Helicoils work great for this problem just measure the depth of the hole so that you can mark the drill bit so you dont drill to far into the forks lowers and ruin the fork. Stripping out threads is not ussually covered under warrenty. Just make sure you drill straight and plumb for a disc brake mount if your not sure you did use a slightly longer bolt and but a v-brake pad conical washer set under the head of the bolt so it sits flush with the caliper keeping it alighned right.