I used two of these last weekend. I can't find an image online of the exact model I used, but it has two vertical slits on opposite corners (looking down on it) with a bolt & nut to tighten each slit.
I bought two of them for 75x75 mm posts, & two posts of the matching size. Are they supposed to make an "interference fit" with the bolts loose? I had to shave a bit off all 4 sides of one end of each post just to get the post started in the top of the spike (then I used a block of wood & a sledgehammer to get it the rest of the way in).
Having used met posts, I think Bill is right. Not all fence posts are created equal, and a bundle of posts, each nominally, say, 4 x 4, will not be exactly the same. Close enough, but even a difference of 1/8th of an inch is the difference between a post slipping easily into the foot, or not. You would think the foot hole would be large enough to accept slightly oversized posts, with the smaller ones being held by adjustment, but apparently not.
The reason for the question marks was that I did not have the foggiest of what you were referring to till someone mentioned Met posts, still had no idea till I looked it up.
I thought that was more of an issue in the USA, where "2 by 4" really includes a fairly wide kerf. Most of the timber I buy in the UK seems to be "actual size".
I suppose a lot depends on where timber is purchased. Last time I bought fence posts, they came from a local supplier who offered various sizes. As far as I could see, they used one machine, which was adjusted as needed to produce batches of, say, 4x4, 3x2, 4x2 or whatever. I bought a number of posts, all supposedly the same size, but they were not exactly the same. I could only conclude that they came from different batches, and the initial setting up was not always to the same exact dimensions. The posts were fine, but some dropped straight into the met post, and some needed a little help with a large hammer. Thinking about it, I didn't check the met posts, which may have differed very slightly too.
I am building something at the moment, using lengths of PAR 2x1 rescued from a pile in the shed. They are not all absolutely identical. Very close, but a joint created using two lengths not cut from the same length can create a very slight lip. Hardly enough to see really, but enough to feel. Thinking again, I suppose some could be older, and have swollen very slightly.
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