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

[Wrambler242@xxxxxxxxxxx]

Uh, I for one don't agree that any of this is a "crock"...
Just not been proven or dissproven. 


I'd also hesitate to attempt to "mill" the block with a router.
Pistons and or aluminum would be fine. I'd still not do it.
Trying to do a clean cut on a cylinder bore with a hand held router and a carbide bit?
You can do it as quickly with a diegrinder and have better control.




--
Mark Price
Morgantown, WV
1969 AMC Rambler, 4.0L, EFI, T-5
2004 Grand Cherokee Laredo, 4.7L, Quadratrc II
" Chronic Pain Hurts"

 -------------- Original message ----------------------
From: 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!)
> 
> 
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