What is a fish plate, I have seen the term used when shortening a frame but I dont know what it is?

 

The term Fish Plate refers to the repairing of a cracked frame rail by first welding the cracks and then reinforcing the rail by welding another plate that covers and extends well beyond the repaired area.

 

When repairing or modifying a frame be sure to NOT weld the patch completely around; instead leave a slight open area on all 4 corners. A complete weld creates new stress risers that could eventually crack.
Note I say "could". Lots of frames done the wrong way survive.

Also when splicing a frame for shortening or lengthening do a Z cut. Weld, grind smooth and then fish plate.

 

Carl,

Could you define "Z" cut?

Thanks

 

John its hard to put it into words.

Looking at the side of the frame the ideal cut for max strength would be at a 45* angle, 30* or a tad less is OK if its not a BIG truck or a real long stretch. The same for the tops and bottoms. Use wide masking tape to line/mark up all the cuts, measure 3 times, cut once. Use a new, quality Sawzall fine tooth blade; if the blade binds and bends get another.

By keeping away from 90* cuts you spread the stresses and leave no new risers to cause problems. The fishplate should extend 12" either side of the cut so choose the location carefully.

Take a look at a stretch limo, airport Explorer or similar.

 

Carl,

That's what I thought, but wanted to be sure. (have we talked about this before?)

I just pictured more of a zig-zaging cut than an angled one. Zig-zags would be hard to do, angles aren't that bad.

Thanks.