Head-bail closed gap width?

[Putangitangi]

We have a headbail built 35 years ago from a plan, now mislaid. With some enthusiasm for getting on with the renovation job, my fix-it partner has pulled it apart, but it would have been sensible for us to do some measuring before taking it down. :roll:

Does anyone have anything similar, where the gap has only one width option when closed on the neck of an animal? (The timbers need to be right down or the animal can jiggle around and move them up to escape.)

The new timber on the right side (animal's left) was added to account for the rotted-out wood on the other side. It has worked for emergency cases, but wouldn't have lasted much longer. It's been a great design for occasional use. We have a new animal tracing/tagging scheme here and I'll need to hold the cows to get their new tags in the right place. We'd buy a fancy new one, except we bought a fancy new tractor and farming incomes just aren't that good.

We now need the measurement for the best gap between those timbers. Thanks to anyone who can help.

 

[Putangitangi]

Please anyone, just the closed gap for an ordinary sized cow will help a lot. I don't have a set of calipers big enough to measure my cows to make sure we get it right.

 

[Banjo]

I would think six inches would do it. I would try filling it with 3/4 to 1 inch boards screwed on top of each other and try it ....if its too tight unscrew a board.