Question for Engine Builders/Engine Mathletes
Thread Starter
Instructor
Joined: Aug 2012
Posts: 113
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Question for Engine Builders/Engine Mathletes
When building an engine, doesn't deck height minus stack height give you the piston-to-deck clearance?
For example, for a stock L72 engine, the deck height is supposedly 9.8" NOMINAL.
The compression height is 1.770" (+/-.002"), the rod length is 6.135" (+/-.002") and the crank arm length is 1.883". Adding these 3 together equals 9.788" (+/-.004). This is what's known as the stack height, correct?
Subtracting the 9.788" stack height from the 9.8" deck height means the piston-to-deck clearance is .012", i.e. .012" in the hole.
Am I doing this right or am I forgetting something, like bearing crush, high-rpm elongation, etc.?
I'm asking because I have heard that unmolested big blocks left the plant with 9.800" deck heights and pistons that were .020" in the hole, but when I did the math, it just didn't add up (pun intended).
Thanks to any and all who can shed light on this academic exercise.
Sam
For example, for a stock L72 engine, the deck height is supposedly 9.8" NOMINAL.
The compression height is 1.770" (+/-.002"), the rod length is 6.135" (+/-.002") and the crank arm length is 1.883". Adding these 3 together equals 9.788" (+/-.004). This is what's known as the stack height, correct?
Subtracting the 9.788" stack height from the 9.8" deck height means the piston-to-deck clearance is .012", i.e. .012" in the hole.
Am I doing this right or am I forgetting something, like bearing crush, high-rpm elongation, etc.?
I'm asking because I have heard that unmolested big blocks left the plant with 9.800" deck heights and pistons that were .020" in the hole, but when I did the math, it just didn't add up (pun intended).
Thanks to any and all who can shed light on this academic exercise.
Sam
Drifting
Joined: Apr 2001
Posts: 1,926
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From: Portugal
I believe you are doing the math correctly. As for the piston-to-deck clearance, I don't know what they were from the factory. Nowadays, I think folks try to get it much tighter than that 0.02 number. You also need to factor in the gasket compressed thickness. Adding those two numbers give you the quench/squish which is kinda important. Of course, the cam lift and duration and valves vs the actual real world contour of the piston dome matter too.
I know enough to be dangerous. I pay a good engine builder to assemble my engines because what I don't know can hurt my engine.
I know enough to be dangerous. I pay a good engine builder to assemble my engines because what I don't know can hurt my engine.
Drifting
Joined: Apr 2001
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From: Portugal
Instructor
Joined: Oct 2021
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From: Space Coast
Your math looks right but it’s more important to actually measure. The few I’ve measured vary front to back and bank to bank due to various casting / machining imperfections. This guy explains how with an accent.
Race Director
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From: Corsicana, Tx
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Your math is correct....it's all those + and - that impact things.
"Nominal" blueprint deck height on a BBC is 9.800". Most are taller out of the factory and when blueprinted are cut to that number. Very common to find pistons .018"-.025" down in the hole. And many aftermarket pistons are made "short" to account for additional decking someone might do. Without actually measuring it's easy to end up with a piston .040" or more down in the hole...and THEN a composition head gasket is added that is .040"-.050"+ instead of the .018"-.020" steel shim it left the factory with and somehow people are amazed when they end up with an 8.5 compression engine thinking they have 10.25 or something. This is why some engines outperform others. Details.
My turbo'd 555" engine has the pistons .009" OUT of the hole....and my recent 408" supercharged project has pistons I custom ordered .020" TALL to get zero deck without having to cut the block. I wanted to leave it as thick as possible.
JIM
"Nominal" blueprint deck height on a BBC is 9.800". Most are taller out of the factory and when blueprinted are cut to that number. Very common to find pistons .018"-.025" down in the hole. And many aftermarket pistons are made "short" to account for additional decking someone might do. Without actually measuring it's easy to end up with a piston .040" or more down in the hole...and THEN a composition head gasket is added that is .040"-.050"+ instead of the .018"-.020" steel shim it left the factory with and somehow people are amazed when they end up with an 8.5 compression engine thinking they have 10.25 or something. This is why some engines outperform others. Details.
My turbo'd 555" engine has the pistons .009" OUT of the hole....and my recent 408" supercharged project has pistons I custom ordered .020" TALL to get zero deck without having to cut the block. I wanted to leave it as thick as possible.
JIM
Thread Starter
Instructor
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Thanks Jim--great info. I've heard of unmolested BBC deck heights as high as 9.815 from the Tonawonda factory. When you combine that with the .021" compressed steel shim gasket and the absolute minimum stack height of 9.781" (1.768 compression height + 6.130 rod length + 1.883 crank arm length), one might be surprised to learn the compression ratio leaving the Tonawonda plant is still well above 11:1. Using the median/average specs for these 3 components, the compression ratio is 11.6. No wonder they recommended using the highest octane fuel available!
Race Director
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From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
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Thanks Jim--great info. I've heard of unmolested BBC deck heights as high as 9.815 from the Tonawonda factory. When you combine that with the .021" compressed steel shim gasket and the absolute minimum stack height of 9.781" (1.768 compression height + 6.130 rod length + 1.883 crank arm length), one might be surprised to learn the compression ratio leaving the Tonawonda plant is still well above 11:1. Using the median/average specs for these 3 components, the compression ratio is 11.6. No wonder they recommended using the highest octane fuel available!
What you're describing is how you truly blueprint an engine. The vast majority came from the factory nowhere close to 11.0/1. Head CC's are all over the board too...and when valvejobs are done and valves get sunk....gets even worse.
All in the details.
JIM
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From: Sarver Pa
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Jim is right. The smart way to do it is to assemble with no rings BEFORE you cut the deck. With the parts you are going to use. Crank, rods, bearings , pistons. You quit often find its different front to back side to side. Get yourself a good depth gauge and carefully measure , double check it . THEN take it back apart and give it to a competent machine shop. You can calculate all you want , but there's no substitute for careful measurements. The old saying, measure twice cut once. Have fun !