Swapping 360 or 340 Heads Onto A 318

Swapping 360 or 340 Heads Onto A 318
By: Josh Francis a.k.a. HotRod

	Lately in the #mopar channel on MoperPages.com we've been
getting lots of questions about swapping 360 heads onto a 318.
Everybody seems to want to do this swap that has been common for
years. The answer to the question of can it be done is simple, sure
it can. All smallblock non-magnum heads are a direct bolt on to any
other engine in the A engine family. For those of you new to mopar
this means that the heads from the 273, 318, 340, and 360 are
interchangeable between these motors.  Simple answer huh? If it
seems simple that's because that's not all there is too it. The
heads are a direct bolt up now we get into the other issues involved.

	The first choice to be made is what head to be used. Generally
I would recommend sticking with the smaller 1.88 /1.60 valve
combination for a 318 but several people have successfully used the
later 2.02 intake valves. One of the most important aspects of the
head selection should be the combustion chamber size and I'll get
into why shortly.

Here are some heads and casting numbers to help you find what you want:

Year      Engine        Casting Number  Combustion CC's
68-71     340		2531894	        63.0-73.5 cc
70        340 6bbl      3418915		63.0-73.5 cc
72	  340 	        3418915		65.0-73.0 cc
73        340 	        3671587		65.0-73.0 cc
71-72	  360		3418915		65.0-73.5 cc
73-74 	  360 	        3671587		65.0-73.0 cc
73	  360 AP	3671587		65.0-73.0 cc
75	  360		3769974		65.0-73.0 cc
76	  360		3671587-3769974	65.0-73.0 cc
77-79	  360		4027596-4071051	66.0-72.5 cc
80	  360		4027596		66.0-72.5 cc

*AP-Denotes Air Pump...these heads have holes below the exhaust ports
which must be tapped and plugged if you wish to use them.

	Most of the time when someone is speaking of these heads the
whole casting number isn't used. They'll simply refer to them by the
last three numbers by saying for example 915's. The 915 heads are also
referred to commonly as the J heads. These heads came with both
1.88/1.60 and 2.02/1.60 valves and use open chambers. They have large
port volumes and work nicely on a 318.

	The 596 head is another one that you might want to look for.
These were actually put on the 318 by the factory for the 318 4bbl
"police interceptor" package. The valves on these are the 1.88/1.60
size and they also use an open chamber design. The 596 also use a
slightly smaller exhaust port volume but it is only around a 4 cc
different.

	This is a VERY brief rundown on the cylinder heads that you
might want to use. I could sit here and type pretty much all day on
just cylinder head specifications but that's not the aim of this 
document so now we'll move on.

	The biggest problem about swapping the 360 head onto the 318 is
the larger combustion chambers will yield a much lower static
compression. Let's look at this for a second. A typical early model 318
with an 8.5:1 static compression uses a combustion chamber volume in
the 61.0-67.0 cc range. When you take this same engine without any
changes and just bolt on a standard 360 head with combustion chambers
in the 65.0-73.0 cc range you lower the compression to something around
7.5:1 which is unacceptable for performance applications.  There are
two approaches you can take to correct this and in some cases you might
need to use both. The first approach is to use a set of pistons to
raise the compression ratio of the motor by using a longer compression
height. The compression height is the distance from the center of the
piston pin to the top of the piston. Another way to solve this is to
deck the heads. Decking the heads removes material from the surface of
the head and makes the combustion chamber smaller in volume thereby
raising the compression. To actually build a performance engine with
9.5:1 static compression and above you will most likely have to use
both of these methods. When I did this swap on my 318 about 4 years ago
there weren't many performance pistons available at the time for the
318 so my choices were very limited. Today you can find performance
pistons from virtually any manufacturer for this application. I
personally used the Mopar Performance 9:1 cast 318 piston part
#P4529419 and decked my 360 heads to 63 CC combustion chambers to
achieve a 9.6:1 static compression.

The formula to figure your compression ratio is:

Volume Of Everything At BDC/Volume of everything above piston TDC 3D CR

This means that to figure your compression you would use the total
volume of one cylinder in cc's + the head gasket thickness volume in
cc's + valve notches cc + combustion chamber cc. Then you would divide
that number by the total of the head gasket cc's, valve notch cc, deck
height cc, and the combustion chamber cc. This would yield your static
compression ratio. As you can see from the ratios in this formula you
can raise the static compression by removing volume from above the
piston when the piston is at TDC.  Decking the heads to removing
combustion chamber volume can do this and by increasing the height of
the piston which is the compression height.

 The maximum that you can actually mill an A engine is .060 inches.
Anymore than that and you will start to actually create sealing
problems in the heads as well as throw off the valve train geometry.The
basic rule of thumb for decking these heads is that for the larger
chamber 340/360 heads you have to mill off .0048 inches to take 1 cc of
combustion chamber volume out. So if I wanted to drop 6 cc from my
combustion chamber I would have to cut .029 off the head. Simple math
6x .0048.  Also to prevent sealing problems and make sure that the
intake bolt holes line up you are supposed to mill .0095 off the intake
side of the head for each .010 taken from the head. I didn't do this
when I did mine and have had no problems from it though so you can if
you want to.

Since chances are you will have to deck the heads we'll also go into
the problems that arise from that. As the deck height is lowered closer
to the centerline of the block that also means that the rocker assembly
gets closer to the centerline of the block. This means that the stock
pushrods are now too long and your lifters might run compressed. This
is definitely not a good thing. There are several ways that you can
address this problem. Adjustable rockers are probably the best way to
go but are relatively expensive for a solution as are the custom
pushrods that you could use to solve the problem. The cheapest and
easiest way to do it is to simply place shims under the rocker shaft
pedestals. This will shim the rocker shaft up and give you more
clearance.

Another problem with decking heads and putting a taller compression
eight piston in the 318, or any engine, is that you may experience
valve to piston clearance problems. You should always check this
clearance after constructing ANY performance engine. The minimum intake
clearance should be .100 and the minimum exhaust clearance should be at
least .125 in.

Any questions or comments you can contact me on www.moparchat.net in the
chat room.