevin's recommendation without question. He has been there and done it and his advice could save you a lot of grief down the road. 

Tom




Posted By: Trader
Date Posted: Dec/13/2018 at 9:03pm
Posted is the 4.250" bore i.m presuming. From the numbers this block max would be 4.195" and that would be a push.
I'll be doing a 73 block this spring at 0.030" over and hope never to push this far. 
Some of those mid numbers are just plain scary. Distortion after torquing the head bolts would be significant to say the least.
This thing survived? How long? Assuming the block was filled for race application???


Posted By: shootist
Date Posted: Dec/14/2018 at 12:25pm
Originally posted by Trader Trader wrote:

Lowry didn't say #7 needed a sleeve. I stated IF there is core shift, #7 seems to be the usual problem when over boring is required, and a sleeves are viable options.
Have the block inspected, there may be plenty of material in all cylinders.

The original post stated #7 was the one with rust so I believe some folks assumed the damage is at a level where it needs to be bored out. Putting in a sleeve and reusing the pistons he has would be a good and cost effective option. There is a lot of assumption in that but certainly a viable approach for someone on a tighter budget. I suppose some other valid questions would be how did water get into the bore? Are you building for performance or longevity? Is budget a concern? Checking out what you have completely would be a wise move before doing anything with it.




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Posted By: 74Bubblefender
Date Posted: Dec/14/2018 at 3:39pm
What also affects the thickness of the cylinder walls is the coolant you use. The last 401 went in a 1979 jeep and that was 40 years ago. If the block ran a low mix of coolant and mostly water or low on coolant...letting it freeze or sit to rust the casting will shed into the coolant over time. If you ever get a chance to cut a few blocks in half you will see this. It really should be minimal...maybe .03-.04" but that be the difference between crack and no crack. Always best to sonic check the block from 1:00-4:00 and from 6:00 to 11:00 positions. 12:00 and 5:00 will never be thinner than those other areas. I should say...shouldnt ever be. Whenever possible a sleeve should not be considered.


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We are just about to forge new AMC V8 crankshafts.. please check here
http://www.bulltear.com/forums/showthread.php?19564


Posted By: farna
Date Posted: Dec/15/2018 at 8:01am
PROSTOCKTOM -- on the block diagram you posted there are three measurements for each position. Am I correct in assuming that the largest is the sidewall thickness of the block "as is" and the smaller measurements are the sidewall thickness of some overbore? Looks like 0.020" each time??


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Frank Swygert


Posted By: farna
Date Posted: Dec/15/2018 at 8:06am
Small pits in a cylinder wall won't hurt a thing. Won't leak compression around them, not enough to be measurable anyway. I've run several engines (sixes, but doesn't really matter) that had pits where rings had rusted in the cylinders. These were stock rebuilds, but there was no difference when some of the pits were left. Compression readings were up where a stock rebuild should be. The cylinders were bored due to wear, but just bored the next common size (usually 0.030, one old 196 was bored 0.040 though). That cleaned half or more of the pits, but still left a few. Machinist didn't really like it, but the option was to get another block or sleeve three cylinders on one of them. He didn't think they would make a difference either, just didn't like seeing them... neither did I! But we went ahead and it ran like a champ for years... might still be going (sold).


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Frank Swygert


Posted By: Boris Badanov
Date Posted: Dec/16/2018 at 8:01am
You have to think that the machine shop that took it to .03 over
may have shifted the bores. Most shops used a simple three fingered
probe to locate the bore on center. the chance of a error
is rather big. That's is partly why AMC recommended no more than .02 over.

Setting it up in a mill and picking up the datums, so called Blue printing
is expensive. I bet most automotive machine shops today cant do it.


Sleeve the bad cylinder and be done with it.

Or maybe sleeve all 8 and go to 4.25 bore :-)




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Gremlin Dreams



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