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Thread: Preventing precup cracks, or dumb idea?

  1. #1
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    Default Preventing precup cracks, or dumb idea?

    Hey everyone, I just recently acquired a new set of 6.2 3 dot Inconel precups. After the initial magnet test and thorough inspection I noticed the edges of the prechamber opening are razor sharp. I've never seen a brand new set before, so this got me thinking... is it possible these sharp edges are part of the reason these cups crack so frequently? From what I've heard cracks like this are somewhat normal for diesel engines in general, and minor cracks aren't considered too big a deal, as long as they don't extend into the fire ring. That being said, would it be worth a try to use something like a small jewler's file or other abrasive to take the sharp edge off and eliminate the possible stress riser? Nothing crazy, just enough to soften them up a bit. Not sure if this is wishful thinking or if it's just a flat out stupid idea that will cause more harm than good. Any thoughts? I'm no expert in the area, so as always I appreciate any and all input.
    Thanks in advance!

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    The only concern, I think, would be flow characteristics. I don't see that smoothing edges of the opening would change that any, or enough to really matter. I'd probably do it. I don't recall what a new set looked like, other than normal, but I don't recall seeing any with sloppy edges like yours. I think I'd notice. That said, most all high-mileage precups I've seen had cracks, and I've never had to discard any due to flash-hole cracks.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  4. #4
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    Yeah, I thought they looked a little off... I might try running a deburring tool lightly over the edges that need it. 6 out of 8 of mine from the engine I'm currently rebuilding look pretty good, most have cracks but nothing over spec. The two in question were beat up pretty bad from some debris in the cylinder. Ended up getting a new set just because I wanted them to all match. Wasn't sure if they all looked like this new, maybe quality control was out to lunch on this batch. Thanks for the advice, I'll go easy on them!

  5. #5
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    Those cups are very hard. I don't think I'd go too easy on them. I've had to clean up burrs on the skirts a time or to, and I think they were harder than my file. I'd use a Dremmel, and start with a small dia. fine drum or jeweler stone.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

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    Would you recommend a silicon carbide or aluminum oxide bit for Inconel?

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    Either would be fine. I'd start with whichever is finest.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

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    A stone could work or it could load up, but I think I'd try using a carbide Dremel bit. Don't need to take much off. The info about "if the crack doesn't extend to the fire-ring it's OK" came from GM during the research I did about cup cracks back in the late 1990s. Generally, if a set of cups need to be replaced because some of the cracks extend past the fire-ring, there is usually other damage due to excessive combustion temperatures (i.e. piston scoring, head cracks, etc).

    As a sidebar... we're seeing more chinese heads and head parts nowadays. Seems most of the cups associated with these are magnetic, where the genuine cups aren't. So, there is a definite metallurgical difference... Does that make a difference? I dunno...

    Jim

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    Didn't know you were involved in that, pretty cool stuff! Would you say the smaller, less concerning cracks tend to stay put if not in a harsh environment? Or do they continue to creep toward the fire ring once started?
    I'm not very well versed in metallurgy, but these cups are non-magnetic. I'm assuming this means they are Inconel, but I have no way to actually verify this. I was burned once once by a cheap eBay set... They were listed as GM NOS, but a magnet stuck right to them. Luckily I was able to return.
    I'll try out the carbide first and see how they turn out. Thanks for the input!

  10. #10
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    Quote Originally Posted by alanjd93 View Post
    Didn't know you were involved in that, pretty cool stuff! Would you say the smaller, less concerning cracks tend to stay put if not in a harsh environment? Or do they continue to creep toward the fire ring once started?
    I'm not very well versed in metallurgy, but these cups are non-magnetic. I'm assuming this means they are Inconel, but I have no way to actually verify this. I was burned once once by a cheap eBay set... They were listed as GM NOS, but a magnet stuck right to them. Luckily I was able to return.
    I'll try out the carbide first and see how they turn out. Thanks for the input!
    This: "cracks tend to stay put if not in a harsh environment..." (exposed to a worse environment than it's already seen)

    Usually, "non-magnetic" can mean the alloy is some form of non-magnetic stainless steel. There are varieties of stainless that are magnetic and some that aren't, depending on the alloy.
    Some of the most common are type 304, type 316, type 410, and type 430.

    https://www.globalspec.com/learnmore...s_steel_alloys

    Today, you'll find diesel exhaust systems advertised as "stainless", but it'll still rust. The best stainless is very resistant to rust, but it costs more. Who knows what the cheaper import 6.2/6.5 diesel precup stainless is...

  11. #11
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    There are different grades of Inconel which are high temperature, exotic alloys used in the hot section on gas turbine engines. Turbine wheels and blades are made from it in some cases. I'm currently working on reverse engineering a large (50 MW) industrial gas turbine that uses Inconel 718 for the low pressure turbine rotor shaft.

    The 400 series stainless steels are magnetic. I have engineered magnetic carbon seals into helicopter engines before. The rotating seal ring which supports the carbon is made from 420 stainless. The magnet is the stationary ring and magnetic attraction is what provides the interface force.

    300 series stainless steels are not magnetic. Some of these are difficult to machine.

    The precups are made from Inconel castings to help handle the temperature that they endure as they have better high temperature strength retention than other materials. Yet, they still crack. This is most likely due to low cycle fatigue due to accumulation of heat up-cool down cycles each time you start then stop the engine. Rounding the edges will likely help.

  12. #12
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    Wow that sounds fascinating! Also way over my head... Honestly this is my first diesel rebuild, I mainly work on gassers. Didn't even know what a prechamber was tbh until I tore into this and noticed something was wrong. Now I know more about them than I ever would have imagined.
    Turbine engines are something else. I saw the old Firebird 1 prototype from the 50s when I visited the GM heritage center a couple years ago. That thing was insane! Would've been wild to have been on that project.
    Thank you all for all your expertise!

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