how do you know it's "about 155"? you could have got a defective stat [not uncommon] or a failing sender or the gauge could be off, not bad, just wrong .... on a new-to-you old truck always use a laser pointer digital thermometer to check over the system, read near each side of the stat housing as it warms up and find out what temp the stat opens, read the top and bottom of the rad and check around the engine, can learn a lot that way and by comparing all that to the reading of the gauge you'll know what it's telling you, the old gauges aren't real precision, just meant to indicate when things get too far away from "normal", which is why most years don't have actual numbers
and I've understood the rings might move from the staggered position you start them at, but I've never heard they "spin" life-long, which would be pretty tough with them pressing out against the cylinder walls and being subject to normal compression
Bill
