How important is it to have your block decked? Like is this something I should be doing for a high performance street engine? And what is the optimal clearance for the quench area between the top of the piston and the bottom of the head? Like is .50" too much or too little?

Thanks guys

I am running .005" piston to deck clearence, this is only because my 454 is only .030" over and it is a thick walled block that is good to at least .100" maybe .125" over, so I wanted to leave myself a margin in case I run into problems. I am also running a .039" thick gasket, so that puts my quench at .044." My is a race motor, I would say .050" is fine for a hipo street motor

Decking is a truing up operation. Many factory decks are not square to the crank. They can, and often are, .003-.005", or more, off front to back. Also the banks can be way off. With one often being several thousandths taller than the other. It is not necessary to "zero" deck the block, although many do this. Once you decide on a quench distance and select the gasket you want to use, then deck to get the quench you want. My deck is .006" and I run a .026" gakset for a .032" quench.

.050" (you meant this and not really .50", right?) is considered a bit high for the quench distance but shouldn't be a problem. .035-.045" is usually considered best for a street engine. Many run tighter than this. At .060" all quench benefits are lost.

Yeah thats what I meant graemlins/clonk.gif
So when I buy a head gasket for my .30 454, I'll take my compressed gasket thickness and figure out what deck height I'll need.

New question now....since my block has already been bored .30 over do I need to specify that when I order a head gasket? Or are they all pretty much the same diameter around the cylinders?

The gaskets are the same.

Just to add to what Pat said they can be off end & side to side but he is being very consertive in his estimate of how much.
I just finished a 350 block that was off .017 from the lowest corner to the highest on the opposite bank. :(
Also be sure whoever is doing your block is doing it off the mains & not the pan rails.
People who set up off the pan rails will tell you this is the way the factory does it. This is true but is also how the damn block got off so far in the first place.
Decking off the pan rails is assuming the main bores are straight to the pan rail place which just ain't so in a lot of cases.
Now if it is being done on properly on CNC equipment & the mains are being redone too than using the pan rails as a reference is fine as you are straightening everthing from one data point.
If Carl from CNC Blocks reads this I am sure he can explain this better as it appears this is what he does.
Point is, Decking is not just to make the surfaces flat it is to make them flat & parallel to the mains.
Every machining operation should normally start with the mains as a reference assuming the mains are straight to start with. :D

I agree Mike when doing performance work everything comes of the main line, I think thats why you see blocks off from front to rear. I've taken a dial indicator and swept the pan suface and there not always parallel with the main line. I had to prove this to another shop that bores of the pan rail, This gave him a whole no outlook on things.

There are a lot of shops that don't square and deck the block before boring, I feel the the 45
degree can be off a little the piston can follow that due to the wrist pin, but on the other hand if the block is off from front to rear and you bore it then the cylinder is not 90 degrees to the main line which would make it hard on the rings tring to do there job. Its not uncommon to find decks of alot. Some people put to much faith in GMs work. I have noticed that the new blocks form GM have been a lot closer to blue print then the older ones.

On a blue print of a GM block everything comes off the center line of the cam and the crank not the pan rail.

Originally posted by CNC BLOCKS N/E:
I agree Mike when doing performance work everything comes of the main line, I think thats why you see blocks off from front to rear. I've taken a dial indicator and swept the pan suface and there not always parallel with the main line. I had to prove this to another shop that bores of the pan rail, This gave him a whole no outlook on things.

There are a lot of shops that don't square and deck the block before boring, I feel the the 45
degree can be off a little the piston can follow that due to the wrist pin, but on the other hand if the block is off from front to rear and you bore it then the cylinder is not 90 degrees to the main line which would make it hard on the rings tring to do there job. Its not uncommon to find decks of alot. Some people put to much faith in GMs work. I have noticed that the new blocks form GM have been a lot closer to blue print then the older ones.

On a blue print of a GM block everything comes off the center line of the cam and the crank not the pan rail. =
Yea, I get blocks quite regularly that have been bored .030 that I can't straighten at .060 because someone has hamburgered it up :mad:

And in your last sentence is where the problem with the factory blocks used to be as the blueprint is off the centerline on the cam/crank & somewhere in production someone decided to use the pan rail with those two holes on the left side to reference off of.
This would be fine if they had gone back to the blueprint & made sure the block ended up in the machines properly every time but instead probably for cost reasons it was assumed the two holes would magically locate the block in the machining equipment & everything would come out straight :rolleyes:

Or maybe it was some sort of master plan so we would have something to do & could pay for our equipment straightening all this stuff out :D :D

Now I have noticed that on the Bow-tie blocks have a machined pan rail and seems to be parallel with the main line and I think thats due to the fact on the front and rear 4 bolt caps are stepped and the step has to right for the caps to fit.

I got all my machining completed and did a trial assembly to check decks on my SB. They were pretty typical, pistons were between 0.023" and 0.031" down the hole.

Square decking was completed using one of those fixtures that sets up on the mainline/camline. The passenger side ended up about 0.007" down the bore. I was shooting for 0.005", but this was ok.

Drivers side ended up at 0.000" on the lifter valley side and 0.015" below on the outside. When we began cutting the deck it was clearly off so we stopped, re set-up and started over with the exact same results.

I went ahead and assembled it. Yeah it's not perfect but at least the quench side is 0.000" and not 0.015". Runs fine and with square decks exactly 90° apart the intake manifold seals very well.

Carl,
Yea, the bowtie stuff is really nice to work with. The mains are straight with the cam, the lifter bores are where they belong, the decks are close,,,nice stuff, saves a lot of work to get it right ;)

Roadknee,
First, I don't consider .008 difference "normal" in an engine that was squared up.
You will find slight differences in deck hgt. because of the tolerances in piston/rods & crank but 8 thou is in my opinion not an acceptable "tolerance"
Unless we are talking a $700 parts store rebuild. :(
If I square a block & you can measure more than .002 between all four corners I will return your money.
>.001 on the four corners is what I strive for & almost always get.
I have some customers that just would not even consider accepting those kinds of differences.

Call me dense but I am completely missing what you are trying to convey :confused:
Are you saying the block was first bored off a crooked deck & then decked square?
That's the only way the bores are going to not be parallel with the deck from side to side.
If this is the case I would suggest finding someone who knows how to square a block properly.
If I am missing something please explain??

Originally posted by Wolfplace:


Roadknee,
First, I don't consider .008 difference "normal" in an engine that was squared up.
You will find slight differences in deck hgt. because of the tolerances in piston/rods & crank but 8 thou is in my opinion not an acceptable "tolerance"
Unless we are talking a $700 parts store rebuild. :(
If I square a block & you can measure more than .002 between all four corners I will return your money.
>.001 on the four corners is what I strive for & almost always get.
I have some customers that just would not even consider accepting those kinds of differences.

Call me dense but I am completely missing what you are trying to convey :confused:
Are you saying the block was first bored off a crooked deck & then decked square?
That's the only way the bores are going to not be parallel with the deck from side to side.
If this is the case I would suggest finding someone who knows how to square a block properly.
If I am missing something please explain?? You're not dense. I'll clarify.

The block had not been squared up the first time I measured, so the 0.008" difference was from GM. That's what I refer to as "normal".

I believe the block was bored/honed using a Sunnen CK-10. Don't these just follow the bore alignment from the factory???

A very compentent machinist and personal friend (someone other than who bored the block) performed the square decking. I was present for the whole process and am absolutely certain that the decks are 90° from each other and oriented correctly with respect to crank/cam centerline.

The passenger side came out perfectly, within 0.001" as you say with the exception of one cylinder where the stock rod appeared to be about 0.003" shorter than the rest.

The drivers side is perfect front to back, but the pistons at the lifter valley side are about 0.015" higher than the outside.

I was trying to convey that the pasenger and driver side cylinder bores are not 90° from one another and appear to have been that way from the factory.

Thanks, that explains it completly.
Yes a CK10 or any hone will only follow whatever direction the hole was in the first place.
This is why if you want it straight you bore it & deck it off the mains then hone it.
It sounds like had your machinist done it he would have bored the block before honing in which case everything would have been straight.
It is not unusual for the bores & decks to be going in all different directions from the factory.
And sometimes even worse & damn near impossible to fix after someone has been there before you :D