The RF and R2 diesels all use Torque to Yield head bolts.
I was interested to find out why, as I have not had much experience with TTYs.
It may have something to do with using an MLS (multi layer steel) headgasket which does not compress like a composite one coupled with 22.7:1 compression and the corresponding cylinder pressures (dynamic).
There have also been a few cases that I have heard with RF-CX motors with increased boost stretching the headbolts causing a massive headgasket failure.
As far as I can determine tho, the head bolts are the same on the RF-CX as they are on the RF-T, RF-5C and RF7 ( the later model direct injection and common rail turbo engines). In theory they should be able to at least take the same pressure as you can generate in an RF7 which is the 2.0 diesel in a Mazda 6.
I need to do some more research, I'd like to find a way to increase the boost on the RF motor without stretching all the head bolts and blowing the headgasket.
Here are some good explanations I have discovered on TTYs :
Threaded Fasteners Torque-to-Yield and Torque-to-Angle: Engine Builder
You can even buy them on ebay :
MAZDA SUZUKI 2.2D 2.0D 2.0TD 87 on HEAD BOLTS - eBay, Ford, Car Parts, Vehicle Parts Accessories. (end time 01-Jun-10 05:55:18 AEST)
According to the WSM you can re use them if they are under the recommended length, but other manufacturers recommend throwing them away and never re using them. I would be inclinded to er on the side of reliability and replace them anyway even if they were under the recommended length