Deck height measurements driving me crazy, Need advice Options

[stugray]

Ok, so I bought the deck height measurement tools from the group buy.
But I cannot figure out exactly how the OP intended to use the middle plate.

I have been trying to measure my deck height accurately for weeks now and I either am not doing it right, or I have a problem.

I have the EMW AA cylinders & KB Hypereutectic pistons - brand new.
I am measuring with no shims under the cyls.
I bolt the holddown tool on two cyls with pistons installed and torque to spec (24 ft-lbs)

I set TDC on the cyl I am about to measure with a mag base & dial indicator using the middle of the piston as reference.

I then set a VERY straight edge across both cylinders and clamp it down.
I then measure the top of the cylinder mating surface down to the very edge of the piston using a dial indicator/depth gauge.
I repeat the measurement at least 5 times and average the results.

I am getting measurements that vary from .022 to .032 and they can vary by up to .0067 across a single piston (front to rear). (IMG:style_emoticons/default/WTF.gif)
(measurements at the same spot are repeateable within less than .001)

When we are trying to get these deck heights to match to within a few thousandths, I cannot see how that can be done when the piston can be off by more than 5 thousandths front to rear.

Does this mean I have bent rods or misaligned wrist pin bushings?

I had the entire rotating assembly balanced, so I would hope they would have caught something bent.


I guess my next steps will be to shuffle the pistons around and see if the measurements follow the pistons.
If they dont, I might need to move rods.
Can all 4 rods be removed & replaced without splitting the case again?

Stu

[VaccaRabite]

Shim the base of the cylinders.

Don't start moving pistons and rods. Move the cylinders out slightly with base shims - some would have come in your rebuild seal kit usually.

Zach

[rick 918-S]

Just a guess but sounds like maybe collapsed registers. If the measurement is off across the piston in the same plane as the wrist pin. Check to see if the block is true across the bores where the cylinders slide in.

[Valy]

Are the rings on?
The pistons will rock a bit in the jugs when the rings are off causing inconsistent measurements.
Also make sure the jugs are fastened well and use a shim under them.

[yeahmag]

Are you using the tool I built?

-Aaron

[McMark]

Did you have the cylinder spigots machined?

[stugray]

1- Just curious, why would shims make a difference. They cannot make it any flatter.

2 - Aaron, Yes I am using your tool, but I cannot make sense how to use the plate. So currently I am just using the holddown bars.

3 - McMark - what are the cylinder spigots? You mean what everyone else calls registers? No I did not have anything on the case machined.

When I put a VERY straight edge across the tops of the cylinders, I do not see any gaps where the straight edge touches the cylinder mating surfaces. I assume within .001 flat acroos two cyls.


Stu

[yeahmag]

The plate simply sits on top of the cylinders located by the C shaped notches. Do not clamp it. It must be free floating. Use the center "hole" to find TDC. Zero your depth mic on the far side with the slot against the cylinder you are measuring. Drop the depth mic in the center hole and measure. You can now also measure the adjacent cylinder via the "slot" to see if there are any height differences.

[r_towle]

I measure at the side of the piston directly above the piston ring.
Use just the top ring in place.

Pistons rock when you touch them.

rich

[yeahmag]

Rich,

I think you mean above the piston "wrist pin". You get the least "wiggle" there.

[VaccaRabite]

When I did mine I measured from the exact center of the piston to minimize the rocking, and did it w/o any rings on the pistons.

What is your target compression ratio?

Deck height is important because that is one of the variables in your compression ratio. Shims make a difference because that is how you get your deck height the same between all the cylinders. Measuring w/o shims is kind of pointless.

You start with the target deck height to need to get to the ideal compression ratio. Add shims to get that deck height on all 4 corners.

If the .022 to .032 delta is between cylinder of opposite banks, you are probably fine. Once you shim the bases of the cylinders they will be the same. If the .022 to .032 delta is on the same bank (IE - the 1,2 bank or the 3,4 bank) you may have head sealing issues on that side. The cause will be the case registers, which should have been checked while the case was at your machinist. Its one of those things that many machinists don't know to do unless we ask them. The registers on my case had to come down quite a bit to get them even, and were the cause of a head leak which made me tear down the engine for rebuilding in the first place.

Zach

[stugray]

I have all of the rings on and I measure in line with the wrist pins so I should have minimal "wiggle".

Stu

[yeahmag]

Do you understand how to use the tool now? If not PM me and I'll give you my phone number.

[r_towle]

Rich,

I think you mean above the piston "wrist pin". You get the least "wiggle" there.

yup.

[stugray]

If the .022 to .032 delta is between cylinder of opposite banks, you are probably fine. Once you shim the bases of the cylinders they will be the same.

The .006 difference is across the same piston without moving the crank.

What is your target compression ratio?

10:1

Deck height is important because that is one of the variables in your compression ratio. Shims make a difference because that is how you get your deck height the same between all the cylinders. Measuring w/o shims is kind of pointless.

I understand that.
My very next step was to have the heads machined to increase my CR.
Before I can even determine how much to take off the heads, I need accurate deck heights.
So my initial measurements were to be with no shims.
Then machine heads
Then remeasure everything and pick the shims.
Then I can shim to where I want.

Stu

[yeahmag]

Which direction are you measuring the piston? Across the pin (horizontal) or vertically?

That's a pretty big number for CR. What cam and gas are you planning on using?

You should only shim if you have too little deck height. So I'd suggest a slightly different order:

* Measure deck height.
* Shim if necessary to get safe margins
* CC heads
* Calculate CR
* Fly cut heads to raise CR/Add shims to lower CR.

The only meaningful deck height number is along the wrist pin. Ideally in the center hole.

[wndsrfr]

[quote name='stugray' date='Oct 15 2012, 07:19 PM' post='1753775']


The .006 difference is across the same piston without moving the crank.


(IMG:style_emoticons/default/idea.gif) I'm going to close my eyes and imagine what would cause this....only picture I can imagine is that one register is lower than the other. The top plate is actually holding the jugs in alignment at the top but if you could get a feeler gauge under the bottom of the cylinders all around you'd find some clearance on one side or another of the cylinder bases. That would yield a cylinder alignment cocked with respect to the perpindicular of the centerline of the crank/rod throws. If this is the case, the high side of adjacent pistons would show up either toward the front or the back of the engine....solution, deck your case!

[r_towle]

If those measurements are at the top of the piston, directly above both sides of the wrist pin then you need to deck your case.

If you are measuring the top and bottom of the piston, those measurement are actually normal due to rocking of the piston.

Rich

[stugray]

That's a pretty big number for CR. What cam and gas are you planning on using?

This is a vintage race engine that is carbed and will race at >5000 ft and run race gas (if required). I have Jakes full race cam package.

If those measurements are at the top of the piston, directly above both sides of the wrist pin then you need to deck your case.

They are measured directly above the ends of the wrist pins.

But when I put a straight edge across the tops of the cylinders they appear to be flat to within .001. To get a tipped piston, I can only imagine a bent rod, or a bad wrist pin bushing. I have a local Porsche expert that told me he has found wrist pin bushings installed incorrectly that would give me this symptom.

Stu

[yeahmag]

Measure using the slot on the inner edge of each cylinder:

)(

Let us know if there is a any measurable difference. Is it only one piston giving you grief? Can you take a picture of how you have the deck height tool set up just to idiot check it?