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Is it about the size of 1/4"npt? Just a tad too small? If so, you can use a 1/4"npt tap to slightly enlarge the hole to fit your hose.

There are adapters available, and your thread is probably bpt straight. Look and see if it tapers into the hole, or is constant diameter. Sorry, don't have a source, but it should be on here in the form of oil pressure sender fittings.

It doesn't appear to taper and all the adapters I've seen are tapered thread adapters. Will those hold ok in a non tapered hole?
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