My Porter-Cable broke spirals when tightening them, so I never used them. I tried an Olsen spiral in my EX-21 and it worked just fine. I broke one while cutting, but it got through 100+ cuts before braking, and it broke in the center (teeth area).
Make sure the set screw in the blade clamp is not back in the thread area - that lets the thumb screw bend the blade when tightened. On the EX-21 I added Blue Loctite to hold both set screws in place, with the blade held at center-clamp.