Milling for compression

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Trevor B

Trevor B

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Warning: math is definitely not my strongest suit, even though I triple-checked it...

I just took a bunch of measurements in order to calculate the compression for the 225 slant (0.040" over) I'm rebuilding. I came up with the following, using krazykuda's formula:
The piston needs to be 0.050" below the deck for my goal of 9.5:1 compression on a naturally aspirated engine. It is currently 0.200" below the deck. That means milling a combined 0.150" off of the block and heads.

Does this sound right?
What should I do?
0.100" off the block and 0.050" off the head?

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Here are my numbers:
Cylinders are 0.040" over = 3.44" diameter
Stroke is 4.125"
Swept volume calculated at 628.36 cc
Piston top to deck ~ 0.200" = 30-31 cc
Head combustion chamber volume: 60 cc (averaged across 6 cylinders with block off plate, graduated cylinder, and patience)
Head gasket thickness = 0.040" = gasket volume = 6.3 cc
Fire ring = 3.5"
... Swept volume = 628.36 cc
... Clearance volume = 97.6 cc

These numbers yield stock compression of 7.5:1

How to Check Compression
 
if the piston is .200 in hole and you need it .050 in hole then top of block needs .150 off the deck! nothing to do with head, cc it, what is it? whats it need to be? what that difference? gets the head done! machining head ant ever gonna bring piston up,..just bring combustion chamber down! good luck!
 
dirty white boy said:
if the piston is .200 in hole and you need it .050 in hole then top of block needs .150 off the deck! nothing to do with head, cc it, what is it? whats it need to be? what that difference? gets the head done! machining head ant ever gonna bring piston up,..just bring combustion chamber down! good luck!
Click to expand...
will a slant 6 take that much milling ? I think I`d look at some new pistons . May be getting ready to do one myself.
 
If someone doesn't answer here the guys at slantsix.org can help you out. I remember that taking a 10 th of an inch off the bloc is not uncommon. But I also seem to remember there is only so much meat on the head to use up. I think I have that right.
 
dirty white boy said:
if the piston is .200 in hole and you need it .050 in hole then top of block needs .150 off the deck! nothing to do with head, cc it, what is it? whats it need to be? what that difference? gets the head done! machining head ant ever gonna bring piston up,..just bring combustion chamber down! good luck!
Click to expand...

Argh! You're right. I was so excited to have come up with a logical seeming answer to math that I forgot to think about the physics so carefully. But it will still raise compression with a smaller combustion chamber, wouldn't it?
 
Wouldn't it cost less to buy some tall pistons so you don't have to machine the block or the heads?? There are many companies out there that don't charge that much for a custom "stock type" piston.

Treblig
 
Treblig said:
Wouldn't it cost less to buy some tall pistons so you don't have to machine the block or the heads?? There are many companies out there that don't charge that much for a custom "stock type" piston.
Treblig
Click to expand...

Could very well have been a cheaper option but I already have the pistons - the block was bored for them. And the head has to go to the machine shop anyway to finish the valve job and clean up (I got bigger valves and had the head hogged out for them, then brought it home to do some light porting/cleaning).
 
The head will take that much milling. I am not sure about the block. Charlie can probably answer your question. I know he has a head with more than .100 milled off......I think it was .125, but it was an accident. He had forgotten it had been milled before, I believe, and then had it milled again. It worked fine though.

I would take a guess the block can probably take a .150 cut, but that's just a guess.

You could also have the crankshaft offset ground to give a little more stroke and that would raise compression as well.
 
I think there is the additional issue of whether decking the block .1" would change the valvetrain geometry too much. I don't know this of my own experience, but I was told by a respected engine builder that this is often the limiting factor in decking a block.
 
Bill Crowell said:
I think there is the additional issue of whether decking the block .1" would change the valvetrain geometry too much. I don't know this of my own experience, but I was told by a respected engine builder that this is often the limiting factor in decking a block.
Click to expand...
Offset grinding the crankshaft is starting to sound better!!!

treblig
 
Bill Crowell said:
I think there is the additional issue of whether decking the block .1" would change the valvetrain geometry too much. I don't know this of my own experience, but I was told by a respected engine builder that this is often the limiting factor in decking a block.
Click to expand...

It does not change valve train geometry one iota, but you might need shorter pushrods.
 
Is it really .200 in the hole ? Almost 1/4" that sounds way off to me but I could be wrong.
I would double check that reading. Stock compression should be 8.5 : 1
 
brian6pac said:
Is it really .200 in the hole ? Almost 1/4" that sounds way off to me but I could be wrong.
I would double check that reading. Stock compression should be 8.5 : 1
Click to expand...

Yeah that's about right.
 
How about some custom longer rods? That would skirt all the issues.. Don't forget to keep track of the quench. If you get it into the ugly zone of .050 to .080, you might just aggravate the chambers right into being detonation prone.
 
AJ/FormS said:
How about some custom longer rods? That would skirt all the issues.. Don't forget to keep track of the quench. If you get it into the ugly zone of .050 to .080, you might just aggravate the chambers right into being detonation prone.
Click to expand...
the rods outta a 198 sposed to be longer than a 225 by best i remember .200
 
My almost 30yr. old built 225 is .060" over w/a deck ht. of .130" & the head whacked .055", cc'd at 9.35:1 the pistons were stock cast .060 over replacements.....they saw
7K on a few ill advised uhmmm, exuberances, and over 6 regularly,often,& frequently!! The exact amount the block was decked is hard to say, but I wanted .060" off each,
and the head checked .055", so .070" off the deck is possible. That may have been an "executive decision" made by the guy I was using at the time(He was good for those).
 
famous bob said:
will a slant 6 take that much milling ? I think I`d look at some new pistons . May be getting ready to do one myself.
Click to expand...
Yes it can,.....go ahead & look for those pistons...& think how much they'll weigh.....they already look like a can of baked beans!!!
The reason the 198 long rod swap is popular is the ease of reducing the piston mass & putting it up high all at once....................
 
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