Sunday, March 15, 2009

The Best-Ever 6.5 Chevy Diesel Rebuild

A while back, Ted Rich, of Granite Falls, Washington contacted me about a rather unique 6.5 Chevy diesel engine build for his 1998 Chevrolet 3500 4x4 dually truck. Ted, like many of you, is a GM diesel enthusiast who loves his truck. He had spent a lot of effort and money over the years getting it built up exactly like he wanted. Unfortunately, after about 150,000 miles of happy service, his engine failed. Instead of buying a new Duramax replacement, Ted challenged me to build the most durable, smoothest running and efficient 6.5L Chevy diesel engine that I could imagine. A few extra horsepower wouldn’t hurt, he said, but sheer power definitely was not his aim. Diesel Depot – with our 10 years of experience building these engines for a wide range of applications from mild to wild – accepted Ted’s challenge and began this unique engine build. My thanks to Ted, he allowed me to use my imagination and to utilize some relatively new and exciting technologies to achieve our goals. Durability
The 6.5 L diesel has always had certain structural weaknesses. Cracking in the main webs of the blocks and in the number seven and eight cylinders, blown head gaskets and broken crankshafts have all been common issues over the years. In choosing which components and processes to use for this build, we sought those that would yield the strongest possible combinations to mitigate or eliminate these known weaknesses. That is, we used parts and procedures with proven track records of reliability in our custom built engines over the years.
The first decision we faced was which block we would use as the foundation of our engine. It may surprise some of you that we decided to use an earlier 6.2L engine block with the casting number 660. In our opinion, these early 6.2L blocks are stronger in many cases than the later 6.5 L diesel blocks. Of course, there are newer, improved original equipment (OE) blocks; but we have seen even these latest blocks exhibit the same cracking as other 6.5 L diesel turbo engines in critical areas. Mostly, I have seen cracks in the main bearing bolt hole areas as shown in the photo below. New aftermarket blocks, produced in China, are also available. For me, these are buyer beware: they are still unproven in long-term, real-world conditions. We have not used any of these imports and we have not been able to find any other engine builders who have used them in any volume. For this project, we chose to use a crack-free and seasoned 1990 6.2L block.
In order to prepare the block, we cleaned, magnetic particle inspected (magnafluxed) for cracks and checked line bore distortion as well as torque plate boring and honing, surfacing the deck to even the deck heights (square decked). We made sure that the deck heights on both sides of the block were as close as possible. Ultimately, the block ended up 0.020 inches over on the bore with 0.004 inches taken off of the deck surface.
A standard bore 6.2L engine is .080 smaller in diameter than a standard bore 6.5 L diesel engine. By starting with a 6.2L block and boring it .020 oversize, we did lose some engine displacement versus a true 6.5 L diesel engine. We figure that Ted’s hybrid engine ended up with approximately a 6.35 liter displacement. In our experience, this slight loss of volume does not sacrifice engine performance. With diesel engines, most of your performance comes from the amount of fuel and air that goes into the engine. As long as the same fuel and turbo systems are used, I would say that the seat-of-your-pants performance of a 6.2L and a 6.5L is identical. We have sold hundreds of these rebuilt 6.2L engines to replace 6.5L and I have never had a customer complain about a loss of performance.
An electromagnet (A) is used in a process called magnafluxing to reveal cracks in the block. Colored iron filings are attracted to the cracks. In this photo, one of the cracks is highlighted and enlarged. There is another crack not quite as visible, can you see it?
When building an engine based on an early 6.2L diesel bottom end (with a two piece rear main seal), there are no crankshaft alternatives other than using a standard or re-ground crankshaft. We chose to use a standard OE crankshaft that had been cleaned, polished, inspected for cracks and measured for straightness.
Throughout the production of the 6.2L and 6.5L, there has been a tendency for cracks to develop at the outer main cap bolt holes on the center three main caps. Sadly, we have made many trips to the local scrap yard with loads of scrap blocks to prove it. When a crack-free block is located, we feel that the best way to combat cracks from developing is to install a stud and girdle kit.
This process uses six ARP studs and a girdle to tie the main caps together. In our application and in general, the use of a stud converts the typical twisting force on the bolt hole to more of a stretching force on the stud itself. Changing this load on the block is, in our opinion, the best and most cost effective way to prevent block cracking at the six locations where the studs are installed.
Next, we installed our own reconditioned connecting rods. Each rod goes through an extensive procedure of:
  • Cleaning and relieving stress with a process called metal shot blasting or peening,
  • Installing new rod bolts and nuts,
  • Re-sizing the big ends,
  • Installing a new pin bushing and
  • Boring the bushing parallel to the big end. 
Perhaps the most critical of these steps is the sizing of the new pin bushing. During this procedure we are also setting the actual rod length (center of big end to center of small end distance) to within 0.0005 inches on each rod in the set of eight.
Quality pistons are, of course, another significant component. We have seen and tried many piston manufacturers’ products for these engines over the years. It has become clear to us that Mahle pistons are the best available and we use them exclusively. We wanted to lower our cylinder pressure slightly so we installed .010 topped 6.2L pistons to lower the compression ratio to roughly 20:1. Also, to make the pistons tougher for this turbocharged application, we coated the domes with a thermal barrier ceramic coating and the skirts with a dry film lubricant coating. I’ll go into more detail about the benefits of these coatings later.
Another of the more common problems with both the 6.2L and 6.5 L diesel engines over the years has been leaking head gaskets. The factory installed TTY (torque-to-yield) bolts in all GM diesel engines. TTY bolts are taken to a certain stretch point during assembly rather than to a final torque value. While this bolt design works well in some applications, the extremely high compression and cylinder pressures of a 6.2L or a 6.5L, especially when turbocharged, dictated that we use a stronger alternative to prevent gasket failure.
ARP cylinder head studs provided this engine with the extra clamping force needed to hold the cylinder head in place under the most extreme conditions without fear of stretching. Again, the benefits of a stud are the same here as in the bottom end. This time, however, the elimination of the twisting forces should prevent cracks in the cylinders just below the deck surface. In conjunction with the studs, we also surfaced the mating areas on the heads and block to make sure that the sealing surfaces were in perfect condition. Finally, we installed Fel-Pro 6.5L head gaskets that utilize a unique steel reinforcement between the fire ring and a large coolant passage at the end cylinders.
The cylinder heads were the final components addressed in making the engine as durable as possible. We chose to use 1998 6.5L cylinder heads. The 1997 and newer GM castings have better coolant flow characteristics and are less prone to cracking than earlier heads. As an extra line of defense, we installed bronze coolant passage liners into each cylinder head. These liners isolate the area between the intake and exhaust seats so that cracks occurring in these areas will not affect drivability. Once installed, coolant should never cross over into the combustion chamber and compression should never cross over into the cooling system.

Caution about Oversized Engine Boring

When we build our 6.2L hybrid engines, we never bore the blocks more than .040 oversize. It is physically possible to bore an early 6.2L block .080 over in order to create a standard bore 6.5L. While I have heard of people trying this, and even have seen some businesses selling these engines online, I am concerned that the cylinder walls become too thin and that the chance of cracking in the cylinders is too great to do this.

It is a well known fact that the smoother an engine runs the longer lasting and more efficient it will be. An unbalanced engine has torsional vibrations or harmonics. These unbalanced forces multiply as engine speed increases and cause not only vibrations, but also crankshaft wobble that damages main bearings. In preparing to build Ted’s engine we balanced the entire rotating assembly down to what are widely considered race engine tolerances. The pistons were taken to within one-half gram of one another as were the connecting rods on both the small end and big end. Then, the bob weights (weights to simulate the actual weight of the pistons, rings, rod bearings, and bearings on each journal), the harmonic balancer, and a new flex plate were installed on the crankshaft.

Block with crankshaft, stud girdle, and gear drive fitted.

The stud girdle kit installed utilizing the six outer main bolt holes where cracks are a common problem.
Reconditioned rods with custom-coated Mahle pistons installed.

The engine with ARP cylinder head sztuds installed.

6.2L diesel crankshaft with bob weights installed during the dynamic balancing process.

Since the GM diesels are externally balanced (they have counterweights on the balancer and flex plate), these external components had to be installed when the crankshaft was dynamically balanced. By removing weight from the crankshaft we were able to take the unbalanced condition of the rotating assembly down to less than 0.30 ounces at 4,000 RPM.
While balancing to this degree would be sufficient for most engines, we decided to take one more step with the rotating assembly in order to make the engine smoother across a larger RPM range. About a year ago, we assisted Horschel Motorsports with the development of a Fluidampr for the GM diesels.
This unit takes the place of the factory harmonic balancer. The Fluidampr uses a viscous silicone fluid and an internal inertia ring to control torsional harmonics across a much broader RPM range than a stock balancer can handle. This gave us the best protection for the rotating assembly.
Another key to a smooth-as-possible engine is to make the compression as consistent as possible on all eight cylinders. The squaring of the deck surfaces and the setting of the rod lengths that I discussed earlier, enabled us to achieve a piston protrusion (the distance the pistons sit below the deck surface at top dead center) within 0.002 inches on all cylinders.
Other factors that play a part in keeping the compression ratios consistent were controlled in the remanufacturing process for the cylinder heads. They were built to our own proven specifications. All valve face protrusions (the distance from the valve face to the flat cylinder head surface) and valve stem heights were set within the tightest of tolerances. This focus on the details assured us that the compression would not vary much from cylinder to cylinder.
In trying to achieve our goal of best-possible engine efficiency, we focused on both thermal efficiency and airflow efficiency in order to optimize wherever possible.
In addressing these issues we were able to utilize several proven modern technologies: specialty engine coatings and extrude honing. The wide-ranging use of these products in this engine is what really set it apart from others. The specialty engine coatings fall into three basic categories:
  • Dry film lubricants – provide a lubricating film that reduces friction and aids in dissipating heat
  • Thermal barrier coatings – internal or external, these thin layers of ceramic are designed to reduce the movement of heat either into or out of components
  • Thermal dispersants - designed to move heat more quickly than the metal surfaces they coat
We used each of these in different areas of the engine in order to control and regulate critical engine temperatures. Here is a list of the coatings and the components to which they were applied:
  • Dry film lubricant (DFL) – Piston Skirts
  • Internal Thermal Barrier Coating (ITB) – Piston Crowns, Valve Heads, Precombustion Chamber Skirts, Combustion Chamber, Exhaust Ports
  • External Thermal Barrier Coating (ETB) – Exhaust Manifolds, Underside of Intake Manifold, Turbo Exhaust Housing, Turbo to Intake Plumbing, Fuel Injection Lines
  • Thermal Dispersants (TD) – Top of Intake Manifold, Top of Intake Plenum, Turbo Center Cartridge, Turbo Inlet Housing
In trying to explain the use of the various coatings, perhaps it is best to take you step-by-step as the airflow travels through the engine and out through the exhaust.

Jamie Avant checks piston protrusion on cylinder #1.

At the turbo inlet, the TD coating takes heat out of the housing and then out of the intake manifold while an ITB coating isolates the lower intake manifold from the heat of the engine below. We are trying to keep the air as cool and as dense as possible while passing through the turbo and intake manifold and on into the combustion chamber. Once there, the ITB coating on the piston dome, valve faces and cylinder head surface blocks combustion heat from soaking into the surrounding parts and the cooling system.
Also, ITB coating on the outer skirt of the pre-cup traps heat in this pre-combustion chamber. This is where the fuel enters the engine and we want this area to be as hot as possible so that the fuel will burn faster and more completely. This combination allows for a hotter explosion that pushes the piston down with more force, creating more usable power. The exhaust valve then opens to expel the exhaust. The ETB coating covering the exhaust port prevents heat from radiating into the cylinder head as the burnt fuel passes by. Finally, the ETB coating on the exhaust manifolds and turbo keeps heat passing through these parts rather than letting it radiate out. This extra heat energy forced through the turbo causes faster spool-up and more throttle responsiveness. As an added bonus, under hood temperatures in general are reduced.

Airflow Efficiency

Simplistically, all engines can be viewed as air pumps. For optimum efficiency, you need to get as much air as possible into and out of the engine as smoothly as possible. As the air transitions from component to component in the intake system and, then, out though the exhaust components, there are irregularities that disrupt and slow the flow through the engine.
We started improving our airflow by gasket matching the intake and exhaust ports of the intake and the manifolds. Then we had every part that has airflow – either into or out of the engine – extrusion honed by Kennametal Extrude Hone. This amazing process pumps abrasive putty, under high pressure, through each part, following the same path as air flow during engine operation. The result is a naturally smart porting job. Following the principles of hydraulics, when this putty encounters restrictions, velocity increases and so does the cutting action of the abrasive. This process reduces restrictions and leaves the ports highly polished. Hopefully, the component photos on the next page help to demonstrate the efficiency advantages gained by this process – it looks even better in real life.
6.2L diesel combustion chamber and exhaust port with TBC applied.

6.2L diesel intake manifold with ETB coating applied.
6.2L diesel precombustion chamber with TBC applied around outer skirt area.
6.2L diesel intake port gasket matching in milling machine
6.5L diesel cylinder head being tested on a flow-bench
Extrude Hone Process Results
Since this was our first time using the Extrude Hone process, we decided to test the heads on a flow-bench. This machine enables you to test the airflow volume and air velocity as it travels through the intake and exhaust ports of the cylinder heads. Tests are performed at different valve lifts. We started at .050 and then compared the performance at .050 intervals up to the maximum valve lift of .450. We compared the flow numbers of Ted’s custom heads to a stock 6.5L cylinder head to see exactly what he gained in the honing process. While I don’t want to go into a great deal of detail here about our flow-bench findings, I will make a few comments. There were improvements in both airflow and velocity across the board. Our biggest gain for the intake port was a 28% increase in airflow volume and a 22% increase in velocity at .100 valve lift. On the exhaust side, our biggest gain was a 15% increase in volume and a 26% increase in velocity at .450 valve lift.

6.5L diesel lower intake

6.5L diesel upper intake plenum
6.5L diesel turbo flange
6.5L diesel left side exhaust manifold. 
6.5L diesel intake port.

Here is a list of parts that were extrude honed:

  • Upper Intake Plenum
  • Lower Intake Manifold
  • Turbo Exhaust Housing
  • Cylinder Heads (Intake and Exhaust Ports)
  • Exhaust Manifolds
Also, we built a custom intake tube to go between the turbo outlet and the new upper intake plenum. This tube gradually tapers in size from the turbo outlet to the plenum to eliminate any bottlenecks in this plumbing, contributing to improved airflow efficiency.
Finally, we installed a Heath Diesel Turbo-Master adjustable boost controller along with the custom GM-8 turbocharger. This will allow Ted the ability to adjust the boost pressure of the turbo. I would think that he will eventually be running around 15 PSI.
The engine is currently in transit from Georgia to Washington. I am hoping that the combination of our intimate knowledge of these engines, a lot of hard work, and some advanced technology have enabled us to produce a truly remarkable replacement engine for his truck. I am confident that our mission to build the most durable, smoothest running, and most efficient 6.5L Chevy diesel engine has been accomplished. Down the road, Ted will be giving us some feedback about the engine and how it performs. I will try to share that information with you. Thanks for following through this one-of-a-kind build with me.
Jamie Avant can be reached at the Diesel Depot: (478) 552-9510 or


  1. Replies
    1. Good luck getting an answer to that. Small fortune just in shipping there.

    2. Good luck getting an answer to that. Small fortune just in shipping there.

  2. What was the names of the various coatings?

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  5. this reminded me of my grinding and polishing the heads , off the 327 motor..guess thats not new....but call old school practice...Charlie

  6. This comment has been removed by the author.

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  11. Why not a liquid intercooler to reduce cylinder temps?

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  14. One of the best articles that I’ve read in a very long time! I Took notes and surely gonna implement and test bunch of stuff you talked about.
    You’re a beast! Cheers, Ash
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