Deck height variance

Wondering what acceptable deck height variance is after machine work is done?

505" stroker and the variance is .004 between lowest and highest. Good or no good??
Another question, I have the icon 842 inverted dome pistons, eagle crank and 7.1 scat Rods- pistons in the hole .010-.014 with 440 source heads- Question is what Head gasket thickness recommendation?
I'm guessing use the .027 or .030?

Thanks

Deck height is the distance from the crank center-line to the deck. Are you saying your deck is not parallel with the crank center line? If this is the case, have the machine shop fix it.

If you are talking about differences in the piston to deck clearance, then you can live with what you have (not that big of a deal IMO), or look at where the highs and lows are, (is it one crank throw, or is it a rod or two) and start swapping piston and rods around to minimize the cylinder to cylinder variance.

It depends on how they measured the deck. If it is cut off of the crank c-line or if it was measured with the crank, 4 rods and a piston in 1/7/2/8 using a bridge and height mic. In short you either blueprint the block arbitrarily to 9.980 or whatever or you deck it with respect to where the pistons are in the block. One of the professionals can probably explain this better.

FWIW on my stock, undecked 400 blocks with Diamond 1.32" CH pistons every corner hole had a different measurement from the block deck to piston height. The fellow that did my machine work cut the deck based on those measurements. Make sure to label the pistons to the correct cylinder number in case you want to remeasure, etc.

Yes the difference between each piston to deck clearance is what I was concerned about. Thanks for the reply.

If you really want to find out where the differences in your piston deck heights are use one piston in the four corner cylinders and rock that piston back and forth in the bores and use your depth mike or dial indicator directly over the wrist pins and see how much variances you have
Please let us know what you find out
I see this often with all after market parts, usually it is the rod

Ideally you want the finished deck height the same on all four corners. If not, the valve to piston clearance, quench distance, etc. follows suit.

As Cab said above when you put a piston/rod in the corner holes (1/7/2/8) you can zero deck it to the piston/rod you are actually using. The rods can be moved around if you want to try and find the longest by a few thou. I have two 400 blocks done this way and none of the holes were above deck after zero decking using 1/7/2/8 as a reference. Definitely use a dial indicator to make sure you are getting an accurate measurement taking into account for the piston rocking on the wrist pin. I set up my magnetic dial indicator on the deck to get the piston as square as I could in the bore and then used a deck and height mic to check the piston to deck clearance. Make sure the oil rings, and appropriate rod/main bearing are installed, the deck surface is clean and that you label the parts for future reference.

I enjoy doing this kind of stuff!

This is right as you have to use the same piston and rod and try it in all 4 corners and see if it comes out right. Using the same piston and rod in all 4 corners makes sure its the same as you could have small varience in the rod bushing in the rod and throw if off a tiny bit. Using the same piston/rod lets you know if its off its the deck and not a difference in the rod/piston. Ron

It sounds like he's already put all the piston in. From that you can usually tell what is going on by the distribution of clearance variations. Front to back, side to side, rods lengths and/or the throws. At this point I would simply look at the data he already has, see what its telling him, and then make a decision on next steps, if any.

Like Cab, I find its usually the rods.

I have yet to see one perfect. Most vary around .005" . Want to really confuse your self? Check each piston front to rear. That won't be the same either. There's many factors that contribute. Stack them all up and you get what you get.
Doug

I agree. First every process has variation so even though the goal is to have the same measurement from bank to bank and cylinder to cylinder, there is a tolerance or variance that you will see in this number. Next, your measurement is a process as well and that is highly prone to variation so it's quite likely that if you're getting (for example) a .005" difference between cylinders, .003" might be the actual height variation and .002" is a variation "adder" based on your measurement method, measurement tool and the human interaction with both the method and tool.

- EM

Your quench #s, seems tight.


Im doing one right now thats a mild street build that is so near perfect its unreal. Oem offset ground crank, corrected DC small block rods and domed kb hypers. My machine shop guy is in his 30-s and spent 10 years under his dad learning the trade. I marked the lowest spot on the deck and asked him to square and take XX off my marked spot that would put me at zero. It came in .002 under on all 4 corners measuring the same way.

I feel hes good and maybe in this case just a hint of luck played it, but the irony of its a 318 based build.

.......and that is an example of the machine work that everyone should strive for.

Thanks for all the replies! Yes, the motor is already together and I was looking at the spec sheet and noticed that they were not all the same. The following is how it measured out:
1-.010
2-.013
3-.010
4-.010
5-.012
6-.014
7-.014
8-.014

I really appreciate all of the help and input from you guys. This is a motor that I got along with a car I bought and the short block was already assembled by the machine shop. Any more input so far as the head gasket thickness?

Even moving one piston around to every cylinder is going to depend on the stroke being exactly the same.

I would find someone with a very acurate mesuring device and check the length of each rod, check the pin height of each piston. Use one piston and rod in every cylinder to check stroke (not just deck clearance but stroke) on all 8 cylinders. Then start doing a little math to finger out how to get em all as close as possible.

Put a .030 gasket & let it EAT !!!! it will be ok !!!

Are these your measurements? If I did anything at all, I would first double check all the numbers two more times and convince myself of what the numbers are. Then for fun, I might switch the #4 with #8, and switch #3 with #7 and see how the numbers change, if at all. If it changes for the better, switch #1 with #6, but you might need to turn the piston on the rod.

At the end of the day, you're plenty good to go with it the way it is now. And it's unlikely to swap around to get much better than 0.003 difference.

I use .035 minimum piston to head clearances which includes measure every piston at TDC the edge of the piston and block on the cam side with that piston pushed over tight to the cylinder wall by using a small narrow screw driver blade push down between the other side of the piston and cylinder wall to force that to piston rock over which should lower that side down and measure it and then switch sides with the screw driver to the cam side of that piston and rock it back the other way to make that side come up all the way, that is the highest edge on that side to use for the .035 minimum piston edge to head clearances. then do the other side of that piston and use the highest measurement to get your P to H clearances
You can check two pistons at TDC without moving the crankshaft also

I'd say that is normal for a typical bracket race type of engine. When you use off the shelf parts you'll have tolerances for stroke, rod length and piston deck height. Add up those three tolerances and you'll get a few thou of variation from top to bottom. This is assuming that your deck is square. If the deck isn't square then you have another tolerance in the mix. I wouldn't worry about it unless you're a class racer. If you're a class racer then you probably should've had the crank indexed and you should've bought 3 or 4 sets of rods so you could pick the 8 that matched.

That’s what I’d do.

Looking at your numbers again it looks like the block has a slight taper on both sides from the rear to the front
I would do the checking myself on the way it is now to find out if it is tapered or not, if you get the same results I would remove the corner rods and pistons and use #1to check all four holes to see if it reads the same or not
If it is the same I would have the block straighten up, but that I me being anal on getting every thing perfect .
You won't see any differences in the way that motor will run with all the pistons at -.014 compared to them at -.010
On the head gasket to use I would find and use a gasket that is at least .025 thick compress or thicker Maybe even a Fel Pro 1009