any v6 configuration of engine where the rear bank is hard to get to you will find that a platinum type plug is used for its longevity. i have seen a few cases where 1. question asked of a mechanic/ workshop. how much to do a timing belt (they quote labour and a belt) . if it was to be done properly the tensioners , idlers and seals ,water pump need to be inspected. expect a pair of cam seals and a crank seal would need replacing at 150 000klms too. i have had a family friend ring up and ask for a 4cyl camry belt replacement price and he was quoted worse case scenario. belt ,seals idler bearings etc. he went with the ole $300 mate job from joe blogs local garage. Guess what ? it was back with in 3 months , oil all over the new "dayco timing belt" and a waterpump bearing shot. so he got a bill for $950 then plus the $300 for the dodgy job. back on track .. as far as a clunk then needing platinum plugs , i say WTF