This is part memory, part speculation, so someone please correct me if I'm wrong.  If Evelrod reads this and replies, accept his word as gospel.

I believe that aluminum cylinder bores without liners were pioneered in the 70's in the Chevy Vega.  By making the interior of the bore have a higher silicon content (by casting cooling rates?), the dissimilarities between the piston and cylinder prevent galling and seizing.  That being said, it must have been a tricky process because junkyards were soon full of seized up aluminum block Vegas.

The other thing to remember is that these are CHEAP and DISPOSABLE engines.  Probably loose sloppy tolerances play as big a role in preventing seizure as anything else.

Are you sure? Briggs have a metal liner for the cylinder bore and an aluminun housing poured around them. Just like your modern aluminum car engines. They've changed from the original design in that you can't remove the cylinder bore from the crankcase, its all one piece.

As far as the plating I don't think you can hard crome plate aluminum. The Chain Saw motors also have a metal liner and them and are crome plated to help prevent wear. However you can't bore out a hard cromed cylinder, you just replace it. Briggs however have a cast iron liner just no crome so you could bore it out and rebuild it. However since you basically have to dismantle the entire engine to be able to bore out the cylinder (one piece construction) I would guess that Briggs have made the liners to thin to bore out, and rebuild. After all to get it stripped down and rebuilt by a shop would cost more money than a new motor. The lesson i've learned is just to get old stuff and keep them running, as the new ones are disposable.

I can't speak for what they are building now, but when I was at MTD (Yard Man, White, Cub Cadet) 4 years ago this was the case.

Will

Will, I'm very sure that the Briggs engines have aluminum bores, basically, you could scratch the bore with your fingernails, and I've seen this on two Briggs engines and a Tecumseh also. As a selling advertisement, Briggs offers "Cast iron Cylinder!" as engines alternative to aluminum ones, obviously these last longer. I was really conserned on how they're designed to not wear out rather if the bore was actually aluminum (I have no doubt that they are). Try finding a disassembled lawnmower engine and see for yourself. I was quite surprised when I discovered it.
DJ

I've dismantled and rebuilt a lot of Briggs over the years, I've never ran into one that didn't, but I rarely rebuild any that less than 10 years old. Heres the official line from from briggs.
 All engine models named Vanguard, I/C, Industrial Plus, Diamond Plus, Intek Edge, Intek Pro, ELS (Extended Life Series), 3LC (Three Cylinder Liquid Cooled), Animal or Europa, have a cast iron sleeve in the cylinder bore. All other engines are dependant on the model, type, and code numbers to determine whether or not the engine has a cast iron sleeve installed.
I had always assumed it was a marketing ploy. I know for a fact that horsepower's are often under reported. They do this to show slightly different models and prices. I will say that if you match the two engines up side by side and they look the same and the smaller engines vary by some fraction or the larger ones vary by one or two horsepower they are the same engine.
However I guess I was wrong, it didn't used to be this way. I would guess they just started matching up life expectancies of the motors with the equipment. Some of the low end roto-tillers only have a design life of 120 hours. I would imagine an aluminum would only last that long.

Will

I think I just answered my own question.
Below are links to Tecumseh engines where they actually specify that the cylinder is aluminum.
Also if you go into the products section and then select utility engines then Formula®, they compare all their models, where you can actually see that some engines have aluminum cylinders and other are specifyed as "aluminum w/cast iron sleeve"

http://www.tecumsehpower.com/Tecumseh.nsf/Second%20Level?OpenFrameSet&Frame=FrameB&Src=_l5ta6aorldlpmaq1edppmcbqge9nm8lq28la5eh1v9to6ari6dtp6q9i1elq6uhjic5mmap00_

Regards,
DJ