Spark plug Thread inserts

We installed a set of plug inserts on my brothers 69 Shovel, can not get them to stop leaking around the plugs. Is there a set of inserts that work well for this application? we purchased them from NAPA

Keenserts work best.

Thanks Chris, I'll check them out. bugs the heck out of me that they're still leaking past the threads.

New set of heads from a 68, has steel thread inserts. I screwed the plugs in an filled the well with gas, half hour later no gas leaked out past threads. Installed heads an oil leaks out of plug holes after an hour of use, checked plugs both tight.
I am assuming the aluminum heads are expanding an allowing leakage between head and plug insert. The sealing ring on the spark plug is only large enough to contact the insert, with no overlap to the aluminum. What if anything can I use to seal an insert when installing it?
Chris you said the keenserts work best, do you seal them? This troublehead is driving me nuts:)

Hi all, after several attempts to seal the leaking inserts in my brothers heads with no success. I purchased an insert kit from Time sert http://www.timesert.com/html/bigsert_sparkplug.html. This kit was an easy install, I used the aluminum annodized inserts, timesert says they use them to replace the helicoil style inserts we had. With my old inserts out the hole was .660 in., they have an insert thats good for a .720 in. hole. You just install the 14mm insert into the larger insert. Anyway 220 miles so far an no sign of leaking, I have also taken the plugs out several times to see if the inserts would come loose. So far perfect I would highly recommend this fix.