Hi pulled down my 7.3 after a overheating incident . Well actually tore down 3 engines one dropped valve (head destroyed) . overheated one had one head cracked , ok so now i have 4 usable heads BUT all of them have hailine cracks in the prechambers, Is this something i should be worried about or common what do ya think. thanks mike

 

Cracks in the prechambers are EXTREMELY common. Every IDI (IH, GMC and mutsu) diesel head I pulled had them. They are acceptable as long as they DO NOT extend past the fire ring of the head gasket.

 

If the cracks are getting close to the fire ring contact point, I would replace them.

When the crack reaches the fire ring, you will loose compression.

 

Ok , second ? i know there were some slight differences between the 6.9 and 7.3 chambers but if you changed the whole set could you use 6.9 chambers in 7.3 heads OR can the 6.9 head bolt holes be drilled out for the 7.3 bolts . thanks mike

 

Castings are very similar or even identical between the 6.9 and 7.3 IDI heads, but they should not be considered interchangeable. I saw a machine shop try to adapt 6.9 style heads for a 7.3 because they couldn't locate any 7.3 cores that were salvageable. Turns out the seat for the head bolt is a different height between the two heads. If you want to swap one to the other, you will need custom head bolts/studs.

I have no idea about interchangeability of prechambers....

 

the hairline cracks are the result of the higher combustion pressures of a diesel
it is normal

 

Here is a little info on 6.9 and 7.3 heads.

Head design changes -
6.9 features "centered" glow plugs, while 7.3 are "off-center"
Injection nozzles have been retracted on the 7.3 .060" to facilitate Federal and state emission certs.

Precombustion chamber volume -
6.9 = 18.34 cm3
7.3 = 20.42 cm3 (increased only for emissions certs)
This is also why the 6.9 has 22.5 to 1 compression and the 7.3 is 21.5 to 1.

Precombusion chamber inserts are physically interchangable, however the throat has been redesigned to distinguish them apart, and the 7.3 prechamber cub has an embossed "88" on it.

Valve Stem -
The 7.3 valve stem oil sheild is larger to reduce hydrocarbon emissions. Intake and Exhaust sheilds have been color coded, even though they are identical and interchangeable.

Coolant passages -
The lower corner coolant passages on the cylinder head on the 7.3 have been plugged. This change is introduced to eliminate lower corner gasket coolant seepage during cold start engine warm-up. Mating passages in the block are also plugged. (could be why we hear more 7.3 overheating issues).

Exhaust valves - The 7.3 exhaust valve has had its alloy composition changed, with more nickel, chrome, and silver content, and decreased manganese content. The 7.3 valves may be used in the 6.9, but not the other way around.

Use 6.9 heads on a 6.9.
Use 7.3 valves, valve springs, rocker arms and valve stem seals on the 6.9 head for a better head.