Setup for measuring deck height?

[304065]

For those of you that have measured your deck height, how did you set up to take the measurement?

I have a few dial indicators, a magnetic stand etc. What I'm interested in is how did you attach whatever fixture you use for positioning the indicator to the engine, and where?

Has anyone fabbed a special fixture for doing this? Seems simple enough but I'm looking for ideas. Photos would be a huge benefit.



Chris, how did you fabricate the above? It looks like a semicircular area was milled out of the rectangular base.



Here is what the Chebby guys use.

[cgarr]

I just use a Vernier caliper for a quick check or you can use the solder method, I like the solder because everything is torqued down real nice when checking.

[tom1394racing]

john

If your pistons have a flat around the circumference, the vernier method works better than the solder method in my opinion.

The key is to get real TDC which requires a dial gage on the piston crown.

[304065]

Tom,

Mine don't have the flat area, the dome starts at the skirt, illustrated below.




Now, I do know the dome height, this was a published figure from JE, so it's easy enough to compute the deck height by measuring on top of the piston in the flat area you see (my "massaging" didn't remove any metal there, just smoothed the edges), subtracting the dome height (after converting it from decimal-inch to metric, JE uses inches) and then comparing that "deck height" to the top of the cylinder.

So how would you set up to do that? I suppose you could set up a pair of gage blocks on the cylinder top with a depth gage laid across them, then zero the dial indicator at the base of the depth gage (the same plane that the gage blocks touch) and do the math that way. The nice thing about the red "bridge" style gage is you can zero the indicator on a flat surface then rest the ends of the bridge on the cylinder top and measure directly. But the depth of the bridge had better be greater than the dome height and a long-stroke indicator would be required.

Decisions decisions.

[tom1394racing]

John

Your proposed method should work. You will be relying on the JE data for dome height.

I'd also do the solder method. I'd do it both ways for all your P's and C's to see how much variation you get cylinder-to-cylinder and between the two methods.

We call it gage R&R in my neighborhood. The measurement variation should be less than 25% of the target spec.

Deck Ht is such a critical measurement, it's probably wort the extra effort.

Good luck.

[euro911sc]

John,

Chris lent me his tool once and I put it in CAD. Basically, its a standard depth gage fitting for a dial gage with a ~98mm diameter curve hogged out of it on the bottom. Then the corners were ground to fit between the studs. Note, also that the tip of the gage is ground as well such that it would contact the small flat lip on his 3.4's pistons. I don't think this kind of block will help you here with your piston style.

Your idea with the gage blocks will work fine. Parallels would also work, I suspect, if your blocks are not long enough to span the cyl at the right spot. I think the Chevy style would actually be best for your piston configuration.

As for actually getting the measurement you want I'm not sure you are on the right track... Deck height is a measurement to make sure that the piston does not hit the head... right? If you simply do the math i'm not sure you will be checking correctly. What I'm trying to say is you don't know the curve on your piston and it may be that the closest point between head and piston is not at the edge where your calculation will be measuring. This could just be my inexperience in engine building showing caution here. Solder method would catch this I think.

Best regards,

Michael


Best regards,

Michael

[304065]

Michael,

It's important to distinguish deck height, which classically is defined as the distance between the point where the piston dome begins and the top of the cylinder, from piston-to-head clearance, which is what you are measuring with the "solder method."

In my case, it's impossible for me to precisely locate the point where the piston skirt ends and the dome begins, due to the continuous curvature from the top ring land to the top of the dome. But by measuring how far the piston sticks up above the top of the cylinder and subtracting the dome height (determined by JE as they made the pistons) this will give me the reference point of where the dome starts, and it's this point that is compared to the top of the cylinder to determine whether deck height is in spec or not.

The point of deck height is to verify that the cylinder height, shims, spigot depth, rod length and rod bushing bore are all correct-- that the piston is not sticking too far out of the hole. It's also a factor in the compression ratio calculation insofar as there's a small cylinder with a diameter equal to the bore, whose height is the deck height, that's a factor in the calculations. Of course the dome protrudes through this in the real world, but ensuring that it's at the right level is the key to ensuring the right combustion chamber volume at TDC.

With the solder method, you're measuring clearance, but there's nowhere to plug the clearance into a repeatable formula to determine volume at TDC. You would have to know the contour of the dome, and the contour of the head, and the space between them at all points, etc. So this is a practical, if somewhat theoretical, calculation to help you figure that out.

I'm probably going to use the solder method also, but to verify that clearance is adequate. . . you never know if the pistons were made after the CNC machine was out to lunch on a Friday. . .

[tom1394racing]

John

Actually, the solder method can be used to measure deck ht. It is important to make sure the solder extends all the way across the piston dome so that the edges get squished into the deck area. You then measure the squished solder thickness at the ends to get the deck ht.

I think Wayne describes this method in his book

[J P Stein]

Plumbers putty rolled into a string would be good for them pistons. Vaseline or sumthin' on the opposing surface to keep it from sticking.

For the dome volume, set the piston down in the bore a known amount....like 1 inch. Calculate the empty volume. A graduated beaker feeding a light weight oil
into the bore/piston using plexi glass for a top....subtract the 2 & there is your dome volume. I betch'a you can finger out the C/R from there....