Spark Plug Inserts and Varied Spark Plug Distances

[Fitz98]

I am working on putting new Cal-Van inserts into my heads on my 98 Expedition 5.4 with the heads still on the block.
I have noticed as I am putting in the inserts, that the spark plug seems to be seating at different heights between cylinders when reaching torque. I have done 4 on 1 head so far.
I measure from the top of the head to the top of the plug and the distances vary from one cylinder to the next by about 1/8".
The Cal-Van has a cap at the top of the insert that seats against the original spark plug seat that is machined in the head. Is it possible that the original seats vary between cylinders in their machined heights?
More importantly, how critical is the distance from the spark plug to the piston? I am trying to determine if there is an insert problem causing the difference in spark plug heights, but more importantly, if 1/8" difference in heights between spark plugs would even matter.
Unfortunately, I have no way of seeing the spark plug position from the bottom of the head.
If anything, the plug would be further away from the piston from the original, not closer.
I will try to get some more accurate measurements.

 

[clinebarger]

Something is wrong....Assuming you're measuring accurately. The tops of the spark plug well is not square to the seat.

Are you sure you're tapping all the way through? If you leave any material in the hole.....The insert will hang up on it.

 

[Fitz98]

I am measuring using the Cal-Van insert tool reamer and tap guide and sliding the reamer into the guide until it sits on the top of the plug, marking the bit and checking the line between cylinders.
1/8" variances between lowest and highest.
Definitely tapping through all the way. Cal-Van's kit makes it fool-proof. Threads are clean. I thread them with the tap a couple times, flush the threads with brake cleaner and blow with compressed air, multiple times and inspect with a borescope before inserting.
In April, Cal-Van had an issue with the machining on their inserts, they have since corrected the issue, but it's possible I have an older kit, even though I just ordered it a couple weeks back. I will call them tomorrow to see if it might be an insert issue.
More importantly, how could the 1/8" plug distance variances affect my engine and heads, because at this point, I don't think there is any way to get them out. I am going through all this to make the heads better and prevent the spitting plug issues. I did one out of necessity and decided to do the rest as preventative measure and the intent was to have consistency between the cylinders both in metallurgic properties and plug distances. I know I have at least 8 threads contact in the head now, even with the highest insert. Cal-Van has 9-10 threads on the head side of the insert.

 

[Linctex]

I wouldn't worry about the height, the plug isn't quite THAT close to the piston in these engines.

 

[Fitz98]

Originally Posted by Linctex
I wouldn't worry about the height, the plug isn't quite THAT close to the piston in these engines.

The plug will be further away from the piston in all cases because of the design of the insert. It's the inconsistency in a precision machined head that bothers me. Measurements to follow

 

[Fitz98]

I measured with a micrometer.
All the spark plug tips on the head I have not touched are 1.94" from the top of the head
The head I have finished with inserts are as follows from front to back top of head to tip of plug
1.80
1.875
1.862
1.725

 

[Fitz98]

Does anyone have any insight if these new spark plug heights are still within tolerance compared to original spark plug My intention was to make an improvement by re-tapping and adding more threads on heads with inserts, but want to be sure I am not creating a different problem before venturing into the 2nd head.
Cal-Van is sending 4 more inserts that are known good. They think it was possible a couple of the April bad batch of inserts could have found their way into my kit.