hello everyone!

just like to say how impressed i am with your forum and have a few questions about replacing the tired 7.3 in my 89 2wd f-250.

i have a 6.9 out of a 85 or 86 f-250 i believe it is out of a 86 but its under my work bench and will be pulling it out today to start tear down and evaluate.

as for the casting numbers ans serial numbers are they under the oil cooler meaning the cooler must be removed to see them?

i do know that the oil dip stick area of the block has a specific area in the casting for the steel tube.

so i am hoping that this is the newer of the 6.9 blocks.

the block will need to be bored as the cylinders are rusty. so how far can i go? would 60 over be too much as i can get a good price on pistons and rings that size.

as for cracks in the blocks webbing is there any particular web that is prone to this? or any other areas to check?

are fel-pro gaskets a must?

as far as the heads are concerned upgrade to 7.3 valves but are the 6.9 valve seals still available any where. from what i've read the 7.3 worked too good sealing out oil.

are the crank and rods interchangeable between the 2 engines?

thanks for any input

Rich

 

Welcome to the forum, good handle (user name).

I don't know about the numbers on the block, but some one else will be along
shortly with that info. The block heater should have a counter boring around it if its the newer casting.Late 1985 could alreadt be the newer block

You are on he right track. The crank and rods will interchange as long as you are not dealing with a factory turbo 7.3 IDI (they have bigger wrist pins). Valve seals should be available for the 6.9. I haven't noticed much oil consumption since the rebuild, but they are supposed to be the 6.9 style seals. Valves and rocker arms are better on the 7.3 and you should try to swap those over (I'm still running stock 6.9 parts on mine).

Bore only as much as needed to clean up the cylinders. I went 30 thou over. 60 is still safe, but in the interest of long term resistance to cavitation, I would go only as much as needed to get a good seating for the new rings. A 7.3 IDI is a 6.9 bored 110 thou over. If you want more power, a turbocharger is the better way to go than boring over.

Look for cracks in the bottom main bearing area of the block. Hot tank and magnafluxing is recommended to be completely sure.

Felpro are good gaskets, I've used them and am satisfied. Most parts stores don't seem to stock much else anyway in the way of brands.

 

thanks for the warm welcome david.

i got a set of cast 60 over pistons with rings for $200.00 delivered, the place where i got them from is clearing out old stock, and not going for massive horsepower just want to get my truck back on the road.

and believe it or not under the grease the sticker is still on the valve cover, and the date is Oct of 85 and all the outside markings indicate the preferred 6.9 block, i just have my fingers crossed on the main bearing area.

the 6.9 truck came with a banks turbo but not sure i'm going to install it, the impeller on the compressor end is a bit nicked up to my liking, i'm sure banks can fix it for me but we are goin low budget as i am on a fixed income. maybe install it later or may sell it.

thats how i finance my projects sell off one of the MANY incomplete ones to finish others. lol

well i'll be tearing down the lower end tonight and will report with results, wish me luck!

 

well i've been searching and reading about rebuilding cyl. heads and have yet to find any info on sinking valves to obtain proper valve to piston clearance.

after resurfacing (if needed in the first place) say 0.003 has been removed from the head to block surface, why cant you just cut the seats and or valves to compensate?

like they do in some racing applications where you aren't allowed to remove any chamber material to cc combustion chambers.

i know removing material from the exhaust valves isn't a good thing but should be able to get some out of the seats.

just a thought

 

the way I understand it the pistons get milled to get the proper valve to piston clearance...you can buy them allready milled for just that purpose...

I read you already have the pistons and I'm guessing there stock in height...I'm pretty sure you can have them milled to get the clearance you need. A good machine shop should be able to tell you the best way to accomplish it...but I'm no expert, lol, I dig dirt for a living...anyone else have any thoughts?

 

why pay for the extra machine work? any machine shop worth it's weight in salt should be able to do a 3 angle and keep the valve where it's supposed to be i would think. but thats just me

 

good point...

 

i would have to imagine thats what the re builders must be doing, i don't think that they would send you a set of heads with instructions telling you that you have to have your pistons cut for their heads to fit.

the spec is distance the valve extends from the head surface not distance the valve is to the piston i think.

 

The rebuilt heads I got were resurfaced and had 10 thou taken off the top ends of the valve stems. Although I still had my pistons shaved off by 20 thou. There are pistons available that are already "milled" down 10 thou from the factory in the event of havinf to resurface the head or block.

 

pistons cut 0.010 will bring back the spec for a decked block or running a turbo for more chamber volume. it shouldn't have anything to do with the heads.

cutting the valve stem tips is usually done to to put a new flat surface on an old valve for the rocker arm to slide on or to make up the distance that the valve was sunk into the seat.

you can cut as much as you want off of the valve stem and the lifter will adjust it self to zero lash.

there are only two things that make the valve lift, one is the amount of distance the cam lobe rises from base circle and the rocker arm pivot ratio.

taking 0.010 off of the valve stem would be like taking 0.0075 off of a push rod (using 1.5 ratio rocker arms) but the lifter will make up the distance by adjusting it self to 0 lash.

if the valve hits the piston becaus the valve is sticking out of the head too far that head/valve is out of spec.

now with sinking a valve into the seat to get the right hight you would have to shim the valve spring to get the right installed spring hight to keep the correct spring tension.

anything after the spring retainer is irrelevant

imho .

 

The problem you are looking at is why there is a minimum head thickness.

If you go below that thickness, you replace the heads.

 

still doesn't tell me why

anything more than 0.005 will what?

1) be more prone to cracking? doubt it

2) change in chamber size? could be

3) valve to piston clearance on a head that had no valve work done? can be compensated by cutting valve and valve seat.

 

Combustion chamber size is 18.34 cubic centimeters on the 6.9 which yields a 22.5 to 1 compression ratio.

So surfacing the heads below the service limit will increase the compression ratio rather fast.

Valve to piston clearance is around .035".

So once again, you are already close to valve/piston contact.

Just milling the pistons will not do much to help with valve clearance issues due to the valve reliefs in the pistons.

Cracks in the heads between the valves is already an issue.
The more metal you take out there, the more the issue increases.

 

thats what ive been saying, i could not see the point in messing with the pistons OTHER than getting the compression ratio down for a turbo application.

and cutting the valves and seats will get your chamber volume back but you can only go so far with that. but it can be done.

as far as cracked heads 99% would seem (as i've been reading in pretty much every idi forum on the web) that heat is the number one cause. usualy your blown head gasket broken water pump/ hose etc, and even less so on the newer castings. and mostly here in florida nobody wants to invest in a new radiator.

my rule of thumb here is if it's over 10 years old it needs a radiator leaking or not. The salt air and micro fine sand here eats them up on the outside and when i pulled the old one out and laid it down on the ground there had to be a half a cup of very fine sand that came out of the fins, and unless your running a filter system the inside is pretty much coated with scale.

when rebuilding any motor i tell my customers unless you can show me a receipt for a radiator within the last 2 years you will get the "tail light guarantee".

 

The 6.9 doens't have valve reliefs in the pistons. At least mine doesn't

The only thing that sinks below the crown of the piston is the prechamber dish and its too small for the valves to fit into. If a collision happens between the pistons and a valve, the valve will only make contact on the top of the piston.

I can take a pic if you like, I still have the old heads and pistons.