Thread: Chassis repairs - patch or butt into the frame?

I would but weld, others won't, but don't use right angles, cut your patch in a diamond.

have a read of this.
Tying frame sections together - OFN Forums

It's an interseting question, the rules suggest you shouldn't weld the chassis, but then all you have to do is remind them that the chassis is already welded.

The usual practice is to fit and weld a patch to size and then weld a reinforcing across the weld line usually in a diamond shape at each end of the repair.

You should not cut both sides of the chassis rail in the same plane on both sides of the box, i.e. have an angled cut from top front to bottom rear on the outside and top rear to bottom front on the inside preferably with the cuts offset fore and aft.

Avoid vertical welds that continue from top to bottom.

All rust repairs I have done on series chassis I like to make appear as if they have never been done.

i.e. cut out rust, make plate to fit exactly in hole. Weld in, grind welds flush.
I have never had a (chassis) weld break, despite heavy loads and offroading that would have stressed the chassis.

I use 3 mm for the repair sections. Original is ~2.5 mm.

The BIG problem with cutting out rust & then welding in a patch is that you get nasty surprises when you stitch up viz. the surrounding metal is not as thick as you thought and you blow holes thru it.

If possible, I would weld a backing plate via a few plug welds on the INSIDE of the chassis member bigger than the patch. Your patch now sits in a recess making the stitch up a breeze and only you know it's there! Plus it adds to the strength.

You could hold it in place with self drill screws, drill a couple of holes, plug weld then progressively remove the screws plug welding as you go. Slap your patch on with magnetic clamps & "Robert's your father's brother!"

Best advice for any chassis repair. Tell no one. When I bought my 130 cab chassis new it was not registered as I wanted to make and fit my owm steel tray. I didn't want the flimsy ally tray. The dealer gave me a month to make the tray, return the vehicle then they registered it. I cut off all the superfluous mounting brackets and metal I did not need. I had to make sure nothing was obvious as an engineer report would have been required.

One problem with doing welds on a lap-joint is that damp then accumulates between the two sheets and rust of course, finds it very easy to propagate.

If you do butt-welds, leave a gap at least as wide as the metal is thick; tack together at regular intervals (about 30mm) and when you weld it do it in stages allowing the chassis to cool, before doing the next weld on the opposite side of the repair.

Before welding the chassis cut several test pieces of the correct material (about 75mm x 30mm) and practice your welding joining two pieces together; this also allows time to get the machine set to the correct amperage. A good weld will penetrate completely to the back of the join and the test pieces when bent through 180 deg., will not crack on the weld,

Cheers Charlie

Now I'm confused.
It seems butting the patch in is the go. I'm using 3mm mild steel.

One of the patches will be just above where the exhaust goes under the chassis rail. The bottom of the rail and up the inner side of the chassis have some holes. So its going to have to be an L shaped patch.
If I do an internal fish plate as Geodon suggested I have the advantage of reinforcing the existing metal, but the issue ( as Chazza said) of lap joints and capillary action causing rust, as the fish plate will only be attached to the existing chassis through plug welds and the butt/lap weld around the outside of the plate. Leaving the outer edge of the internal fish plate unwelded and open to water ingress.

If I dont do an internal fish plate, but butt the plate in, then fish plate over the top, I've probably got a stronger fix (assuming good chassis metal) as I can plug weld the fish plate to the chassis and the plate and seam weld the outside. However you can see it.

Am I over thinking this?

Welding seems to be 90% thinky, cutting, prep, 10% worky.

As an aside how likely am I to set fire to stuff/ the car? I was thinking of using a fire blanket to wrap/cover any wiring fuel/brake lines in the region of the welding.

There will be much practise of the type and manner of welds I need to do before I get anywhere near Bill, and if I don't cut the mustard as a welder I have a mate who can who's volunteered, but I'd like to do it if I can.