Sealing a 5/32″ wide gap left in a slit Smokebox

[GreensandsParticipant @greensands]

I am in the process of modifying a slightly undersized smokebox in terms of diameter to fit an existing copper boiler and have been able to achieve a satisfactory fit at the expense of course of having to now fill in a 5/32” width gap left by the sawing operation. Initial thoughts are to mill a length of top hat section from 1/8” thick mild steel to fill the gap from the inside of the smokebox using small rivets to hold the strip in position followed by silver soldering from the outside to make a proper job of it. The line of the cut was arranged to fall within the area covered by the saddle to maintain external appearances. Before cutting any metal I thought it might be worth putting the issue to the forum to see if there might be any other possible solutions out there. Incidentally, I do not have access to any welding gear which be the more obvious answer to the problem. All thoughts and suggestion most welcomed.

[Nigel BennettParticipant @nigelbennett69913]

That sounds like a decent plan for your circumstances; I think it’s what I’d do.

Make sure that the 5/32″ wide bit sticks proud of the smokebox OD slightly, and then it can be filed/sanded or whatever back to leave a virtually invisible joint. Use Tenacity 4A or 5 flux, (Or CuP HT%) not standard Easyflo when you do it. Don’t be too enthusiastic with the riveting or the solder won’t penetrate round the rivets.

[bernard towersParticipant @bernardtowers37738]

Personally I would look for a welder local to you and have a chat he might be interested enough to do it for a drink!

[Clive Brown 1Participant @clivebrown1]

Agree it seems good plan but I’m not sure that you even need to mill a tee section. A thinnish inner strap held with a few rivets plus a separate 5/32″ wide filler strip of suitable thickness in the gap would be basically self-jigging for silver soldering. Dare I suggest brass for the filler strip would be a doddle to file down to profile.

[Nigel Graham 2Participant @nigelgraham2]

Why the silver-soldering?

I think I’d use Clive’s suggestion of two strips, with countersunk rivets or screws, but no soldering. Heating the unit could risk distortion and is not really necessary.

I’d rivet the two strips together firts to form a stubby T-profile, (no reason you could not machine the profile though), and clamp that within the slot to drill through for the two rows of fasteners.

Screws or rivets? Structurally similar. If screws, make the inner strap thick enough to tap for them. Don’t countersink the outside quite to full depth, so that once assembled you can file or abrade the heads down flush, removing some of the slotted portion while still leaving enough head for strength. The paint will fill what’s left, and as you say it is far down enough to be scarcely noticeable.

 

Consider that some model locomotives’ smokeboxes are made in two parts split along the centre-line and held together with straps and countersunk screws. This allows removing the top half for better access to the pipework and fittings, though raises having to disguise the fact, unless the locomotive is a class with big smoke-deflectors.

So simply using screws or rivets should be adequate. Make the filler-strip a good fit for alignment and to help sealing, and once painted the joint will be air-tight. No harm in a spot of high-temperature sealant along the joint as well, before assembly.

 

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Had you not already drilled for the two pipes you may have been able to exploit that slot as on my loco, making it the blast-pipe diameter wide and along the bottom so the smokebox can be drawn off completely – the boiler has to be temporarily supported across the frames. This system does need a continuous saddle, not the “roofless” pattern, with match-plates to suit.

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