I have an 85 block in my 89 Spirit with the older 10mm headbolts. Anyone who's listened to me talk over the last 10 years i've been here shouldn't be surprised to hear i RARELY buy new headbolts. I've reused the 11mm headbolts over and over with success. Recently i've been setting the torque wrench to 90ftlb and checking that it clicks during the last 90 degree turn on the headbolt. Over time i've noticed the 10mm seem to 'feel' as if they've failed far more often than with used 11mm bolts. Since i started doing the 90ftlb check, I dont think any of them have clicked the wrench during the final turn. This is all pretty vague but over time i've become more and more uncomfortable with them.
I had to take my spirit engine apart because i cracked a piston running the engine with no coolant (long story involving being parked over winter and forgetting something i did several months ago) and when i took it apart i was a little miffed by how easily the 10mm headbolts broke loose by hand (long handle 1/2" ratchet, but still..), so i decided i would just tap the block for 11mm headbolts while it was apart.
This turned out to be way easier than i for some reason imagined. The 10mm headbolts are a 10x1.5 thread, while the 11mm are 11x1.5. This is very convenient for tapping purposes. I think my tap chart called for a 9.8mm drill bit to tap for 11x1.5. Well, the closest thing i had was 3/8 which is ~9.55mm. Drilling a hole with 10mm threads with a 9.5mm drill bit leaves you some thread left, and since it's the same thread pitch as the larger 11mm headbolt, this makes it very easy for the tap to 'thread' into what's left of the 10mm threads and tap a perfectly straight hole.
I was able to do all of the headbolt holes with the ac bracket and passenger mount brackets on just fine, in about an hour.
Can't wait to go back together with USED 11mm headbolts.