Am I going to do any inherit harm by only replacing the one side that went bad??(I found one cylander full of rad fluid).

I mean I realize leaving a head gasket in that has 120k+ miles on it is not ideal, but this is my daily driver and the wife is tired of carpooling....

Thanks

 

No, you can get away with it; but it's probably not a good idea on a motor that old.

Whatever happened that caused that one to fail, happened to the other one too. Whatever bad conditions (overheating, pre-ignition, whatever it was) that overstressed the one, stressed the other too. However many miles the one has, the other does too.

If they were fairly new and something happened, then it would be less risky; but at that age, you're just asking for it to disable itself a second time.

It's probably no more than 10% extra work, and zero extra money if you buy a gasket set, to do the other one. I think you'd be wise to go ahead and do them both so there's less odds of future malfunctions.