Sako dovetail ratio

Discussion in 'General Sako Discussions' started by tilleyman, Jul 26, 2021.

  1. tilleyman

    tilleyman Well-Known Member

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    Was looking at some Sako factory videos on YouTube and stumbled across this one about the integral dovetails:


    Lo and behold there it is!
    1752C788-7774-412D-8518-DD266DCF0DFB.png
    The Sako tapered dovetail ratio is 1:17 or 3.37 degrees... been bugging me for ages!
    Yet it is not to Morse, Jacobs, Jarno, Brown and Sharpe or ISO/DIN standards.
    Jarno is closest at 1:20 taper.
    The only other reference to a 1:17 ratio machine taper I can find is the reamer used for cello string tuning adjustment pins.
    Now unless back in the day some Sako design engineer was also a keen cellist I cannot for the life of me work out why they chose 1:17!
    Any explanations gratefully received...

     
    Last edited: Jul 28, 2021
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  2. kevinlg

    kevinlg Well-Known Member

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    No explanation.....but maybe an observation.

    I would think the taper/ratio would need to be "shallow" enough to retain ring/mount positioning, AND not "steep" enough to let the ring/mount slide off(rearward). A too shallow draft might allow movement, under recoil or rough handling, depending on how tight the large knobs(or driven on base) were turned.

    I'll add a fly to the ointment.......
    Ever notice how one, and ONLY one, Redfield base will slide onto the Sako tapered receiver to a given position.........but the SAME base will slide onto a different receiver( say an L461 receiver versus a second L461 receiver), to a markedly different position??

    Hmmmmmmm.........

    I've seen this many times over the years, and can only surmise one cause.
    It is the receiver dovetail "height" that can vary. It may be worth measuring the checkered flat height above the lower dovetail flat, for variance between like actions.

    One other thing.......
    It seems as though the rear receiver ring dovetail has a shallower taper than the front receiver ring dovetail.
    Another one of my optical delusions???
    edit: proved otherwise.......see below.
     
    Last edited: Jul 26, 2021
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  3. Old Hippie

    Old Hippie Formerly known as bloorooster

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    Kevin, if you place a straight edge along side both front and rear dovetails , it appears to follow the same taper from extreme front to back, maintaining the same angles.

    Hippie
     
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  4. kevinlg

    kevinlg Well-Known Member

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    Agreed......good test.

    It's just when you look at the bottom of two Redfield bases, side by side, the rear one seems a bit shallower in taper.
    Again.......my optical delusion. :)
     
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  5. Old Hippie

    Old Hippie Formerly known as bloorooster

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    I had the same condition a while back, which I refer to as my own optional illusion…grabbed the straight edge to ease my worried mind!

    Hippie
     
  6. kevinlg

    kevinlg Well-Known Member

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    Yep......

    Put two 6" machinists' rules on same side of both tapers..........rules parallel??......yes.....same taper. :)
     
  7. tilleyman

    tilleyman Well-Known Member

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    Hmm... interesting!
    Apart from optical illusions part of the problem seems to be that the dovetails have not been cut accurately!
    Now I'll probably be tied to the bronze moose at Riihimaki and publicly flogged for such heresy, but the dovetails on my ~1975 L579 are noticeably off-centre by 0.5mm :(
    Making a sketch model of my L579 receiver and dovetails in 3D CAD also reveals the dovetail included angle to be 2.53 degrees not 3.37 degrees and a 1:17 taper ratio!
    Back in the day the jigs and fixtures must not have been robustly indexed OR the dovetails were largely set up and milled by hand, allowing significant production variation. Brno ZKK 600 series parallel integral dovetails feature similar variation, but the Sako dovetail taper adds complexity.
    No wonder different manufacturer's mounts sit all over the place, and the reason original Sako mounts had the capability for a lot of windage adjustment.
    Modern CNC milling would now be far more accurate and repeatable for the 75-85 series receivers, possibly the reason Optilocks no longer feature windage adjustment?
     
    Last edited: Jul 28, 2021
  8. icebear

    icebear Sako-addicted

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    To add to this observation, years ago I fitted a set of Conetrol bases to an AIII in 7 mag. I had to do quite a bit of filing on the front base to get it in exactly the right place on the dovetail. Rear base was fine as it was. I sold the gun but kept the bases for possible future use. Recently I checked the front base on a few different L61R/AIII/AV guns and on every one of them, it slid well forward of its proper position. So, it seems that there was a manufacturing variance (out of tolerance) on that one particular gun.
     
  9. Foxhunter223

    Foxhunter223 Well-Known Member

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    I have two Sako's fitted with Leupold QR bases and mounts, they have no windage adjustment. An S491 and L461, both times when zeroing the scopes the windage was no more than a few clicks from centre.

    Pete
     
  10. stonecreek

    stonecreek SCC Secretary Forum Owner SCC Board Member

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    There were (and still are) a number of one-piece bases made for Sakos. I've never trusted the one-piece bases to fit well because of the very slight differences which can occur in the "pre-CNC" machining of the receivers. If you'll notice, all of the one-piece Sako bases use set screws. The set screws are superfluous on two-piece bases (which should be driven on), but when you have either the front or the rear of the one-piece base gripping the tapered dovetail firmer than the other the only way you can keep the base from moving around is to use a set screw.
     
  11. icebear

    icebear Sako-addicted

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    Generally true, but here's an alternative thought - carefully filing the tighter of the two dovetails (on the base, not the gun) could bring the base to a correct fit. If I were doing that, I'd probably still use the set screws, but I'd substitute brass screws or try the trick of putting a piece of lead shot under the screw to cushion the screw tip and keep it from marring the base.

    Currently I have two Sakos with one-piece scope bases. One, an L579, has a Kuharsky external-adjustment base for a Bausch & Lomb scope. I installed that one with brass set screws. It fit well enough that I didn't see a need to file the dovetails. The other is an early wing-safety L46 with a Stith base and rings and a 6x Kollmorgen scope. That setup is a perfect complement to the rifle, and I believe it dates back to the original buyer. It's staying right where it is as long as I own the gun. I have several more Kuharsky and Stith scope mount setups, awaiting a call to service that may or may not ever come.

    308-3.JPG 1951 L46 Action R.JPG
     
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  12. stonecreek

    stonecreek SCC Secretary Forum Owner SCC Board Member

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    Icebear, good comments on ways to make the one-piece bases fit well and be secure.

    Also, I've run across an occasional Redfield base that comes to rest too far forward. Careful peening of the underside grooves of the base will tighten it and cause it to come to a firm rest in the proper place.
     
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  13. icebear

    icebear Sako-addicted

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    Conetrol recommends that as well. I tried it when I was trying to fit the base mentioned in a previous post to a couple of different guns. Seems like Conetrol bases are made of pretty hard metal, as I was unable to get any significant change in fit, even with some hard banging with a big hammer. It's certainly worth a try, though.
     
  14. kevinlg

    kevinlg Well-Known Member

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    Conetrol bases are a bit tougher than Redfields.

    I use a small machinists' vise on my workbench. I clamp an inverted base at the top of the jaws.....while being padded by old "business" card stock, and supported by a block of aluminum. All quite solid.
    I then use a polished small hard ball peen hammer(note: ball peen hammers are NOT equally hard) to slowly peen the rails for a proper fit.
    I've refined my method over the years, to insure that the base fits solid and FLAT(if not flat....the anchor screw can tilt the base).
    I peen the rear of the two rails until the base slides on to about 1/16" rearward of desired final position.....THEN peen the front of the two rails until the base is about 1/8" rearward of desired final position.
    Press a lead shot into the anchor screw hole(it has little effect, since I just barely tighten the screw into the lead) .
    Lube the rail grooves with FP-10......and drive base on with a leather mallet(barrelled action OUT of stock.

    Note that this is for a loose base, that slides on too far.

    Hope this helps.....

    (Edit: BTW.....and it's just me.......I dislike bridge-mounts.)
     

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