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Thread: 1960's Japanse Bolt Grade System Question

  1. #1
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    1960's Japanse Bolt Grade System Question

    I have not been able to find any resource regarding converting the bolt grades of Japanese cars to modern bolt grade systems. There are many grading systems, ISO, ASTM, SAE, etc. and none of them are consistent with the bolts found on Japanese cars, even through 1993.

    I am finding bolts marked with the number 7 and the number 9 on the heads in the suspension of a 1969 Japanese car. I have already read one warning to use strong bolts from someone who came close to a serious problem when his car started steering funny, and upon inspection, found half of the shiny new bolts he had recently installed were broken or missing.

    The best explanation I have found so far is from 1984-1989 shop manuals:
    4 = 4T, Low Carbon Steel

    7 = 7T, High Carbon Steel

    9 = 9T, Alloy Steel (The number nine has a straight line for the back/lower part, so that it is not confused with a number six).
    This does not give any conversion, rating, or detail at all.

    A second explanation from 1990-1993 shop manuals:
    4 or No Mark = Class 4.4

    8 or Long Slash From Hex Point And Dot = Class 8.8 Refined

    8 Dot or Short Slash From Hex Point And Dot = Class 8.8 Non-Refined

    9 or Slash From Hex Flat Side And Dot = Class 9.8 (Again, the number nine has the straight back).
    I know that this system is not SAE or the American system of bolt classing.

    And the lack of a 7 and 9 together in the more recent classing system with translation to what appears to be the modern decimal based "property" classing system (8.8, 10.9, 12.9), worries me.

    In my experience, I have not seen many bolts come off a Japanese car with a number 9 printed on the head, and this seems to be rare and indicate a rather extremely high rating. I'd like to get a solid conversion of the rating system, because fasteners get pricey in the modern 10.9 class, and even more expensive as well as unavailable in a lot of fine thread sizes in the modern 12.9 class.

    Is there a chart or web page that I have missed in months of research? Or anyone who has investigated this subject more successfully?

  2. #2
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    I know this isn't conversion info you're after, but may help somehow?




  3. #3
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    Theosophy, thanks for throwing in a hand to help.

    Imperial looks like another name for SAE, American, or fractional.
    Metric is ASTM, or the modern decimal property system.

    I read a couple places that the Imperial/SAE/Fractional system does not have a class 9.
    The scale with 4, 7, and 9 worries me. The descriptions of low carbon, high carbon, and alloy steel don't really match anything modern.

    Why am I getting the feeling that this adventure is going to end only after a very expensive metallurgy test of a 40 year old grade 9 bolt...

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    I would assume a class 9, if it's a metric bolt, would be 8.8/10.9 and go with it.

    Do you know what your friends 'shiny new bolts' tensile strength were? Find out because zinc/nickel plating is far more common in low tensile 4.6 style mild steel. Or if they were 8.8 just get 10.9 etc, both commonly available in standard metric/imperial-black/zinc (although not so in 10.9 metric fine). You won't find much/anything in 12.9 standard hex head, most hex key bolts are though and they are also common in both metric/imperial-black/zinc, again, not in metric fine though. I seriously doubt any of the bolts are over imperial 8/ metric 10.9 (or even metric 8.8 for that matter). The only 9 graded bolts I've sold (though I'm in the industrial side not auto) are plow bolts for farm machinery etc which are all imperial grade 9, but they're a different thing anyway.

    Any load bearing bolt I've ever replaced has been no stronger than 8.8 (all metric, I own old Mazdas) and have never had a problem. Although if you're looking to get it spot on this probably is no help, or information you don't already know anyway haha.

  5. #5
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    Quote Originally Posted by Theosophy
    I would assume a class 9, if it's a metric bolt, would be 8.8/10.9 and go with it.
    I'm starting to think that 10.9 might be as good as can be done, regardless of what the scale might actually translate over to, for the same reasons you mention. 12.9 has a very narrow choice of thread pitches in hex head and seems to default to socket headed cap screws for a lot of sizes.

    It's 40 year old bolts. The metallurgy has to have improved over four decades. Unless the Japanese auto makers drafted sword makers to make their bolts and the grade 9 bolts are made from recycled katanas.

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    Bahahahaha that's gold! :lol: :lol: :lol:

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    Somewhere, and it is really bugging me that I can't remember, in all of the Mazda RX-4/929 manuals and stuff that I have, Mazda gave a breakdown of the strength of their numerically marked bolts.

    FWIW: I use the same type of bolt off a better condition car when putting stuff back together (Im lucky, I have access to a few parts cars).

  8. #8
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    Titanium bolts

    If you really have a critical bolt and want the toughest posible bolt, go to the Carr-McMasters [or is that McMasters=Carr] catalog and order Titanium bolts. Much stronger that steel. Their store is in the LA area San Fernando valley near the North end of Tampa Boulevard.

  9. #9
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    mitsubishi still uses 4 7 9 11

    4 is pretty weak stuff, usually used on non support type stuff like interior. & seems to be th standard hardened I believe. Then you have some stuff like 9 10 11.

    I've seen 10 on driveshaft bolts, I've had both 9 and 11 head bolts

    As for titanium bolts, last I checked they are not as strong as most high grade steels, just lighter.

    Everything on your engine should be 7 minus the head bolts crank bolt cam bolt, and flywheel bolts. Accessories n what not should be 7 as well.

    For me buying metric in thestates drives me nuts. I hat ehow teh head diameter is changed. I already have a massive bolt collection from old mitsubishi's so it's usually ont a problem. I'd go to a jyard and collect proper looking bolts before I got anything fromthe hardware store if I had some time.

    Also since you are doing suspension. You are no talking about any special body fit bolts right?

  10. #10
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    Datsun Bolt Torque Guide

    Datsun Bolt Torque Guide

    Some service info from Nissan on their bolts. Too big to post here so I put it at the HybridZ site:

    http://forums.hybridz.org/index.php/top ... que-guide/

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