Masking a dam for braze repair

Dumpster_diving · Apr 26, 2021

I’ve got a cast iron flywheel where the hub broke away at the key way. Due to the age of the unit, it’s not practical to obtain a replacement flywheel, so I’m going to attempt to TIG braze back the missing material. I turned a male mandrel of stainless bar stock with the same taper that I’m going to use as a dam, to keep the braze from filling up the unbroken portion of the flywheel bore. I need to mask the stainless somehow, to keep the bronze from sticking to the male plug. Ordinarily I would use soot from an acetylene flame to coat the plug, but I don’t have access to an OA torch.

Suggestions for a simple masking agent (i.e. readily available at a hardware store) to use in place of acetylene soot? That won’t off gas too much to ruin the adjacent braze build up?

Dychem? Spray paint? Sharpie?

Phil in Montana · Apr 26, 2021

Your question leaves to more questions, a flywheel can be a dangerous thing, so the repair needs to be good, most of the time you would just weld it up and MACHINE it back to size, looks like you want to skip that part...Any repair that does not use the Machining part will end in disaster. The flywheel will not run true on the shaft do to the welding..Ports a pix ...Phil

Dumpster_diving · Apr 26, 2021

This is the mandrel I turned up that shows the taper

Dumpster_diving · Apr 26, 2021

This is the flywheel

Dumpster_diving · Apr 26, 2021

The braze is in place of welding, because it’s cast iron. The bore will be cleaned up on the lathe and a new tapered key way cut. So there will be machining.

Part is a Briggs and Stratton engine and the flywheel (even as broken) is still captured. So no safety risk involved.

rklopp · Apr 26, 2021

Bic Wite-Out correction fluid works really well as a stop for silver brazing. I brush it on and then bake off the water/solvent before final high heat. I’ve never tried it for bronze brazing.

Phil in Montana · Apr 26, 2021

Ok,its a low value casting when made, (cast will be crap) vee it out, braze the part, along with the keyway and machine, I would not be concerned where the brass flowed, but you can shape a carbon block to act as a dam if you can hold the heat away from the carbon...I might even machine the out side and shrink a band around it...Phil

Scottl · Apr 26, 2021

Know anyone with a carbide lantern or sight smoker?

As I recall, even soot from a candle is mostly carbon.

BT Fabrication · Apr 26, 2021

that whole bore will change diameter with all the welding heat and make it oblong. not a great repair, just V it out and stick it back in. it will most likely break as soon as its mounted back on though as the flywheel dampens the pulses from the combustion cycle.

only other way is bore it all out, and machine down a sleeve and lightly press fit it back in, but anglular alignment is key in that part.

good luck, gonna need it.

Dumpster_diving · Apr 26, 2021

Thanks for the input folks. I’ll give the white out a try, and candle soot is a good back up. I’ll check the clearance around the hub as a steel band would add a lot of strength. But I suspect other parts above it (fan shroud ) will prevent adding dimension for a band. But we’ll see.

I hear you about the distortion. I’m thinking I’ll have to re-machine the taper to restore cocentricity once the brazing is in place.

atomarc · Apr 26, 2021

With all the bazillions of B&S engines made over the years it seems there should be a used, intact one kicking around somewhere.

Stuart

Scruffy887 · Apr 26, 2021

Buy a fat carpenters pencil and scribble where you want as a release agent. Graphite. Test on scrap first.
Black pencil please. Lumber crayons will not work.

true temper · Apr 26, 2021

What model of engine are you working on.

steve-l · Apr 27, 2021

I read a lot of dumb ideas here, but this one is truly insane. The OP does not need luck, he needs his head examined. He is endangering himself and everybody around that engine when it is run.

wgnrr1 · Apr 27, 2021

Preheat and post heat. That cast will crack from heat variances in a different spot. I've been doing a ton of brazing repairs lately. Usually something someone else tried first and cracked it elsewhere.

Sent from my rotary dial flip fone

woodchuckNJ · Apr 27, 2021

copper does not weld, I don't know about brazing with bronze, but generally I use copper to back up a weld that I need to protect.

Dumpster_diving · Apr 27, 2021

Atomark and true temper-

It’s a 20 year-old lawn tractor. Went down the road of trying to find a replacement flywheel first. No joy. OEM part is discontinued and not stocked new. EBay turned up only one similar, but with the timing key not oriented the same way to the magnet that triggers the coil. So timing would be off.

So I’m stuck with repair.

Steve-l, I guess I’m not actually stuck with repair. I could scrap the tractor and buy a 2,000 replacement. Or I could spend 1,200 and do an engine swap. But in my opinion, the flywheel is repairable without compromising safety. Only the upper boss is broken. The entire perimeter of the flywheel (including the entire bore/taper the full thickness of the flywheel is intact. Even if my repair fails, the flywheel is still retained on the motor shaft with the 7/8” bolt and washer and 1” of undamaged cast iron flywheel. So whether the repair holds or no will have a lot to do with my skill in executing the repair. But dumb or insane? I think that’s hyperbolic. I’m comfortable with my plan. You comfortable sitting with your criticism?

BT Fabrication · Apr 27, 2021

20 year old, time to junk it and buy something else.

woodchuckNJ · Apr 27, 2021

BT Fabrication said:
20 year old, time to junk it and buy something else.

Hell no... we are machinists right? Repair...

steve-l · Apr 27, 2021

Dumpster_diving said:
Atomark and true temper-

It’s a 20 year-old lawn tractor. Went down the road of trying to find a replacement flywheel first. No joy. OEM part is discontinued and not stocked new. EBay turned up only one similar, but with the timing key not oriented the same way to the magnet that triggers the coil. So timing would be off.

So I’m stuck with repair.

Steve-l, I guess I’m not actually stuck with repair. I could scrap the tractor and buy a 2,000 replacement. Or I could spend 1,200 and do an engine swap. But in my opinion, the flywheel is repairable without compromising safety. Only the upper boss is broken. The entire perimeter of the flywheel (including the entire bore/taper the full thickness of the flywheel is intact. Even if my repair fails, the flywheel is still retained on the motor shaft with the 7/8” bolt and washer and 1” of undamaged cast iron flywheel. So whether the repair holds or no will have a lot to do with my skill in executing the repair. But dumb or insane? I think that’s hyperbolic. I’m comfortable with my plan. You comfortable sitting with your criticism?

I absolutely am. If a used one is not available, make a new one out of steel. It is not rocket science and it could save a life.

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