Hi @markwatts,
Thank you for your question about how to secure your new step treads to your stringers.
Before heading into your local store, I'd suggest removing one of the existing coach screws so you can measure the diameter of the hole in the stringer. You can then use this measurement to confirm compatibility with the hardware I mention below.
I prefer using a nut and bolt as it has greater holding power than a coach screw, which is what you currently have . The downside to this is you have a visible fixing on the top of the step.
If you want to use a nut and bolt, these ZENITH M12 x 75mm Hot Dip Galvanised Cup Head Bolts & Nuts should work fine, but you'll have to confirm they will fit through the holes in your stringers. If not, then an M10 bolt such as these Zenith M10 x 75mm Hot Dipped Galvanised Cup Head Bolt And Nut might work.
The cup heads are designed to dig into the timber and sit fairly flush, so they won't be overly intrusive on the top of your step.
If you wanted to replicate what is already there so you don't have visible fixings from the top, then either these M12 x 50mm Galvanised Coach Screws or these M10 x 50mm Galvanised Coach Screws should be fine. Being 50mm long, the thickness of the bracket on your stringers will prevent them from poking through the top of the tread, meaning you get the maximum embedment possible.
Predrilling with a suitably sized drill bit is a must. When you've selected your hardware, head to the Tool Shop so you can compare.
If using a bolt, the drill bit should be just larger than the outside diameter of the thread so it can pass through cleanly.
If you are using a coach screw, grab a drill bit that is roughly the same diameter as the core (the part the thread runs around) of the screw. This will ensure there is space for the screw to fit, with the thread still digging into the timber. Use a piece of tape on the drill bit to set a depth gauge for your pilot hole so you don't poke through the top of the tread.
Let me know if you have any further questions.
Jacob
