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Author Topic: Head Bolts  (Read 123 times)
Rod
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« on: November 10, 2019, 11:09:14 PM »
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Hi All,

In cleaning up my motor and looking in the spare parts department I thought I would share some useless trivia that I have searched for on this forum but havent' found reference to.

As we are aware there are two different lengths of head bolts . I looked in my master parts catalogue which goes up to EK and there is only reference to one part number for each length. This is what I have found:

Short Bolts. I have found three different types.
      1. Slightly Domed bolt head with Ajax 70 on top. The shaft of the bolt is uniform thickness.
      2. Flat bolt head with a Star Pattern on top. The shaft of the bolt is uniform thickness.
      3. Flat bolt head with a Circle on top. The shaft has a shoulder under the bolt head (as if to assist in the centering of the head on the block) which is the same diameter of the bolts above. However, the majority of the shaft is slightly smaller in diameter.

Long Bolts. I have have found two different types.
      1. Flat bolt head with a Star Pattern on top. The shaft of the bolt is uniform thickness as with the first two short bolts.
      2. Flat bolt head with an ever so slightly different Star Pattern. This bolt has a shoulder under the head like the third short bolt with the majority of the shaft slightly smaller in diameter.

I know this is useless trivia but it has me thinking. The bolts may have changed once the EJ motor came out (I haven't got a parts catalogue for the EJ). However, I don't think that this would be the case considering there are three short bolts versus two long types. Possibly different suppliers. I wonder if the bolts with the shoulder and slightly smaller diameter shafts made way for a "stronger" uniform lengthed diameter shaft.

Anyway thats enough from me on bolts. I am now paronoid about bolts, especially broken ones.

Cheers

Rod
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Harv
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« Reply #1 on: November 11, 2019, 07:27:15 AM »
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GMH made both “long” cylinder head bolts (part number 7401119), and “short” ones (part number 7401120) for the grey motor. The GMH cylinder head bolts have a very short thread length. The idea behind this is that you only cut enough thread to do the job – any unengaged thread acts as a localised stress riser, leading to fatigue failure of the cylinder head gasket joint. The bolts are also reduced in diameter (they should be 7/16” diameter, but are less than that). This reduces the bending stiffness of the bolt, increasing fatigue strength.

Cheers,
Harv
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Rod
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« Reply #2 on: November 11, 2019, 10:20:36 AM »
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Hi Harv,

Thanks for offering additional information. Based on what you have provided would indicate the third example I had for the 'short' bolt and the second example for the 'long' bolt are GMH ones. This makes sense as the marking on the head of each bolt (circle and star) and shoulder under the head of the bolt, being 7/16 with the rest of the shaft narrower, being consistent with the markings on the Main Bearing bolts as I have found.

Interestingly, the threads on all of the bolts are relatively short for the reasons you indicated but the GMH ones are slightly longer. The shoulder under the head is 7/16 in diameter (a length of about a 1/3 of an inch) which runs into the main shaft which has a diameter of 7/18 (if there is such a diameter).

Thanks for your post. I have learnt something about the strength of these bolts. I was going to run with the bolts that had uniform shaft diameter thinking these would be stronger.

The other bolts I have listed must be after market ones ie: Repco etc...

Cheers

Rod
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ardiesse
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« Reply #3 on: November 11, 2019, 10:44:59 AM »
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Rod,

You've just described the history of Holden cylinder head bolts.

There are "early", "mid" and "late" head bolts.
"Early" head bolts have stars on the heads, for both long and short bolts, and full 7/16" shaft diameter.  This style was used for FX and FJ.
The "mid" head bolts were made by Ajax, have domed heads, and I think the full 7/16" shaft diameter.  I have an FC motor with these head bolts.  Can't recall now if the long bolts also have domed heads.
The "late" head bolts (at least B and J motors) have O on the short bolts, * on the long bolts, and reduced shaft diameter.

I'd say all your head bolts are OEM.  But GMH would have had at least two suppliers.  And years of engine rebuilds will have randomised things further.

Rob
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mcl1959
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« Reply #4 on: November 11, 2019, 04:59:06 PM »
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I’m sure I’ve seen head bolts with GMH on the top, I’m going to go look to see if I have any.
Ken
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ardiesse
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« Reply #5 on: November 11, 2019, 07:30:58 PM »
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Ken,

Head bolts with the GMH logo - that rings a bell with me, now that you mention it . . .

Rob
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