Replacing dropped spark plug

This can be a two way debate or sword fight (LOL) and I don't think either side is wrong it's just you should go in with some knowledge. 

First the correct answer to the test question is to replace it. You see that written all the time.

I might catch some flack for this one.

One thing you can do is. Check for any porcelain cracking and make sure the gap is correct. Then put it in. Then run the engine and check for a higher than normal mag drop. Then just keep an eye open for an issue as you put time on the engine. If it's good for a short test or flight it most likely will stay good. 

So if you drop a plug the answer is yes to replacing it by the book, but many times they aren't damaged. You can't always know if it's truly damaged, but you can run the engine and test it, you can pull it later and look at it and you can watch the mag drop and see if you have a new higher rpm drop on one mag.

Your choice.

Hello, yes replace it as Roger noted.  A crack in the porcelain might not affect it right away and might not be visible to the naked eye.  You could argue that you still have one good plug in that cylinder however it is still just a guess until the plug fails.  You and your aircraft are, in that order, more important than the cost of a plug.

Cheers

Okay, thanks for these real world responses.  If it is bad, or I decide just to replace it, is it okay to just replace this one?  It's about 1/2 way through the book life of 200 hours burning 100LL.

Thanks.

Well you can't mix NGK and Rotax plugs but as long as you are replacing with the same plug, then yes just the one is OK.