Back in the 60-early 70s I turned and ground many many pistons on a piston grinding machine, we had different cams for the machine A-B-C-D-E-and special E cams, we use to have semi pistons that would clean .060 I turned a lot of used pistons like .060 to .040 ect. For some people that don't know, pistons are not round, they are cam ground and tapered, the ring lands are about .025 smaller than skirts. I'm sure it's hard to find a shop with a piston grinder now days. If you could they could grind a .060 to .045 easy. Walt btw, pistons in a Briggs - Stratton are round.
At the risk of 'being on my soapbox'....
Walt almost addressed his own comment, or rule, automotive "pistons are not round", and "btw, BS pistons ARE round." There are other examples around, for example, I keep a forged TRW piston sitting here that IS round.
Yes, the ring land area is smaller.
I interpret the original OP to be inquiring about a 'repair' or possibly a 'patch-job', since it is being asked about ONE piston. That means to me that the repair is 'compromised'. Taking a used .060, and making a .045, is, in my opinion, fair game. As for balancing, yes, I would make sure that the single 'replacement' piston matches the other ones in weight.
As for turning one on a lathe, in my opinion, fair game. Especially for a 'compromised' repair. I've done it, many of us have done it, it can work.
Also, using a .040 piston in a .045 bore, would be fair game to at least look at. That's only .005 off, and there are a whole bunch of engines running with at least .005 or more wear. I'd be looking at skirt design, and clearance, and would think to myself, 'in my experience, would this work?'
I also mention, that for my own personal engines, I'm of the school-of-thought that 'runs them loose'. For me personally, my preference is to match piston-to-bore, at the loose end of the spec. So, carefully picking a .040 piston out of the junk barrel would be something to look at, and see if I think the skirt clearance (and style) is acceptable. I'm ok with a little 'cold-slap'.
I'm going to partially credit this thinking to rumbleseat (Paul). You can look up all of this in his notes. He built his own, 'loose'.
Also, I reference numerous magazine articles that deal with 'compromises' and running them 'loose'.
And, finally, the set of volumes by Smokey Yunick, where he references racing one night, having a burned piston, welding it up and hand-shaping it, overnight, and racing it the next day. (and sometimes winning, or at least placing.) IMO, if it's good enough for Smokey to race it, then it ought to be good enough for me, to get an old flathead down the road.
Now, caveat... I would never do any of this for someone else. Only when I'm messing with, and responsible for, my own stuff.
Only opinion, and Off soapbox...
I recognize there are numerous opposing opinions here. I came from an era when the norm was to patch them up, used-car-style. At the time, that is generally what the customers wanted.
. Some Rugby fans luggage is a better idea though, Someone will know Marty, he is famous in NZ.
...or he will know someone who is over there.
