Advice On 6.9 Overhaul

[hidyusa]

I will try and make this brief but I want to paint a picture of what I am up against, appreciate any advice.

1986 F250 6.9L. I am the 2nd owner of this truck, bought in 1991. It dropped a valve, or they told me so, and was rebuilt, in 1992. I do not have any knowledge of what was exactly done.

In 1998 I broke a glow plug off and ended up overhauling the engine with 1 new piston and a hone job. I ground the valves myself, I am a diesel tech and had access to this equipment. Reused the valves and springs. No machine work was done as it all looked fine. I had relations with the local IH dealership and everything that was used in the engine was from them.

Today, My son and I decided to make this truck a project by completely going through everything. This engine has ran flawless from my overhaul in 1998, so this engine tear down was more of a refresh than anything and a learning tool for my son. Upon pulling the heads I have found that the head gaskets have imprinted into both the block and the heads to where it will require machine work. #2 Intake valve and #6 exhaust valve kissed the pistons just enough to leave a imprint with a line you can barely feel with your finger.

Before I get a machine shop involved, I am looking for advice from others in this forum on there experiences of decking the head and block and valve clearance issues and why #2 and #6 hit, and others did not. I have not done any of the things I need to do yet, like measuring piston protrusion and checking valve guides. I know that the head gasket fire ring was smashed and flattened out in several spots on both sides, but still sealing. I am wondering if this engine was decked in 1991 and I put a standard headgasket back in it in 1998. If this is a headgasket decking problem I would think all valves would have touched.

The pistons are standard and the holes look decent with no cross hatch left on power side of cylinder wall. Would I get away with another hone job? Engine has 300k on it. I have not measured the bore yet.

Being parts are very hard to find and labor costs are through the roof, am I crazy to even think about building this thing? I called one local machine shop and they asked me what an IDI was. I politely told them I am going to keep looking.

I would hate to spend the money on machine work if others have been here and know its a lost cause. It seems finding pistons and having someone cut them down is going to take some fairy dust, unfortunately. If you deck the heads and block, will you have to cut pistons? Anyone know of a good machine shop in the country that can help me? I know it will take measurements and machining to know for sure, but if someone did this stuff all day every day they would know just by looking, as I can with other engines I am familar with.

A donor 7.3 would make the most sense, but I could end up right back to this same spot with it in regards to machine work, and the lack of knowledge of current machine shops.

 

[FrozenMerc]

I would suggest you start off by cleaning the deck and measuring piston protrusion. No more than 0.031" out of the hole. Fel-pro head gaskets compress to 0.063", giving you approx. 0.032" of quench clearance. I am not sure if there are any other gaskets out there that are thicker. 7.3 pistons can be purchased from Mahle that have a 0.010" dropped piston pin location, unfortunately, there is no option like that for a 6.9, other than a custom piston. It probably would be fairly reasonable to have a custom piston house take a set of 7.3 slugs and turn them down to 6.9 size if worse came to worse.

If your piston protrusion is OK, the valve imprints are probably from an engine over speed event, and the springs on 2 and 6 are probably a bit weaker than the rest, causing those valves to float. Or those holes have slightly more protrusion than the rest due to manufacturing tolerances, or even a combination of the above.

As far as a hone job being OK, you would have to measure the bores and see where they are at. You described the cross-hatching be worn away on the thrust side. That tells me that the cylinders have at least some out-of-round, or oval shape to them. A hand hone may not be able to correct that.

 

[IDIBRONCO]

First off, there's no more new 6.9 pistons being made. You might be able to find a dusty set somewhere, maybe not. The general idea is to use standard sized 7.3 pistons. Before doing a final engine assembly, I would recommend checking your valve to piston clearances. It's an old hot rodder's trick and works really well. Another option to gain valve clearance is to have someone machine some material off of the tops of your new pistons. Several members on here have had that done, but I'm not that lucky. Any machine shop that I talked to around me acted like I had just asked them for the winning lottery numbers. Your engine or heads could have been milled slightly, but I've also seen quite a few engines that ran perfectly fine and had valve marks on the pistons. Without knowing any background on them, I have to guess that they were original and that the close tolerances that these engine have weren't always checked before assembly. There goes that assembly line thing again.

 

[hidyusa]

I would suggest you start off by cleaning the deck and measuring piston protrusion. No more than 0.031" out of the hole. Fel-pro head gaskets compress to 0.063", giving you approx. 0.032" of quench clearance. I am not sure if there are any other gaskets out there that are thicker. 7.3 pistons can be purchased from Mahle that have a 0.010" dropped piston pin location, unfortunately, there is no option like that for a 6.9, other than a custom piston. It probably would be fairly reasonable to have a custom piston house take a set of 7.3 slugs and turn them down to 6.9 size if worse came to worse.

If your piston protrusion is OK, the valve imprints are probably from an engine over speed event, and the springs on 2 and 6 are probably a bit weaker than the rest, causing those valves to float. Or those holes have slightly more protrusion than the rest due to manufacturing tolerances, or even a combination of the above.

As far as a hone job being OK, you would have to measure the bores and see where they are at. You described the cross-hatching be worn away on the thrust side. That tells me that the cylinders have at least some out-of-round, or oval shape to them. A hand hone may not be able to correct that.

Thank You. After spending most of the day looking for simple items, like piston rings, I am going to declare the 6.9L DEAD. Really sad. I will be building up the small block Chevy of diesels to put in this thing, the 6BT. Finding a 7.3 donor would work, but why? It will be next on the chopping block.

 

[IDIBRONCO]

Well that's too bad. Not that I have anything against the Cummins, but it doesn't need any help in the fans department.

 

[Nero]

As a Cummins employee, I agree.
Doesn't help they made the 6.9 for what... Two years, then made the 7.3 for SEVEN. Still plenty of 7.3 parts out there, and lots of support.

 

[IDIBRONCO]

Doesn't help they made the 6.9 for what... Two years,

83, 84, 85, 86,& 87. Either my math is off here or yours is.

 

[hidyusa]

As a Cummins employee, I agree.
Doesn't help they made the 6.9 for what... Two years, then made the 7.3 for SEVEN. Still plenty of 7.3 parts out there, and lots of support.

The world I live in, the Class 8 market, I do not like Cummins. Mainly due to their competitors doing a way better job on warranty and moving trucks through shops. There were a lot of 6.9 engines made and as far as I am concerned had some pros over the 7.3. In the diesel world the inline 6 is king. There is a a lot of reasons every single Semi truck you see going down the road, has an inline in it. V engines were phased out a lot of years ago. Ford is just too dumb to figure this out. (I only own Fords, BTW). (but not their diesels)

I have had this pickup since I was 17 and I am north of 50 now. If there were easy to obtain parts at market price, I would do it again and not worry about it till I was 80 or 90. My boy is 16 and after thinking this through, the best thing to do is put an engine in this thing that he will be able to maintain as I will not be around for ever. The Cummins 6BT will be around for his life time and probably his kids lifetime, as they are still making them in several countries. The IDI will not.

IHC has not made an engine worth a crap since the IDI and DT era. Still don't in the heavy market. So watching the only thing they made worth a crap, go to the wayside is sad, but doesn't surprise me as IHC is a very different company today. They have sister companies make their AG engines and their own designed current engines have cost Fleets in this country a lot of money.

I am Gen X, so fanboy is not in my vocabulary. We simply put things how they are and use stuff that works. If it is a ****, its a ****. Some of us just call this common sense.

 

[Black dawg]

So what are your plans as far as trans behind the 5.9?

 

[hidyusa]

So what are your plans as far as trans behind the 5.9?

I suppose the ZF5 to keep it in my German roots. Not too keen on GM or Dodge stuff.

 

[Nero]

83, 84, 85, 86,& 87. Either my math is off here or yours is.

Can't be right all the time Still, 5 years vs 7....

 

[Old-Red]

Here's a good article on all of the Cummins

6B, 6BT, 6BTA, 4B, 4BT, AND 4BTA ENGINES​

Understanding Cummins 6B, 6BT, 6BTA, 4B, 4BT, and 4BTA Engines | Diesel Pro Power

Cummins B Series engines are among the most reliable and widely used diesel engines in the world. Known for their mechanical simplicity, high torque output, and long lifespan, these engines have been the backbone of marine, construction, trucking, agricultural, and industrial applications for...

dieselpro.com

 

[TahoeTom]

More Information for MAHLE / CLEVITE 2242190

I might be late to the party. Just checked Rock Auto and they have Mahle .040 oversize in stock. They seem to come and go, in and out of stock. I was stuck last year with no oversize pistons available. I kept checking and one day they were listed so bought the same set. Be aware that rings for these are NLA, as the oil control ring is not a common size. I talked to Total Seal and got a set of rings to fit top and second groove. The oil control ring required the groove to be slightly larger. I found a machine shop to do that. I was pleasantly surprised to find that the pistons were destroked .010. I had piston to valve contact on a previous rebuild so felt I had a better chance with these. Valve recession is another part of the clearance equation. Best to keep near max at .060 exhaust and .050 intake.

It's too bad the 6.9 is no longer supported by OEM or aftermarket. I was thinking of buying a set of 7.3 IDI pistons and boring my block to that size, but figured there was a good reason IH went to 1/2" head bolts with the 7.3. Another expensive option is to sleeve the block back to standard. I believe Justin at R&D IDI is sleeving 7.3 IDI blocks to use standard 6.9 pistons.