Swingarm crack, 1976-7 MC-5

[Lloyd Boland]

I finally got the swingarm bearing races out and the new bearings pressed in.  In prepping to repaint the swingarm, I noticed a small crack in the swingarm, about 1 - 1.5 inches in length.  It is located just in front of the rear axle slot on the left side.  It is only on the inner side of the swingarm, therefore I never noticed it from the outside.  The crack runs in a somewhat vertical fashion.  It looks like someone in the past had also welded the swingarm just above the current crack.  Is this very common?  So I know what to tell the welder, if it is ok to weld, what type of metal is the swingarm made out of?  It is a stock swingarm.
Thanks for all the help.

Lloyd

[G Ellis]

It was common. The swing arm is made of chrome moly. Hope this will help. Later Gary

[Britt Boyette]

On airplanes with CM tube frames, we had to normalize the weld repair by heating up the weld and the surrounding metal with a rose bud tipped torch. Just heat it up until it turned red then let it air cool.

Britt Boyette
1976 125 MC5
1975 250 HS

[garrettccovington]

My 6-day had a little sticker on it printed 4130 chrome-moly. I beleive it was chrome with red lettering about 1/4 x 3/4 on 1 of the down tubes.  Anyways

  I believe, and I'm no expert, 4130 likes to be pre-heated before welding then slow cooled.  Any good "welding" supply store (not harbor freight) sells "temp-sticks" good for a specific tempature range.  You mark the area "around" that needs welding, heat it up with a torch until the temp-stick disapears and now it is the correct tempature to weld. I think this is used to minimize the shock of the high temp hitting the cold metal. Like I said I'm no expert and maybe things have changed over the years.

G

72 six-day
79 KTM MC80 250

[garrettccovington]

Lloyd
I did a internet search for "pre-heat chrome-moly" and alot of good stuff came up. Lincoln, Miller all had good info on the topic.  When and what Temp to pre-heat, what rod to use, tig, mig questions.  Quick read and really good stuff.

G

72 six-day
79 KTM MC80 250

[David Laite]

Lincoln does have a good read on the subject, no pre-heat necessary for thin wall. I would clean and strip the area, then stop drill the crack, grind a narrow V-notch on the crack and mig w/gas. You can then grind it smooth and re-paint. The weld will be much stronger than the material surrounding it. Please let us know how you make out.

1973 Penton Six Day
1973 Penton Jackpiner
1982 Yamaha XT200
1982 Yamaha XJ650J Maxim
1987 Yamaha YZ490
2005 Honda CRF450R