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Main cap girdle question...

Discussion in '6.5 Performance' started by great white, Jun 16, 2010.

  1. great white

    great white Well-Known Member

    So, it appears all the 6.5 main cap girdles are of this sort:


    where as I've always used (or seen) girdles similar to these on gasser builds:




    It seems the 6.5 girdles offered are not very robust, only tying the 3 per side outer studs together. The others I've shown would seem to offer much greater lower end strength and stability.

    Why the diff?

    Is there limited room in the pan? Or is it the "kick up" at the timing cover, which makes it an odd shape? or the rear seal/cap area?

    I would think even a girdle like the "donut" style ones above would offer better stability that the available 6.5 ones, even if not tied into the block skirts/pan rail like the Mopar one above.

    I don't think they would do much for the bolt hole cracking, but I would think it would prevent the caps from walking around in the cap register and cracking the web in that area.

    Hmm, anyone on Vancouver Island have a trashed 6.5 with the cover and pan on it I could use as a "mule"?

    Might try my hand at building a better girdle....

    Yeah, i know, that sounds kind of funny doesn't it?

    Last edited: Jun 16, 2010
  2. Woody35

    Woody35 Active Member

    Apr 2, 2009
    Chi-Town Suburbs
    theres some people on here currently trying to design a better girdle. im anxiously waiting for there results
  3. ak diesel driver

    ak diesel driver 6.5 driver

    Feb 7, 2009
    there's been some debate as to whether those flimsy girdles do any good at all. chevyinline6 has the plans all drawn up for a girdle which looks similar to the p400. Bison is also playing with one. I was kind of toying with the idea of a structual oil pan
  4. Turbine Doc

    Turbine Doc Just Another Diesel Guy

    The 1st one has been around a while, & not one I'd use, the latter full piece is way I'd go if adding a girdle, IMO most 6.5s don't need either style as they aren't pushed to higher power levels.
  5. great white

    great white Well-Known Member

    I would think the greater benefit for a 6.5 would be another stress loading path in an attempt to prevent cracks in the main cap register area on the web....
  6. Turbine Doc

    Turbine Doc Just Another Diesel Guy

    I hear ya GW and I'm a little mixed on the idea of the cause of cracks in the 6.5 and what we are attempting to fix with a girdle. I'm of the opinion if a block has never been overheated, the injector balance maintained with healthy injectors, and HB kept healthy the 6.5 will survive as is for most 6.5ers out there.

    Up the ante with more power demand without preparing the engine to take on extra heat load and things turn bad.

    One of my 6.5 mentors GMCTD had an additional theory/observation about block bore consistency, if you look at some of the failed blocks some of the cyls appear to be cut deeper into the web risers than others, and this delta, side to side cylinders sets up an uneven thermal growth and causing thermal cyclic fatigue over time.

    There may be some merit to that as P400 latest & greatest adds more metal to it, to provide endurance to the Hummers, what caused Hummers to "go bad" all of a sudden & need a beefy block.

    IMO "up-armor" added weight on a vehicle designed for high mobility (Lightweight) now being tasked to to heavy work in a hot ass place with the same cooling system set up for it when it was a high mobility (Lightweight vehicle).

    Our trucks if kept to the duty they were designed to run at should be okay, but no it's a Diesel so it should be able to pull a house right ??? yes they can pull more once set up for it. How many posts back in early days before forums such as this were there horror stories of overheating trucks, billowing black smoke.

    We now know radiator fins have to be clean so air can flow thru them, when vac pumps/solenoids fail you have no boosted air to keep the smoke ay bay, black smoke = high EGT and EGT need to be monitored, high IAT limps the truck and cuts fuel but you still try to bully your way up that mountain side dragging that 30' trailer causing even more over heat issues.

    That is TD's wholly opinionated observations of 6.5 life, girdles have to add strength if a full on girdle of that I have confidence, flat bar ??? ehhhh I'm not impressed, does every 6.5 need to wear a girdle :thinking::thinking::thinking: I don't know.

    If I was in rebuild mode and a affordable full piece was available I might consider it, I went with scat crank-splayed mains-and fluid damper after doing a blueprint/balance just because, on my project engine. Did I over build who knows but it is a better one than what GM put in the truck, that said the original engine in it now is a "at risk squirter" is now @ 213K on the odometer
  7. great white

    great white Well-Known Member


    I often wish manufacturers gave EGT ratings for reciprocating engines for automotive use.

    When running turbines on the test stand and operating them in Aircraft, we use EGT for everything from heath monitoring to power production.

    That. and oil pressures and temperatures, are the only real indications of the engines heath and usage.

    Obviously, anything operated outside of it's design specs will be stressed and fatigued to the point of failure. Aerospace components are designed to have an "ultimate failure point" in excess of the maximum usage on the gauge. We operate military equipment (Mil Spec) so the manufacturer must release to us where the failure point is by the gauges and how long it can take it and what the implications of it are. I doubt there's much of an "excess of capacity" in most automotive designs, nor would they tell you how long it could survive such abuse. Probably just enough to survive a bit of abuse during the warranty period but not much more...no way the OE's would ever want (or let) that info outside the corporate doors......:rolleyes:

    Which is not to say design and manufacturing defects (core shift, poor tooling, casting irregularities, etc) won't also bring about a premature failure. Mass produce X hundred thousand of anything and there's gonna be some duds....

    The benefit I see in a girdle is to assist in distributing the side loading on the cap registers as a result of crank "rocking" in response to power pulses. Instead of feeding the pulse into the one main cap, web and register the load is somewhat spread about the entire web area of the block. The register is essentially a "stress riser" due to the 90 degree machined surface and as such, highly susceptible to stress cracking failure. In my mind, anything that reduces stress in the area (at all) is a good thing.

    As you mention, heat (and operator misuse) is probably a big factor here too. The block is subjected to faster heat swings than the cap material. I'm theorizing here now, but the expansion rates of the block and caps are most likely different due to their composition and location. This most likely leads to stress on the registers as they try to compress the main cap and the only option they have in a rapid heat gain situ (towing, foot to the floor, ignoring the rolling coal) is to relive the stress by cracking the register at the weakest point:

    The machined 90 degree stress riser, also known as the main cap register.

    Just a theory though...
    Last edited: Jun 17, 2010
  8. uniquediesel

    uniquediesel 1/4 mi. Fan

    Jun 15, 2009
    I think the girdle helps, as shown above.

    That's what I've run since Day 1 in the race truck, it hasn't broken yet.

    one of the biggest problems with girdles, especially those pictured on gas blocks is that you're simply betting that the height of the caps are all the same. Different cap heights on the pad where the girdle mounts will cause more problems than any added strength from the girdle.

    You'd have to carefully fit and check for straightness when installing a girdle.

    The DSG one is small enough to not cause any major alignment issues. By alignment issues I'm mainly concerned with altering the final align bore of the block.

    Just splay it and be done with it.
  9. Turbine Doc

    Turbine Doc Just Another Diesel Guy

    That was the direction I went after MUCH thought on it, I filled the old outer bolt holes with a bolt and milled flat before doing the splay bores. IIRC one of the 6.5ers got a block with no drilled outers then splayed it, that would be a solid combination.

    I'll look around to see if I still have the pics of the build of mine as it happened, if not there is a post on it at the old site somewhere I'd link it but I forgot my pw for over there to go find the link to it.
  10. great white

    great white Well-Known Member

    I would be interested in seeing those pics if you find them....:thumbsup:
  11. Matt Bachand

    Matt Bachand Depends on the 6.5

    Jun 10, 2008
    Worcester, MA
    GW you have a gift of finding pictures on the web :) Thanks for sharing.

    Also your heat cycle thought,

    "As you mention, heat (and operator misuse) is probably a big factor here too. The block is subjected to faster heat swings than the cap material. I'm theorizing here now, but the expansion rates of the block and caps are most likely different due to their composition and location. This most likely leads to stress on the registers as they try to compress the main cap and the only option they have in a rapid heat gain situ (towing, foot to the floor, ignoring the rolling coal) is to relive the stress by cracking the register at the weakest point:"

    That is the reason I think the 190/195 T-stats are better than the 180's to avoid the additional heat swings, and more importantly, less dramatic heat swings. I think over time, cycling between 192 and and 210 is alot less fatiguing on the block, than cycling between 180 and 210.
  12. great white

    great white Well-Known Member

    It's just plain ol' google images search....:thumbsup:
  13. Turbine Doc

    Turbine Doc Just Another Diesel Guy

    If you are into searching much as I hate to plug that site, the old site has the photos and discussion where I posted the start of the project engine build, alas my pictures of the build aren't with me they are in Ms on my home computer, I won't be home until 4 July weekend, so if you don't find it over there, we will have to wait until then before I can search my home computer.
  14. bobbiemartin

    bobbiemartin Author of The DB2 Conversion Guide

    Sep 25, 2009
    Jacksonville, Florida
    I hate to differ with you Tim, but on our 1995 Suburban the main webs were cracked all the way through at 180,000 miles. This was a non oil squirter block. You would have to look pretty hard to find a 6.5 that led an easier life than that engine. We are the original owners and for most of its life, it was the wife's daily driver. I can assure you it never overheated and had regular maintenance along with periodic oil samples to monitor the condition of the engine. We very rarely towed anything and when we did the trailer weighed less than 3500 pounds. It was basically used as a station wagon. So while overheating and heavy use may take its toll, its certainly not the only cause. I think some (most?) blocks are just prone to cracking and there is not much you can do to stop it. I don't think the DSG girdle will help, in fact I don't see how you can maintain proper torque on the main cap fasteners if you use it.

    BTW, the oil analysis showed high levels of iron and chromium. The lab thought it had a broken piston ring but they were all good. They had no explanation for the high chrome, so I guess it came from the crack surfaces wearing? I sold the lump to my machine shop. They told me everything except the block was usable and the crank only needed a polish - it was fine.

    Those cracks went all the way through the webs. Other webs were cracked as well, but this one was the worst. I'm sure it would have made a big boom if we had kept driving it.
  15. Turbine Doc

    Turbine Doc Just Another Diesel Guy

    NP Bobbie to the disagreement I certainly don't have all the answers, GM apparently didn't either as even the vaunted Dmax has it's issues.

    A Friday cast block ??? guy ready for the end of the week, or a monday pour and operator is still hung over :D

    As with any casting process there will also be some bad cast, maybe you got one of those; I think we'll never know THE Cause/s for the 6.5's that have failed, yours is a 95, GM made changes to cooling flow and added squirters later on down the line for some reason, I have to suspect it was temp related.

    Some say the squirters put in by GM for piston cooling hurt the webs, possibly; thus far mine with all the extra capacity & cooling I have added beyond what GM did I'm still okay, your block pic non squirt looks interesting, pics of bad squirt blocks I've seen; crack in a different area of the block, I suspect GM fixed one fail mode and maybe moved it to a different mode/location.

    Temperature stress/creep is a theory of mine, an opinion worth what you paid for it ):h

    My generic comment to the overheating and beating is really more directed to possibly of ones that were borderline from being cracked or not; that may have held it together were they treated to a kinder/gentler life such as your yet it still failed.

    Problem is with the span of years and sample variation due to as built/sold configurations across the years.

    I don't think we can make apples/apples comparison to nail down the root cause. Some could be batch lot casting related, which then drove GM to come up with a fix that created a different fail mode.

    It happened more than one would imagine/prefer when when I was a field engineer at GEAE; what worked in flight engines did not work so well in the land based versions, quality control for flight sometimes more stringent, things were at times for better production cost things eliminated; and maybe deemed not to be necessary because it wasn't in flight biting us in the behind later.

    Some examples I remember vividly we could have premature turbine blade fails from something as simple as not holding consistent oven curing temps over time in the life extension coatings application phase. Or the guy that used to QC the correct wire being used in LPT (low pressure turbine) case fabrication retired, his job no longer needed, we never have had a fail in those we don't need to replace the headcount.

    Our LPTs were built 2-3 yrears in advance of going into service, so it took about 3 years for his absence to be noticed by fails in the field, that wire was important as it controlled the thermal growth of the cases, every 2" was a segment weld basically a "hard weld" adjacent to a "soft weld" so the case growth would be at a controlled rate that would not crack the case.

    IIRC there was a lot of "bad" cases for about 200 engines (big $$$ to correct) worth of production, yet same design engines before and after that batch/lot were fine. Different issues, but that problem that made them all "suspect" did not show up in all engines as THE problem.

    It's far too easy to broad brush what went wrong here, and apply to there so you are very much welcome to disagree with my hypothesis.
  16. Turbine Doc

    Turbine Doc Just Another Diesel Guy

    I did a bottom end inspection of mine @ 100K and noticed no cracks, I'll be interested this fall for what I may find @ 200+ K, as there is one school of thought that a "seasoned" block if it made it to 100K should not crack, yours did not fit that assumption so it will be interesting,

    Since I'm 1400 miles from home until middle of Aug I'm praying each day that I'm not added to the statistical pool next time I go to pass somebody.
  17. great white

    great white Well-Known Member

    I've been toying with the idea of pulling the pan off my block and having a look.

    And the original squirter block was swapped out for a "seasoned" 599 block by a PO.

    But, I'm also sort of thinking I'd rather not know.........:ninja:
    Last edited: Jun 25, 2010
  18. ak diesel driver

    ak diesel driver 6.5 driver

    Feb 7, 2009
    I've been toying with the idea of freshening mine up but like you I'm not sure I want to look.
  19. great white

    great white Well-Known Member

    Yup, ignorance is bliss...........sometimes.

    Last edited: Jun 25, 2010
  20. bobbiemartin

    bobbiemartin Author of The DB2 Conversion Guide

    Sep 25, 2009
    Jacksonville, Florida
    Good points Tim. I was just pointing out that any block can crack, not just the abused ones.

    Interestingly, there were no outward indications of the cracks in the block. It ran great, no funny noises, no oil in the coolant or vice versa. A little smoke when you started it up, but that was about it.

    The only clue was the elevated iron & chrome in the oil sample. I even send a second sample in several thousand miles later to rule out that maybe my sample got mixed up with another one or contaminated or something like that.

    Without the oil samples to alert me that something was going on, we probably would have driven it until it let go (in a big way I would bet).

    I really thought it had a broken piston ring, still haven't figured out where the elevated chrome came from.
    Last edited by a moderator: Jun 25, 2010

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