AMC 258 Cylnder Head

I have a two part question...
First i noticed on the 258 specs on the internet that the compression ratio increased over the years, how was that achived?

Second can i mill the head on my 258,i know it has non ajustable rocker would the reduced head hight cause problems?
thanks

 

have no idea on either.

for the first, it seems that one way to accomplish increased compression would be to make the combustion chamber smaller. without changing the bore/stroke, the way to do this would be by reducing the clearance in the head, or by using pistons that are taller/domed/whatever.

in essence, carbon/deposit buildups in the chamber create a higher compression ratio by reducing the volume of the chamber.

zero answer for the second question.

 

If you mill the head and you have rocker on a shaft, you can put shims of same amount you mill off the head,under the rocker arm shaft. that raises the rocker arm shaft back to spec.

 

The change in compression could be done by either changing the combustion chamber design or changing the piston. More likely that AMC changed the pistons - that seems the easier route.

With hydraulic lifters, you may not need to shim the rocker pivots, depending on how much material you remove. Most of the 258 heads used bridged pivots anyway, and I'm not certain that you can shim those.

<edit> There are also several different pushrod lengths for the 258 - you may be able to use a shorter pushrod instead of raising the pivots. If you take a lot off the head, be careful that the valves don't collide with the pistons (unlikely on a 258, but something you need to check). <end edit>

You can also use a thinner head gasket - there's at least a 4.0L gasket available from Mopar Performance Parts, as well as the usual Felpro, Victor, etc.

I wouldn't spend a lot of money on a 258 head, considering how much better the 4.0L HO head is ... but you already knew that

 

I just put the head back on my 73, 258 after completing a cam swap. I too noticed that the compression listed for my engine was lower than others. In my engine the piston tops are recessed to give the lower reading. If I had realized that when I had the engine bored, a few years ago, I would have installed flat tops to pick up some compression. Oh well, maybe next time.