How much off the block?

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Sportman

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I am taking my block to the machine shop on Thursday. My question is how much should I have him take off the block. My combo adds up to 9.583 that should make the pistons down the hole .017 considering stock height of 9.600??? I am running the 71cc j heads. I talked with the machine shop and he wanted to know how much I wanted off the block.....help! Should I have him take off the .017, or less?

Thanks
 
Technically, you should mock up the engine with the crank rods & pistons & then measure. Or you can just fudge the numbers & hope you're close. Then get it where you need it to be with head gaskets? Maybe find a machinist who knows the answer? It's probably more critical that he gets the decks squared with with the crank bores.
 
Take the measurements in 4 corners .. use 1 rod piston combo for all measurements. Prolly not a bad idea to have the crank checked for stroke while the blocks in for machine.
 
My machine shop line honed the block then they used special equipment that could measure the deck height from the crank center line and cut from there. Not all shops have that equipment though. I would not assume your block is a certain height, all blocks are different due to very sloppy factory tolerances.
 
340sFastback said:
My machine shop line honed the block then they used special equipment that could measure the deck height from the crank center line and cut from there. Not all shops have that equipment though. I would not assume your block is a certain height, all blocks are different due to very sloppy factory tolerances.
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Exactly. Some blocks are taller than the 9.60 spec. You need to measure things.
 
many times more is taken off of one side than the other. Like my 318 I had .020 off of one side and .023 off the other to "square up the block" I am not at zero deck, just wanted a bit more compression.
 
70aarcuda said:
BHJ block squarer is what is needed
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Isn't that whole process commonly called "square decking a block" ??

It cost more that just trueing up the decks. You gotta ask yourself if that 2-4 thou variation is worth the extra cost.

What is the common cost for square decking a block?
 
In my case the machine shop is located 5 hours round trip from where I live, and the block needs bored prior to mocking up the motor....so. I am in an area where there are a few machine shops but not any that are specialised in performance type motors. The one I am taking my block to told me he was a machinist not an engine builder. He said it was up to me to tell him what I wanted done as far as decking. I am heading there today with some measurements to discus what I want done maybe the block will come home with me! I think I will just have him clean, line bore & square deck the block.
 
With J heads you are not dealing with any worries about quench and as you say there's some logistical reasons to not worry so much. Have him cut .020 off it and call it good.

As far as the value of square decking... Yes, it's much better in most cases, a little better in some, but always better overall. It's much more than a few thousanths... It's more like .002" on one end, and .017" on the opposite end because of errors in the factory and subsequent milling attempts. Square decking brings the head surfaces into exactly parallel witht he crank centerline. That's critical when you set up a quench distance, or are looking to balance power on all cylinders because they can be tens of thousanths off square for a variety of reasons. Factory machining of all parts was really horrid.
 
1wild&crazyguy said:
the last 2 la blocks I worked with were around .006 off catty corner/across
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Same here.

But I don't think I'm going to get $200 worth of HP gain from it. Or I could spend $200 on something else and get more gain.
 
The machine shop shouldn't be asking you how much they should take off. They should be asking you where you want the pistons to sit in relation to the deck. Then they should be measuring the deck height (they are more often than not higher than the nominal 9.600") and squareness and tell you what needs to be done.

And to determine where you want the pistons to sit you need to know what you want for a compression ratio and to do that you will need to know the actual chamber volume of your heads as well as the piston volume (dished, flat top, valve reliefs, etc) and the head gasket you plan to use (compressed thickness and bore). J heads could be anywhere from 68 to 72 cc, so you will need to meausure.

One of the online compression ratio calculators will help, like this one http://www.kb-silvolite.com/calc.php.

BTW, if they are using a machine like this with the same type of fixturing and they remove material from one end to the other the deck will be square.

BlockDecking.jpg
 
think u need to find a shop that is going to tell you what you need to have taken off,,,
 
You have to measure it and get the exact deck height, because not all blocks are the same, they could have been shaved before for all you know. Take a dial gauge and measure the piston to deck clearance with one installed to get a ball park figure.
 
autoxcuda said:
Same here.

But I don't think I'm going to get $200 worth of HP gain from it. Or I could spend $200 on something else and get more gain.
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I can't disagree if those are yours and 1Wild's findings. That has not been mine. typically decks are .020-.030 taller than factory blueprint, and if they are milled on a horizontal milling table with the typical multi cutter round head (like the factory) they will be out a couple thou from valley to exterior edge just because of the curve of the wheel, possibly more depending on how the block is held to the table, the quality of the lelvel used to set it up, and the attitude of the machinist.
 
autoxcuda said:
Isn't that whole process commonly called "square decking a block" ??

It cost more that just trueing up the decks. You gotta ask yourself if that 2-4 thou variation is worth the extra cost.

What is the common cost for square decking a block?
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I was just posting as to what piece of equipment is needed...as far as cost to benefit ratio...i agree.....probably not worth it on a street driven engine...

if you are looking for every last ounce of performance...Yes..
 
Just got back home from the machine shop. I think the old guy was jerking my chain, he has been doing this for over 40 years. My guess is that he knows what he is doing. I left him with my figures and he said he would take care of everything! I am keeping my fingers crossed, said it would be finished next week. Thanks everyone for helping me understand all of this.
 
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