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401 compression specs

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htrdkd69 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote htrdkd69 Quote  Post ReplyReply Direct Link To This Post Topic: 401 compression specs
    Posted: Oct/13/2018 at 1:54am
hi I was wondering if anyone could tell me what the compression is in my 77 401? its stock stroke 40 over pistons and also what is max over bore for a 401? ive read 40 is max but I dont know for sure any help would be great thank you.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote purple72Gremlin Quote  Post ReplyReply Direct Link To This Post Posted: Oct/13/2018 at 3:16am
Stock compression? 8.1 to 8.5...
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htrdkd69 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote htrdkd69 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/13/2018 at 7:39am
ok thank you very much
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpnjim Quote  Post ReplyReply Direct Link To This Post Posted: Oct/13/2018 at 8:20am
AMC said max over bore on the 401's was .0225*,
that's a very conservative number and I've never heard of a real problem at .030".
.040 or .045" seems ok too, though maybe not ideal.
but beyond that opinions vary.

Some wouldn't touch a .060" 401 with a 10 foot pole,
others have gone +.080" to 4.25 and say it's fine.

The best answer is to check for core shift and match the wall thickness to the application.

I might not want to build a .060 over 401 for a rock crawler, or a daily driver,
but there's people drag racing half filled .080" 401's and saying it's great.

I'd try to keep a STD 401 at 4.165 as long as I could,
then I'd only bore it as much as necessary.
The next standard size piston in +.030" but custom pistons are available in any size, so going +.020" instead could save material for another rebuild down the line.

For a .040" 401 I'd sonic test the wall thickness to see what my options were.

Wiseco makes great .045" pistons in two different compression ratio's for under $450, 
if your walls are good, and the cylinders are still straight enough you may be able to hone it to .045 and run those.

*fixed, thanks Tom!


Edited by jpnjim - Oct/13/2018 at 11:17am
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was Green/green now T/A red w/blk leather

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PROSTOCKTOM View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PROSTOCKTOM Quote  Post ReplyReply Direct Link To This Post Posted: Oct/13/2018 at 9:07am
This is coincidental that you should ask the question about the bore. I wanted to fill a 401 block with HardBlok and bore it out big, but after talking to Kevin Fontenot (Stickshifter) I got my bubble busted with his reply.  "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." 

So with that in mind your already .005" over what he considers the safe limit. Though I am sure there are lots of AMC engines out there running .040" or more that have ran without issue for years. 

Tom

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Post Options Post Options   Thanks (1) Thanks(1)   Quote PROSTOCKTOM Quote  Post ReplyReply Direct Link To This Post Posted: Oct/13/2018 at 9:23am
Originally posted by jpnjim jpnjim wrote:

AMC said max over bore on the 401's was .225
I think you mean .0225" and not a 1/4" LOL

Tom
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpnjim Quote  Post ReplyReply Direct Link To This Post Posted: Oct/13/2018 at 11:18am
Originally posted by PROSTOCKTOM PROSTOCKTOM wrote:

Originally posted by jpnjim jpnjim wrote:

AMC said max over bore on the 401's was .225
I think you mean .0225" and not a 1/4" LOL

Tom


lol,
 oops,
yeah that little zero does change things a little ConfusedLOL

Thanks Tom
71 Javelin AMX P-code Go pac 360/4spd/3.91's
was Green/green now T/A red w/blk leather

88,89&98 Jeeps
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