I posted this hint given to me by 11 Outback V.
Get a hole saw kit that has the spindle & interchangeable hole sizes. Stack the current hole size inside the desired hole size and the smaller bit will act as a guide/hold it in place.
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Thread: Make hole in hull bigger
08-03-2012, 10:12 AM #11Drew
2015 Supra SG
08-03-2012, 11:25 AM #12
Easiest and safest method is to use a round file. It'll open it up in less than a minute and not risk destroying the gel coat. Put the power tools down for this job. When I put new thru hulls in I had to go a little lager than the 1" hole saw I used to make the initial hole. I used a round file and I swear it only took a few swipes to make a significant different to the size f the hole..2004 Moomba Mobius LS