Classic Inlines
603 W Pecos Ave
Mesa, AZ 85210


 
Small Six Cylinder Head Swaps

First, let start by saying that all 144, 170, 200, & 250ci heads are interchangeable.... and while the valve size, chamber size (cc's), and intake/port volumes (cc's) changed over the years, all of the small six cylinder heads will physically bolt on and function on any small six. However if you were to put a cylinder head from a 144ci or 170ci on a 250ci block, it would suffer from too much compression and it would lack the ability to breathe properly due to the lower intake/port volume, smaller chambers, and smaller valves. Conversely, putting a 200ci or 250ci cylinder head on a 144ci would kill the CR due to the larger chambers (about 6.3:1), and while the intake/port volume is higher, and the valves are bigger, the additional air flow would not make up for the loss in compression. Fortunately this can be corrected.

If your goal is to increase the performance of your small six, the best solution is to seek out a late model "large log" cylinder head, as they have more intake volume and bigger valves, which means they'll flow better than the small log heads. And even though they are a bolt on swap, you'll probably need to make a few modifications to get the best results. For example; when you install a late model ('69-up) 200ci or 250ci head on a ‘66 200 block, there are two issues that must be addressed to take advantage of the increased intake volume, and the bigger valves. First, all 144, 170, 200 & 250ci engines came from the factory with steel shim head gaskets that have a compressed thickness of approximately .025", however these gaskets are no longer available and scarce as NOS (New Old Stock). As such, you'll need to use an aftermarket head gasket (produced by Felpro, Victor, Corteco, etc) which are composite type head gaskets that have compress thickness ranging from .040" to .055", depending on the manufacturer. Composite gaskets seal better and last longer than OEM steel shim gaskets, but to use them you'll need to mill the head to compensate for the difference in gasket thickness. To do that you need to compare the thickness of the OEM gasket that was removed, to the composite gaskets advertised compressed thickness, then mill the head accordingly. For example, if your old shim gasket had a compressed thickness of .025", and the new gasket has a compressed thickness of .050", you'll need to mill .025" off the cylinder head to maintain the same CR, more if you want to increase the C/R.

Secondly all stock cylinder heads produced between 1963 to 1968, had approximately 52cc combustion chambers, while the cylinder heads produced from 1969 onward had approximately 62cc combustion chambers. Please note the "approximately" compromise, as casting inaccuracies often resulted in smaller or large chambers. You need to remember that you'll be working with a cylinder head that is 40-50 years old, so there is no guarantee it will still be in a stock condition. More than likely the head has already been rebuilt and milled to true the mating surface, possibly several times. As such the actual chamber volumes may vary considerably, which means they must be re-measured (cc'd). The key is to measure carefully and record the results, then mill the head accordingly to obtain the desired compression ratio. Typically, milling the head .010" will result in an approximate reduction in volume of 2cc's. For example:
if your cylinder head currently has 62cc chambers, and you want 52cc's, you would need to mill .050" off the head to reduce the chamber volume by 10cc's.

Therefore, to use a late model 200/250ci cylinder head that had 62cc chambers, on a ‘65 or ‘66 200ci block, using a composite head gasket, you would need to mill the cylinder head a total of .075" if you wanted to maintain the stock compression ratio. That’s .025" for the difference in gasket thickness and .050" to reduce the chamber volume to 52cc's.

TIP: Measure and record the chamber volumes, and re-calculate the compression ratios after every rebuild using an online calculator, then keep the results for future reference.

CAUTION: Be sure you thoroughly clean the head bolt holes, making sure they are free from dirt, grease and oil, and then use hardened washers under the head bolts to compensate for the reduced thickness of the cylinder head due to machining. IF YOU FAIL TO DO THIS, THE HEAD BOLTS MAY BOTTOM OUT IN THEIR HOLES, RESULTING IN INACCURATE TORQUE READINGS
AND FAILURE TO TIGHTENING THE HEAD DOWN TO THE BLOCK.

FYI: All small six cylinder heads can be safely milled .090". If you need to go more than that you should do some sonic testing to determine the thickness of the cylinder head deck. You should maintain at least .100" thickness to have a good, true, solid mating surface. If you have to go more than that to achieve your desired C/R, you really should consider decking the block, using different pistons (flat-top), a taller pin height, or off-set grinding the crank.

Many factors limit the compression ratio, such as elevation, ambient temperature, humidity, quench, cam profile, cam and ignition timing, load, piston design, chamber quality, as well as cylinder head material and design. For example, an aluminum head will dissipate heat faster than a cast ion head, there-by allowing a slightly higher compression ratio. When determining the desired compression ratio, we also suggest a comparison to the octane of the gas you intend to use. For example, if you plan to use 89 octane (regular unleaded), plan for a C/R around 9.0 to 9.2. If you can stand the extra expense of 92 octane gas, you can bump the C/R to 9.2 to 9.5. Remember that regular leaded gas in 1965 was 92 octane.
The desired C/R can also vary depending on your cam profile (see Static vs Dynamic C/R).

NOTE: On the typical wedge shaped combustion chamber, 1 point of compression, all else being equal, is worth a 4% increase or decrease in power.

NOTE: The 200ci and 250ci, for the most part, use the same cylinder head after 1977.

If you are planning a cylinder head swap, we highly recommend a late model cylinder head simply because they flow more air. After all, that's what it's all about. When you increase air flow, you increase horse power. The only time we recommend using a small log head is when your trying to restore the engine to its original condition and you need to maintain the matching numbers. Otherwise, do yourself a big favor and find a late model "large log" cylinder head. The difference in performance is well worth the effort it takes to locate one. Most shops that rebuild cylinder heads charge around $350 for a good core, but you may be able to find one online or at your local bone yards for less. Sometimes you can even get them for free, if your lucky. But before you put down your hard earned cash, make sure the head has been cleaned and magnaflux to be sure it's free from any defects or cracks. While cracks can be usually be repaired by a qualified welder or shop, it's not cheap. Finally, make sure the head is suitable for milling, meaning it hasn't already been milled to the max.

In the table below you can compare the differences in chamber size, valve size, and intake volume. It should also be noted that all "small log" heads have a 1.5" carb bore, while the late model "large log" heads have a 1.75" carb bore, making them easy to identify However the best large log heads are those produced after 1977, as they have the largest intake volumes, meaning they will flow better, especially with oversized valves and port work.

Cylinder Head Specs
YEAR
CI
Chamber
Intake
Exhaust
Log CC
64-60
144
44-51
1.467
1.266
850
61-63
170
48-53
1.522
1.266
890
65-72
170
48-53
1.649
1.380
890
65-69
200
51-53
1.649
1.380
1100
69-71
200
62
1.649
1.380
1240
72-76
200
62
1.649
1.380
1240
77-80
200
62
1.750
1.380
1345
69-71
250
62
1.649
1.380
1240
72-76
250
62
1.649
1.380
1240
77-80
250
62
1.750
1.380
1345
80-up
250
62
1.750
1.380
1420

It is also interesting to compare the results we got on a flow bench. As you can clearly see, a small log head with extensive port work and larger aftermarket valves, still didn't flow as well as a stock large log head. Instead of investing hundreds of dollars in your small log head, you'd be much better off buying a large log head. Even if you only did a standard valve job, and nothing else, it's going to flow better than the warmed over small log head.

For more information, please see our tech article "Flow Testing the Log Cylinder Heads"

We also recommend modifying the intake manifold so you can use a two-barrel carb, which we call a "2V Conversion". This modification in itself is good for about 25 horse power. For more information please see our tech article "2V Carb Swaps & Log Modifications".

Another item to consider is the length of the pushrods. When you mill the cylinder head and deck the block, you may need slightly shorter pushrods. Another method is to shim up the rocker pedestals an amount equal to the milling and/or decking. Others simply rely on the hydraulic lifters to absorb to tighter dimensions. However, the best way to determine the proper pushrod length, is to assemble the cylinder head and valve train and check the length with an adjustable pushrod, and verifying proper valve geometry. These can be purchased online from several suppliers, including /Classic Inlines.

Click here for information on Classic Inlines Aluminum Cylinder Head.

Written by: CZLN6
(Fordsix forum member and co-author of the Falcon Performance Handbook)
Edited by: AzCoupe


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