P&W Wasp Jr. in 1/6th scale

[Mike R]

Cast aluminum is the best I can do for material specification.  I checked all my notes and correspondence and can't find any reference to material specifics.

[vtsteam]

Then I'm guessing that's a blowhole caused by faulty homebrew de-gassing, based on the white powder residue. Or could be investment.

A new casting would be the best bet. For repair, you can talk to a professional TIG welder and get an opinion.

Is it possible to bore out and press a plug in? Doesn't look easy from here, but you'll know better with part in hand.

There are some aluminum brazes that might work. How hot can a model head run near the exhaust? Do people run CHTs and EGTs on these? If too close to aluminum melt temp, there's a thin line between braze application and damaging the head. Some so called "welding" brazes and solders for aluminum are much lower temp products, and I wouldn't bother with them

I know what I'd do, if absolutely no chance of another repair type, and that is, I'd fill the interior with greensand, blocking one opening, and then pour molten aluminum from my furnace into the other. But I can hardly recommend that to anyone else, and that's just last ditch. I have done that occasionally in the past and it has worked for me, but I wouldn't take a chance on a casting as valuable as yours.

[Mike R]

To give an idea of the scale of the issue and a different view of the "worm hole" as I'm starting to call it in my head

[Jasonb]

I think I would set it back up in the fixture, face that surface back a bit more then open it up and cut a fine thread for a top hat bush that goes all the way through to the combustion chamber so it closes off the large end of the flaw.

Turn up a suitable bush an fix with JBWeld (550deg F) then when set machine as you were when you found the problem

[Admiral_dk]

Jasons sugestion is probably the best ....

Welding such a small part is moore or less out of the question - but hard-solder could be an option :

https://www.tav3.de/    This stuff works really well - but would probably still be too much on this scale.

Best wishes

Per

[uuu]

I'm voting with Jason also - a sleeve in the spark/glow plug hole will seal things up.  Perhaps then a solder mixed with metal powder?

Wilf

[Vixen]

Hello Mike

Bad luck in discovering a flawed casting after all this time and after all your work. It would be nice if you could find a replacement casting and start again, but I doubt that's going to happen. So, we need to think about the best way to recover the flawed casting.

My take:        Normal TIG welding is out because of the small size and delicate nature if the casting. Forget any of the 'wonder' aluminium solders, oxide removal would not be possible inside the tiny void cavity.

I agree with Jason, an aluminium top hat insert would repair the plug thread and hi Temp JB weld to fill the void cavity. However I think you will need to do some extra preparation and exploration before applying the JB Weld.

Looking at you IMG 5907. I suspect the flaw extends further to the left between the two valve seats. There is a dark stain with what appears to be a crack/ flaw. you may need to open the edges of the obvious flaw and see if it extends towards the left. You may need to open the sides of the hole and explore the extents of the flaw You must to be sure all the white powder (investment??) and rubbish is removed.

A dental amalgam packing tool/ carrier/ gun (search ebay or amazon  one-use packing tools are not too expensive.) would appear to be a good way of packing the Hi temp JB Weld tightly into the cavity. Gaining a good access is another reason for slightly opening the hole inside the combustion chamber. 

I cannot think of any other alternative method of packing the JB Weld into the cavity.

Good luck

Mike

 PS don't be too worried about combustion chamber temperature. Methanol burns significantly cooler than petrol (gasoline)

[Jasonb]

Out of interest what holds the valve cages in place?

[Floating around]

Hi,
that’s annoying! Give me a couple of days and I will pm you the details of the current owner of the dies. He may have a casting or 2 left. There are plans to make more but no timeframe.

[Mike R]

Thanks to all who've supplied suggestions, I'll try and answer here as best possible.


I was probably still in a bad frame of mind when I posted and still recovering from the shock and disappointment of the discovery to have a full think through of the issue.

Floating Around - Please do PM those contact details - I've been attempting to track down known associates and have emailed authenticscale.com who listed Bruce as a contributor to their engineering and consultants in the hopes they can put me in touch with someone.

To those who suggest JB Weld (or equivalent) I may need to reconsider - I had in my haste assumed that the head would be too hot.  But on further review and research it does appear that the head temperature could be within the acceptable range for those high temperature epoxies, especially as I will not be running this engine hard. 
Jason - the valve cages are held in with Loctite 638 - rated to 180 C,  as recommended by the drawings - so if that temp rating is good enough for the valve cages then the epoxy is probably good too from a bulk cylinder head temperature perspective.



I got out my dentistry tools and poked and scraped to find the extents of the defect.  Then I gave it a long cleaning in the ultrasonic bath, so I'm pretty sure I got all the junk out that is contiguous with this void. I have mapped out and drawn on the part the "subterranean" cavern.  It is effectively hemispherical within the bounds I've drawn.  Making a plug that fits well between the valve cages is going to be a real challenge.


I'm actually not that worried about the side of the glow plug hole.  There is enough metal to support the thread of the plug that to me its just a matter of plugging up any gas paths.  I do agree that the proper fix for that portion is the top hat type plug and that is what I'll plan to do. 
I was more worried about the effect of combustion on exposed epoxy for the section that is nearest the valve cage.  But on thinking about it more, there is no reason I can't use both epoxy and a bit of metal facing - as long as the metal is held properly by the epoxy and is maybe keyed.  That also could be overthinking the issue.
Perhaps better would be a mix of JB Weld and aluminum shavings - I'd think that might be the solution for the combustion chamber area, or a bit of larger aluminum pieces back filled by the JB Weld and small shavings...


I'll ponder it some more - there's no hurry - been plucking away at this for more than 10 years - no need to hurry now!

[vtsteam]

Sounds like you have a good plan with Jason's suggestion.

Just for interest's sake, re aluminum braze products, I thought the following might be of interest generally:

<a href="https://www.youtube.com/watch?v=fKIKsDfRAcs" target="_blank">http://www.youtube.com/watch?v=fKIKsDfRAcs</a>

[Jasonb]

Looking again I was concentrating on your comment that the void connects the combustion chamber to the glowplug hole and that the engine won't function.

Actually all it is likely to do is result in slightly lower compression ratio as there is no connection from the combustion chamber to anything outside. So the engine will work with the void and any filling is really only to even up the compression.

Maybe fill a syringe with JBWeld (no needle) and see if you can pump it in until it flows out the side of the Glowplug hole)

[petertha]

Sorry to learn of your discovery. I just so happen to be looking at some Harris aluminum braze products. I had some grand plans & might still try some Al-Braze for personal R&D. But the melting temp is getting up there relative to base aluminum. So aside from visual feedback on the flux state, not too much extra torch dwell time between flowing braze into the cavity & a puddle formerly known as Head. I have seen some of the low temp sticks. I have not heard good things about strength, but maybe for this purpose it might be suitable assuming combustion temp stays under melt temperature by a safe enough margin. Is there any similar scrap part in your kit you can experiment on?

I assume the bronze? valve cages are already in place further aggravating the situation?

https://www.harrisproductsgroup.com/en/Products/Brazing-Soldering-Fluxes/Brazing-Alloys/Aluminum

https://www.harrisproductsgroup.com/en/Products/hpg-na-albraze

[john mills]

what we used when i was an apprentice on full size engines was tapered plugs made of similar metal .drilled a hole then tapped the hole
with the matching tappered thread screwed in the plug tight.then the need plug would be placed touching the last plug .then the plug ground down and peened in then ground to the original shape .plugs were used to repair cracks or any defect holes re tapped as required .
screwed plugs peened in might be good .
John

[vtsteam]

So John, what I'm understanding, and please correct if wrong, is, two tapered threaded plugs, one above the other, in a single tapered threaded hole. Both peened to tighten them further?