Deck Height Question - Options?

[Trophy]

I am currently rebuilding my 993 engine with new p&c's (Je Pistons and Nickies) During the normal measurement process I put cylinders 1 and 4 in place to measure the deck height, and ended up with #1 at 0.681 mm and #4 at 0.899 mm

When the difference was so large I ended up installing the rest of the cylinders with the following results:

#1 - 0.681
#2 - 0.732
#3 - 0.758
#4 - 0.899
#5 - 0.792
#6 - 0.790

All measurements are in mm and were done with a dial gauge on a height measurement bridge.

What sort of options do I have to get these back in line, as the 993's don't use a cylinder base gasket?

Any help, direction or chastising is appreciated.

[docrodg]

The case may be the problem, needs to be machined to get all the spigots at same level.

[Trophy]

When the case was machined for 109 bore the spigots measured up spot on.

[docrodg]

If the case was recently machined to flatten out the spigots then I would have to say it may be the rods are not all the same length. new bushings and an offset honing will take care of that. Check the spigots with a straightedge if you want to ensure machining was right. Would suck if was machined wrong and you got the rods rebushed to only have same problem.

[Lapkritis]

Is the bridge square? They appear to be running for each side slightly.

[Trophy]

I will get myself a straight edge (need one for cam alignment anyway) and check the case completely.

[911pcars]

Originally Posted by docrodg

If the case was recently machined to flatten out the spigots then I would have to say it may be the rods are not all the same length. new bushings and an offset honing will take care of that. Check the spigots with a straightedge if you want to ensure machining was right. Would suck if was machined wrong and you got the rods rebushed to only have same problem.

... or the cylinder heights aren't equal on the one bank. Measure and compare. if still unequal and the crankcase is squared away (spigot-wise), the con rods might have been rebuilt incorrectly. The only other variable is a piston (custom made?) with a pin height not to spec. A more bizarre condition would be a crank with incorrectly reground crankpins.

Sherwood

[Trophy]

Thanks Guys, I will work through all these suggestions and keep you updated.

[Green993]

Not sure, but would barrel shims help here?

[304065]

I think the issue is in the measurement setup. I mean, if you had somebody machine the case, part of the process is checking your work afterward. Who did the machining?

How did you mount the bridge? I used the standard measurement bridge and had to space it up with some gage blocks on either side.

Where did you measure? I assume the JEs have a ring around the outside of the piston. Or did you measure to the top of the crown and subtract the dome height as furnished by JE?

How did you zero the indicator, how did you establish that you were at TDC when you zeroed?

What kind of clock, digital or analog?

Not trying to grill you but I would double check the metrology of the deck height measurement before moving on to the rods.

Craig Garr machined up a steel checking bar that mounted in the mains for checking spigot height, I think there is a photo in the forum.

[ChrisBennet]

My thoughts are along the lines of 304065's (measurement process).

I measure deck height to flat band around the circumference of the piston face (what I call the "ant racetrack") using a tool I had made. Since the piston can and does tilt in the bore, I measure at 4 places (in line with the pin axis and at right angles to the pin). That gives me the worst case value and an average deck height for the piston.

A possible source of error would be tilt of the cylinder. Are the cylinders held down with 2 cylinder hold down "nuts" each so they are flat?

Use 6 hold down nuts to hold 3 cylinders down on one side (2 nuts each). Then place a machinists flat edge or piece of ground stock across the top and check for flatness.

If you measurement technique is valid, your measurements will be consistent i.e. if you measured those pistons again, you would get the same values. If your measurements are inconsistent, you simply can't trust your numbers.

[PFM]

Steven,

I tried to resist but had to respond. FWIW my first flat 6 rebuild started with rebuilt rods from a source I have used for years that ended up with wrist pins that were not parallel to the crank bore, the result was a deck height measurement that was about 2 thousands off front to back of piston, remove the rod rotate it 180 degrees re-install and test the 2 thousands did not quite repeat the other way... odd. Grabbed a "stock rod" tested again had about .005" difference in deck front to back, repeated with rod rotated as above.

Long long story short after the rods were done for the THIRD time they were OK but some deck height taper existed like yours, a straight edge proved nothing. We blocked the case up on a plate and measured with a good height micrometer and yes when the case was machined for stock bearings it must have been clamped on some shavings as the case cylinder bases were not square to the crank bore, however they were "flat" from cylinder to cylinder. Both sides to some degree had this problem. At the end of the day I had to re-machine the case my self. The end result was a cylinder base parallel to the crank bore, even deck heights from cylinders 1-3 and 4-6 and flat across a single bore.

Trust nothing measure everything and write it all in your build book.

[Trophy]

Originally Posted by 304065

I think the issue is in the measurement setup. I mean, if you had somebody machine the case, part of the process is checking your work afterward. Who did the machining?

The machine work was done by Ollie in Arizona. They only did the boring of the case to 109mm

How did you mount the bridge?

I used a dial indicator mounted in a bridge, with this I could get consistent measurements even after rotating the crank and resetting tdc for each cylinder.

Where did you measure? I assume the JEs have a ring around the outside of the piston. Or did you measure to the top of the crown and subtract the dome height as furnished by JE?

JE's have the ring around the outside of the piston, like others. I measured along the centerline of the pins. I was able to determine centerline by rocking the piston and having the measurement not move while the dial indicator was in place.

How did you zero the indicator, how did you establish that you were at TDC when you zeroed?

I zeroed the indicator while on the bridge on a window. I tested this by zeroing, measuring a known thickness on a flat surface (feeler gauge) resetting the gauge, re zeroing and repeating. This gave me confidence that the gauge was truly zeroed.

What kind of clock, digital or analog?

Digital gauge, sure made it easy for zeroing and measuring actual heights.

Not trying to grill you but I would double check the metrology of the deck height measurement before moving on to the rods.

Craig Garr machined up a steel checking bar that mounted in the mains for checking spigot height, I think there is a photo in the forum.

Grilling is good, your comments have made me go back and redo them more than once, I have now measured the heights at least 5 times for each piston. This really made me check my methodology to ensure what I was measuring was being accurately measured and consistent.

I will see if I can find the pic of the bar, otherwise I will pick myself up a starret straight edge

[Trophy]

Originally Posted by ChrisBennet

My thoughts are along the lines of 304065's (measurement process).

I measure deck height to flat band around the circumference of the piston face (what I call the "ant racetrack") using a tool I had made. Since the piston can and does tilt in the bore, I measure at 4 places (in line with the pin axis and at right angles to the pin). That gives me the worst case value and an average deck height for the piston.

A possible source of error would be tilt of the cylinder. Are the cylinders held down with 2 cylinder hold down "nuts" each so they are flat?

Each cylinder was held down with 2 hold downs (stomski racing hold downs). Everything was cleaned completely before putting the cylinders in place etc.

Use 6 hold down nuts to hold 3 cylinders down on one side (2 nuts each). Then place a machinists flat edge or piece of ground stock across the top and check for flatness.

As soon as I pick up the straight edge I will do this, great idea.

[/B]i.e. if you measured those pistons again, you would get the same values. If your measurements are inconsistent, you simply can't trust your numbers.

Measuring and re measuring has been something I have been doing slot, initially my technique was not good, however I have honed this to a place I am very confident.

Thanks Chris, I appreciate you taking the time to respond to my questions...

[Trophy]

Originally Posted by 911pcars

... or the cylinder heights aren't equal on the one bank. Measure and compare. if still unequal and the crankcase is squared away (spigot-wise), the con rods might have been rebuilt incorrectly. The only other variable is a piston (custom made?) with a pin height not to spec. A more bizarre condition would be a crank with incorrectly reground crankpins.

Sherwood

Hey Sherwood,

Checked the cylinders tonight, they are perfect all at 82.82mm. I measured on all 4 corners as well and were identical.

Compression height on the pistons is also spot on perfect.

Crank is brand new from Porsche so no machining has gone on at all here.

[911pcars]

Originally Posted by Trophy

Hey Sherwood,

Checked the cylinders tonight, they are perfect all at 82.82mm. I measured on all 4 corners as well and were identical.

Compression height on the pistons is also spot on perfect.

Crank is brand new from Porsche so no machining has gone on at all here.

What's the remaining variable? Con rods?

If possible, swap a couple of con rods and see if it makes any difference in the deck ht. You already have reference figures. If con rods are all the same, your measurements won't change.

I wouldn't suggest shimming individual cylinders as that changes the cylinder height and affects the clamping force on the heads

Sherwood