Thursday, 30 April 2020

FH Dry Build - Fitting & Repairing the Rear Mudguard

Updated: first published 27th April 2019

The whole point of a dry build is to trial fit parts before committing to an expensive paint finish that you might ruin with any remedial work you have to do, and the rear mudguard was a good example. Aside from the moth damage at the join, the mudguard wouldn't fit with the oil tank in place.

You can see the back of the oil tank bulges and pushes the mudguard to the nearside:

The centre bolt won't line up:

And here is the problem - the back of the oil tank is supposed to be flat:

Five minutes with a dead blow hammer, working around the edges has it back in shape. There is clearance between the back of the oil tank and the mudguard now, and the mudguard's centre bolt is in place.

Having got the mudguard to fit, we now need to repair it. It's not the worst I've seen.

This is the rear end of the main section showing the rust damage to the bridging piece, to which the tail section is bolted:

This is what it is supposed to look like:

And here is the front end of the tail section:

Again, this is what it is supposed to look like:

I cleaned this up with a wire brush a while ago to get an idea of what I was going to be dealing with - those holes are in the main blade section as well as this joining panel. Notice how the joddle that lives in the mudguard rib is rather deformed - it's flattened toward the rear end. It's quite square at the front and is clearly supposed to fit inside the rib, with the rear light cable below it - the cable does not go between the mudguard and the joining rib. We'll need to remove that, make a new one, and repair the guard as well.

Here's the blank, sliced from a quarter sheet of 20 SWG CRS. It's 3" wide and about 15" long.

Here it s again, with a datum edge trued up on the linisher. It's marked out with a Sharpie against the dimensions of the old, moth-eaten one.

Here are the first two bends going in:

And joddling the top hat section in the middle:

Next step is to fold the two lower channels that fit around the rolled edge of the mudguard, then we will make the main bends. When that fits, we will cut out the old one and repair the main blade.

Here's the first bend of the two side channels:

To make the next bend, the work has been inverted from the previous shot, and it is clamped to one of the folding bars using a piece of 25 x 4mm flat bar. The second folding bar has been raised by the depth of the bend:

Here the work has been folded down over the raised folding bar, producing a joddle:

And here, the joddled section has been clamped behind a 1/2" bar and bent again, producing a shallow channel:

That done, we joddle the last bend and repeat the whole process at the other end to produce this.

I'm very pleased that it sits flat on the bench, with it's datum edge remaining straight and proving that all the bends are square.

Next we need to make the curves, and for that we need a former which is where this empty gas bottle comes in. These were a very expensive way to buy Argon that I used to use before I rented a refillable bottle:

A line drawn parallel to the axis of the bottle, and another square to the datum edge ensures the bend is square:


And it even fits. The marker line you see in the holes is drawn 1/2" from the datum udge, copying the hole positions from the original:

I've drilled the top two holes 9/32", to clear 1/4" screws which we will use for the next steps. These screws will ultimately be 3/8" (for my 5/16" threaded bobbin), but drilling them smaller allows me to move them a little if I need to. The bobbins look like this:

Having a quick look at the side pieces and the holes reveals I have been generous with the length of the blank, and the rolled edges of the tailpiece prevent the over-long bridging piece sitting in the correct position.

The next job therefore is to cut away the old bridging piece from the front section of the mudguard, so we can edge a bit closer to having the bridging piece in the right place and trimming it up. The old one was not too difficult to remove as it was extremely weak; I could see where a couple of the spot welds were and I removed these with a special spot-weld cutter which has been knocking around in my sheet-metal toolbox for decades:

It's a small, double ended saw blade (imagine a 5/16" hole saw) mounted on a mandrel. You can cut right around the weld:

Here's the end of the front section with the bridging piece mostly gone:

I'm going to cut it away back to that black line and let in a new piece of steel, like this:

I'm very happy that I've finally found a decent bi-metal blade for the bandsaw, as it makes life very easy when you want to cut out bits of sheet. This is the new panel - I used a similar technique to before for the centre rib; the curves are bent by hand:

Tacking up. I have a narrow dart in either side to let the new panel curvature match the old guard:

And here is the joining piece, with similar darts. Here it is bolted to the rear section:

These are skin pins - I have about 15 of these 3/16" pins, used for holding sheet metal parts together before welding:

Here's a couple of pictures of the mudguard bolted up:

From the inside, the bridging piece is approaching the final shape. We need to machine the weld nuts on the lathe and get those in next.

This view shows the mudguard with the bobbins in place and bolted up. I made them 1/4" BSC, to give myself a bit more wiggle room. I may make up some special 1/4" bolts with 5/16" size heads, domed and polished.

Here is the bridge piece again, held in place with a few skin pins and with the rear section bolted in place with 1/4" BSC bolts. This all looks good and the bobbins & bolt holes all align, so we can weld the bobbins in place:

Leaving the skin pins in, we can start closing the various joints. Secondly, one at a time, we remove the skin pins and make a plug weld in the hole, which fastens the bridge piece permanently to the main section of the mudguard:

Turning our attention to the other half of the mudguard, we draw around the mangled ears onto a small piece of 20 SWG, and cut it out with snips and the band saw. I've left  14 - 5/16" edge to allow for turning over:

We can raise the edge using various forms - lifting the whole edge gradually to avoid unwanted stretching:

Eventually we can raise it to 45 degrees - and then the end can be beaten flat.

We need two of those, one LH, one RH. We'll use the snips and a hacksaw to remove the old ears:

We need to be very tidy about this, so the new piece fits in the right place:

Welding against old metal is often tricky, but this has gone surprisingly well. Not so neat, mainly due to the fact that I dipped the tungsten in the weld pool part way through...

And the other side - there are four holes to fill, two holes to drill and then we are done:

I'm not sure what these two holes were for - a rack perhaps. To repair them, first clean the area thoroughly:

I tack a little patch on the back - this will be ground off eventually:

Fill the hole from the outside:

We'll grind it flat in a bit. We can do the same with this one:

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