Head locating dowel question

Discussion in 'Workshop Forum' started by Dr.Jeff, May 3, 2019.

  1. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    I notice the dowels that locate the head onto the block are available in at least two sizes. Some listings say M10 vs M12, other listings say 1300 vs 1500. What's the story? What dowel sizes are there and what are the differences?
     
  2. andreav

    andreav True Classic

    Location:
    Croatia
    I think that difference can be only m10 vs m12, but probably someone will confirm this.
     
  3. fiatfactory

    fiatfactory Steve Cecchele

    Location:
    Western Australia
    difference is in overall length... 1500 is overall longer as the recess in the block is deeper. OD and ID are the same

    SteveC
     
    Dr.Jeff likes this.
  4. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    So is it a matter of all 1300 vs all 1500? Meaning, do all 1500's have the longer recess independent of what size bolt they use?
     
  5. fiatfactory

    fiatfactory Steve Cecchele

    Location:
    Western Australia
    yes AFAIK all 1500 (even the M12) use the longer dowel (so a deeper recess in the block)

    which I guess raises another side issue... my recommendation of 6mm/11mm spot face is for a 1300 block, with the shallower dowel recess... I guess for an M12 1500 block the spot face would need to be a couple of mm deeper again to allow for the deeper dowel recess.

    SteveC
     
  6. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    It just occurred to me. When I read your previous comment about machining the additional depth on those two holes, I was not completely clear to me how the dowels would play a role in the stud length. But after thinking about the dowels in this latest context I get what you meant. However it seems to be two different things; one is the external spot face for the shorter stud length to extend higher above the head, and the other being the internal depth of bore within the block/head recesses to allow the dowel to fit. The spot facing is done on the outside (top) of the head, so the height of the head relative to the block isn't changing when installing the shorter studs, therefore the position (relative length) of the dowels hasn't changed? Although I'd have to go actually look at a head and dowels to visualize this better, so I might be seeing it wrong at the moment.
     
  7. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    I recall there have been instances when heads were milled significantly to increase compression and the length of the bolts (bottoming out) and dowels (also bottoming out) created issues with the head not seating fully. As for the dowel issue, I suppose one solution for a 1500 application is to use 1300 dowels? I'm not milling a head so this does not apply to me, just thinking out loud.
     
  8. fiatfactory

    fiatfactory Steve Cecchele

    Location:
    Western Australia
    The studs screw in until they (lightly) bottom out on the first thread...if the dowel recess is deeper, then the stud length above the block deck height when seated will be less with the deeper recess of the 1500, so the spot face on the head fastener washer pad will need to be deeper.

    the dowels themselves play no role in the stud length, but the fact that the recess in the block for said dowel is deeper in a 1500 block, then it will also affect spot facing if ARP m12 "kit" is used

    SteveC
     
  9. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Ahhh, I get it. Thanks for the explanation. I think you may have said that before but I'd forgotten the rationale.

    By the way, I noticed the factory bolts leave a lot of threads exposed above the surface of the block. And the short bolts (M12 engine) only go into the block a relatively short depth (much less than the longer bolts). No real relevance here, just commenting on observations.
     
  10. fiatfactory

    fiatfactory Steve Cecchele

    Location:
    Western Australia
    Bolts will always have exposed threads at that junction, one of the reasons they are not as strong as studs.

    TTY bolts have thread that extends nearly the full length in some examples, but done like that for a reason.

    M12 kit with equal length studs after spot facing is simply the most technically correct available option... other options include different thread pitches (technically very imperfect as the coarse pitch thread the nut tightens up against doesn't provide as much clamping force as a fine pitch thread (for the same torque valve) and leaving thread exposed at the block interface, again very technically imperfect.

    Seriously Jeff, for the baby boost your aiming for... stock fasteners and a stock genuine Astadur head gasket will not fail unless your tune is miles out and you get detonation.

    SteveC
     
  11. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Back to dowels...thanks for the info. I am assisting another Fiat owner with this and wasn't able to give a full answer about the available dowels.
     
  12. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    A little follow up about head locating dowels.

    I have three 1500 engines torn down at the moment; a '79 carb version (with M12 hardware), a '80 FI, and a '85 FI (both with M10 hardware). All are 10 bolt heads. I discovered the '79 has the shorter dowels, while the other two have the longer ones. So despite all three being 1500 blocks, there is a difference. Perhaps the dowel length is related to hardware size more than block displacement? Or it might just be one of those variable things with Fiats.

    As it has been noted, the blocks have different depths of recess to accomodate for the length of the dowel ('79 has shallower recesses). All of the heads have the same depth of recesses. Another odd finding was the short dowels in the '79 were extremely brittle and fell apart (crumbled) upon removal (this engine appears to have never been opened before). The longer ones in the other engines did not.

    No significance to any of this, just passing along what I've found.
     

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