Rear Axle Strip Down - Part 8 - Trimming Rear Brake Backplates

Before I can start reassembling the rear hubs there are a couple of components that I still need to clean and fettle.

I had completely forgotten about the ABS sensor rings that press onto the rear of the hub axle.  These were still in my box of removed parts.  These just needed the usual clean up, sandblast and powder coat to get them ready for reassembly.

Before and after cleaning and sandblasting...

...and after powder coating

The other components that needed attention were the rear brake backplates.  To be honest I had been putting these off a while.

The original XJ40 backing plate covers the whole of the rear disk with a slight lip that wraps around the edge of the disk.  In the AK set-up, it is recommended that the backing plate is trimmed down, although there is no guidance in the build manual on how far to trim (and the picture in the manual is based on an older style backing plate and not the XJ40 type that I have).

The backplates that came with my donor axles had, frankly, seen better days and the edges of the plates were rather chewed up and had succumbed to an attack of rust - although in theory given that the plates need to be trimmed this was not, initially, a concern.

XJ40 style brake backplate - this is the better of the two...

The first step was to give the plates a clean-up and see exactly what I was dealing with.  I gave them both a quick brush off to remove loose dirt and grime and then gave them a bath in some citric acid solution for a couple of days to remove as much rust as possible.  Given the size of the plates and my bucket, I could actually only immerse half of each plate in the acid solution at a time - but that was enough to see that I had a problem.

After half an acid bath - tidemark is clearly visible

It was obvious at this point that the lower plate in the above photo was generally sound including around the handbrake shoe mounting points.  The upper plate, however, was showing signs of tin worm in some critical areas around the lower brake shoe mounting block.

Signs of tin worm...

This corrosion was well within the area of the backplate that I figured needed to be retained after trimming, so this was not a good sign at all.  Holding the plate up to the light confirmed that there was even more bad news, with evidence of the dreaded rot around the handbrake retaining pin mounting holes as well.

Only one of those holes is supposed to be there...

The other side is not much better...

I was already concerned that the retaining pin holes were oversized and were likely to require some cunning plan to fix them to be able to hold the pins - but this discovery basically confined this backing plate to the scrap bin.

I dropped Ben at Simply Performance a note to see if he could send me a replacement pair of backplates.  I have to say I can't fault the customer service of Simply Performance one bit.  They guarantee that all the donor parts they provide can be refurbished and reused.  There's no quibbling over this and again, in this case, Ben said he would pop a couple of replacements in the post and, sure enough, two new(er) backplates arrived a couple of days later.  Ben had even sandblasted the centres of the plates to check that there was no rot present. That is service!

Shiny 'new' brake backplates courtesy of Simply Performance

The replacement backplates came without the lower brake shoe mounting blocks in place so I needed to remove these from my original components.  Following the dip in the acid bath, undoing the two bolts that secure each block was relatively straightforward.  On the rear side of the block, there is an additional cable guide formed from a short section of tube brazed to a (very) thin plate.  Again on the more corroded original backplate, this plate had been reduced in thickness to practically transparent.

Brake shoe mounting block and rear cable guide after removal

Finally monsieur - a wafer-thin mint!

I consulted with my local fellow AK builders Richard and Dave.  Richard had elected to keep this guide plate on his build on the basis that it gave a smoother path for the handbrake cable.  Dave had his backplates trimmed down by AK and they were supplied without the guide plate; the conclusion is, therefore, that it's optional!

Backplate installed with handbrake cable - Photo courtesy of Richard Chippendale

I kind of concur with Richard's view that it assists with the routing of the handbrake cable but it seems a bit odd that the guide tube leaves a part of the handbrake cable exposed.  For the moment I think I will proceed without using this component; although maybe later might consider fabricating a replacement plate with a longer tube to cover the full path of the handbrake cable in this area.

So the next job - the one I hadn't been looking forward to - was to trim the backplates down.  As noted above, the AK Build Manual doesn't really give any guidance in this regard.  But thankfully Dave sent me a couple of photos of his which had been cut down by AK.  

Cut down backplate as supplied by AK - photo courtesy of Dave Rich

So basically the plates need to be trimmed back to the edge of the raised circle as shown below.  Luckily I now have a couple of spares to practice on, so out with the angle grinder!

Extent of the backplate to be trimmed

Using a cutting disk I trimmed close to the edge of the raised flange.  I then switched to a grinding disk and took the excess steel back flush with the edge of the raised flange.  

Initial trimming with grinder...

First pass of trimming complete...

Edge dressed with grinding disk

Feeling pretty pleased with the trial run, I took a deep breath and took the angle grinder to the two replacement backplates!  I followed exactly the same technique as for the trial run, but finished off the edges after the grinding disk with some gentle filing and then a touch of linishing on the belt sander to remove any sharp edges and to make them super smooth.

Final cut-down backplates

I was quite happy with the final result and it wasn't as traumatic an operation as I had feared.  I treated the backplates to a quick sandblast for a final clean-up as well as the brake shoe mounting blocks before another session with the powder coat gun!

Looking better than when I started!

Brake shoe mounting block was treated to the candy red powder coat!

With those jobs done I am now very close to being able to start reassembling the rear hubs.