8 Reales 1770 Fm Mexico , Is This Abnormal ?

wonghinghi Is correct as far as the data presented goes.

The weigh of 27.1 grams is at or near the mint limit. I really would like to know an accurate weight to 1/100th gram. A weight rounded to 1/10th gram could actually weight anywhere from 27.05 to 27.15 grams. That may not seem like a lot but 27.05 represents NO problem but 27.15 could represent a problem. The reason is because there is some slight wear on your coin based on the picture. I would really expect the coin to weigh closer to 27.00 grams. The problem with tolerance in this case is that it can excuse potential counterfeits.

The diameter of 39 mm is correct in some contexts - particularly with reference to Mexico City issues of 1770 and 1771. That said I would prefer to see the coin measure 37.5 to 38.5 mm.

So you have a coin at the limits of tolerance (potentially) which could taken together be an issue.

The top of the crown on the right of the obverse is opposite a fully struck area on the opposite side - that is incorrect and to me indicates there MUST BE ACTUAL WEAR at that point on the coin - an abrasion that may have been intentional that removed metal. There is only a filled die left as a potential reason for the weakness. Filled dies in open sided presses ARE NOT COMMON.

This inappropriate weakness must be explained or it becomes a point of suspicion.

Until there are better pictures I can not comment much further.

Moroccan Friend,
As stated by Bob, the 1770 and 1771 Mo Pillar dollars (and the first two years of portrait dollars in 1772 and 1773) were generally struck on very broad planchets allowing for full image and well defined denticles all around. They can usually be picked out of a stack of milled dollars based on the size alone. Your coin at 39mm would not surprise me at all. However, personally, I would be very concerned by the strange lack of strike on the side of the crown as you pointed out, that does not look right in my humble opinion. I am also interested in seeing more pictures of this coin before rendering an opinion.