400 block number 1 cylinder thin area.
Not wanting to be an a$$ but I just looked up some random sleeves on Summit, and they seem to average .094 wall thickness. if that is the case, what are you gaining by sleeving it?
Cast iron is not as strong as steel, nor as dense. There's a material property called modulus of elasticity which is lower for cast iron too. It varies with the grade of iron, but steel tends to be stiffer, harder, and stronger and so it can be a thinner wall without risk of failure.
Many sleeves are also made from cast iron to avoid a dissimilar material situation. Cast sleeves are likely made from a higher grade with properties closer to that of steel, or at least in excess of what a typical block is made from. They're also typically going to be very consistent since there's no cores for water passages as such. That consistency means a lot when it comes to strength.
Also, a thin spot in a block could be an indicator of corrosion or other inclusion. There's no guarantee the 'thinness' is the result of just a thinner or shifted wall. It could be a bubble, entrapped sand, or a shallow crack which has preferentially corroded to a deeper 'pit' than other areas. Short of x-ray, it's hard to determine the cause of the thin wall - sleeving it avoids having to lose sleep over it or taking the 'wait and see' approch, which may result in a failure sometime down the road.
I've had blocks sleeved before, and after seeing the process and the results, it's nothing to try and avoid IMO. Done properly, the block will be better/stronger than before sleeving.
