Cracks in 6.5 Blocks

[wdewar]

Why do 6.5 Blocks Crack #7 & #8 Cylinders.

We have found 6.5 Blocks to be cracking in the rear cylinders and pressurises the cooling system.
The cracks seem to be occuring after the engines have travelled 15,000 or more miles.
We are aware that some blocks have a weakness in the top deck area and cant tolerate more than 100 lbs. tension in this area.
Does anyone have a solution to this problem.

[markrinker]

Poor casting techniques in early 6.5 blocks.

[Mainous]

Not sure what year engine you are speaking of but in 1997 GM drilled passages for oil spray jets to cool the underside of the pistons. This created a weakness in the rear cylinders and many crack in the number 4 main journal and it travels up the back four cylinders. I got to experience this first hand. The problem was corrected when AM General took over the engine plant in 1999. The number 4 main bulkhead is now .005 thicker than previous years and the main caps are quite a bit beefier. If you are rebuilding one, look for an "Enhanced Block". You can find threads in this forum that will help you to learn how to identify these blocks.

[Kennedy]

From what I've heard, it is primarily the Hummers and some 99-ish trucks that crack here. The trucks I see cracking in the main webs more.

The later (2001+) blocks have more beef in the deck area to help prevent these cracks.

[Dvldog 8793]

Howdy
I have a very nice "enhanced" block that is doing duty as a card table. It cracked as stated above through the piston oiler holes and up into the 6 & 8 cyl walls. This crack was HUGE, the block was shifted about 1/64in with the main cap still bolted on. The truck was 50k miles out of warrenty but GM stood behind it for 30% (still waiting for the check). I replaced it with a long block made by Navistar and assembled by AM General. When you go block shopping make sure it is a Navistar blockm the logo is visible in the valley. They not only beefed up the block but added some moly and changed the casting process.(stress relieving???)
Hope this helps!
L8r
Conley

[tom.mcinerney]

J. Mainous:
>The number 4 main bulkhead is now .005 thicker than previous years...

.050" ?? Don't see how 5 mills could help!

[tom.mcinerney]

Dewars:
The earlier engines (pre-97) tend to crack main webs. The 97-99(2000??) engines tend to have the web crack extend thru cylinders as you note.

>Does anyone have a solution to this problem.

The best medecine seems to be a new 2001+ Navistar/AMG enhanced block. The next best deal is reputably a (92-96) "599" block, which hasn't yet sufferred web cracking, to be rebuilt.

Two innovations are employed by experienced rebuilders, primarily to prevent web cracking --
1.) A "Stud Girdle" kit. These are machined elements (bars/rods/beams) which interconnect (tie) all the main caps on each side of block, improving the rigidity {decreasing fatigue-inducing flexing} of the web-cap assemblies.
2.) "Splayed Main Caps" . These are aftermarket main bearing caps (central three) which need to have block drilled and tapped to accommodate them; they apply the bearing cap tension to the stronger outer area of block.
It is generally agreed that the 97-99(2000?) blocks have the weakness of the 92-96 blocks, and additional cracking propensity due to piston oil-spray holes which significantly magnify the cracking tendency because the holes further weaken the webs.

John Kennedy of Kennedy Diesel E-mail kennedy@tznet.com http://www.kennedydiesel.com/
Phone: (715) 255-9433 Fax: (715) 255-9499

and Jamie Avant of AvantSalvage avasalv@mylink.net
www.avantsalvage.com/ (http://www.avantsalvage.com/) 478-552-1901 Toll Free: 800-553-8192 Fax: 478-552-6179

are probably the most experienced rebuilders on this board. Kennedy Diesel stocks both girdle and splayed main kits.

[Mainous]

Tom Mac 95,

I'm not an metallurgical engineer so I can't answer as to how much rigidity an additional .005" adds. I just measured it with a micrometer and reported the difference. Then I ended up reading about it on a Hummer link.

Dvldog 8793,

You and I are talking about the same block. When I talk about "Enhanced" block, it's the whole deal. Again, here is the link that I put together for identification as I was dealing with my cracked block:

http://home.cinci.rr.com/mainous/EnhancedBlkID.htm

I'm not exactly sure when the Navistar logo began appearing in the valley but my block was date coded in 2000.

Sorry to have misled anyone.

[Dvldog 8793]

Howdy
The good-old navistar "double-diamond is the best way to ID as my old block had some of the features that Jeff listed but still had the huge piston oilers. I think maybe there was some transition blocks, and it would seem to play right into my luck to get one! :D
L8r
Conley

[tom.mcinerney]

J. Mainous--
Thanks for response. I thought must be typo. Now it sounds like the engineers wanted to insure the thickness was beyond a safe threshold/boundary...

[Kennedy]

http://www.kennedydiesel.com/images/Splayed-caps-Installed.gif

[ace58]

Were all blocks "upgraded" by 2002? I have a 2002 long block that does not have the 506 in the lifter valley. I will see if I can see the Navistar double diamond with the wiring, injector pump, etc. installed or the motor mount boss differences pictured.

[ace58]

I also should have noted that the engine is a van style with the lifter valley turbo set up. Could that be the reason for lack of 506 marking?

[Tough Guy]

I've got the good ole 599 block with over 150,000 miles on it now and still running strong.

Cheers