Gluing them on the surface without inserting the pins into the holes will likely not be very successful. They are molded delrin plastic which is hard to glue.
You need to measure the pin size and drill the holes to match, or just very slightly larger.
Kato grabs can be a pain for sure! Time and patience. I have drilled some out, but then they need to be glued. If you can get them in the holes, they will stay with no glue.
Some engineering plastics don’t respond well to many types of glues and adhesives. If I were adding grabirons to cars and locomotives, I’d use phosphor-bronze wire, available from Tichy in a wide variety of diameters. Tichy also offers pre-formed grabirons, with both straight- and drop-style ones.
I’d suggest that you buy a set of pin vises (usually in four sizes) that will accommodate the full range of drill bits needed for most HO scale model work.
I have sets that will accommodate small drill bits from #80 to #61, and also from #60 to #1, and others for lettered drill bits (A-to-Z) and, of course, the usual fractional sizes that are widely used with power tools.
A handy tool for create holes for grabirons and other small details is a draughting compass…not for making a circle, (or creating a hole) but rather to create a small pin-prick to make a starting point for using a drill bit, which will prevent the bit from wandering.
Once you have the holes drilled, the neatest way to install the grabirons is to removed the body shell (whether it’s a locomotiove or a freight car), then add the grabiron(s), with a small piece of appropriately thick material (usually plastic) to keep the grabiron at a reasonable distance from the body shell…simply apply finger pressure to the grab iron, while using your other hand (along with a suitable tool) to bend-over the portions of the grabiron that are inside the body shell.
You can then add some ca on the tip of your X-Acto knife, and touch it to the folded-over wires, inside the bodyshell, at the points where the wire comes through the holes. This will create a strong bond with no visible mess on the outside of the body shell.
Apologies for the long-winded explanation, but I have done hundreds of cars and locomotives with such details, and have learned what works and what doesn’t work, too.
A safety pin, medium-sized, might work. From the point there will be a gentle taper to the full width of the shaft. Enlarge the hole a little at a time till the grab iron fits snugly…
I had the same trouble with mine. After using a 0.5mm drill bit to clean out the holes, the grab irons were able to press in much more easily and still be tight enough to not require glue.
If you have steady hands and good eyes, you can use a single-edge razor blade to cut a small angle on the end that fits inside the body. Then it will slide in, stay tight, and you won’t need to fool around with glue. All you need is that little bit of help getting it into the mounting hole.
