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the cure to the uncommon bore.

Discussion in 'The Garage' started by colbystephens, Jul 14, 2005.

?

Should I...

  1. Use oversized piston rings.

    3 vote(s)
    15.8%
  2. Purchase the proper size pistons.

    16 vote(s)
    84.2%
  1. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    so i'm rebuilding a 6.2L diesel motor out of my '83 k5. i recently purchased it for 500 bux and am doing a complete rebuild and build up. anyway, today i was measuring the bore of the cylinders, and found that someone who previously owned the vehicle bored the cylinders .030 over (which makes for 384 ci) to 4.008, but the pistons are only at 3.985. This makes .023" of room for the pistons to play in the cylinder. My diesel manual allows for only about .004" of play - this is a huge difference. The motor had proper piston rings so that it held compression and whatnot, but there was a slight bit of wear on the piston skirts - not much, but some. My uncle, a mechanical engineer, said that it was minimal wear and that he wouldn't hesitate to just get a new set of the proper rings and use the pistons again, but I'm always interested in other opinions. :) So what would you do? Let me know your reasons and suggestions! Thanks for all your help!
     
  2. Big Red

    Big Red 1/2 ton status

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    Been a while since i last posted, but here goes... starting with your measurement of the bore, you must measure in three places. (IMO that is) close to the bottom of the bore, mid-bore, and of course, just before the ridge. Now, assuming you did that, coming out with a 4.008" bore is indeed an odd size, but remember its not far from a 350 chevy size. (not saying ya go swapping 350 pistons in there!! :haha: ) That being said, i would say that at least one company out there has made oversized pistons for it, probably a compan like JE or Federal Mogul. And after some research @ northernautoparts.com, they list Federal Mogul Pistins in .030 Over sizes w/ matching rings. (still working on actual part numbers, gimme a wee bit to find them for ya. dang ol federal mogul site is a pita to search thru) As of late i have become a firm believer in never doing things half-heartedly after seeing me and my friends trucks motor chew itself apart. (we don't gotta worry bout bearing sizes he said. and me being to eager to get it done wne t along with it, uber doh there) NA Parts list a master kit for just over a grand. Im sure AAP carries a similiar kit for less. If not, I know they can get you the main and rod bearings, as well as the pistons/rings/gaskets that are necesary to do this well. believe me when i say doing the job half way isn't worth it. Good Luck!!!
     
  3. royjones

    royjones Trucker upper Premium Member

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    Hello iwas wandering i have'nt heard anything good about these engines even with a turbo . could you inlighten me on why. I have changed a smoker over to gas. and the gas motor had more power . :doah:
     
  4. Kiwi John

    Kiwi John 1/2 ton status

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    Not Boring

    I'd take notice of Big Reds advice. I'm not up to speed with the bore measurements as such but here's how i see it.
    It's a diesel engine and that relies on compression for combustion. The more u squeeze, the bigger the bang. So you want to fill the holes in your block as fully as possible with minimal chance to let gas leak by. It follows therefore that you should have the best fitting pistons you can, within manufacturers specification, to hold the piston rings in place properly. The compression ratio for the diesel engine is, on average, 3 times higher than a petrol engine. There's an awful lot of pressure inside and you dont want pistons slapping around in your bores or knocking the conrods, crankshaft etc. Build it properly like Big Red says and you'll minimize the chance of engine failure.
    I've got the 6.2 in my '83 K5 and i love it. Good Luck.
     
  5. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    My decision: Replace the pistons.

    Here's a piece of info which was not noted in anyone's posts incase anyone is interested. This actually plays into my decision to go ahead and replace the pistons. In asking around about this whole deal I learned something I had not considered. I was not too concerned about the pistons wearing away since, but I learned that if you have too much piston slap you have the likelihood of wearing the cylinders out of round. It was referred to as ovaling. Don't know how technical that term is, but it makes sense. Anyway, just for anyone who might care.

    Colby Stephens.
    www.web.pdx.edu/~colbys
     
  6. dave_90_blazer

    dave_90_blazer 1/2 ton status

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    just FYI, most of the top name piston manufacturers awill make you custom pistons, any size you want(within some limits, but that should be fairly easy for them
     
  7. big_truxx

    big_truxx 1/2 ton status

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    because some people are opinonated sob's? :D :dunno: :haha:

    seriously I own an 89 6.2 powered suburban w/ a th400 even and I do notice more torque in the seat of my pants dyno than my dad's (used to be mine) 82 crew dual w/ mr goodwrench 350... granted the 350 will get you to speed faster but the 6.2 will outwork the 350. also getting 18mpg w/ burb and 10 w/ crew dual. both having 3.73's ;) the 6.2 is a great motor and you try out a turbo w/ propane injection in a crew dually sometime hauling a nice trailer with a good load up a hill and then come back and tell me there is nothing good about em ;) :waytogo:

    now that we are done jacking this thread i would think we should start a part # list and info post somewhere for the 6.2. obviously in the diesel section. lol it would be so handy as a resource to have.

    and as a side note I am jealous that you bought a 6.2 k5 for $500 :D :waytogo: grats! I will have me my M1009 next month so I will be less jealous then ;)
     
  8. project84k5

    project84k5 Registered Member

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    sorry about the jack, but my blazer has the 6.2 and i would never put anything else in it. it is an awsome motor and has never let me down, it has never died on me and you cant beat the fuel mileage off road!
     
  9. dirtwarrior17

    dirtwarrior17 Banned

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    The 6.2 has less hp than a 305 and a little less tq than a 350 but like big truxx said it will pull a little harder than a 350 when it has a load on it.
    If you want it to be worth your while you have to turbo it and inject propane... But then again, put a turbo on a 350 and see how it pulls.

    Back to Original post...

    either you didn't measure right or those are some very rare piston rings. . If they were made for an .030 over piston then there would be alot of room for the ring to move around on the stock piston. Either way get some new pistons.
     
    Last edited: Jul 17, 2005

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