251 Head Thickness

[grubby65]

I have the head from my S-15 at the machine shop.  I was going to have .050" taken off to raise the compression ratio, but the guys at the shop thought it may have been cut before.  I was wondering if there was a specification on the thickness of the head and where the measurements should be taken.

[JBNeal]

in lieu of measuring the head thickness, it might be more accurate to measure the chamber volume by clamping a sealed piece of plexiglass (clear acrylic) with a hole drllied in it to add water to the chamber, then measure the extracted water to get an estimated volume of the chamber...with the head gasket thickness, this can give ya an idea of your CR, and then ya can go from there...

[Matt Wilson]

On 10/29/2020 at 1:05 PM, JBNeal said:

in lieu of measuring the head thickness, it might be more accurate to measure the chamber volume by clamping a sealed piece of plexiglass (clear acrylic) with a hole drllied in it to add water to the chamber, then measure the extracted water to get an estimated volume of the chamber...with the head gasket thickness, this can give ya an idea of your CR, and then ya can go from there...

I agree.  I see people asking for stock head thickness info all the time, but after 60 - 80 years, I doubt there are very many that haven't been cut already.  Also, I'd be a little wary if the accracy of any numbers I might find, unless they were from a factory manual or similarly reputable source.  Measuring the combustion chamber volume is more tedious and time-consuming than measuring the thickness, but it's also a much more reliable way to get the answer you're looking for.  Ideally, you would measure every cylinder.  Once you get these volume(s), you'll have to do a little math to see if the compression ratio matches (or is close to) the original number published in your manuals.  If so, then it hasn't been cut, or at least not much.  Let us know if you need help with the math side of it.

Edited October 31, 2020 by Matt Wilson

[RobertKB]

I would just make sure the head is even. I never have seen the point of shaving heads on our flatheads to get a couple of extra horsepower from an engine only producing 100 hp on a good day. 

[plymjim]

  Just because this is what you requested I measured a P-25 head

 in my possession for 50 Years & to the best of my knowledge un-cut.
From highest point (head bolt landing) to bottom is 1-31/32". I agree

with others though that chamber volume is the way to go.

  

 

[Frank Gooz]

I have an factory aluminum head that is at the machine shop know being cleaned. I can see that it is much shorter then an iron head. The repair Manuel shows that my 1948 Desoto C.R at 6.6. This is a 25 inch engine.

Is there anyone that has CCed their head. I would like to know what Numbers our member are getting. I plan on CCing the aluminum head a spitfire head and a plan old run of the mill head. I want to end up at just about 8.5 to 9.0 at the max.

While I am here has anyone had there engine fully balanced. 

[Tony WestOZ]

As others have said the head will have to be CC `ed. It`s the only way to know exactly what you have got.

I have never found any direct relationship between head thickness and compression ratio.  

 

Tony

 

 

[grubby65]

I decided to just have the head trued.  It took .009".  Thanks for the thoughts, the article above really surprises me.  Appears you can remove a lot of material and not affect the compression ratio very much.  

 

[Matt Wilson]

6 hours ago, grubby65 said:

Appears you can remove a lot of material and not affect the compression ratio very much.  

 

Also, based on what I've read, an increase in compression ratio only results in a power increase of a few percent.  For example, I think I read that a change from 8:1 to 9:1 only increases the power by around 5% for an overhead valve V-8.  I suspect it would be similar for a flathead six.  Such a small power increase is probably not noticeable to most of us.

Edited October 30, 2020 by Matt Wilson

[James_Douglas]

This is covered in old threads. Search for them. Also, valves have been known to head a head shaved too much.

[greg g]

With higher compression you can advance the initial timing. While the horsepower gain maybe small the throttle response seems quite a bit snappier than stock. Go to carnut.com car specs section and look at the 218 and 230 HP numbers in relation to the increased compression ratio.  I know other changes may have been made in cam specs but it seems compression ratio, more HP and more modern fuels contributed to the power increase more than otherfactors.