True, the 4BT's do not have liners but their cylinder walls are made of some of the toughest stuff I know of. I routinely do "in-frames" at 200K and I spend a good 20-45 minutes on each cylinder wall just honing the glaze and truing the bores.

Now mind you, I use an industrial hone run by a 1/2" super torque drill. In comparison, I can true and hone an American V-8 in about 4-5 passes of the hone.

In the 25 years I've been running the "B"s in my fleet I have had only 1 which I had to over-size and that was because the operator ran it hot and welded a piston ring to the cylinder wall which left a "dink" in the wall. Spent about 4 hours on that one just to take it .010" over.

Danged tough li'l power plants.

Haven't had to do a 6BT yet. So I can't really speak for their block materials. All of my 6BT's have well over 250K on them but they are mostly highway miles.