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AMC 401 Rebuild Path Forward Sanity Check

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LowryCJ View Drop Down
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    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!           



     

  

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69 ambassador 390 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 69 ambassador 390 Quote  Post ReplyReply Direct Link To This Post 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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LakesideRamblin View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote LakesideRamblin Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post 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.




Edited by FSJunkie - Dec/13/2018 at 2:22am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jcisworthy Quote  Post ReplyReply Direct Link To This Post Posted: Dec/13/2018 at 4:10am
They get much thinner where the water circulates between the bores
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post 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.


Edited by Trader - Dec/13/2018 at 8:11am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote White70JavelinSST Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Shawn_Watson Quote  Post ReplyReply Direct Link To This Post 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?


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Post Options Post Options   Thanks (1) Thanks(1)   Quote DragRacingSpirit Quote  Post ReplyReply Direct Link To This Post 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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