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GMC 270 Military engine - weak points?

Discussion in 'The Hokey Ass Message Board' started by retromotors, Mar 20, 2010.

  1. retromotors
    Joined: Dec 10, 2008
    Posts: 1,045

    retromotors
    Member

    Yesterday I picked up a GMC 270 inline 6. It's complete but internal condition is unknown ... haven't got to fiddle with it yet.

    A HAMB search brought up some good info on older threads. I now know how to date the engine and where the casting numbers are when I manage to scrape down to them!

    It is a military engine, and has a rebuild tag from the Red River Arsenal dated 1951. (see pics).
    The head casting number, 6107412, puts it in the 1939 - 52 category, so I guess it's safe to assume a late '40s production date.

    A web search took me to several WWII military vehicle sites, primarily dealing with CCKW variants, (the venerable "deuce and a half"), and also the DUKW amphibious vehicles.

    On more than one of these sites, owners are strongly cautioned not to exceed maximum rpms for any length of time, promising dire consequences.
    Max rpms in this case being about 2750 for earlier engines, and a little over 3000 on later ones.

    So my question is ... is there something inherently weak about the military engines as opposed to the civilian versions, and if so, what?
    I would assume (ha ... there's that word!), that the rotating assembly and rods would be pretty sturdy, and that any possible weak points would involve the pistons, or possibly the valve train.

    Any input would be appreciated .... thanks!

    [​IMG]

    [​IMG]
     
  2. THE FRENCHTOWN FLYER
    Joined: Jun 6, 2007
    Posts: 3,895

    THE FRENCHTOWN FLYER
    Member
    from FRENCHTOWN

    Pose this question over on www.inliners.org .You'll get knowledgeable help.
     
  3. retromotors
    Joined: Dec 10, 2008
    Posts: 1,045

    retromotors
    Member

    Well yeah, that's true.
    But I live here! :D
     
  4. krooser
    Joined: Jul 25, 2004
    Posts: 4,584

    krooser
    Member

    Pretty cool to see the Red River tag... they are still in biz rebuilding HumVees and the like.

    I have a source for several of those old jimmy's... been thinking about building a vintage jalpoy racer and a Jimmy would be pretty cool... I'm sure you'll be able to spin that motor faster than 3K... someone will tell you how,
     

  5. Deuces
    Joined: Nov 3, 2009
    Posts: 21,627

    Deuces
    Member
    from Michigan

    Nothing weak about military motors!! Those things are built tough just like our armed forces!!! :)
     
    Last edited: Mar 20, 2010
  6. retromotors
    Joined: Dec 10, 2008
    Posts: 1,045

    retromotors
    Member

    Thanks krooser,
    From what I gather, the 270s & 302s could be made to spin up pretty tight. I'm just wondering what the limiting factor is (if any) on the military version.

    BTW, lest anyone think I'm gettin' too far off topic for the HAMB, this engine is slated to go into my '50 Chevy sedan.
     
  7. Revhead
    Joined: Mar 19, 2001
    Posts: 3,027

    Revhead
    Member
    from Dallas, TX

    does that say .040 on the mains!?
     
  8. retromotors
    Joined: Dec 10, 2008
    Posts: 1,045

    retromotors
    Member

    Yeah, I caught some of that loose flywheel action on some earlier threads. Also, if I'm remembering correctly there were 4-bolt and 6-bolt flywheels?
     
  9. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 5,670

    Ebbsspeed
    ALLIANCE MEMBER

    I have a friend with a 302 GMC in a 50 Chevy Fastback, and he regularly exceeds those RPM numbers. Stock crank and rods, Flywheel is gone in lieu of a flexplate, so the bolt loosening mentioned above probably wouldn't be relevant. Not sure why the warning, unless it is carryover from what the military decided should be the maximum. It would speak to the fact that back then many government agencies did have some regard for how the taxpayers money should be spent, and repairing engines that were abused was not one of them.
     
  10. retromotors
    Joined: Dec 10, 2008
    Posts: 1,045

    retromotors
    Member

    Sure looks like it!
    To be honest, the pic you're looking at is the closest I've come to actually reading the tag.
    Got the engine home after dark yesterday, then sat it next to my 235 Cheby, too close to get a good look at the tag.:confused:
    Just stuck the camera down in there and fired away!
     
  11. gas pumper
    Joined: Aug 13, 2007
    Posts: 2,956

    gas pumper
    Member

    Yellow Coach became GMC in 1943. So your engine would be 1943 or earlier. Probably casting codes will tell you.
     
  12. CONNMAN
    Joined: Jul 19, 2006
    Posts: 1,297

    CONNMAN
    Member
    from Lampe,Mo.

    >>>>>,First ,,,i'm sorry i don't have any pics ,,been too long ,,,,
    back in '59-'60 ,,,my buddy Cal Specht & I put a Military 270 in a '39 Chevy 4 door ,,his daily / hi school driver ,,we used a 3 speed GMC truck flywheel/clutch n trans ,,a "Honest Charley" cam n Kit ,,Hooker hedders ,w' three one barrel carbs,,,spun it to the moon ,,
    then ,,in '60, we took a '31 Full Fenderd Modal "A" 5 window coupe & put a Sprint Car 302 Jimmy Six in it ...w' a Wayne 12 port head ,,Howard Five Duece intake w' 5 Stromberg 97's ,,Mallory Dual Point,,Heddman Hedders ,,hooked to a junkyard Cad LaSalle three speed trans w' a homemade floor shifter w' a reverse shift pattern ,,,,,it was a TRUE "Red Lite Bandit" ,,we'd Cruze Des Moines on Fri & Sat nites lookin' for the Rich Kids in their '59-'60-'61 Corvettes ,,with their pretty little blonde "Honeys" sittin' beside em ,,we kicked their butts ,,,many times Cal and I got the "Honeys" too ,,
    We spun that 302 to over 8,000 rpm ,,that 270 to over 6000 rpm ,,
    those good ole Jimmy Six Bangers will out live YOU ,,with the right stuff in em ,,GREAT MOTORS ,,
     
  13. retromotors
    Joined: Dec 10, 2008
    Posts: 1,045

    retromotors
    Member

    Looks like I'm in luck there. The 235 Chevy next to the GMC is from a '57 Apache truck. Appears to have the same transmission bolt pattern as the Jimmy (haven't verified that, though), so that would give me an open driveline option.

    Maybe the bellhousings even interchange? I need to start pulling parts and measuring.
    I'm kinda stoked about the 270 engine ... always like to be different!:D
     
  14. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,280

    Truckedup
    Member

    The bellhousing will interchange,but a 235 flywheel isn't even close to fitting on a GMC.
     
  15. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 5,670

    Ebbsspeed
    ALLIANCE MEMBER

    It's got a GM 700R4 behind it.
     
  16. krooser
    Joined: Jul 25, 2004
    Posts: 4,584

    krooser
    Member

    Yep... not unusual in older engines to see rods and mains cut .050 under...
     
  17. RichFox
    Joined: Dec 3, 2006
    Posts: 10,020

    RichFox
    Member Emeritus

    The early engine may have a 4 bolt flywheel. Mine did. I drilled the crank for the 6 bolt pattern and used a Chevy bellhousing and a 12 volt GMC flywheel and starter. Ran up to 21 pounds of boost and mostly 17 lbs. Went 158 mph in the shown car in '78. Was still running in '98 when I sold the car. (GMC went one way, car another) Is still a record holding El Mirage motor. 270 military motors weak? I don't think so.
     

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  18. krooser
    Joined: Jul 25, 2004
    Posts: 4,584

    krooser
    Member

    How good are the 228/248 engines? There's a '52 Jimmy nearby that's for sale...
     
  19. ironfly28
    Joined: Dec 22, 2003
    Posts: 969

    ironfly28
    Member
    from Orange, CA

    you should be fine this is a not really for the engine so much as it is to prevent an over speed of the transfer case on the early war trucks with banjo axles. I guess given production demand they had some issues getting the high range to run at 1:1 instead it ran at 1:.690 (note the point) so too many revs made stuff break
     
  20. ironfly28
    Joined: Dec 22, 2003
    Posts: 969

    ironfly28
    Member
    from Orange, CA

    and by the way ALL military engines warn of over revving...it's good not to be too hard on something that can bring you back or push you through the fighting.
     
  21. Kinky6
    Joined: May 11, 2003
    Posts: 1,765

    Kinky6
    Member

    You could figure a 228 or 248 to be just as sturdy as the 270 or 302, but these engines would cost you even more to rebuild, in terms of parts and machine shop work, than a 235 or 261 Chevy, with no advantage in cubic inches.

    The 270 or 302 would be worth your while due to their larger displacement and sturdier construction than the Chevy engines, but if someone offered me a 228, I would try and buy it cheap, then strip it for parts. Some stuff would make good spare parts for a 270 or 302 (sheet metal, push rods, valve train), and the connecting rods could be used to build a stroker version of a 235 or 261. The width would have to be shaved down to match the Chevy rods, but they are much sturdier. This would also require cast or forged pistons for a stroker Chevy, but you would want to replace the cast iron pistons in the larger GMC's, as well.

    Later, Kinky6 :cool:
     
  22. krooser
    Joined: Jul 25, 2004
    Posts: 4,584

    krooser
    Member

    The guys got the whole '52 1/2 ton complete and it ran a few yaers ago... I was just wondering if it had insert bearings and other more modern parts like the bigger ones...
     
  23. povertyflats
    Joined: Jan 8, 2007
    Posts: 8,283

    povertyflats
    Member
    from Missouri

    I have a couple of 270's----I like them too.
     
  24. BIG PORT JIMMY 6
    Joined: Dec 7, 2004
    Posts: 333

    BIG PORT JIMMY 6
    Member

    My 302 will turn 5500 but likes 5000 alot better, it has a fair amount of work in it. Light weight pistons .125 over and some head porting. If I take it back down I would like to try to lighten the crank and rebalance. Mine is a late 56 or 57 civvie block. I also have a 53 military 302 as a spare. Enjoy the Jimmy. Jay
     
  25. motion guru
    Joined: Oct 18, 2009
    Posts: 159

    motion guru
    Member
    from yacolt, wa

    My 270 has lightweight forged pistons in it and dual Zeniths on a Nicson intake and dual exhaust, porting, oversized valves on a 270 small port head w/ 9.5:1 compression and it runs smooth to 4200 rpm before it starts wheezing for air. I have a 3-carb intake ready to go on it to see if I can coax it to 4800 rpm tops.

    8000 rpm = you better have your scatter shield in place!
     
  26. old bone
    Joined: Mar 5, 2008
    Posts: 312

    old bone
    Member
    from maine

    i have a friend ..a fellow hamb r' who has this motor...so will the 235 flywheel work?...what is a readily available trans. for these motors....sam where are you?..these guys are talking about your engine
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  27. RichFox
    Joined: Dec 3, 2006
    Posts: 10,020

    RichFox
    Member Emeritus

    No deal on the 235 flywheel. You need a GMC flywheel
     
  28. krooser
    Joined: Jul 25, 2004
    Posts: 4,584

    krooser
    Member

    You have a couple of everything!!!
     
  29. retromotors
    Joined: Dec 10, 2008
    Posts: 1,045

    retromotors
    Member

    I'd be interested in hearing the answer to that, too.

    Also, I wasn't aware that a 235 bellhousing would accept a T-5. Any particular year bell? How about a '57 truck bell?

    Guess I need to go back and read the T-5 and 235 threads again. It's been awhile and those threads almost offer too much info ... almost an overload!:D
     
  30. RichFox
    Joined: Dec 3, 2006
    Posts: 10,020

    RichFox
    Member Emeritus

    I bought a Billet flywheel as required by the SCTA for what I was doing. GMC dosn't have the same wierd rear seal as the Chevy and I really think I remember trying a Chevy wheel and it didn't work at all. Remember we are talking 30+ years ago.
     

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