Swapping plugs around

once a new spark plug match with cylinder and ignition coil , next turn  if we change the location, it can disturb the strength of  load and could lead to faster plug fouling.

one more understanding , if any spark plug found some issue , whenever mix up, it can not identified by owner that which cap , leads or insulator ceramic hairline fracture , Unless you inspect it closely and or tap on the plug in an inverted position. you might miss it.

for proper inspection and diagnosis of spark plug conditions  , use the 8-spark plug hold tray. 

Jim Asked:

... Why does the Maintenance manual state we should not interchange spark plugs between cylinders? 

More than anything it is a matter of convenience.

The Continental/Lycoming Folks swap/Rotate plugs all the time.

The reason for rotating the plugs is to get a little more life out of those expensive plugs. (Hold this thought!)

The current in the spark flows from Negative to Positive and tend to carry a little of the electrode metal with it.

This causes the Negative Electrode in the plug to erode faster.

I can't tell from the circuit diagrams which plugs have the Positive polarity but it seems from the diagrams that All 4 plugs in Cylinders 1&3 have the same polarity and all 4 on the other side have the other polarity.

So if you want to swap plug you would need to swap a Left Plug with a Right Plug and ideally the plugs that were paired to the same coil.

So swap 1Top with 2Top and 1B with 2B and so on...

After the swap, the plugs with the eroded center electrodes will start eroding the outer electrodes and vice versa.

Just make sure you keep them all straight.

Does/did it make sense to write up a detailed explanation of the proper procedure to rotate $3 spark plugs, or was it just simpler to say, "Just replace them!".  Rotax correctly thought so!

. . .

Up until now it just did not make much sense trying to stretch a little more life out of an easily replaceable $3 plug. ($24 for all 8)

Now that the latest dual-electrode non-re-gapable plugs are $30 Each, ($240 for 8) doing the mental exercise of keeping track of where each plug came from and where it needs to go back to is starting to make more economical sense. 

If you have the older single electrode NGK Plugs, Just re-gap them and put them back where you found them, or if they look worn, replace them.

If you have the newest dual-electrode plugs...  Well, we all know what you are going to do no matter what we say!  

So why, exactly, are the new $30 plugs now recommended?

I was told by a respected Rotax authority that the reason for the plug change was that NGK no longer wanted to be associated with aviation.

Here's the official line from the Service Instruction (912-027): 

1.3) Reason.  In the course of continuous development and standardization, a new spark plug (part no. 297656) and new spark plug connector (part no. 265249) has been introduced.

I guess maybe that's just the "reason" for putting out the SI, but they seem to be saying it's "all in the name of progress..."

I notice that all mention of the old plugs has been removed from the new Line Maintenance manual.  In fact, one of the new checks is to make sure you have those $22 Rotax plugs in there.