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Technical Is this a big bore or small bore 6-71

Discussion in 'The Hokey Ass Message Board' started by 64 SS, Nov 2, 2015.

  1. 64 SS
    Joined: Sep 10, 2012
    Posts: 97

    64 SS
    Member

    I picked up this little jewel not really knowin alot about it. I've done some digging with conflicting information on the large and small bore. The thickness on the inlet is .80 and it has the 3 supports across top with just the centered one machined down. Anyone??
    [​IMG]
    [​IMG]
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 49,836

    squirrel
    Member

    the small rounded cross supports are what the larger bore cases had, according to one source I have. Kind of hard to measure the inside diameter of the case bores when it's assembled, but the large ones are 5.778" and small are 5.505"
     
  3. 64 SS
    Joined: Sep 10, 2012
    Posts: 97

    64 SS
    Member

    Gotcha! Thanks
     
  4. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 8,924

    mgtstumpy
    Member

    Well you learn something everyday, I had a 6-71 many years ago that I sold with that cast webbing on the inlet. I had no idea there were small and large bore cases.
     
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  5. wrenchbender
    Joined: Sep 5, 2007
    Posts: 2,142

    wrenchbender
    Member

    That appears to be a small rotor case the cross bars have nothing to do with whether it's a small or large case but if you look at the top of the second pic in the middle of the opening there is a hump that protrudes down between the rotors if that has not been machines off flat that is a quick way to tell if it is a large rotor if it's just a cast finish it will be a small rotor
     
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  6. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 8,924

    mgtstumpy
    Member

    A quick Google established that there are two different 671 blowers commonly used, case depth measured at inlet:-
    1. Large diameter with 1/2" case thickness (11% more volume); and
    2. Small diameter with 3/4" case thickness.
     
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  7. 64 SS
    Joined: Sep 10, 2012
    Posts: 97

    64 SS
    Member

    Is that a big performance difference? This will be going on a sbc street car. Thanks for the info!
     
  8. squirrel
    Joined: Sep 23, 2004
    Posts: 49,836

    squirrel
    Member

    It would make more a difference on a big block car, since a 6-71 is a tad large (or just right, depending on how you look at it) for a small block street car.

    Does the blower case have a casting date on the bottom?
     
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  9. 64 SS
    Joined: Sep 10, 2012
    Posts: 97

    64 SS
    Member

    Casting number
    5155866 demi 12 r 8
    No date on casting
     
  10. 64 SS
    Joined: Sep 10, 2012
    Posts: 97

    64 SS
    Member

    The few I did find with the same casting number said they are a big bore.
     
  11. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,474

    landseaandair
    Member
    from phoenix

    Could be wrong, been a while since I studied up on it. This is what I use as a quick identifier of a small rotor case. The recesses on either end always seem to be present from my experience. Filled on big bores.
    s-l1600.jpg
     
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  12. 64 SS
    Joined: Sep 10, 2012
    Posts: 97

    64 SS
    Member

    Ok.... I spoke with a couple blower shops and they said you can measure from the top op the rotor to the top of the blower case, 3/4 is a big bore 7/8 is a small bore. I have the large bore.
     
    Last edited: Nov 3, 2015
  13. 64 SS
    Joined: Sep 10, 2012
    Posts: 97

    64 SS
    Member

  14. 64 SS
    Joined: Sep 10, 2012
    Posts: 97

    64 SS
    Member

    [​IMG]
    A guy from the blower shop said if you can fit a 7/8 wrench on the case, like so, it's a large bore.
     
  15. From all the ones I've had, I believe the casting numbers that end in 866 and 715 are big bore. A casting number the ends with 725 is a small bore. The real way to tell is to measure across the opening with it disassembled. Side to side across the bores. I can't remember the dimension off the top of my head.
     
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  16. GearheadsQCE
    Joined: Mar 23, 2011
    Posts: 2,804

    GearheadsQCE
    Alliance Vendor

    Bringing this back up as I have two 6-71s to pic from. Looks like the wrench test is valid. I also measured the thickness of the top flange, 1.024" on the green case and .891" on the sort of polished one. Calculating the difference (.133"X 2) gives a total of .266" difference. Using Squirrel's rotor diameter numbers of 5.778" and 5.505" the difference is .273". Close enough for the girls I go with.

    I also have a 4-71 case with the rotors out. Will measure it tomorrow. I know it is prior to 1978 because I bought it in 1969 and it wasn't even a little bit close to new. Don't know if there was a small bore 4-71 or not.
     

    Attached Files:

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  17. Boneyard51
    Joined: Dec 10, 2017
    Posts: 5,187

    Boneyard51
    Member

    Do these things have any value?I’m fixing to junk a truck with a 8/71 Detroit in it. Would it be worth my time to remove the blower, the truck ran good when parked!” Lol!






    Bones
     
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  18. Depending on the condition upwards of $500 for a diesel 6 or 8-71 non V
     
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  19. Boneyard51
    Joined: Dec 10, 2017
    Posts: 5,187

    Boneyard51
    Member

    I didn’t know they made an 8/71 that wasn’t a “ vee”
    Just my luck mines a V 8/71!








    Bones
     
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  20. Fordors
    Joined: Sep 22, 2016
    Posts: 3,855

    Fordors
    Member

    There never was an in-line 8 cylinder -71 Series engine. The V8/71 used a completely different blower.
     
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  21. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 8,924

    mgtstumpy
    Member

    I did some research and on a 6-71 the difference apart from the case size is rotor diameter.
    • Small bore rotor is 5.505"; and
    • Large bore rotor is 5.778". Large rotors won't fit in small bore case however small rotors will fit in large bore cases; a waste of time and effort with .273" clearance not providing the required boost. :D
    With 4-71s there appears to be only a single case available unlike 6-71s
    http://www.wallaceracing.com/Basic Blower Measurements.htm
     
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  22. I have the same casting info on the inside of my case...
    D R M 3
    12 R 8
    5155866
     

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