Should I re-use this Precup?

[MTKirk]

I’m deep into my 6.9 rebuild and have one nagging issue to resolve. What to do about pre-cups. All of my precups are original, unmilled, still show the 84 embossing. They do have small, within the fire ring, acceptable cracking. One cup has a small dent (3/16” long X .003” deep) dent right at the fire ring. My initial reaction was to replace this, and all the precups. I went ahead and ordered eight new cups from Diesel Cast Welding. I compared the new pre-cups to the old and realized that my heads would have to be bored to accept the new, larger precups, they are larger in height (.020”) and diameter (.004). So now I’m am leaning toward reinstalling my old precups, and not resurfacing the heads at all (they are within .003 of being dead flat). My biggest reservation is the dent (see pic). I can’t see any way this could cause a compression leak as the gasket and block are solid from the fire ring to past the outer edge of the precup. A careful autopsy shows a punch through in the gasket where this dent is, I’m thinking it is the result of a piece of metal trapped under the head gasket at a previous gasket change, and that it had been this way for a fairly long time. Can anyone think of a reason using this dented precup would be a problem?

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[IDIoit]

i would replace all of them, and have the heads decked.

that .003 depth will give combustion gasses an "out" and will most likely take the HG with it.

i guess an "experienced" person could weld it up and grind/stone flat.


but the heads are off, they would be getting everything done, if this is to be a reliable vehicle

 

[OLDBULL8]

Your new precups could possibly be for a 7.3, the "outlet" port for a 7.3 is a little larger. You could have new cup ground down on a surface grinder to size. Put one in your refig freezer and see if it will shrink that .004.

I would not just put a new gasket on and depend on it. that piece of metal that made the dent blocked any combustion blowby, without it you surely would have.

Are you going to have any valve job done on the heads?

 

[hesutton]

I personally would not trust it. It is right in the fire ring of the head gasket. 6.9's have more head gasket issues anyway. It's your call of course. But, I can tell you from past painful experience, the head and deck surfaces need to be as close to flawless as they can be to prevent failure. I had a head gasket failure right after a bunch of engine work and at the worst time possible. That was a hard lesion for me to learn. Don't want you to have to learn the same.

Heath

 

[icanfixall]

In all my years working on these idi engines I have never seen or heard of a precup bing larger like you discribe. As Bill posted. This damaged cup must be replaced or it will fail. What got trapped in there kept the combustion gas from leaking out.. but you were extremely lucky it did not fail. Do the new precups have the 88 made into them?
The thing about cracks is the larger they get the harder it is for them to get larger. They start off near a very thin edge but grow towards the thickest part of the cup. Its impossible to say how fast or if they will eventually grow into the fire ring and fail.

 

[OLDBULL8]

Got to thinking of how you measured the two for comparison. You must of had one or more precups out to do that.

What did you use to measure with? Four thousandth between the two can easily be an error in measuring.

Edit: looking at your pic again, anyway you look at it, that head has to be resurfaced to take that dent out or the head gasket will blow.

 

[MTKirk]

I have all eight precups out. The new precups are ones that Diesel Cast has fabbed to their specs. I have measured both with a micrometer and the new cups are most definitely 4 thousandths oversize. My thought is that would be much to great to be an interference fit and machining will be required. Maybe another approach would be to turn the 4 thou off in a lathe?

 

[riotwarrior]

First I would encourage a kiss on a head mill the least amount possible to clean that zurface up.

Second your precup is trash....unless you have it professionally welded up. That gash dent whatever will cause premature gasket failure and zubseqent engine issues.

Not sure how uou plan to remove 0.004" on a lathe...those are some super hard material IIRC.

Needs to be ground down. Freeze em in a deep freeze see if they shrink...heat head to bout 500 in bbq and see if u can drop a froZen precup in place

I will see if can get tele no. to place that welds up precups.

Juzt called Pendozi machine shop in Kelowna...they used to ship out precups to Que. for welding.

Mark explained..they would gouge out the cracks...then SPRAY weld molten metal back in....sound familiar guys????? seems that the Eutalloy thermal spray welding is same and does indeed have materials for said repairs....hmmm

Can see a test coming up some time soon....yuppers..

JM7.3CW

 

[icanfixall]

Guys the circle of the precup is too small to shrink 4 thousands. If it was large like 12 inches or more in diameter it will easily shrink that much. Maybe liquid nitrogen could shronk it but not 4 thousands. When I worked in the huge machine shop a 2 thousands press fit was generally felt the standard on steam turbine throttle and stop valve parts. Even so in the nuclear field. If the cup was shrunk and the head heated up enough to :make it fit". The stress on the cup and the head probably will cause cracks in the heads. The cups are a size on size fit in the heads. That way they have better heat transfer. Next time you look at a precup notice that only the outside lip actually touches the heads. The sides touch nothing and the bottom barely contacts the head for depth.. Lesson is over for today. Got some garden block work scheduled...

 

[MTKirk]

Guys the circle of the precup is too small to shrink 4 thousands. If it was large like 12 inches or more in diameter it will easily shrink that much. Maybe liquid nitrogen could shronk it but not 4 thousands. When I worked in the huge machine shop a 2 thousands press fit was generally felt the standard on steam turbine throttle and stop valve parts. Even so in the nuclear field. If the cup was shrunk and the head heated up enough to :make it fit". The stress on the cup and the head probably will cause cracks in the heads. The cups are a size on size fit in the heads. That way they have better heat transfer. Next time you look at a precup notice that only the outside lip actually touches the heads. The sides touch nothing and the bottom barely contacts the head for depth.. Lesson is over for today. Got some garden block work scheduled...

I agree! Even if I managed to force them in through temperature differential and pressure I would probably create a larger problem, possibly even a warped or cracked head. From an ANSI calculator I get a MAX oversize of about .004 (also this max does not take into account the lower ductility of the cast head). I would be adding that much to what is already a very tight fit. I believe machining will be required to use the new pre-cups.

 

[MTKirk]

I took the new pre-cups into a machine shop with considerably more machinery than your typical automotive machine shop. Around here if you call one of these shops they'll tell you they don't work on automotive stuff, so I just took in the cups and kept my mouth shut, except to tell him how I wanted the cups altered. He gave it a hardness test (he said about the hardness of a chisel) and said "No problem, $70 an hour and shouldn't take more than 2 hours to do all 8, also said he could maintain the shoulder angle (it is not 90, the diameter increases slightly as you go towards the combustion chamber).

I'm thinking this is the way to go. If for no other reason than if It gets messed up, or I need to change a pre-cup at a later time, the head will still take an original pre-cup. If I modify the head and have a problem, I'm up a creek. So the plan is to double check the bore measurements on the head and take the cups in for resizing on Monday.

 

[icanfixall]

I suggest you give them the head so "they" can measure it. That way you know be no mistakes. Now also please remember these cups can sit below the height 2 1/2 thous or above the head 2 1/2 thou... But no more and no less than this. Even is the best depth in my opinion. I believe these cups are made from an Inconel material. It has very good heat and erosion careicterfics.

 

[dunk]

Wait there are brand new precups being made? Who is making them and where can they be ordered?

 

[icanfixall]

I recall the original poster said he got them from Dieselcast Welding. I realise its an odd name for a cylinder head service but they are a good place to buy from. So far only one member of this forum had isues with them. Sad part is he lived in Australia. Seems several of the guides were broken. Turns out Dieselcast just bought out another head rebuilder and these were not checked for quality before shipping. Our member got another set of heads and no more problems. I believe he got to keep the other set that needed new guides installed. Not sure now but I know they made it right for him.

 

[MTKirk]

An update,

The new cups from Diesel Cast welding went in fine. I put the head out in the sun and cooled the cups with dry ice, they tapped right in with a rubber hammer. We then milled the head down to 4.795". The new cups are not the same alloy as the old cups. The old cups were not attracted to my magnet, the new ones were. My machinist reported nothing unusual about the way they cut on his machine. I mic'ed the heck out of the old cups and I believe they had "shrunk", they literally rattled inside the heads. Also, the diameters in the direction across the fuel nozzles was more than a thousandth smaller than the other axis. It was obvious that compression gases had been leaking around the entire perimeter of the cup.

I don't think there is any crucial need for the cup to seal to the head or the gasket, as mine wasn't. The truck was running fine and developed a gasket leak in another location. I believe Ford says cracks extending beyond the fire ring is cause for replacement, not because of compression leak, but because there is danger of a piece of the cup breaking off inside the cylinder. Just one man's opinion!