I've purchased a Hobbico ARF. It came with a plastic spinner, which I'd like to know how to enlarge the hole in the backplate to fit a Magnum 61 engine. My question is how to make sure the spinner stays concentric when enlarging the hole? Should I purchase a new spinner with backplate adaptors?

Thanks,

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Uausally this is not a problem because most spinners come with a backplate hole that is too big and sleeves in different diameters are provided to take up the slack.
You could use a tapered reamer or as I prefer, a "Unibit" stepped drill bit, they come in 3 sizes and each bit has 8 sizes on it depending on ones needs.
The tapered reamer would be the easier of the two because you can hold the plate by hand.

You really should use a reamer on spinners and props as it's more acurate than a drill, and it doesn't take being off by much to create a lot of vibration.

This one has no adaptors.

But thanks, I'll use the reamer. Just hope it stays concentric!

Greg