BBC 9.8 low deck block. What is the actual measurement of the factory uncut deck height? Is it actually 9.800" or something else.

I had an individual who claims to build race engines spit out some number like 9.78?" I don't know the guy from adam and question if his information was accurate?

I always thought the factory deck height was taller than the spec of 9.8 at like 9.825"?

What's the story here?

Todd,

I guess I never checked the deck height measurement directly, but when I mocked up a bone stock untouched 427 block a few years ago ... the piston was .030 down the hole at TDC.

My Merlin Block was the same... .030 down at TDC, I milled it .025 and when put back together for Final assembly the pistons were .005 down the hole.

So it seems my crank and rods and pistons were right on.

Hope this helps

Todd,
The nominal deck is 9,800"
Tall deck is 10.200" ;)

The # 9.78 is ok ,He is using pistons with 1.645 pin height and leaving the piston .020 down the hole .


1.645 -pin Ht.
2. -crank stroke 1/2
+ 6.135 -rods
_______

9.78
+ .020 piston down hole
_______

9.8 total block deck heights

9.780 specified deck height
1.645 pin height (-)
2.00 galf crank stroke (-)
6.135 rod length (-)
--------
zero meaning the block is zero decked and piston is flush with the top of the block. This also means there's no room for milling in the future during freshen ups.

Hmmm...only way to know for sure is to measure the block in question.

I guess the real basis of this question is/was if the bbc stock unmodified deck height is a true 9.800" or does is typically measure larger or smaller than that?

I'm not refering to the rotating assembly piston height mocked up in the bore, rather the actual block deck measurement from the crank centerline to the top of the deck.

Todd,
At the risk of repeating myself :D

The nominal deck is 9,800"
Tall deck is 10.200" ;)

OK Mike, point taken...sounds like this fellow was mistaken with the dimension he quoted me.

thanks for the DOUBLE clarification ;) :D

1/2 stroke + Pin Height + Rod Length= Deck Height.

Originally posted by Jp-15:
1/2 stroke + Pin Height + Rod Length= Deck Height. Not so ;)
The deck height is the distance from the centerline of the main bores to the top of the block.
Your above formula will tell you how long the rotating assembly is from the crank centerline which will tell you in theory at least how much to cut off of the deck of the block to get a certain piston height which is sometimes also referred to as deck height but is correctly "deck height clearance"

Yeah,,, I know,, picky, picky :D