Problem With Breaking Pistons

:banghead:
I own a 1990 Geo Metro LSI with the 1 Litre engine. This is the 3rd engine that I have had problems with in the last 5 years. I keep it tuned up, and regularly change the oil. I recently started to hear a knocking noise in the engine, and right away I knew what it was.
It is a broken piston, and it is the center one and the skirt is broken on the back of the piston clear up to the oil control ring. Both of the other engines I had did the exact same thing in the same spot.
I'm not sure if it is piston wear or what it might be. The other 2 pistons are in great shape with little cylinder wear.
I use Castrol GTX 5W20 oil as if I use heavier oil it will hang the lifters open and burn the valves. And my local car dealership recommended that weight of oil as well.
Another problem I have is with the cooling system. In the summer it heats up and runs great, but in the winter it runs really cold, and it never really heats up. I have changed the thermostat several times, I have replaced the water pump, radiator, both hoses, the belt to no avail. I can let it set and idle, and it barely leaves the cold mark and the cooling fan never comes on. It has me completely baffled.
Any help will be greatly appreciated, and I thank you for taking the time to read my post. :banghead:

Solve your cooling problem and then you'll be able to use the proper oil that your engine desperately needs. When they had the bad valve burning, they found it was because the holes in the lifters were too small to drain properly when people used 10W-40, they redesigned the lifters and took away the option of using 10W-40, leaving 5W-30 or 10W-30 as the only choices, 5W-30 preferred. I've had 6 Metros, over 650K miles, and am active on five Internet groups, sites, bulletin boards, whatever you call these things, I've never in my life heard anyone recommend 20 weight oil. Put in new lifters, stainless steel valves, and change the #2 connecting rod along with the piston this time.

Your thermostat isn't closing up for some reason. There are two different sizes of thermostat for the G10 engine, one of them is smaller and requires a rubber ring around the outside of it or else the water just goes around. Get a thermostat from a different source, might even try the dealer, and pay close attention to how it fits in the housing, it needs to be snug. There are also 180° and 195° thermostats available.

I went to the dealer, and several auto part stores for thermostats. I know what you are talking about it having a good seal around the housing. I have tried both 160 and 195 stats and I haven't had any difference in either. When I start it up in the winter when it's cold, it will sit there and idle, and the fan will never come on. The upper hose is cold, and the lower one is warm to the touch, but not hot. The original engine in it when I bought it ran good, ran hot like it was supposed to, and I have no idea what I did wrong when I replaced it. I have been a mechanic for a number of years, but I have never ran across an engine that won't heat up properly. :banghead:

The fan shouldn't really come on in the winter, that's more a summer thing. One thing that you might check is the engine timing. That seems to make a difference in how fast the engine warms.