Should I deck my block?
 

Hey guys... I'm building my 351W at the moment. I'm trying to decide whether or not I need to have my block's mating surface milled/decked for SEAL only.

My machinist said some people have it decked just to be safe, while others sometimes opt not to to save a hundred bucks on the build.

My engine is a mild build only. I'm shooting for only 350-400hp, modest compression, no forced induction or nitrous or anything like that.

I want to do the right thing and build myself a reliable motor, but I don't want to throw away $100 getting it decked if I don't need to for my intended application...

Recommendations? Thx

I deck all blocks to zero or near zero depending on numbers. I feel after spending as much money as I have already spent the cost of decking is such a small percentage of build it is right thing to do. Benifits are many and include proper relation to crank / cam center line, proper seal surface, ability to correct any core shifting and allows CC of chambers ( deck runout is big influence), performance adder etc.. If only decking to zero you will not alter intake to head seal or any other spec with stock stroke and pin height. Only down side is cost and that is small % to insure many possible conflicts removed.
If just a cheapest build possible maybe overkill but is that what you are doing or are you upgrading parts and performance?

What I would be conserned about is quench. To make the most power and prevent pinging you need to run about .035" quench +/- .005 depending on who you ask. Quench is measured from the top of the piston to the bottom of your head. I milled my block to allow the pistons to stick out .004" with a .041 head gasket it comes out to .035 quench. You need to allow room for the parts to expand and bearing cleanances, so running less becomes more risky, running more quench is more likely to cause pining because your not creating as much turbulence that helps cool hot spots.

Well like I said, this is not an ultra high-performance machine by any stretch, but it's not supposed to be an absolute dirt-cheap rebuild either. To give you an idea - I'm using the stock crank, stock rods w/ ARP bolts, hypereutectic pistons, iron heads, probably a mid 9s CR with a street performance cam... that type of thing. Shooting for 350-400hp N/A, maybe a 5500 redline. Nothing astronomical, but not the cheapest stuff either.

So do you think having it zero decked is reasonable for my setup? Or overkill for what I'm doing?

brainsboy,

Well, I'm using stock everything, so 3.500" stroke, 5.956" rod, and 1.772" comp height. That gives 9.478" from crank centerline to the top of the piston. The deck height on my block is 9.503", so according to what you're saying, I should probably have 0.020" to 0.030" shaved?

Or should I just fug it for now, have it bored, order my pistons, bring it home, test fit it, and deck it later?

Dan

OK this all sounds very weird because no one has mentioned the fact the block should be measured for "square" before decking.

Measure deck height at each corner of the block and compare that number to the stock specification. If the block is NOT square it's a no brainer, gotta deck it.

If it is square and it passes a flat test using a straight edge and a feeler gauge there is no "normal" reason to deck it at all.

If you want to get fancy and go for zero deck height (not a bad thing) or some other "race ready" reason, then deck it.

What bothers me about this is:

Why didn't your machinist measure the block for square before recommending he mill the block? I don't like doing things "just because" thats the way the machine shop always "does it".

Deck height is measured two ways:
From the center of the crankline to the top of the block. This is a VERY difficult measurement and requires special "tools". I wonder if the machinist can do it? The other way is measure rod length and piston pin to top length, stroke and then do the "math". BOTH sets of measurements should agree with each other. YOU could measure your rods and pistons yourself and compare that number to what the machine shop said it was.

In his defense, my machinst didn't actually recommend anything. He ASKED me whether I wanted to deck it or not. I asked what he recommended, and he said 'some do, some don't'. He didn't mention measuring it for squareness, though.

I think because of the fact that the motor ran reasonably well when pulled, and the fact that I'm doing a relatively 'minor' build here, he figued the shape it was in was probably sufficient....no?

Dan,
Basicly you are trying to build it RIGHT. The term blueprinting is often used and exploited to a degree but if you have stock parts as ( new parts meeting stock spec ) then taking block to factory spec of deck height to + or - 0.00 is in 75 % of cases a zero deck. I have not looked this one up so am not on hard ground here but easy to check.
With factory spec parts and no big cam change you will be right there but checking all numbers and tolorances is part of a quality build. I would check #'s befor doing anything to be sure. But the question was as I went off with is decking as a general question a good thing and worth the money. YES it is ! too what height is answered by selection of real parts and their tolorance stack up. I then took it a little farther and said ZERO is the happy spot in most cases with stock parts. You must or builder must know all variables to answer finished deck height. What ever that # is will then make all square and this is reason for correction or milling. Decking is a correction of many factors at the same time, parralelism to crank, proper angle or square to bores, proper or equal combustion area of all cyl's, proper or flat surface for gasket, equlizes distance from cam for rocker geometry. for a 100 bucks..

Thanks for all the advice.

According to the CR calculators I'm using, using flat-top pistons will drive my CR right where I want it, to about 9.5. So I don't think I need to remove any material from the block, just ensure that it's the factory spec 9.503" like it should be.

So maybe the best idea is to have him just bore it for now, I'll bring it home, order my pistons, do a test fit as well as measure the surface for flatness, and take it back then for decking?

Dan

Well I feel much better with your machine shop now. He's a sharp guy to bring up the subject and discuss it with you. Which is way different than suggesting you deck it without discussing it.

Your on the right track, carry on. :D

IF you have ever wondered why one motor makes 500hp but another almost identical motor only makes 475hp its usually because of the details. Your block being square or not, you need to deck. First the pistons are probably rated at 9.5:1 based at a 0 deck. This means if your pistons sit at where your saying .025" below deck that now your only at 9.03:1 compression. A little bit of proven info is that a motor with 11:1 and proper quench will ping less then a motor with 9.5:1 and a large quench. On a ford stroker motor you can count on an extra 5-9hp from doing it. As one person suggested you can wait until your pistons come in and square the block first then mill down to a proper height for your quench.

Check the block for squareness, if it's out, deck it just enough to get it true.

Hang a piston and rod, see where it is relative to the deck height at TDC. This is a number you must check, you can't just look up what it should be. If it's still deeper in the hole than it should be, then let the machinist remove some more off the deck.

If the block has been line honed, the crank has moved up a little bit in the block. If your rods have been resized, they are shorter now. God only knows how close your pistons are to what they are advertised.

In short, you ought not guess, but instead see where your pistons are relative to the deck, then make the necessary corrections after you know.

If your buying new custom pistons you could also specify the piston pin height to accomplish a zero deck clearance.

Just thought I'd throw that in there.

If he's buying custom pistons, I doubt he would be sweating the $100 for block machining.

The machine shop I use does not have all the tools to accurately measure the squareness of the block,what I do is install the crank and the four outside pistons and check piston to deck height,then take it all down and have the block "cut" to MY desired amount,depending on the compression ratio I want......sometimes it is a lot as in my last engine,sometimes it's very little...... I have found quite a bit of difference in blocks,depending on when and where they were casts and the mileage and usage it saw before I got hold of it...............a number of years back while rebuilding two 351-Ws using the same pistons/rods/crank I found on one block the pistons were 20 thousands from the top,on the other they were 65 thousands,the only difference was one block was a 1974 block and the other a 1973 block......when you take into consideration the crank/rods/pistons/block variances,it can add up to quite a bit......

It's a little more work,but when finished I know exactly where my compression ratio is and piston to valve clearances,no guessing about these things............

BTW,the machine shop I use only charges 50 bucks to cut the deck of a block..............

David

I suspect my shop won't have the equipment to measure it either. Besides, I'd like to do it myself if possible. So I think I'll just have the shop do the bore/hone and install cam bearings, plugs and stuff for now. I'll bring it home, slap my pistons in, and see how it looks myself.

Thanks for the help guys.

Here's my .02 $, only because it applies to any engine build.

"Should I deck the block?"

If my machinist told me, "Ehhh, some do, and some don't", I'd promptly find another machine shop!

As described above, machining a block deck cures a number of problems in one operation. So, the correct answer from the machinist is, "The block needs decking because.....", or, "The block does not need decking because.....".

Should you entrust a machinist who doesn't know the right measurements to take to give you an accurate assessment, you'll have a decked block, with the problems still present.

Time was, most auto machine shops rebuilt grocery getter motors, and performance work was occasional, if at all. And you had to tell the guy exactly what you wanted.

Today, the grocery getter motor business is drying up, and performance work is a mainstay. Today,the engine assembly area looks more like a hospital operating room than the greasy workbench of yesteryear.

You should be able to take the motor into the machine shop, and the machinist should tell you what it needs with good, better, and best options. And he should know what works best for your application because he sells them every day. And it's bad for his business (which is largely word-of-mouth) if you break one of "his" engines.

I am a little softer on response that it is only way to build. The tolorances that will run and survive for many miles is surprisingly large. There is OK,good, better and best without a doubt. I think OK is just not acceptable for you and that's cool. Having the proper tooling and the proper documentation to confirm baseline numbers allow you to take measurements of each item in the equation and machine too desired specifications The First time. The last 2 or 3 % of HP to be found in an engine is very hard and expensive to find. Like your favorite color--- what works for you ?

Didn't mean to come down meaning it was the only way to build an engine. Hackles raised at machinist attitude of, "some do, some don't". Laying a straightedege across the block deck, and looking for "light" under it should be enough to make an initial assessment. Deck seems to be flat or not.

Most street motor blocks don't need decking.

As the brains trust says HP is all in the details and the precision. Make sure your engine builder is of the same mind otherwise you will be disappointed in the future. Balancing and squaring are like the foundations of a house - You can always add on later but start with sound foundations and you can do anything you want in the future.

Many don't even calculate their compression math properly. It is the total volume divided by (cc'd combustion chamber volume plus head gasket volume plus deck height volume minus piston pop up volume or -minus valve eyebrow volume or piston dishing

As an example on 351 with 40* pistons and a 60cc combustion chamber... 9.5cc of headgasket volume, 3.36cc of deckheight vol, and 4cc of valve reliefs, ARE significant

Do the math 12:1 or 10.6:1?