Besides the displacement differences between the 6.9 and 7.3 idi what is different in the 7.3. did they change the heads injectors, IP.

 

The 7.3L is a 6.9L bored out.

1987 saw a better glow plug system for the 6.9L that was carried over for the 7.3L.

Head bolts on the 6.9L are 7/16", while the 7.3L has 1/2". The length of the head bolts are also different, so the machining of the heads are different to match the different bolt lengths (not sure if there are any other differences between the two types of heads). Prechamber design is also said to be slightly different between the two engines.

Valve stem seals are designed to leak small amounts of oil in the 6.9L to lube the guides. 7.3L seals do not leak, this has been known to cause premature guide failure

Injector pump calibration is different between the two engines, but they are basically interchangable. The 7.3L turbo has a special pump (and injectors) on its own that should not be used on a N/A engine (unless you like making black smoke).

6.9L has seperated fuel filter and water separator. The 7.3L has an integrated filter/separator in the location of the 6.9 fuel filter.

 

id like to ask something too. In my haynes manual it said the back two cylinders on a 7.3 are bigger than the rest. Is the haynes manual full of crap

 

Every hayens manual I ever read was full of crap, I alwayes got pissed of with that generic phrase "due to the technical sohpistication of the [blabla] system, it is beyond the scope of the home mechanic and should be taken to a qualified mechanic". Funny how the systems that are the most expencive to have repaired are always the ones they refuse to help you with. But to your question, I have never heard of any size differnence between any of the cylinders in these engines.

 

Actually there were some engines that used select fit pistons.
And the rear cylinders were larger, more piston clearance was OK in the rear cylinders.

I don't have my book here, and off the top of my head I can't remember what codes the select fit pistons used except the S piston was OK in any of the cylinders.

 

Just wondering is there any reason for it or is this a fix for if a cylinder needs to be borred out a litle?

 

I've read the chev 6.2/6.5 diesels used selective fit pistons, but wasn't aware that the international IDIs had that as well. Then again, that was in a haynes manual.....