Thanks for those suggestions. It sounds like there are perhaps a few options that will be available then.
An in inch longer than the thickness of a block. They are 100mm ish so go for a drill at least 120mm long. You’ll feel when you penetrate into the cavity from both sides.So a 20mm drill bit with what approx length is good to have for the job?
I would ( and have ) drilled from inside the full diameter hole as far as the cavity, then again from the inside a small pilot drill through to the outside. Then drilled the full diameter from the outside as far as the cavity using the pilot hole as guide.
You should also try to make the hole slope slightly lower at the outside, to prevent water travelling in to the building. A slight slope will do.
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