Have been trying to get old Kohler Rialto toilet running again. Need to replace flush valve which has odd looking heads of bolts. Kohler says to use phillips screwdriver but this is no phillips I've seen before. Any ideas what it is called so I can search

A large slotted (flat) screwdriver should work.

Thanks for the suggestion.

Have you seen a fastener similar to this before?

I did try as you mention but wedge shape of screwdriver tip pushes against inside face of flush valve face which pushes screwdriver tip into only one slot and if I twist any it will just deform slot and head possibly making it difficult to unscrew at all. In pic ya can see all head slots have been tightened to just about same position, if head was already rotated 45-90 deg. more, using a big flat screwdriver might just work.

I've considered several options

1. drill out the existing small diameter hole at bottom in bolt head to maybe a diam. I can drive an easy-out into which might allow me to back those bolts out.

2. drill down into small hole until I reach the actual bolt thread portion which should allow brass lug found below tank bottom to just drop off or able to screw off the lug.

3. use a Dremel with grinding disk and guestimate where bolt threads are located and cut horiz. thru plastic valve body and bolts.

4. cut vert. willy-nilly thru plastic valve body each side all bolt heads from flapper seal down to tank seal so valve body can be removed and possibly make it easier to work on LOL it could happen.

I would use a dremmel cutting disc at base of nuts.

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