How so when the OE Montero plug is a heat range 5 and the others are a heat range 6?

Perhaps it isn't a large difference, but there's enough difference between the heat range 5 and heat range 7 that my engine won't start.

I'm ordering a BPR6ES-11 set and will see how they work - I agree that they should be fine since the Diamante SOHC heads are the same. As for the combustion chamber, I tend to look at the piston top as a part of the chamber during combustion and mine is different since I have a full dished top instead of the normal squish area of the 8.9:1 pistons. I suspect the lack of squish area might also be a reason why the heat range 7 plugs won't fire the mixture.

Frank