Compression ratio and deck blocking

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Old September 22nd, 2008, 09:32 AM
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Compression ratio and deck blocking

I have had several guys PM me to further discuss the compression and block decking aspects of engine building. They also said to please try and disregard 88 coupes obvious attempts to discredit everything that I post. So, here goes. I wrote this for OP. I am not saying it is the right way or the only way, but it's the way I do it.
Norm;, you are on my "ignore" list, I will not be reading or responding to any of your posts. Do as you like, but you will not bait me into any more of your nonsense.

http://www.oldspower.com/vb/showthread.php?t=36717
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Old September 22nd, 2008, 09:49 PM
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thankyou capt jim
makes good sense to even a novice like me .
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Old September 23rd, 2008, 12:09 AM
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Thanks, I could actually understand it too, It's really not that difficult once you know what all the steps involved are. I read about this all the time in Hot Rod magazine in my youth (subscribed when I was 12) but they made it really hard to understand if you weren't an engine builder or math wizard.

Allan
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Old September 23rd, 2008, 05:21 AM
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Originally Posted by captjim
........ I will not be reading or responding to any of your posts ........
Excellent decision.


Everyone else:

It is a “given” that most of his article, deals with one individual's personal preference, as there are many parts combinations and/or methods, that can be used, to meet ones goals. Expense and practicality, of each, will be the deciding factors.

I will only be addressing the basics (machine shop 101) as they apply to this topic.

Originally Posted by captjim, On OP
........ To insure you get an accurate and true Cr, you must measure everything ........
This is done when the engine is disassembled, (preferably, after its parts have been cleaned) to help determine the final specs, so the correct parts can be ordered and the machine work planned.

Originally Posted by captjim, On OP
........ When they are finished, have the shop CC them ........
Should have already been done.

Equal chamber volumes are part of the operation along with correct valve stem height, as each is dependent on the other. Since the required chamber volumes would have been already been defined when the final specs were determined, and that number would have been included in the job, you would be paying the shop to "check" their own work.

Originally Posted by captjim, On OP
........ Many of the low Cr problems come into play because the pistons sit down in the hole as far as .055" ........
If the specs called for a .055” piston to deck, .055" would be correct for the application. If not, it is very likely, the wrong piston was ordered.

Originally Posted by captjim, On OP
........ Get your block machine work done. Once you get them, mock up the engine with the crank in place ........
Why, would one want to do this “mock up” after the machine work has already been done?

Originally Posted by captjim, On OP
........ Have the pistons pressed onto the rods. Install 4 piston/rod assemblies into the 4 corner cylinders, #1, #2, #7, and #8 ........ ........ Rotate the engine and measure how far from the top of the block each piston is at TDC ........
Why not do this after the short block is fully assembled? While the cam is degreed, for instance.

Originally Posted by captjim, On OP
........ Say that the pistons are .025 down in the hole on average ........
If that was the planned dimension, that is what it should be.

Originally Posted by captjim, On OP
........ There may be a slight variance, and usually is, not a big deal ........
That would be due to tolerance stack, and would have been addressed, when the initial measurements were done.

Originally Posted by captjim, On OP
........ So, you remove everything and send the block back to the machine shop ........
So, after the machine work is done, the block is taken home and the engine partially assembled, so dimensions, that have already been recorded, can be measured. Then it is disassembled and the block sent back to the shop, for more machining.

Norm
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