I am replacing my cast iron main stack with ABS. The plan was to use a grinder to cut the cast just above the clean out. Is there any possibility of flammable gasses in the stack that could ignite ?

No! If you use a grinder to cut the stack you're not going to blow your house up. I'm concerned with how you're going to hold the stack up once it's cut. Or are you simply going to let it drop? Good luck, Tom

Thanks for the quick reply and your concern is also mine. I was shocked to find that there wasn't a single riser clamp or bracket of any kind. Plan is to add a couple riser clamps and hanger brackets at the bottom and start removing cast from the top - down.

Removing Cast Iron Pipe - YouTube

A lot faster than a grinder.
Can be rented.

If you follow Harolds advice about cast iron cutters remember there are two types of cast iron cutters. Snap and ratchet, You want the ratchet, (see image) Good luck, Tom