Re: [Amc-list] AMC Block Sizes

[Frank Swygert <farna@xxxxxxx>]

Hey Dave, you forgot something! The stroke isn't the only thing that changed between the engines -- bore did also. 290/304 has a 3.75" bore, 343/360 4.08", and 390/401 4.17". The difference between the 290 and 304 (343 and 360, and 390 and 401) is roughly a 1/16" increase in deck height (and stroke) for the larger engines vs. the smaller. You can't bore a 290/304 out to 343/360 dimensions and expect it to last -- if it can be bored that far at all. Same with 343/360 to 390/401. I have heard of 360 blocks bored that far, but they are usually drag engines with partially filled blocks and have been sonic checked for cylinder wall thickness and core shift. I don't know anyone running a 360 on the street bored out to 390/401 size. That doesn't mean there isn't, but I wouldn't. 

As you mentioned, the internal block castings were different, external dimensions the same (with the small deck height exception) -- as long as we're referring to GEN-2 and GEN-3 engines (66-91 290-401). The old 1956-66 250/287/327 block is a very large and heavy block. Some people refer to it as a "big block" based on physical size and weight (it's about the size/weight of a 352/390 FE Ford or 396 Chevy), but it's technically not a "big block" due to the displacement. 

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Message: 3
Date: Fri, 27 Jul 2007 21:12:00 -0400
From: David Crooks <david.crooks@xxxxxxxxxxxx>

> I seem to recall seeing on one of the AMC sites that all the V8 engines, 304 - 401, 
> were made from the same block. So there is no "big block / small block" with AMC's. 
> Did I read that correctly, and if so, how does that work with such a large 
> displacement difference?


Hi Neil,

This is almost, correct...  All AMC V8's have the same external dimensions, but there
are internal differences, so you can't quite say they're made from the same block.  
There are differences in the cylinder castings, and bearing support webbings.  The 
290/304/343/360 blocks are essentially the same internally (with the 304/360 having 
a higher deck height to allow for the longer stroke.  Similarly the 390 and 401 are 
essentially the same internally.  

However from the outside, they're exactly the same, so a 401 will bolt up where a 
304 once stood, or vice versa.  

The way it works is with changes in stroke!  To get a bigger displacement, you only 
need a longer stroke!  If you have the luxury of enough sales, you can afford to make 
two blocks.  If you don't, you stroke what you've got!  The catch is, you're limited in
how far you can increase the stroke of a motor.  I've seen AMC's pushed to ~427, but 
that's about the limit.  It's also starting to lose performance, as the stroke is getting
too long.  

-- 
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
(free download available!)

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