Hi Porkchop . . .

Maybe they re-tapped for 18mm plugs - like used on some motorcycles. A 7/8 plug would be huge - ~22mm - way oversize.

Or they re-tapped for an repair insert like those sold by Pyramid and others. With these you drill and tap an oversize for the insert and it is tapped for a standard plug diameter.

Check out: Pyramid Fasteners

I fixed a lawn mower head with this type of gizmo - one that I cross threaded myself the day before

regards,
Keith

[Linked Image from 49fastback.com]