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Old 09-05-2007, 12:53 PM
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Mig Brazing Cast Iron Cylinder heads

What do you guys think of mig brazing for cast iron cylinder heads. I have a head and want to modify the combustion chamber and short side radius of the inlet port.

After a bit of research I found that mig brazing has a working temperature of approx 1000 degrees (1850 F) rather than 1600 (2900 F) degrees for mig welding. Also the melting temperature of the braze is around 800 degrees (1472F). While this is lower than the cast iron it is much higher than the melting temperature of aluminium (630 degrees (1200F)) so should not melt during combustion.

Is this possible?

Best Regards
Tom Murphy

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Old 09-05-2007, 04:12 PM
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Whynot just arc weld them with a cast rod? preheat weld aneal
Shane
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Old 09-05-2007, 06:48 PM
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Quote:
Originally Posted by Tommurphy73
What do you guys think of mig brazing for cast iron cylinder heads. I have a head and want to modify the combustion chamber and short side radius of the inlet port.

After a bit of research I found that mig brazing has a working temperature of approx 1000 degrees (1850 F) rather than 1600 (2900 F) degrees for mig welding. Also the melting temperature of the braze is around 800 degrees (1472F). While this is lower than the cast iron it is much higher than the melting temperature of aluminium (630 degrees (1200F)) so should not melt during combustion.

Is this possible?

Best Regards
Tom Murphy
Welding or brazing on cast iron is difficult at best with a fairly low success rate. Even the "low" temp of braze causes extreme heat for cast iron to handle. The heated area tries to expand but is contained by the surrounding cooler material, this results in crack formation. The only reasonably successful method is to heat the whole head till in glows a dull red. Then build up the section with weld. Braze is similar to solder in that it flows into the pores of the parent material, it doesn't lend itself to building thick surfaces. The flow into the pores of the parent material also tends to propagate cracks as now there's a greater volume of material than originally there. This really gets guys who try this on 4130.

You will have cooling problems if otherwise successful as the thickness of the material would become an impediment to heat transfer. Doug Roe made a Vega head with a Ricardo chamber many decades ago by building a squish/quench pad opposite the spark plug and valve side of the chamber. To get this to cool properly he had to first mill a large hole in the combustion chamber into the water passage, then weld over this such that coolant was never more than a quarter inch away from the fire side of the chamber. His success rate with welding on these heads was about 1 in 6, and he's a pro with really good facilities and equipment.

When the welding is complete the head needs to be covered and allowed to slowly cool for a day or two. Then it will be warped and will need to be re-machined on every surface.

However, pro racers like Bill Grumpy Jenkins did a lot of port reshaping back in the good old days using sheet metal and braze, but these were week-end heads at best before the water leaks were so bad they had to be scrapped.

Aluminum is much more successful to weld or braze on as it's a lot softer and the cold material around the heated area will give to the strain. However, any hardening like a welding on 356T6 will make 356T0 out of it and the material then needs to be re heat treated to establish a uniform hardness. For the street you can probbaly get away with welding and using, but for a competition part, failure will be right around the corner if heat treat and re-machine is skipped.

Bogie
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Old 09-05-2007, 07:55 PM
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Ok guys I have been doing this for years with great success.

In the shop I have a bake and blast setup.

First I clean the cast iron subject piece, than place in the oven. Usually around 500 degrees. Then, when hot. I fire up the MIG welder with Bronze wire in it. (.035). I weld the damaged areas. Then quickly put the welded piece into the Shot Blaster/tumbler. This stays in for about 45 min. Then slowly cool.

I buy quite a few damaged heads/blocks on e-Bay and Use most all of them for street/racing applications.
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