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Optimus 9 stress-cracked tank repair - the story so far

Discussion in 'Fettlers Master Class' started by presscall, Jun 25, 2016.

  1. presscall

    presscall United Kingdom Subscriber

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    The Optimus belongs to @Doug L and he's happy for me to post details of the project, which I need to do to get the job completed, frankly. I've had the tank for weeks with just sketches of a possible repair technique to mull over and need the public exposure of sharing the project here as an incentive to make the transition from concept for the repair to taking the fettle to the metal.

    Doug broached the problem, sent me pics and offered the sweetener of a slightly sickly Borde to keep once fettled. We swapped more emails as I outlined my thoughts and together with the Borde he packed the Optimus tank and pump off to me, the latter to enable me to pressurise the tank - repair completed and fuel outlet blocked off - to test the soundness of the repair.

    You know the model type, it heads up the Stove Reference Gallery on the CCS title page.

    image.jpeg


    Invariably pricy due to its rarity and desirability. Its rarity is down to low production numbers and a notorious proneness to fuel tank stress cracks.

    The fuel tank has a hard life.

    It has weighty burner, pot rest and windshields bolted to one end of it and there are mechanical stresses arising from that alongside the stresses caused by pressurising and de-pressurising of the tank.

    Additionally, in spite of an attempt to isolate it from radiant heat from the burner (in close proximity to it) by interleaving an asbestos disk between the tank and the pot rest/windshield mounting plate, heat is conducted through the mounting bolts. There's an asymmetry to the hot-spotting of the tank also, unlike the radiant heat across the top of a discus-type stove for example.

    So, the project.

    image.jpeg


    The crack's on the bottom of the tank when the stove's assembled and is end-to-end.

    image.jpeg


    The tank wall bows out at the fault due to the crack too.

    image.jpeg


    I wanted a good peer inside and made up this device from AA bateries and an LED cluster.

    image.jpeg


    image.jpeg

    image.jpeg

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    My thoughts on the repair

    The high temperature required for a silbrazed repair, which would be strong enough to put things right, would destroy the plating and disrupt the end seams and pickup pipe joints. The idea of protecting those parts with wet rags is workable for soft solder repairs but not for silbrazing, where too much cooling of a component in close proximity acts as a heat-sink and robs the silbraze of the heat needed to melt it.

    Soft (lead) solder wouldn't hold the joint, particularly when the bulge created by the gaping crack were pulled back into place and relied on the solder to hold it there on top of the thermal-related, mechanical and tank-pressurising stresses the soft solder would have to cope with. It couldn't.

    I hope to utilise a benefit of the tank design and that's the access possible to the back side of the crack. I'll be making a 'splint' of brass to the design pictured below to bolt into place with small set screws, pulling in the bulge, pulling the edges of the crack together and creating a base to run soft solder onto. The set screw nuts will be chamfered to fit countersunk tapped holes in the splint so that when soldered up the part of the nuts and set screws standing proud of the surface of the tank can be ground down and polished to create a relatively invisible and mechanically strong repair.

    image.jpeg


    Well, that's the plan. I'll let you know how it works out.

    John
     
    Last edited: Jun 25, 2016
  2. Rickybob United Kingdom

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    if it can be done Maestro - you are the man for the job!
     
  3. Doug L

    Doug L United States Subscriber

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    You will deserve knighthood for this job John
     
  4. shagratork

    shagratork United Kingdom Moderator Subscriber

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    Good grief!!! :clap::clap::clap:
     
  5. presscall

    presscall United Kingdom Subscriber

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    Reassuring comments, Doug, Rickybob, Trevor. Much appreciated.

    I've made a start on the 'splint', which I intend to install in the tank with just the two end set screws then with the nuts on those tightened up and the bulge in the tank cramped flat (crack edges brought together) I'll drill and tap for the central two set screws on the right-angled axis. The curvature I've put on the upper surface (not obvious in the photo) matches the interior curvature of the tank.

    Although the splint could be installed as it is, shaping it roughly to the cruciform pattern of the sketch wouldn't reduce its strength appreciably but would reduce the weight and keep the fuel tank closer to stock net weight.

    To install it I'll remove the set screws of course and manoeuvering it into place to get the tapped holes to coincide with the holes I'll have drilled at either end of the tank crack will be a bit fiddly I expect, but do-able.

    image.jpeg

    John
     
  6. Spiritburner

    Spiritburner United Kingdom Admin Subscriber

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    This is going to be great to follow. I wouldn't know where to start! Or if I didn't wouldn't have the bottle! :clap:
     
  7. Metropolitantrout

    Metropolitantrout United States Subscriber

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    Wow, quite the undertaking but worth it to save a #9.
    It looks like you changed the design of your splint from your original drawing. Did you opt for a simpler design or are you going to braze on some "ears"?

    Nice flexible light idea to illustrate the inside of the tank John. Would love to see that technique again once the splint is installed.

    Looking forward to the next post! Good luck, Jerry
     
  8. presscall

    presscall United Kingdom Subscriber

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    @Metropolitantrout
    Hi Jerry. I covered that with ...
     
  9. Wim

    Wim Belgium Subscriber

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    @presscall Hi John, it looks like your idea is very workable! One small thought ; once the front & rear screws are in situ, you can maybe use one or (better) 2 jubilee clips (with leather strips to protect the fount) to close the gap and then solder it. When reaching the first clip it can be taken away, same with clip N°2. Given the size of the splint I think the repair should be pretty strong, so there would be no need to drill more holes in the fount (and weakening it). Just some thoughts from my wobbly mind.

    All the best,

    Wim
     
  10. Doug L

    Doug L United States Subscriber

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    So the straight bar will be ground down to the 4 screw cross from your original post John?
    Man I wish I had your skills.
     
  11. Rickybob United Kingdom

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    to me it looks like building a model of the cutty sark in a guiness bottle while wearing welding goggles - but on the plus side you have mad skills
    i am betting on a positive result!
     
  12. presscall

    presscall United Kingdom Subscriber

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    Last bit of work for the day completed before I'm off on a training run - I've a 24-hour 65-mile charity walk to complete next Friday.

    Final form for the repair splint.

    image.jpeg


    ... and the matching holes in the tank for the set screws. Holes put a stop to any further cracking on that line, though it couldn't have gone much further than it did already!

    image.jpeg


    An indication of the extent of the bowing outwards of the tank along the line of the crack. That's to be pulled back into shape, much as you suggested @Wim before I drill and tap the final two holes. I hear what you say about the sufficiency of fixings with the two end screws but tightening those won't pull in the bulge or mechanically secure together the tank on either side of the crack, which is needed to make the solder just a sealing medium and not something required to take any forces other than the moderate tank pressure.

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    Ready for the next phase ...

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    ... feeding the splint through the pump/filler hole and jiggling it into place to get the set screws into the tapped holes. Metal puzzle time. Used to like those as a kid.

    image.jpeg


    John
     
  13. snwcmpr United States

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    A fettle I do wish to follow. I cannot add to the already made comments.
    A question. Is the crack I see in this photo an illusion? Or does it extend beyond the hole that you drilled?

    Ken in NC
    image.jpeg
     
  14. presscall

    presscall United Kingdom Subscriber

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    @snwcmpr
    Trick of the light, Ken, thankfully.

    image.jpeg

    John
     
  15. ArchMc

    ArchMc United States Subscriber

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    Will be following this avidly. As with Ken, there's nothing I can add except :clap::clap::clap:.

    ....Arch
     
  16. Matthew92

    Matthew92 Subscriber

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    Very nice work indeed @presscall will you be going into the business of repairing no.9's from now on?:D Such a shame that these stoves are prone to cracking, I wonder just how many have been scrapped though the years when they've been dug out of grandparents lofts and found to be cracked.:cry:

    image.jpeg

    This is of course only the second cracked one I've seen, but interesting they both have only cracked in one place unlike other pressure items I have which have many cracks all over.:-k
     
  17. presscall

    presscall United Kingdom Subscriber

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    Some thoughts about the soldering in prospect.

    M4 (metric) set screws and nuts in stainless steel feature in the photos, a material I saw as contributing to an 'invisible' repair when made flush with the tank surface, blending in with the plating of the tank and the solder.

    The ease with which stainless steel bonds with silbraze using the usual borax flux, which I've done often, distracted me from thinking about its compatibility with soft (lead) solder.

    It requires an acid (phosphoric) flux and although I don't relish getting that on the tank plating (acid resist would control that) the main issue is that I'm unfamiliar - and therefore mistrustful of - the effectiveness of the brass to ss solder bond.

    On balance, I'll ditch the stainless screws and substitute brass, relying on the rosin flux And solder combination I trust, masking the brass in the final finishing with a skim of solder.
     
  18. snwcmpr United States

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    I have used Harris Saftey Silv 50N brazing steel and brass.

    Ken in NC
    (A beginner)
     
  19. presscall

    presscall United Kingdom Subscriber

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    @Matthew92
    Like I said in my initial comments, the stress cracks are a reaction to the unique combination of forces - mechanical and thermal - on these tanks.

    You'll remember I quizzed you when we spoke at Newark the other day about the stress crack in your tank. I had this repair in mind and was formulating my theories about the No.9's Achilles Heel.

    Drilling those two holes in the tank this afternoon reminded me how thin the material is, less than most conventional brassies. Pity.

    @snwcmpr I'll look that up, although I need to soft solder this job and not braze to keep the heat moderate. Thanks Ken.

    John
     
  20. teckguy_58

    teckguy_58 Canada Subscriber

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    Hi John,

    Great post and fettle you have going and I'm looking forward to seeing your finished work.
    In this picture it looks like you are going to use a 2 piece splint system, but it may be just to show how badly the fount is bowed.
    No9project.jpeg

    The 2 piece splint should work much better. I was also thinking maybe using a thin piece of viton gasket material for the inside of the fount. Cutting the viton gasket material would be easy just use an exacto knife to cut the gasket to the exact shape of your splint. Then use some thread sealant for the threads. With the solder, viton gasket and thread sealant there should be no fuel leaking from the fount. Of course I'm sure you have already thought about all of this and scraped it for a better idea, but it is only a suggestion.
    I do know this, John, if you can't fix it no one can.
    I'm looking forward to seeing your repair job completed and seeing the stove up and running.

    John, I hope you have a fantastic day.

    Cheers,
    Norman
     

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