Head bolt differences

Discussion in 'Workshop Forum' started by Dr.Jeff, Aug 10, 2019.

  1. Ulix

    Ulix True Classic

    Stuttgart, Germany
    Regarding thread engagement: more won't necessarily help.
    In aluminum, you should have an engagement of twice the fastener diameter, so 20mm in our case.
    Unfortunately, the top two threads carry most of the load.
    If you now go to 30 or 40mm and increase the torque on the bolt, the top two threads will start to strip, no longer carrying and load.
    The load will now be on threads 3 and 4 which will also soon strip and so on.

    Obviously we are not at this point, otherwise the TTY bolt would also strip out the aluminum, but your finding of 20mm engagement fits perfectly engineering guidelinies.
    Tavalin likes this.
  2. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    I may not have been clear, but the thought I wanted to convey wasn't to increase thread engagement so the torque could be increased. Rather the other way around; to provide additional thread engagement if the torque was increased (I had the understanding the recommendation was to use a higher than factory torque spec with the class 10.9 bolts). With these engines being old and worked on countless times, the threads in the block may be quite marginal - repeated thread cleanings (perhaps with a improper tool), repeated threading and unthreading of the bolts, repeated torquing of them, etc. Especially if you consider the suggested lengths of class 10.9 standard type bolts only allow 15mm or engagement. Therefore I was thinking the use of a little longer ones to allow closer to 20-25mm engagement, and some nicer threads in the block. But it may not help, particularly with the block being cast iron and not aluminum. Keep in mind I am not approaching this from the perspective of an engineer, so these are just thoughts for discussion.
  3. fiatfactory

    fiatfactory Steve Cecchele

    Western Australia
    Fiat M10 bolts on the dohc which are not TTY are 61.5 lb/ft torque from memory, so non TTY methods using "regular" 10.9 metic H.T bolts that would be an appropriate torque value.

    Be aware that head bolts from Fiat (and many other manufacturers) are often a little different in small technical design differences, to a "regular" H.T bolt... look at the underside of a Fiat head bolt and you will usually see a flat raised portion under the head, and often an undercut right at the shank diameter under the bolt head. Most H.T. bolts do not specify a "ground flat" washer, as in "most" engineering applications bolts spacing and number can be precisely determined for best clamping force, but in an engine the head fasteners positions are not always "optimal" and in the sohc most fasteners are quite a distance from the gasket fire ring they are clamping down on...

    A correctly fitted M10 stud can easily take 70 lb/ft torque, a correctly fitted M12 stud can easily take 90lb/ft.

    once the amount of thread engagement into the block (with a stud) is equal to the overall diameter of the fastener in question, there is no additional strength or torque ability to be had with more thread engagement.

    With a bolt it's a little different as you're rotating the bolt into the relatively soft cast iron of the block, so more thread engagement reduces the chance of the threaded hole failing, but at the same time provides additional friction to overcome and this will affect the actual torque and clamping force achieved, compared to the torque shown by a wrench that's tightening it...i.e. there is more chance of the additional friction leading to improper torque values if correct method / lube isn't used.

    Last edited: Aug 14, 2019 at 4:30 AM
  4. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    Great info, thanks.

    This was what I was trying to convey, at least in part, with my suggestion of using a little longer bolts IF standard class 10.9 bolts are used. I'm referring to longer than the ones that were in one of my engines, which I understand came from a vendor for this application as a head bolt replacement option. In particular on the "short" side they are 80mm bolts and (by my measurements) offer 15mm of thread engagement. I thought using 85 or possibly even 90mm bolts might "reduce the chance of the threaded hole failing" - as Steve stated. However I had not considered the additional resistance and its effect on torque. So maybe 85mm would be better than 90mm.

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