Checking deck height

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coloradohill

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Looking for opinions from the engine builders.
I purchased a 1970 340 recently. .030 over, while checking deck height I found the following.
Checked using a deck bridge / dial indicator. All numbers are pistons above deck.
Planning on Cometic mls head gaskets and wanting to take advantage of optimal quench.
My question(s) are; are these normal numbers? would you leave it as is or mill the block?
Thank you for the advice. Feel free to add any information I may have not known to ask.

IMG_3122.jpg


IMG_3127.jpg
 
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You really just need to check it inline with the wrist pin, top and bottom of piston will rock on the pin.
 
coloradohill said:
Looking for opinions from the engine builders.
I purchased a 1970 340 recently. .030 over, while checking deck height I found the following.
Checked using a deck bridge / dial indicator. All numbers are pistons above deck.
Planning on Cometic mls head gaskets and wanting to take advantage of optimal quench.
My question(s) are; are these normal numbers? would you leave it as is or mill the block?
Thank you for the advice. Feel free to add any information I may have not known to ask.

View attachment 1715681488

View attachment 1715681489
Click to expand...

Easy way out would be , buy 2 sets of gaskets , one .060 thick , and one .055 thick, cometic is reported to custom make anything.
 
Those look like stock style 10.5:1 pistons and that was with shim gaskets .025 thick.
 
brian6pac said:
Those look like stock style 10.5:1 pistons and that was with shim gaskets .025 thick.
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Yes, From what I can tell they are TRW 2332P pistons. I am going to install Edelbrock rpm heads and would like to keep quench safe, but as tight as possible. Stock rods.
Thanks for the reply.
 
Call Cometic Gaskets you can get any thickness, I used .080 before. Edelbrock are closed chamber so you could even go .100 - .120 if you wanted. That would be wide on the quench.
 
They SHOULD be out of the deck like that. You can be out .054 or more. Do not screw with it other than to make both sides equal and to increase compression.

Looking at your numbers, I’d deck the block to get both sides equal and the amount of take off would be what ever it takes to get the CR where you want it.
 
yellow rose said:
They SHOULD be out of the deck like that. You can be out .054 or more. Do not screw with it other than to make both sides equal and to increase compression.

Looking at your numbers, I’d deck the block to get both sides equal and the amount of take off would be what ever it takes to get the CR where you want it.
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Thank you YR. Thats what I'm looking for. Advice of what you would do.
 
Just so you know, what you're checking there is deck clearance, not deck height. Deck height is the distance from the crank center line to the deck surface. Deck clearance is the measurement of the relationship of the piston at TDC to the deck surface.
 
RustyRatRod said:
Just so you know, what you're checking there is deck clearance, not deck height. Deck height is the distance from the crank center line to the deck surface. Deck clearance is the measurement of the relationship of the piston at TDC to the deck surface.
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Thanks Rob, Clearance is what I meant. I would like to increase the compression since I'm at 5k feet with aluminum heads. Is it reasonable to just give the numbers to the machinist and have him figure how much to mill?
 
coloradohill said:
Thanks Rob, Clearance is what I meant. I would like to increase the compression since I'm at 5k feet with aluminum heads. Is it reasonable to just give the numbers to the machinist and have him figure how much to mill?
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I don't know. I would be skiddish. I would want my machinist to measure for himself. Not questioning your work, but what he might do. Some machinists are lazy and would set up off your numbers.....this is important enough to set up off measured numbers from the crank center line. That's your reference. You want the decks to be parallel and squared to the crank center line, not to the deck clearance. Does that make sense?
 
RustyRatRod said:
I don't know. I would be skiddish. I would want my machinist to measure for himself. Not questioning your work, but what he might do. Some machinists are lazy and would set up off your numbers.....this is important enough to set up off measured numbers from the crank center line. That's your reference. You want the decks to be parallel and squared to the crank center line, not to the deck clearance. Does that make sense?
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Sounds like a good plan, I have a very good machinist, retired still doing work from home. I would like to save some costs by doing the disassembly / assembly myself. I'll visit him next weekend.
Thanks, Have a great week.
 
coloradohill said:
Sounds like a good plan, I have a very good machinist, retired still doing work from home. I would like to save some costs by doing the disassembly / assembly myself. I'll visit him next weekend.
Thanks, Have a great week.
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Sho nuff! You too!
 
You need to find your low piston and your high piston and swap them on the rods and see if you can get your pistons close as possible. I have done this on many occasions and it does work most of the time.
 
brian6pac said:
You need to find your low piston and your high piston and swap them on the rods and see if you can get your pistons close as possible. I have done this on many occasions and it does work most of the time.
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Yup. This shows up incorrect crank indexing, a different length rod, different compression height piston or a piston head difference. When you get down to measuring THOUSANDTHS of an inch, there's no telling what you'll find.
 
brian6pac said:
You need to find your low piston and your high piston and swap them on the rods and see if you can get your pistons close as possible. I have done this on many occasions and it does work most of the time.
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Going by what the OP posted, it’s off side to side and the even bank has a slope in in it. Changing parts around won’t correct that. Unless some stooge resized the rods and changed the lengths by .005 or something, that’s not in the rods. I don’t even see that being tolerance stack up.

When they get sloped like that it’s almost always because when the block went into the mill the goofball didn’t indicate in the bar, but instead used a level on the deck. You need to indicate in the bar and then make the block flat.
 
brian6pac said:
You need to find your low piston and your high piston and swap them on the rods and see if you can get your pistons close as possible. I have done this on many occasions and it does work most of the time.
Click to expand...

Thanks for the idea but, I forgot to mention that this rotating assembly has been balanced.
 
yellow rose said:
Going by what the OP posted, it’s off side to side and the even bank has a slope in in it. Changing parts around won’t correct that. Unless some stooge resized the rods and changed the lengths by .005 or something, that’s not in the rods. I don’t even see that being tolerance stack up.

When they get sloped like that it’s almost always because when the block went into the mill the goofball didn’t indicate in the bar, but instead used a level on the deck. You need to indicate in the bar and then make the block flat.
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I don't think I would hang my hat on that piston being perfectly square in the bore when taking those numbers. Just a little rock significantly affects the reading top and bottom. Now inline with the crank or wrist pin, I would expect essentially the same on either side of the bore. I know when I was measuring mine for quench, I got a lot different number at the top and bottom of the piston relative to the center-line because of the clearance and light bore wear.

So how are you supposed to measure for quench? Highest point on the edge of the piston when it is rocked? Or in the central area above the wrist pin? I thought is was "worst case" which would be with the rock and you want to have a gasket that will give about.035-.040" gap/quench between the piston and the head deck.
 
gzig5 said:
I don't think I would hang my hat on that piston being perfectly square in the bore when taking those numbers. Just a little rock significantly affects the reading top and bottom. Now inline with the crank or wrist pin, I would expect essentially the same on either side of the bore. I know when I was measuring mine for quench, I got a lot different number at the top and bottom of the piston relative to the center-line because of the clearance and light bore wear.

So how are you supposed to measure for quench? Highest point on the edge of the piston when it is rocked? Or in the central area above the wrist pin? I thought is was "worst case" which would be with the rock and you want to have a gasket that will give about.035-.040" gap/quench between the piston and the head deck.
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According to this article, you average the piston "rock" readings.
Thanks for thought.

EngineLabs Blueprint Series: Verifying Piston-to-Head Clearance
 
coloradohill said:
According to this article, you average the piston "rock" readings.
Thanks for thought.

EngineLabs Blueprint Series: Verifying Piston-to-Head Clearance
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Put the dial indicator over the center of the piston as far inboard as you can before you fall off the quench pad and measure it.

All that average rock and crap will just add to the confusion.

Zero the indicator, roll the crank over twice (so you get two readings) and that’s what it IS. Then correct it from there.

You will drive yourself bat crap crazy doing all the averaging and all that. If you get your piston to head clearance to .050 you won’t find a single HP getting it closer. So let’s do some math, assuming you have open chamber heads.

Let’s just guess and say your heads came from the factory on the low side, meaning that the castings needed more materiel machined off to get them flat and clean (relatively), and your heads came through with the recess only .080 deep. Most I’ve measured are .100, but you got lucky. BTW, you have to measure that recess and make the both the same to start with. Can’t just assume they are the same.

Ok, to get your chamber volume where you want it, you deck another .020 off, and the recess is now .060 deep. Throw a typical rebuilders head gasket on there and that recess is effectively .108-.110 deep!

If your piston is out of the hole .020, you have .088-.090 clearance. And yet, if you have everything else right, they won’t rattle at 10.5:1 on cheap pump gas.

That also means if you want to use that thick gasket, you’d need the piston out of the block .050 just to get your .040 quench clearance. And that’s IF it doesn’t raise the CR too high.

Obviously you can use a thinner gasket and not get the piston so far out of the hole.

At any rate, you have to find a machinist who gets tha it’s the RIGHT away to build an engine like this. By this I mean getting the piston out of the hole.

Most guys THINK they have 10.5:1 with those pistons but they aren’t close.
 
I will measure again tomorrow center of piston when I get a chance and post the findings.
I am using closed chamber aluminum heads and would like to keep compression to the upper limits due to the power loss at altitude.
I’ve been told to add 1 point for aluminum and .5-1.0 static for 5000 ft. For pump gas.
does that sound reasonable to you?
Thanks again
 
coloradohill said:
I will measure again tomorrow center of piston when I get a chance and post the findings.
I am using closed chamber aluminum heads and would like to keep compression to the upper limits due to the power loss at altitude.
I’ve been told to add 1 point for aluminum and .5-1.0 static for 5000 ft. For pump gas.
does that sound reasonable to you?
Thanks again
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If you have closed chamber heads, put the pistons in a piston Vice and knock the quench pad off of it. Run them at zero deck and a .040 gasket and be done with it.

I won’t make a CR recommendation because I run way more CR on pump gas than most do. IMO at you elevation 12:1 would be where I’d start.

That is not a recommendation, it’s my opinion and those two things are not the same.
 
I had time to measure
yellow rose said:
Put the dial indicator over the center of the piston as far inboard as you can before you fall off the quench pad and measure it.

All that average rock and crap will just add to the confusion.

Zero the indicator, roll the crank over twice (so you get two readings) and that’s what it IS. Then correct it from there.

You will drive yourself bat crap crazy doing all the averaging and all that. If you get your piston to head clearance to .050 you won’t find a single HP getting it closer. So let’s do some math, assuming you have open chamber heads.

Let’s just guess and say your heads came from the factory on the low side, meaning that the castings needed more materiel machined off to get them flat and clean (relatively), and your heads came through with the recess only .080 deep. Most I’ve measured are .100, but you got lucky. BTW, you have to measure that recess and make the both the same to start with. Can’t just assume they are the same.

Ok, to get your chamber volume where you want it, you deck another .020 off, and the recess is now .060 deep. Throw a typical rebuilders head gasket on there and that recess is effectively .108-.110 deep!

If your piston is out of the hole .020, you have .088-.090 clearance. And yet, if you have everything else right, they won’t rattle at 10.5:1 on cheap pump gas.

That also means if you want to use that thick gasket, you’d need the piston out of the block .050 just to get your .040 quench clearance. And that’s IF it doesn’t raise the CR too high.

Obviously you can use a thinner gasket and not get the piston so far out of the hole.

At any rate, you have to find a machinist who gets tha it’s the RIGHT away to build an engine like this. By this I mean getting the piston out of the hole.

Most guys THINK they have 10.5:1 with those pistons but they aren’t close.
Click to expand...


I had some time to check at the center of the piston at the edge of the quench pad.

Would you say it is safe to call it .020 out of the hole and order Cometic gaskets to give a .040 quench? Which would be .060 gaskets?
Thanks

IMG_3132.jpg
 
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