having a bit of experience in repairs of cast steel, cast iron and cast aluminum i can offer a few thoughts
the use of sodium silicate is good generally only in low pressure area's, it needs to be in a clean cooling system , allowed to get hot
where it will mix, fill the crack and provide the seal. you then drain the engine and allow it to air dry.
the stuff works very well in alot of cases. the problem with this sort of crack is it is very difficult to get the engine hot and have it fill the crack
when there is so much pressure from compression,,, it just won't work there very easily.
the only good repair is a welded repair.
now then we need to understand what causes a head to crack in this area to start with.
we think of this sort of crack being caused by overheat,, but this is only part of the cause or rather the lead up to the cause.
when the head gets hot/overheated it expands much further than normal, because of a lack of coolant at the deck.
this can be low coolant, a blown head gskt allowing air to become trapped etc.
once the head is overheated and overly expanded, then comes along a slug of coolant which causes the thinner sections of the head to shrink far
faster than the thicker parts and waaalaaaa it literally pulls the thinner section apart much like a piece of taffy.
the thin part is generally between the valves, just as seen in this head.
to effectively repair this head it will need to be stripped and cleaned and heated to around 1000 Degree's F
for a maybe an hour to cook out as much oil and crap from the head casting, and allowed to cool
then the crack will need to be V notched out to the root of the crack to prep it for a weld.
the head is then reheated and the welding begins, then reheat the head and allow to cool very slowly
often times the first pass will have to be reground out, because the casting is porous and some oil will boil out and foul the weld.
so you grind it out, reheat and start over welding again.
if the weld is peened very well it will do two things,
1. it will remove the stresses imparted in the welding process, and
2. it will expand the weld metal which will place a bit of outward force at the crack area,
some outward stress is useful as it allows for the thinner section to be compressed and contained by the thicker sections of the head.
after an effective weld repair the head will have to be cut and valve seat inserts installed, because the repair will overlap the original seat area.
there are various arc rods that are good choices, and perhaps Hotater can recommend one that is machinable for the cutting and installation of new
seats.
there is no other effective repair for this sort of crack on a cylinder head (that can be performed by a small shop), the pressures involved in the combustion/cylinder area is just too high and the vibrations also are very harsh, let alone the temperatures involved.
this covers the subject as i know it and based on many successful repairs to cylinder heads and the like over many years,, both cast iron, cast/billet aluminum .
bob g