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Cracked cooling fin on my cast iron cylinder.
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nickjaxe
Runcorn Cheshire UK
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August 1, 2017 - 9:48 pm
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 Noticed a zinging noise on my B175 of late...looking arouind the engine tonight I found a fin crack on the cyclinder its the top one under the head.

Hope the picture comes up ok...if it does the crack starts between the chalk marks...and runs back to near a stud hole.

I can move the edge of the fin with my fingers.

I know cast iron is not easy to weld...all I have is a gas mig welder an arc and can use gas welding gear in work....with which I could braze it...not sure if it gets to hot for brazing...wonder what temp braze melts at.

Seen a few vids on youtube of guys using a mig to weld some cast and it looked ok...but what will it be like a few moths down the line.

Could do with some advice on which why I need to go with this guys.DSCN6382.JPG

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BASIL
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August 2, 2017 - 8:28 am
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Hi, brazing would be best and strongest in my opinion. Regards Basil.

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The Artful Bodger
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August 5, 2017 - 10:09 am
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I've mig welded a number of broken fins, I wouldn't call it a structurally sound weld but plenty strong enough for fins or filling holes. The secret is not to allow the weld area to get too hot, the problem with welding cast iron is as with any weld the material expands with the heat. As soon as it starts cooling again the cast hasn't got the flexibility "give" (ie. stretch) needed to allow for the metal shrinking. This usually leads to the cast cracking alongside the weld as the metal shrinks.

 By V-ing along the crack, or the edges of two pieces being welded together, the weld is made with a series of "bare minimum" spot welds. These spot welds are literally a spot but the spot needs to arc enough to allow penetration into the metal being welded. Because of the V-ing the weld joins the thickness of the fin without having to burn its way in, the only way this works is to keep the local heat to almost nothing.

  With plenty of gas flow to help keep things cool, weld a spot then stop, move far away from the first spot and do a second spot and allow a few seconds for the heat to disperse between spots. Keep doing widely spaced spots with the cooling few seconds between until the V is totally filled, now it can be ground back. There's a judgment call with the spot timing, the spot needs to get hot enough to actually weld but not heat the fine any more than the minimum.

The usual method of cast brazing or welding is to pre-heat the whole job so it has expanded before welding, then it all cools and shrinks evenly preventing the stress cracking caused by the local heating around the weld.

 I wouldn't use this spot method for a job I was relying on for any strength, but I've done half a dozen or so fins this way. As a by the by for fin repairs, if a missing piece is replaced with mild steel instead of cast iron it helps. Using the same spot method the advantage is the mild steel will "give" as the weld cools so some of the shrinking stress will be lost in the mild steel.

Colin

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nickjaxe
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August 5, 2017 - 10:24 am
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Thankyou for that Colin...very interesting.

When I get the barrel off today I hopefully will be able to see exactly what I need to do.

If I can get away from applying and heat I will...

Nick.

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johnsullivan
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August 5, 2017 - 11:37 am
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The epoxy method works on cast iron reasonably well .Since the fin is not a stressed member cleaning and puttying will stop the the resonanceJB weld high heat works well on stoves {usual disclaimers} and is still sealing an outboard inlet casting .For about 7 gbp might be worth the punt.

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nickjaxe
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August 5, 2017 - 11:41 am
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Come to think of it John...I have an unopened tube of the stuff...and a roll of that 2 part putty stuff that you roll between your fingers...sets like metal.

Thanks for reminding me.

Nick.

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johnsullivan
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August 5, 2017 - 11:47 am
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cheers

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nickjaxe
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August 5, 2017 - 5:26 pm
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Ok took the barrel off today...crack is quite long...still not decided what to do with it...see pic....the end of the crack is under the black dod...far as I can see.

Thinking of drilling a hole at the end of the crack...a blob of mig weld to fill in the hole...maybe well the outer end on the crack and nut bolt big washer in the middle.

I wonder...is it likly that this crack has been making a noise...I have been hearing an odd one of late....sort of a tinny sound.
DSCN6385.JPG

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Mags 1
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August 5, 2017 - 5:34 pm
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You could drill a small hole in area of black dot to stop crack getting any longer.

Especially if you use bike regularly, but I think it's off the road at present?

Four now on the road and at least several in bits.

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