Re: [Amc-list] 4.0 #7120 Head to 258 Swap

[Frank Swygert <farna@xxxxxxx>]

Well, if you have a heavy duty router and an expensive carbide bit you can do it. I'd make a template from 3/4" plywood that will bolt to the head that would guide the router. How depends on the type of router and what's on it to use as a guide -- might have to make the template large enough to fit the entire base in, meaning you'd need a piece on each side of the block as well. I've done light "machine work" like this with a 1/4 hp plunge router and carbide bits. The bits don't last long, how long depends on the hardness of the material and how much is being taken off. On cast aluminum pistons they last fine, on hard cast iron you better have at least two on hand to make sure you get all six cylinders done. 

Actually, the recommendation to do this is pretty much a crock. It's definitely not a necessity, the engine runs just fine without it (yes, I know for a fact, I'm running a 258 w/4.0L head now). The theory is that the slightly larger area above the bore (there's a 0.05" "shelf" around the bore) messes up the swirl and/or flame travel pattern in the combustion chamber, and tapering the bore will improve such. In reality it's not a noticeable difference. There might be a 1-2% increase in efficiency and or power, but it's certainly nothing anyone would notice. I don't really think it matters at all, because the "shelf" is all the way at teh top but below the flame front. On a fire-breathing, built-to-the-hilt engine there might be a slight power increase, but not a hopped up "driver" six. Same thing with spending $1500+ to get the quench height just right on a 4.6L stroker. You get 3-5 hp for a $2000 price hike in building the engine, but it's "correct"!! Leave the piston 0.080"
  down and you'll never notice the diffe3rence in power or economy as long as the compression ratio isn't over 9:1 (or you have a high overlap cam). The "shelf" area is definitely above the quench zone, so there's no danger of the edge getting hot and causing detonation. On a high compression engine that's run for long periods of time (10:1+, something other than a drag car) there might be the possibility of the edge getting hot enough to cause detonation, but again, not on a street built six. 

------------------
From: Brien Tourville <hh7x@xxxxxxxxxxxxx>
Subject: [Amc-list] 4.0  #7120 Head to 258 Swap

Because of the larger combustion chamber in the 4.0
it's recommended to taper the 258 cyl. bore out beginning
above the area occupied by the piston top ring @ TDC.

What type of tooling will permit not having to remove
the engine to perform  this mod ?

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


_______________________________________________
Amc-list mailing list
Amc-list@xxxxxxxxxxxx
http://splatter.wps.com/cgi-bin/mailman/listinfo/amc-list