Brazing cast iron

[Tweegs]

Last Sunday the vac pump seized.
Found I could turn it over easily by hand a few times and then it would bind.
Seemed like something came loose or maybe a bit of crud rolling around inside was the cause so I had a minor rebuild kit and new bearings shipped overnight for delivery on Tues.

Cracked the pump open Monday night after work and found the problem was much more serious. One of the pistons broke.

Of course, this had to happen during the week we were doing a major software upgrade at work where both my presence and undivided attention were required for some very long hours.

No luck so far finding a replacement piston, but haven’t exhausted every lead yet …still working on it.
Probably no chance of getting a new piston prior to the season ending. Plan B would be to get another pump off e-bay for parts.

With nothing to really lose, I figured I’d take a shot at trying to braze this thing back together.
Never brazed anything before, but after watching a few videos and reading up on it, kind of reminds me of soldering, which I can do.
No worse off if I fail.

The pics show what I’m up against. The cylinder portion is 3/16 thick and the piston itself is ¾.
Figure I’ll grind off a little from both the piston and the cylinder but leave enough original metal to register. Clamp it up and spend more time heating the piston because it’s thicker, then have at it. Let it cool naturally.

Any words of wisdom before I fire up the torch?

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[maple flats]

While I'm not a welder or do I have any experience brazing anything that had to be exact (and I don't play one on TV, but I think it is to stand a chance you need to pre-heat the pieces, braze it, then put it in a hot oven to let it cool very slowly. Then you may well need a machinist the bring it back to spec. if that can be determined.

[prairietapper]

you really do need to preheat all the parts in the oven, the hotter the better. shoot for around 500 F. putting it in sand to cool slower would be good also

[Super Sapper]

You can spend the night at a Holiday Inn Express the night before.

[Haynes Forest Products]

Now its still early and I haven't eaten yet so give a little leeway here but isn't that the cylinder??

[Tweegs]

Now its still early and I haven't eaten yet so give a little leeway here but isn't that the cylinder??

Nope, it’s actually kind of weird.
Main shaft goes through the hole in the piston there.
Attached to the main shaft is a cylinder which has its hole for the main shaft drilled off center.
This cylinder acts as a cam and causes the piston to move up and down.
I’ve attached the parts breakdown for better clarity.

So here’s the story…
I heated the whole piston up using MAP gas. When I thought I could just start to see a little color I switched to the torch and brought the repair area up to cherry red.
Used lots of flux and got a nice puddle going and kept it going for the length of the repair. Did this on both sides, then just let it cool.
Had what I thought was a pretty good bond. Couple of hours with a die grinder and a flat file and the piston ran free and easy.
Reassembled the pump and tested. Everything looked good.

Put it back in service and it ran beautifully…strong, smooth, quiet…for about an hour.
Clink, clank, bang. So much for that. It was worth a shot.
At least I got the main lines out in the woods cleared.

Playing e-mail/phone tag with the regional rep for these pumps. Should be able to get something going come Monday.

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