Taking the block to be bored this week and the machinist wants the pistons but I haven't bought them yet because I don't know what overbore size to buy yet so it's kind of a catch 22. Hard to believe the final bore diameter isn't a known value for common over bore sizes and would thing the pistons are machined down to the tenths and don't vary so not sure why he wants the pistons. Surely I'm not the 1st person in this situation and I can't screw this up. I Have been looking on and off for weeks and came up with no docs or specs. The closest I've found is .0055" min measured ,850" from the bottom of the skirt but that requires having a piston to measure.

Can someone point me to a formal spec by Mahle or Ford or quality 7.3 builder.. I'd like to give him a final bore diameter and tolerance.
If I'm not asking too much, also the bore RA number for the rings sold by RifRaff and other reputable vendors as I assume they are all the same.

Thanks

 

Here is a for instance, last BMW M30 I built was spec 92mm bore and the pistons were all about 91.97mm About is the key, as they were all a hair different...Just how machined parts are made due to tooling wear.

I would start with the smallest at 0.020" over and check the bore taper, which is what I was taught why one typically overbores. Which would mean you would be probably around 0.018" over before much else as you can hone the bores to match the final piston dia. If bores are good, then can order pistons and wait until they arrive. How are the bores currently, maybe you could get by with a hone.

Guess could also measure your old standards to compare to published standard bore diameter for an idea for piston to wall clearance. The Dave Graham CDs from Rockauto may have a 7.3DI section

Be concerned about ring end gap, might want to be a hair looser to allow for thermal expansion due to the larger injectors and the higher associated EGTs you are running. I would probably contact Hastings or Total Seal for their input on the RA you seek or if Diesel rings need a specific cylinder wall.

Just my observations

 

The ridge is enough to catch a fingernail and bores are polished smooth so it's going to be .020 or .030 for sure. You can also see some slight distortion in the shiny wall where the 2nd ring sits at TDC during combustion. I'll try and see if the machinist can rough them out to determine a final size, then I'll try to get him pistons ASAP so he can hone them to spec, whatever that is. He not going to be happy about waiting but I don't see any options and don't want to just let him go .030 if it's not needed. Regardless, he wants the pistons before final hone,

Another thing is the deck surface id horrible along with the heads. I really can't believe what a crap job Ford did on machining those and amazed they sealed, must have been done on a Monday. I have a hunch he'll need .010" off the deck to get them flat so I might need .010 off the pistons, which they se,ll...more reason to wait.

As for piston/wall clearances, a few vendors have specs of 0.0550 piston to wall .850 up from the skirt which is insane and must be a typo but I saw those same numbers on 2 other sites so they must all have copied and pasted the same wrong info. .0550 is 50 thousandths clearance and from the few videos I've seen it's 5 thousandths, not 50, and most builders I saw make it tighter. Mind boggling Mahle doesn't spec this since it's piston material and rate of expansion that determines the clearance.

Another bizarre find is my pistons actually look great, no skirt wear but they barely fit down the bore when pushing them past the ridge and a .003" brass shim is a tight fit between the worn bore and skirt so this thing is tight even with the wear.

 

I would just do the math looking at the specs in the service manual for Cylinder Bore and Piston Diameter. Don't know if that's the right way to do it but it's gotta get you close right? Just doing the math for standard bores and pistons looks like about .0048 to .0055 piston to bore clearance.



https://www.diesel-dave.com/vehic/ma...vo/svomain.htm

 

I do not believe Ford assembled this engine, Navistar did...these were days before MLS gaskets as I know, so RA for the heads and deck would be less critical.

At least you have some good info and places to start.

 

Perfect... Thanks

 

While I;m at it asking questions:

What makes some of these motors knock? My present motor has 180k miles and knocks at idle, worse when cold. A guy came over a while ago with the same truck but 320k miles and it sounded perfect, like brand new. I've asked a lot of people who had 7.3s throughout the years and got the same answer: "Some knock, some don't, don't worry about it".

The knock I hear is only at idle and worse when cold, and sometimes doesn't knock at all. Oil pressure never goes under 21, even hot with 15-40. Doubt it's a rod and doesn't sound as light as piston slap, just an irritating knock. Possibly crank end play?

Just want to do my best to minimize it on the new build and will pay attention to crank end play along with the usual clearances for mains and rods. , also surprised at the large amount of clearance (side play) between the crank and rod big end faces but there's not much I can do about that.

Below is the deck surface machined by Navistar back in 1996, looks like someone was in a rush:

 

Injectors is what I've experienced, my 95 with reman 160/0 always had a slight knock specially when hot, after installing brand new, not rebuild 160/0 the knock went away.

Same with the other trucks I had in our fleet, swapped problem injectors with used/shimmed injectors or FoMoCo injectors and the knock would go away, and the crazy part it always was the passenger side bank IDK why but it always was.

 

I was hoping the same but just put 180/0 remans from Rosewood, absolutely no difference.
Thing is haunted and just busting my chops.

I'm just going to build the new motor and set it to the tight side of the specs, along with balancing the crank, rods and pistons.
At this point, I'm convinced this "new" motor will knock too so it is what it is

 

Could it be the same as fuel knock in a gas engine? Especially with a high compression diesel? That would lend to a motor knocking more when warm too. Just preignition of the fuel before TDC on a warm engine. If you build a super tight motor I would expect it to knock more . I don't know though that's just me shooting from the rear-end.

 

I'm just understanding how diesel fueling works now and it's totally different from gas which is injected before TDC and can pre-ignite causing knock. Diesel fuel gets injected either at TDC or later when the cylinder pressure is at the highest so it basically can't knock. I guess you can really do some damage to the motor if injection is started before TDC but the stock tune and most aftermarket tunes don't do any of that.

It's still mind boggling that the motor can take 35psi boost @ 17:1 compression without really pushing it

 

pre-ignite causing knock happen in gassers
Diesels just run this way
hot spots on piston, nozle pattern, injection timing, injection amount, compression, fuel pressure and temp all factor in making a knock
Cam walking, piston slap, crank run out, loose manifold bolts, loose valve guides, harmonics and outside forces can make knocks too
Mine knocks on passenger side at idle like most of the others I have heard and SD uses a different injector in 8 to help with the cackle