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heads, combustion chambers, compression ratio

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I have a couple of extra heads that came from 218 engines.  Both are the external water bypass design.  My third head is a 56 plymouth 230.  Verified by the casting number.

Comparing the older  combustion chambers to the 230 head shows that the 218 had a lot smaller chamber.  Very little open area over the piston.  Otherwise they look identical in shape and depth over the valve area.  My surprise was when I measured the thickness of the head at the bolt bosses.  I expected to find the 218 to be thinner as a result of removing stock to arrive at the smaller chamber.  It's not, it is actually thicker.  Explains the different casting number.


Now I need to cc the 230 head to see how much to remove to get to 8.5 compression ratio.  Blasted it clean today and found a piece of plastic to use to seal the surface.  Probably use a syringe instead of a burette to measure the fluid.  Probably be 60something cc's so more than one syringe fill.


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I went through and cc`d some heads years ago. You are right, head thickness does not correlate to chamber volume. 

I used some 1/4" acrylic that I clamped down with a smear of grease to seal.

I worked out the gasket area with chalk marks and some maths but it was a lot easier to put a used gasket between the acrylic and the head to measure the gasket area.

A bit of food colouring is good to add to the water. Makes it a bit easier to see  and brakes down the water surface tension.

Put the head on a very slight slope with the hole at the top end.









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That last pic looks like my 218 heads.  On the 230 the chamber extends almost all the way over the piston top.  I'll be using diesel for the measurement, no surface tension issues like water and a slight amount of color to make it easy to see.    I plan to lay a gasket in place, trace the chamber on the head with a Sharpie and use that to guide my grease sealer location.


Did you arrive at a material removal amount to gain a specific ratio? 

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Wow!  I don't know if I'd want to remove 100 thou from my head on the 251 engine, but it looks like that's what they can do on the Chrysler 6s.  But what's interesting is only 65 thou on a DeSoto head. 


I really wonder about head part numbers/casting numbers and the differences between them.  I know that the part numbers for the two heads available in the 1960s Dodge trucks was:


2202771--Internal by pass, low compression ratio 6.4 to 1

2264240--External by pass


I'm not sure on the industrial engines.  That would be interesting too.

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