Primus 51 silent caps

Discussion in 'Fettling Forum' started by MrAlexxx, Apr 8, 2020.

  1. MrAlexxx

    MrAlexxx Subscriber

    Offline
    Joined:
    Oct 21, 2015
    Messages:
    1,201
    Quick question...can't seem to get a Primus 51 fitted with a silent burner to fire properly. I can get it fired and let it warm up then after a few more pumps it flashes into an underburn situation. I've tried all sorts of different caps I have but...just wondering are there silent cap sets specific to the 51? If you look at the picture the 51 on the left. Outer cap almost covers the holes in the burner head while on another stove on the right the burner head holes are uncovered. I know some educated person on here knows?

    HELP PLEASE... :)

    Alex

    DSC05773.JPG
     
  2. kerophile

    kerophile United Kingdom SotM Winner Subscriber

    Offline
    Joined:
    Sep 23, 2004
    Messages:
    11,829
    Location:
    Far North of Scotland
    Hi @MrAlexxx . The holes in the base of the “cup” of the silent burner should not be fully or partially covered by the outer cup of the silent burner.
    If even partly covered, there is a potential leak path for the flame to dive under the outer cap and initiate under-burn.

    Check with any other caps you have, to see if you have one which sits properly and doesn’t overlap the holes.

    Best Regards,
    Kerophile.
     
  3. kerophile

    kerophile United Kingdom SotM Winner Subscriber

    Offline
    Joined:
    Sep 23, 2004
    Messages:
    11,829
    Location:
    Far North of Scotland
    Hi @MrAlexxx . Check your silent burner cap dimension against these dimensions:

     
  4. MrAlexxx

    MrAlexxx Subscriber

    Offline
    Joined:
    Oct 21, 2015
    Messages:
    1,201
    @kerophile

    Hi George...I tried all that I have and they all seem to be the same diameter. Underburn is exactly what happens. I'll measure them up and post the measurements. The only way I can see it working is if there is a cap set that is smaller in diameter but I wasn't aware of that being an issue with the 51? I thought silent caps were all the same?

    Alex
     
  5. MrAlexxx

    MrAlexxx Subscriber

    Offline
    Joined:
    Oct 21, 2015
    Messages:
    1,201
    @kerophile

    Okay...makes sense. I guess all silent caps aren't the same. Just my luck...lol
    Thanks George.

    Alex
     
  6. kerophile

    kerophile United Kingdom SotM Winner Subscriber

    Offline
    Joined:
    Sep 23, 2004
    Messages:
    11,829
    Location:
    Far North of Scotland
  7. presscall

    presscall United Kingdom SotM Winner SotY Winner Subscriber

    Offline
    Joined:
    Aug 25, 2009
    Messages:
    9,402
    @MrAlexxx Looks like a Primus No.4 burner might have been fitted, requiring its own (smaller) pair of inner and outer caps. Maybe the No.4 inner cap is already in there?

    John
     
  8. igh371

    igh371 SotM Winner Subscriber

    Offline
    Joined:
    Jun 23, 2013
    Messages:
    1,463
    Well spotted John. It does look like that could be possible. But if that is the case then it raises the question of how, given that a No.4 burner would have smaller base threads?
    Ian:thumbup::-k
     
  9. MrAlexxx

    MrAlexxx Subscriber

    Offline
    Joined:
    Oct 21, 2015
    Messages:
    1,201
    @presscall

    John, I was going to measure it up but battery dead in both of my digital calipers! Can you believe that? lol

    Regardless it's on this unit. It's the one Ben linked us to last year I guess? Both dated S so 1928. One a roarer and the other a silent. I thought maybe it came that way with one roarer and one silent originally from the factory? I have great faith in you guys. You'll figure it out for me. lol

    Alex

    DSC05775.JPG

    DSC05776.JPG
     
  10. Nicola Francesco Elia

    Nicola Francesco Elia Subscriber

    Offline
    Joined:
    Nov 13, 2016
    Messages:
    77
    Hi older 54 or 51 stoves used a one piece riser/burner that used no 4 size burner caps.

    You, for sure (I can clearly see the wrong proportion burner/cap in your photo, are trying a no 5 cap on a burner that requires the smaller no 4.

    Anyway looks like that you don’t have the inner cap that most probably is part of the underburning cause.

    Nicola
     
  11. Nicola Francesco Elia

    Nicola Francesco Elia Subscriber

    Offline
    Joined:
    Nov 13, 2016
    Messages:
    77
    Regarding your other question usually these cast iron beauties came with twin silent burners (2 54s) or with twin roarer (2 51s, less usual).

    Being your stove a 51 probably the previous owner replaced the roarer with a silent, probably from the beginning, just because that style of silent burner didn’t last too many years so is less probable a later replacement... roarer burners last forever, so no reason to change it so soon.

    My thoughts.

    Nicola
     
  12. igh371

    igh371 SotM Winner Subscriber

    Offline
    Joined:
    Jun 23, 2013
    Messages:
    1,463
    Good info., that explains all:thumbup: All clear to see in this old post from Iani.

    Ian:thumbup:

    (@MrAlexxx, now we know what they should be, pm if you have difficulty sourcing the correct caps)
     
  13. presscall

    presscall United Kingdom SotM Winner SotY Winner Subscriber

    Offline
    Joined:
    Aug 25, 2009
    Messages:
    9,402
    Ah yes, all’s revealed, the No.4-sized silent burner with that pattern of attachment to the tank, like the one on my 1929 steel tank Primus 53.

    The jet size should be 0.23mm incidentally.

    AF7F1666-EF0B-4AAC-A05E-1574C2931823.jpeg


    I agree with @Nicola Francesco Elia ’s reasoning that the roarer burner appropriate to a No.51 was swapped early in its life.

    John