"Slightly less" or "slightly more" than 3/4 inch?

When I asked Janne Gustavsson about dimensions of the keel bolts which they used on Lurinkosters he instantly replied: 3/4 inch.
Hearing such prompt answer I took it for granted.

The first seed of doubt was planted by Björn who, upon inspection of my old, rusty keel bolts, pointed that they do not look like being 19mm originally - the thread is too small for that. Why would they use 5/8 UNC on such thick diameter rod?

The seed was planted then and it started to grow when, few days ago, we have taken away the nut on Lennart’s boat - Andrina, also Laurinkoster like my Meritaten. My tools matched the nut but then the thread was different! To my eyes much more beefy!

Björn has lend me 19mm diameter, long steel rod so I could check on the boat if it fits. Looking onto this rod and Lennart’s keel bolt threads I could see that they match. But not my boat’s threads!

Before Björn can proceed with fabricating my new keel bolts I need to be sure that indeed 19mm is the correct diameter. Funny enough, if one looks on Internet for fractional inch-to-mm conversion it can be found that:

18mm —-> slightly less than 3/4 inch
19mm —-> slightly more than 3/4 inch

Hmmm, how should I treat THAT?
Especially that Björn is asking me now about tolerances for new bolts.
Should I say: more or less 19mm? That ain’t working for Björn for sure!

Whatever was installed by Fritjof and Janne in 1958 - at this point I want to follow Arvid Laurin’s specification which says: 3/4 inch.
In my world it is 19,05mm so 19mm it is (it’s a boat, not Hasselblad after all).

Hammering 19mm steel test rod into keel timber and lots of deadwood would definitely suck-in the rod and I would then need to pull it back with help of hydraulic jacks. Instead I’ve decided to run 3/4 inch drill all the way - and thus checking if oak and iron were indeed drilled to that size.

With oak it doesn’t matter - it is easy to drill - but if iron ballast was drilled to 18mm then it could get tricky with enlarging the hole so deep inside the boat structure. Seized drill in the deep hole is the last thing I want to have right now.

Of course - imperial drill sizes do not just sit “on shelf” in my metal shop round the corner. I needed to order it - and I could find only one source in Sweden which sells such metal drills long enough to reach the whole way.

Ordered on Friday - came today!
450mm long drill bit, with no designation nor manufacturer on the body. I call it “no name” then.

It is -17 C tonight and lots of snow so my visit in the boatyard was brief.
First I’ve made test drills with this new, no-name drill and with my 19mm Bosch drill (which is too short for the job but I use it as reference to “calibrate” the no-name drill). Thick piece of laminated timber served the purpose well.

Both holes turned out almost equal, with circa 500um difference, Bosch being more “on the spot” and no-name being tighter. Still good enough to accommodate test rod so I went ahead and drilled on all three keel bolt holes.
It worked! There was just little friction when I reached the iron ballast but I count it as rust remaining from old keel bolts. The drill went through and left even, 19mm hole - ready to accommodate my new keel bolts with tight fit.
Perfect!