A few years back one of the Inliner Club 261 experts found a factory spec on pushing this plug in and I'm sure it was in the neighborhood of 0.62 inches. Wish I had a book to verify, but I don't. I attempted to drive mine in on my 1961 261 and it would not budge so I left it alone as I didn't want to break the block. Have also heard of machine shops drilling the original plug out if it wont budge or heat.
