Repair Broken Cast Iron Part for Classic Car

A guy who is working on the restauration of a Bentley from 1928 asked me to help him with the restauration. I was very happy with this opportunity, so I immediately accepted.

I have already done several interesting jobs related to the restauration of this car and I am probably going to make an Instructable about some of these jobs if they are interesting enough to share.

Today I want to share the process to repair a broken part which is made of cast iron. This cast iron part is bolted to one of the bearings of the crankshaft and takes care of the oil supply to the crankshaft.

It is not a good idea to weld cast iron just as if it was steel, because it is quite likely the part will break again.

The problem with cast iron is that it contains quite a lot of carbon. There is not enough space in the cristal structure of the iron to give each carbon atom a place where it is completely surrouded by iron. So there are small thin areas in the cast iron where there are flakes of pure carbon present. These flakes are like many little cracks between the metal and they make the cast iron brittle.

The heat and the stress caused by shrinking forces after welding can cause additional cracks between the flakes of carbon.

Especially parts of a car that will get hot during use and that have to endure some forces can rupture again within some months after welding.

It is possible to weld cast iron if special welding rods or electrodes suitable for cast iron are used, but they are very expensive. An alternative for welding is to use a brazing process instead of welding. Brazing can be done with a flame but nowadays TIG brazing is also a possibility and that is the process I used.

I used a TIG welding machine and silicone bronze welding rod. There are different types of silicon bronze rods, some are filled with a flux and others are solid. For TIG brazing the solid silicon bronze rods are needed.

The American code for this rod is ERCuSi-A.

The European code for this rod is CuSi3Mn1.

The material number for silicon bronze is 2.1461.

The TIG brazing process is very similar to TIG welding. The difference is that the heat is lower, so only the added silicon bronze melts and the cast iron with a higher melting point than silicon bronze, does not melt.

(It would also be possible to TIG braze cast iron with aluminum bronze. That is slighly stronger, but it requires a TIG welding machine that produces AC welding current and a lot of welding machines are not able to produce AC current.)

The broken part from the Bentley engine was quite small and it had a thin wall of just 2-3 mm thick (1/8").

I did not want to damage that part or expose it to any unnecessary heat, so I wanted my machine settings to be right immediately.

Fortunately I had another cast iron part with two bolt holes that I could use to practise. I used a grinder to cut the part half open and then hit the part with a hammer so the rest broke off. On the photo you can see that the ruptured side is gray/black and the cut side is shiny metal. This gray/black surface has this color because it snapped where all the flakes of carbon were in the cast iron.

As a reference I welded one side back in the way I would weld steel (The right side of the second photo). After I finished this weld I heard sounds sounding like "ping-ping-ping" coming from the part, so inside the part the heat stress from welding was already making small ruptures.

For the left side of the practice part I used TIG brazing with silicon bronze.

After that I found some 1-2 mm thick steel and made a fillet weld to find the best machine settings.

My settings were:

Pulse mode with 33 pulses per second and 33% on time.

Peak current was 100 Amperes. Base current was 33 Amperes.

(I was inspired by Jody from Weldingtipsandtricks.com to use what he called "the rule of 33" and it worked very well.) Without pulse mode I used 60 Amps for testing and that worked, but the pulse mode was better.

As you can see on the photo the color of the silicon bronze looks a bit like gold.

I used a file to remove the gray carbon layer from the fracture and cleaned the part with acetone.

I alligned the part by placing a threaded rod that fitted exactly inside my part. I used the nuts to determine the right lenght, so my part would not be longer or shorter after my repair. The threaded rod is made of stainless steel, because galvanized steel would form spattering or toxic fumes when heated.

It is recommended to preheat the cast iron to approximately 200 degrees F or 100 degrees Celcius. Pre-heating is actually just a way to prevent rapid cooling. A cold piece of cast iron that is locally heated, will cool down rapidly because the heat will flow to the cold side of the part. When it is pre-heated the heat will stay in the entire part.

Normally pre-heating is done with a propane torch, but I do not have that, so I placed the part in a plastic box and placed that in boiling water for some time. The part stayed dry and was still pre-heated this way.

After my repair I wanted the part to slowly cool down. Some days before I spread some sand on the floor of the garage so the sand could dry. After that I put the sand in an aluminum container. Before I repaired the actual part, I welded some other parts and put them in the sand to raise the temperature of the sand already a bit.

Then I used the settings on my TIG welding machine that I tested before and with these settings I repaired the part.

Immediately after my repair, I place my part in the pre-heated sand and left it there to cool down slowly.

The result is a part that is in one piece again. The silicon bronze penetrated well into the inner side. Actually it filled the treads of the treaded rod, but it did not stick to the stainless steel of the threaded rod, so I could screw the threaded rod easily out of the center hole in the part.

As silicon bronze is less strong than steel I made the layer of silicon bronze a bit thicker than the wall thickness of the part, so the additional material will give some additional strength.