Burner Cap Dimensions for a Prentiss Wabers?

Discussion in 'Fettling Forum' started by Ben Hall, Nov 24, 2021.

  1. Ben Hall

    Ben Hall United States Subscriber

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    Good afternoon all,

    This Prentiss Wabers Auto Kamp Kook Kit was on e-place down in LA. You might think that's Los Angeles, but "LA" is actually Lower Alabama. ;) Being in Northern Alabama, how could I resist putting in a bid on it? I ended up winning the auction and it is currently in the mail to me, probably arriving late next week. It even has the matching pump, albeit the tip is missing from the pump. (I figure I can make one, either from a brass pipe fitting or something else.)

    pw02-red.jpg

    As you can see, it's got a problem: it's missing the upper part of the burner cap.
    pw10-burner.jpg

    Idahostoveguy (RIP) has some nice close-up photos of his P-W stove here which I think has the upper burner cap that would be correct for this stove:
    Prentiss Wabers Auto Kook Kit #4 Single Burner
    1336531218-MyPrentiss-WabersNo4SingleBurner_008-crop.jpg

    I'm sure finding a spare one somewhere is going to be impossible, so I've been brainstorming both a field-expedient replacement, as well as a considering creating a 3D model of one then having it 3D printed by Shapeways in sintered metal a la BernieDawg. :)

    I can probably determine dimensions close enough from scaling the photo, but just curious - would anyone have a similar unit that they'd be willing to measure up for me?

    Thanks much,
    ben
     
  2. cottage hill bill

    cottage hill bill Subscriber

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    I had Dean Williams (deanofid) over on CCF make one for my two burner PW. I dropped on of the cast iron caps and it shattered when it hit the floor. By the way you poached that out of my back yard (Pensacola). I suspect the two-burner uses the same cap as your one-burner.
     
  3. konabill

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    @Ben Hall
    I have the exact stove you are wanting to fire-up
    I will help in any way I can…. Not sure how to get an accurate 3-D measurement though ?????
     
  4. Ben Hall

    Ben Hall United States Subscriber

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    Hi @cottage hill bill - I'll get in touch with Dean over at CCF and see if it's something he can make for me. I know the feeling you must have had when that burner cap hit the floor! I did the same years ago with a cast-iron small engine flywheel...I about vomited. :(

    Andalusia is indeed right in your backyard! It's been a while since we've been down to the Pensacola / Mobile / Eglin / Tyndall area. Very nice area, we've always enjoyed our time down there. :)

    Thanks much,
    ben
     
  5. Ben Hall

    Ben Hall United States Subscriber

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    Greetings @konabill - I think there are only a couple of the dimensions that are going to be somewhat critical. I can take those measurements and create a 3D model.

    I'm a visual person who usually can't think about things unless I've got a picture on paper or in my mind...so I drew up a couple of terrible little drawings to think this through. Since I've only seen photos of the outside, I have no clue what shape it is on the inside, so that's a guess!

    PW_01a.jpg
    In this first terrible little sketch, I've drawn up the burner cap as if I've taken a hack-saw and cut it in half from the top down, going right thru the center of the two screw holes. (the sketch is a thumbnail)

    I think what's critical here is the overall diameter of the bottom half (OD1 on the drawing) and the overall diameter of the widest portion of the of the half (OD2 on the drawing)...as well as the overall height (OH on the drawing) and the height of the bottom half (H1 on the drawing)...plus a measurement of how thick the part is in the area of the slots (THICKNESS on the drawing).

    The center-to-center distance between the holes (HD in the sketch) is something I can measure from my stove when it arrives, so no worries there. :)

    Note that I've completely ignored any measurements on the shape of the inside. I think this is not terribly critical as long as the size and shape don't choke off the fuel/air mixture flow. I can judge this from my stove when it arrives. :)

    PW_01b.jpg
    In this second terrible little sketch, I show a zoomed-in version of the slots thru which the fuel/air mixture passes. The height (shown as h on the sketch) and width (shown as w on the sketch) plus the number of the slots are needed here.

    I think with those measurements plus some measurements I can take from my stove when it arrives I can make a really good 3D model of a replacement. :)

    I really appreciate the help!

    Thanks much,
    Ben
     
  6. konabill

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    4D6B48F1-3663-4F0B-B41D-1F43B2B2708A.jpeg @Ben Hall
    ok, I can do that…. heres my stove. For better and accuy measurements ill grab a small micrometer Saturday……
    i searched through all my parts boxes too but no luck.
     
  7. Ben Hall

    Ben Hall United States Subscriber

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    @konabill - awesome, thanks much! That's a really nice stove you've got there! Looks like you've got a lot of the original maroon paint. :)

    thanks much,
    ben
     
  8. Daryl

    Daryl United States SotM Winner Subscriber

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    Ben, I have read some have made replacement pump tips by drilling the back side of Zerk fitting and removing ball. May need to solder on or tin over threads for snug fit. I made one using small hobby torch tip. Good luck with burner, not a lot of those light weight, ha ha, singles around. Maybe sub 10 pounds.
     
  9. Ben Hall

    Ben Hall United States Subscriber

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    Greetings @Daryl - that's genius! I was thinking last night about ways to repair it and this, by far, beats anything I've thought of so far! Thanks much for sharing! Somewhere in the garage I've got a "zerk fittings kit" that should have something I can make work. Fantastic!

    Thanks much,
    ben
     
  10. Ben Hall

    Ben Hall United States Subscriber

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    Good morning all,

    The stove arrived late yesterday. It's in about the shape I figured it would be in - dents here and there - but pretty much everything came apart without too much of a fight. :) Pretty much all of the bits and pieces are in the ultrasonic cleaner right now. Just finished hydrotesting the tank up to about 60 psi - no leaks and a couple of the dents popped out. ;) It's not a perfect tank, but for sure, it's serviceable. Pressure gauge even works and is accurate according to the gauge on my pump. (I removed the gauge for the hydrotest as to not damage it)

    Judging by the lack of yuck in the tank and in the burner, I bet this stove wasn't used much at all. :)

    I've been thinking about the upper burner replacement. I want to do something to test the stove prior to putting the time / effort into a new upper burner cap. I was thinking I might just put a plate over the top a la Phoebus and make it something like a roarer burner.

    Then I brought the lower burner into the house for measuring and noted that it seemed very similar in size to a silent burner from a Primus No. 1 type stove.
    burner idea 1.jpg
    Hmmmm...yes...very similar...

    burner idea 2.jpg
    The reproduction Primus lower burner cap does not reach down to the cast iron boss on the lower burner. But, it could be held in place using the two screw holes - perhaps studs coming up from the holes?

    burner idea 5.jpg
    So I fished out two 8-32 set-screws from my parts bin and viola! The Primus inner burner cap is a nice, firm push fit on the two set-screws!

    burner idea 3.jpg
    The upper cap fits nicely - almost like it was made for it! It's not perfect, there is some slop, so it won't sit perfectly centered, but for test purposes should be perfectly fine.

    burner idea 4.jpg
    The only issue I see is that on the original P-W burner cap, the slots were very low, so that they'd impinge on the cast iron lip around the lower burner keeping it hot to allow the fuel to vaporize.

    I think what I might do is trim this Primus burner cap off at the bottom set of holes, which will put some of the flame right on the cast iron.

    Very interesting.

    Thanks much,
    ben
     
  11. Ben Hall

    Ben Hall United States Subscriber

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    So did end up modifying the Primus-type upper and lower burner caps to work on the P-W - cut the upper cap off at the lowest set of holes and shortened the lower cap by about the same amount. Added four screws to keep the upper cap centered on the lower cap more or less.

    After a long pre-heat, she lit up and ran really nicely:
    P-W with primus silent burner.jpg

    P-W with primus silent burner 2.jpg

    There is a little yellow in the flame. I believe the Primus-type cap setup is too restrictive, as if I open the needle valve further, I get a very rich burn:
    P-W with primus silent burner 3.jpg

    Once the whole thing cools off, I'll probably open up the holes in the upper cap with a drill bit by about 5 to 10% and see how it works. :)

    Thanks much,
    ben
     
  12. Scrambler

    Scrambler Australia Subscriber

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    @Ben Hall - please make sure you have this sorted before drilling. The tip colour reflects the mixture- which afaik is not controlled at these holes but at the jetting. Discuss with more knowledgeable people than me.
     
  13. Ben Hall

    Ben Hall United States Subscriber

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    Greetings @Scrambler - you raise a good point.

    The nice thing is that if I mess it up, I've got another reproduction Primus-type cap and can start all over from the beginning. ;)

    I actually have already opened all the holes up by about 10% (about 1.4mm diameter now) and tested it out. For sure, I get a better, leaner burn at higher flame settings, but still, if I've got the needle valve all the way open, I still get a lot of yellow flames. Plus, now with the holes opened up, the simmer is very unstable and it goes into underburn quite easily at low flame settings... :(

    I've thought about this a lot and here's my thoughts: If we had a fuel jet spraying fuel vapor into a tube of a short length (a cm or two perhaps) the fuel/air mixture is determined by the size of the jet and the inner diameter of the tube.

    The air is drawn into the tube by the fast flow of the fuel vapor going thru the opening of the cylinder. If I make that tube quite long, say 10 centimeters, I've not altered the inner diameter, but the resistance to flow increases. The jet is still spraying the same amount of fuel, but less air is flowing thru the tube, resulting in a richer mixture with more yellow flames. :(

    The holes in the Primus-type upper burner cap are a restriction to flow, and have the same effect on the fuel/air mix as would lengthening the tube. So I suspect what is happening is that there is too much restriction in them, which is resulting in a rich fuel/air mixture when the needle valve is quite far open.

    At least that's my thoughts at the moment, subject to change when smarter folks educate me on where I'm going wrong here. :)

    Thanks much,
    ben
     
  14. Ben Hall

    Ben Hall United States Subscriber

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    Good morning all,

    Was watching a Bernie Dawg video on his Prentiss-Wabers stoves and he shows an underside view of the cap from his single. Looks like the inside of the cap is a very simple structure:
    Screenshot 2021-11-30 062830.jpg

    It doesn't have the chamfer like I've got in my drawing above. I'll have to redo that drawing to reflect what I think the cap now looks like in cross-section.

    Thanks much,
    ben
     
  15. Majicwrench

    Majicwrench Subscriber

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    @Ben Hall do you need more pictures of a PW cap?? I have several flavors of PW
     
  16. Ben Hall

    Ben Hall United States Subscriber

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    @Majicwrench - I'd really appreciate some more pictures! Especially if you can put a ruler in the photo so I can get an idea of the dimensions. :)

    I'd also be curious to know if all P-W caps are the same size. I've been attempting to get an idea of P-W cap dimensions by scaling photos I've found here and in other places. It appears that they may be different sizes, as one cap photo when scaled was significantly larger than the others. As an example, if the 8-32 screw in the Bernie Dawg video photo capture is about 4mm, the largest dimension is about 69mm. The biggest diameter cap that could be installed on my stove is right at about 49 or 50mm.

    As Kerophile says: "Aren't stoves fascinating?"

    Thanks much,
    ben
     
  17. Ben Hall

    Ben Hall United States Subscriber

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    Good evening all,

    Like I noted above, I've been scaling photos, taking measurements on my stove of what I can (such as the mounting hole center to center distance) and figured I'd post my best guess right now at measurements for a P-W cap for my stove. Dimensions are in millimeters.

    All of this is little better than a guess at this point until I can get more info, but comparing it to various photos, it seems close! :)

    Screenshot 2021-11-30 160856.jpg

    The 34.92 measurement likely will seem strange - this is a product of the stove being designed in inches, me measuring the hole-to-hole center distance in inches, then converting to millimeters. :) Lately, I've been doing pretty much everything in metric. I can work in both without issues. ;)

    Screenshot 2021-11-30 160941.jpg

    I'm very unsure about the slots. In one photo, I'm pretty sure I can count 64 of them...and in another...it appears that I count 52!

    I'm also unsure of thickness. I've assumed 2mm, constant thickness as I'd expect for a cast iron part.

    If we make an assumption that the cross sectional area of the slots should be the same as the cross sectional area of the fuel/air mixture tube inner diameter, it seems that 52 is likely the correct number.

    (IE: the inside diameter of the flow path of the burner is about 1/2", or 12.7mm. That's 126mm^2 if I've done the math right. 52 slots of 0.78 by 3mm is 121mm^2, so perhaps a hair on the skinny side - maybe they should be 0.8 mm in width instead that will yield just under 125 mm^2?)

    Thanks much!
    -ben
     
  18. Majicwrench

    Majicwrench Subscriber

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    I took a cap off of mine, will get you pics and measurement tomorrow (Thur) morn
     
  19. Majicwrench

    Majicwrench Subscriber

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    @Ben Hall IMG_2035[1].JPG IMG_2036[1].JPG IMG_2037[1].JPG

    The OD is 1.9" The height of the area with the slots is .37. The overall height is .64
    If you need more measurements or pictures let me know.
     
  20. Ben Hall

    Ben Hall United States Subscriber

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    @Majicwrench - AWESOME! Thanks much! I'll take a look at these in more detail tomorrow, but I think I've gotten everything I need!

    thanks much,
    ben