lilgirlblue if you check into the 400's and read up on them you will find that they do have that problem. it may have been corrected by now but a friend of mine that i race with has one. it didn't crack immediately it took about 3 or 4 months of track time but the sprocket side arm cracked in two. there was a 1/4 inch gap between the two pieces. then again that is under motorcross style conditions not trail jumping.
trail jumping isn't near as hard on the equipment

 

Hey there 250xRacer-My hubby had not heard about this. Does he need to check the rear swing arm or the lower control on the front. And where can we find the articles about this problem. He also asked where are they cracking? Thanks for letting us know about this, I thought I had done all of my homework, but there is always something new to learn about.

00 400ex
01 300ex (mine)

 

the crack my friend had was in the rear swing-arm on the sprocket (left) side about 1 inch away from the swing-arm bearing case where it welds to the arm that goes back to the axle carrier holder (hopefully my discription didn't confuse you).
4-wheel ATV action, June 1999, page 22 Article name "Top 400ex Tech Tips" , Subtopic "Keep An Eye Out". that was the first article i ran across tonight just flipping though to give you a reference. I hope i helped.

 

Hey, I've got an 86 and a 87 R. Original frames and swing arms. No cracks, breaks at all. Mine was thrashed when I bought it, been raced hard. No cracks or breaks anywhere. Original rear axles.
An old R is still better than any of them for jumping.
For the money, you can get an R, mod it out the way you want, and still have less money in it than a new anything.
So, quit wasting your time. Get the best. Get a R!