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What is the problem with welding cast iron?

The problem with cast iron from out perspective lay in its messy, irregular crystalline graphite / carbon structure. Under a microscope it looks like a random splattering of powder or bacteria growth on a petri dish, with some large blobs of carbon and some tiny. This is fine in some circumstances but not for welding.

When you try and weld using your nice bit of mild steel some of the carbon from the cast iron flows into the pool to form a three part system being:
Cast Iron > Cast Iron/Steel Mixed > Steel (or Cast Iron if you're using the original fin bit

This in itself isn't so much of as a problem as is their different expansion and contraction rates.

cast iron cylinder fin welding

These will each have a different expansion and contraction rate when heated so the theory is that if you heat the cast iron cylinder then weld it and let it cool slowly then the join will hold. This is true, and more so if you use a magnesium filler rod and allow to cool in hot sand for a couple of days.....but there is another way!

How to weld cast iron cylinder fins with a MIG – WITHOUT PRE-HEATING

The secret is that if you can't pre-heat the cylinder then you have to weld it as cold as possible and in a certain pattern to allow for the two metals to find their natural equilibrium at a certain temperature. You can also use stainless steel MIG wire rather than mild steel because this helps create a carbon barrier I believe.

cast iron fin welding pattern

  • Make sure your new fin piece is cut to size and the edge ground to an angle where it will join.
  • Warm the cylinder in the oven so it is not stone cold or leave in the sun
  • Line up the fin on the cylinder and clamp each end leaving the middle clear to weld.
  • FOLLOW THIS BIT EXACTY !! Look at the diagram above

  • Weld a single 1 SECOND or less spot weld right in the middle - DOT 1
  • Wait 3-5 mins minimum! You must NOT hear any crackle or pinging else its to much weld
  • Clean weld with wire brush and gentle picking
  • Weld DOT 2 for 1 SECOND or less if possible
  • Wait 3-5 mins minimum! You must NOT hear any crackle or pinging else its to much weld
  • Clean weld with wire brush and gentle picking
  • Weld DOT 3 for 1 SECOND or less if possible
  • Wait 3-5 mins minimum! You must NOT hear any crackle or pinging else its to much weld
  • Clean weld with wire brush and gentle picking
  • Weld DOT 4 for 1 SECOND or less if possible
  • And so on

    It is important to rush and not to introduce much heat from a bit spot weld. Aim for as small a spot weld as possible, allow plenty of time for cooling, perhaps even 10 - 15 mins between weld spots, clean between welds and listen out for any noise. Any noises are a bad sign, done correctly it will be quiet.

    When grinding flat later, do not be tempted to get an angle grinder and hack it all off in one go; you are better off with a dremel. Again do not get it too hot in one spot!. I'd be inclined to only down of the outer edges where the fins show more.

    To test the cylinder head, place in an oven at 250C and leave for 30 minutes; then place the cylinder outside and let it cool down and inspect for cracks - you won't find any if you followed this correctly and you won't hear any noises either.

    Before final grinding and painting....

    motorcycle cylinder fin welding

    Any amendments or additions; please suggest below.

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