Originally Posted by grumpus

I wouldn't want to use such a short shim in any normal aluminium frame that's used with a long seatpost. You're basically applying the load on the saddle, multiplied by the seatpost lever, on just that short length of seat tube, every time you pedal or hit a bump. Give it any excuse to crack and it will, as is shown in this thread. Google says online opinion is that aluminium frames have a life expectancy of around five to ten years. I'm not sure how anyone has arrived at those figures, they seem reasonable but we mostly only hear about failed frames, not high mileage frames that haven't had problems. So perhaps assume that your frame may last somewhat less than that, depending on use. Having said that, I bought a very cheap used aluminium frame on eBay fifteen years ago and it seems to be lasting fine. I'm not sure I'd be so confident with aluminium forks, which I tend to avoid.