Although I have no experience with these I have heard nothing but bad things about Bosch. I curently have ac delco iridium plugs gapped at .040. And i have no complaints.
I'm thingking of switching to a copper plug because I know copper is the best conductive material on earth although they do not last as long. I am using msd wires. Stock wires will be fine also to.

 

They should be good for 20,000 miles.

I ran my last set for 12,000, and they were still fine, I just changed them out since I was switching header gaskets.

 

That sounds about right, but a platinum or iridium plug has a much longer lifespan than that.

 

just changed all 8 on mine '98 ls1. I also have hookers headers that are 'fatter' than stock and I think it made it so much harder to reach to #8 spark plug.


Best guide is

http://www.installuniversity.com/ins...g_and_wire.htm

I did everything from the top ( I do not even know how to reach the #8 from the bottom of the car without taking some pannes off).



I first took out the AIR pipe, then coil #6 (on '98 there is
no coil rail)

then used huskypro inexible ratched to reach to #8.

http://www.homedepot.com/prel80/HDUS...rchResults.jsp

It took me 2 days several hours per day to take that #8 out.
Had muscle cramps while trying to reach. But finally did it,
and with my luck dropped it when taking out
(my suggestion here is to put something that can catch
the plug before it disapers underneatht the engine)


I purchased in advance NGK TR55, but after the hustle
with #8, decided to loose the 20$ I spent on the TR55
and go with irridium NGK TR55IX,
also got MSD wires.

The wire on my #6 was shot, because the clowns
that installed my headers did not put any heat isolation
around the spark plug wire so it was touching the header
and was eventually barelly working (that's why I had
misfires and low hp numbers...)

So if fatter header tubes are involved change the spark plug
wires and put some heat isolating tape where the wire
touches the header pipes.

I also used huskypro infiniflex to reach to other areas
(including bolts on the #6 coil pack)

http://www.homedepot.com/prel80/HDUS...rchResults.jsp

Another thing: the guide above tells you to barely tighten
the plugs (irridiums are 1/16 of a turn after they are hand-tight) -- but this was not enough I had used that index ratched to tighten it as much as I could (at first I slighly tightened it, but after driving for a day I started hearing the hissing noise so I figured air was forced out of spark plugs wells - so I had to re-tighten all of them ... went through hell again and broke one plug in the process, so had to replace, thank g-d it did I was able to unscrew it, it was #7).