Opinions on the use of seatpost shims? More specifically for aluminum frame and seatpost? Long term use?

I was thinking of shimming a 27.2 to 31.6.

I've used seatpost shims for extended periods with no ill effects. If anything super thin ones are bad. At 4.4mm difference you'll be OK.

2 bikes, I own, both top dollar, have Machined shims between the ID of the seat tube,
and the OD of the seatpost. they are fine .
USE a British Company, make a 25.0 and a 27.2 seat post,
and a series of shims for all the sizes in between.

Thanks!

Normally, a seatpost is a close fit in a seat tube and so is supported over a fairly long length. I the shim is short and only supports the seatpost at the clamp, there will be a lot of stress at that point since the bottom end of the seat tube will be loose in the seat post and free to wobble around. Ideally, the shim should be long enough to support the seatpost in the seat tube over a good length, suggest at least 8 cm.

Read this one again if you didn't. Supporting a length of the seat tube is important for much the same reason following "minimum insertion" marks is important. If you look around the forums you'll find a few threads where people didn't follow that guide and separated their frame at the seat post lug.

The biggest problems with using a shim, is that modern bikes now often have the seat tube extending far above the seat stays, where as in the past the seat tube stopped at the stays. if your frame doesn't have an extended seat tub, a shim will be fine, would use caution if it does as you may end up with too little fully supported lenght in the seat tube.

^For a pop can shim I'd agree, but I think you're still likely to elevate stress in the joint given the diameters the OP is planning to shim.

you know, most shims are long enough to reach down below the top tube joint.

and most seat tubes are butted anyways, meaning the seat posts don't touch the inside of the seat tube after a certain amount of insertion.
Which is usually just below the joint at the top tube.

BEER can shim? better get out the dial caliper..
many frames get damaged by kludging in a too small seat post
then overtightening the frame to compensate.
that is why there are seatposts made in increments to the 2 tenths of a millimeter.