Procedure for measuring how much to deck for zero deck

[edgewood bronco]

I'm trying to save myself some cash in doing the assembly, measurement, and disassembly for this next stage. I picked up the cleaned, line-bored block and the crank today. Here's my question:
After I get the crank in and the pistons on the crank, then how do I actually measure how much deck needs to come off?
Do I throw a straight edge across and put a feeler gauge between the piston and the straight edge, or do I need a dial indicator or some such measuring device?
Also, do I need to measure every piston, or only one, or only numbers 1, 6, and, say 3?
Am I thinking correctly in that this would be a good time to plastigauge the main and rod journals?
Thanks!

 

[Econoline]

Yes definitely a good time for the plastigage before you go back to the machinist. I did it with a dial indicator w/magnetic base. Ideally you want to measure in the center of the piston, the dish complicates this. But you can then measure the depth of the dish and subtract the difference. Or you can measure the edge of the piston but it won't be as accurate. If you don't have rings on the piston it get's even more sketchy. I wouldn't bother measuring every one but you could measure either end. The more seasoned among us may have better advise or tips. Good luck with your build.

 

[autoX65]

I didn't get too technical with it just digital caliper and eye called it and other came out just shy of the deck ...I told the machinists an estimate in thousands came out perfect for me but I know its a tough one because you can't take to much or too little

 

[CNC-Dude]

Ideally you want to measure in the center of the piston, the dish complicates this. But you can then measure the depth of the dish and subtract the difference. Or you can measure the edge of the piston but it won't be as accurate. I wouldn't bother measuring every one but you could measure either end.

This is absolutely the opposite of how you need to proceed to correctly and accurately check the deck. You must always check the piston height at both the spark plug side and quench side(opposite spark plug) by rocking the piston back and forth to get an average of its height. You only need to check it in the center to verify it is at its TDC position. Measuring the deck height in the center only can be very catastrophic when trying to obtain zero deck, because the piston is often .010"-.015" higher at these outward positions as the piston rocks when reversing direction. So if you have a zero height in the center of the piston, the edges can rock out of the top of the cylinder and hit the head resulting in severe engine failure if you don't really know what you are doing. With a .040" gasket and zero deck clearance, you have very little margin for error if you loose .010" or .015" more clearance unknowingly. I have seen pistons hit the head with .032" quench clearance, so you need to be as precise as possible or you will be sorry. You also must check every piston and rod in the actual cylinder they will be assembled in, because even with new pistons, the machining tolerances can vary enough to make a difference. As for the crank throws and rod lengths, they will also vary more than you think they will, so you have to check each and every one individually. At just 1500 RPM, each piston makes 25 revolutions per second, and before you could shut the engine down the damage is done. Many people don't realize how close to a cliff they are standing when they start talking about zero decking an engine and making an error only the thickness of a human hair can be costly. This is one of those instances where a few basic tools are needed and anyone can do it, but be careful.

 

[Econoline]

Thanks for the correcting my methodology cncdude. Thats the advantage of experience, priceless. I did read into the process extensively and measuring the center was what was recommended over the edge. But they do rock in there and what you're saying makes perfect sense to me. Basically, you're saying you're going to end up somewhere around .005" below with fresh bores and new rings if you find you tip .010" either way? So, in that scenario, for C/R measurements you would use .005" for deck height?

I didn't zero my deck b/c it's a 250, fortunately, so it wasn't as critical. Just shaved off .032", used 2.5 HSC pistons and ended up with .020" deck height. Which is right about where I wanted to be, so I would still have the option of going with the aluminum head in the future, can't afford custom pistons and it's not a racer. Had 8cc dishes milled into the 2.5's , had about $200 all in in them.

 

[CNC-Dude]

No, you would still use "zero" as your deck value because after you get through checking all the pistons and decking the block you will need to also cut your pistons to bring the deck to its final measurement target. When you go down the line checking deck, you will be lucky if you have two pistons that are the same, and you may have .010" difference from #1 to #6 piston, and .005"-.010" from side to side.

Most people just don't realize there is a lot more too this that there is. Most internet searches only give you the starting point of bring the piston to TDC and assume that's all there is too it, but as you can see, there is a lot more to it, and with only being given that info its literally enough just to make someone dangerous that doesn't know any differently. Once you deck the block to its final height, you also will need to cut most of the pistons to equalize the "zero" deck on all the cylinders, just cutting the block by itself won't get all of the pistons the same height because of stroke variation and rod length differences. Piston design and piston clearances will also affect how much the pistons rock.

 

[edgewood bronco]

How do you recommend it be measured? Dial indicator, feeler gauge and straight edge, other?
Thanks!

 

[CNC-Dude]

A dial indicator is fine. Just zero the indicator with the top of the deck and then you can read the piston height based on that baseline either as a positive or negative.

 

[Econoline]

But assuming the pistons will rock when pressed hard on either side and the amount was .010" total swing from their lowest point to their highest, then the deck height if cut so the pressed up edge was at the surface would actually be around .005" when the piston was even or "level" in the bore right?

 

[CNC-Dude]

No, you can have the piston rock more in one direction than the other. So if for instance you have the block rough decked and have the #1 rod and piston in the bore for checking. You get the piston up to its TDC position and then move the indicator over to the edge of the cylinder on the spark plug side. You can push down on the piston and get a reading of say, -.025". Then you push down on the opposite side of the piston to rock it upward, and you might get -.008". Since they both represent a minus value, you add the two together and get -.033", then divide by 2, and the deck is -.0165"on this side of the piston. Move the indicator over to the quench side(its always called that even if it doesn't have quench just to differentiate it from the spark plug side). Repeat the same process, push down, then rock upward. This time let's say you get -.020" and -.004(its not uncommon to have a crooked deck reading). So on this side we have a deck of -.012".

Now, disregarding the rest of the pistons for now. How much do we deck off of the block to get zero deck, .012" or .0165"? You deck the block .0165" and cut the one side of the piston .0045"!

Now looking at the whole picture once you check all the pistons, whichever piston is the farthest in the hole, you deck the block to that one to get zero, then you'll have to cut the pistons to adjust for rocking like the example above. You will literally scare yourself the first time you check the deck this way and see how much they vary not only side to side but cylinder to cylinder. Hope this explains better.

 

[edgewood bronco]

So then you must remove material from the connecting rod or somewhere beneath the piston to balance the pistons with respect to each other?

 

[CNC-Dude]

Only if you removed a lot of material from it. Taking just a light cut like mentioned above wouldn't even be measurable in most cases with a dished piston since it doesn't span across the whole flat surface of the piston. Most stock replacement cast piston sets are within 3-4 grams of each other weight wise, and it takes a lot just to remove 1 gram of weight, so even if you didn't balance the assembly this wouldn't upset anything at all. But in the scheme of things, this would be done before the assembly is balanced, and even before the pistons are installed on the rods, so any weight differences could be addressed when that stage of the rebuild is done.

 

[edgewood bronco]

Thanks. I'm going to assemble it this weekend and see what I've got. I reckon this will make a lot of sense when I get into it. How much should I expect to be taking off to get to zero? One last question: I assume I need to put the rings in... is that correct?
Thanks!

 

[CNC-Dude]

Not sure how much you'll need to cut your block to achieve your zero deck target. There is always the chance it could have been gone thru before long ago and had a resurface done then. At this stage of checking, you are really only doing a dry run, so if by chance any of the pistons need to be cut, then you would have to press them back off of the rods to do this. So no rings need to be installed either. For press fit rods, I take a good used wrist pin and polish slightly about halfway down its length so it still pushes into the rods S/E snugly and holds the piston perpendicular to the rod without being sloppy. Hope all goes well for you, its really a pretty easy process, just get a note pad and write down the numbers 1-6 with an SP and Q on either side to represent the spark plug and quench. Also do the same on the piston tops, that way you wont get turned around and wonder which piston went where and have a way to double check your numbers.

 

[edgewood bronco]

Well, I'm delayed by a main bearing with a hole in the silver coating down to the copper. That's not right, is it? I'm hoping I can just replace the one with the divot in it, not all 14. But I don't see any point in plastigaging the engine before I have the bearings I'm going to use in their final positions.
I've also got a rod journal that I can catch a fingernail in. The shop said it was good to go, but I'm not sure how they checked it.

Also, what are folks using for lint-free wipes?