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AMC 401 Rebuild Path Forward Sanity Check |
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LowryCJ
AMC Fan Joined: Dec/11/2018 Location: Denver Status: Offline Points: 2 |
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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
Supporter of TheAMCForum Joined: Nov/22/2009 Location: Peoria, AZ Status: Offline Points: 3539 |
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.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 |
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LakesideRamblin
AMC Addicted Joined: Dec/21/2015 Location: So. California Status: Offline Points: 2682 |
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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 |
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FSJunkie
AMC Addicted Joined: Jan/09/2011 Location: Flagstaff, AZ Status: Offline Points: 4741 |
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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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1955 Packard
1966 Marlin 1972 Wagoneer 1973 Ambassador 1977 Hornet 1982 Concord D/L 1984 Eagle Limited |
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jcisworthy
AMC Addicted Joined: Jul/23/2009 Location: North Carolina Status: Offline Points: 2805 |
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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 |
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farna
Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19610 |
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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
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Trader
AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6754 |
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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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White70JavelinSST
Supporter of TheAMCForum Joined: Aug/08/2012 Location: Minnesota Status: Offline Points: 4867 |
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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
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Shawn_Watson
AMC Apprentice Joined: Dec/05/2018 Location: Carlsbad, NM Status: Offline Points: 117 |
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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
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DragRacingSpirit
AMC Addicted Joined: May/27/2009 Location: Mo Status: Offline Points: 903 |
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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
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