Rear Axle Strip Down - Part 7 - A Fatal Extraction

With the rear hubs now all nice and shiny after their trip to the sandblaster, it was time to tackle one of those tasks I was not looking forward to.

When stripping down the rear hubs, the heads of the two smaller screws securing the brake backing plate to one of the hubs had sheared off.  Now it is time to attempt to remove the remaining parts of the screws which are still stuck in the hub.

Today's mission - removal of two sheared screws...

There are many suggested ways, it seems, to remove broken screws and bolts.

The most straightforward would have been to position a suitable sized nut over the top of the sheared-off screw and using a hot metal glue gun, blob some weld into the middle of the nut to join it to the screw.  Then it should just be a matter of using a spanner to undo the nut/thread combo.  My only issue with this was that the sheared-off thread is just below the surface of the hub and, while the theory is that the steel weld won't stick to the aluminium of the hub, I wasn't prepared to take that chance with my hub.

Next on the list of discarded options was the use of a suitable sized screw/stud extractor.  The theory behind these is that a pilot hole is drilled into the broken screw into which the extractor is inserted; the extractor has some reverse cut threads that bite into the metal and undo the screw.  Now I have never had much luck with using screw extractors even on screws in wood with chewed up heads.  And given that these broken screws had resisted all attempts to remove them initially before breaking, including heat, impact screwdrivers and penetrating oil, I seriously doubted that using an extractor was suddenly going to persuade these screws to give up their grip on my hub.

So the method I decided upon was to slowly try and drill out the broken screws, using drill bits of increasing size until the hole is drilled out to the diameter of the screw shank and then to retap the hole with the required thread size.

Of course, I should have known it was never going to be quite that simple.

The first problem is that actually trying to centre punch the end of a sheared-off screw is quite difficult - the broken surface was uneven which made it hard to locate the punch in the exact centre. In hindsight, I should have used a Dremel or similar to grind the end of the screw flat before using the punch...

The second problem was that smaller diameter drill bits are very short, the chuck on my drill is quite large and the offending holes are very close to the hub bearing housing; the result that I could barely get any depth to my initial pilot holes in the screws using a 2mm bit.  Worse to get any depth I had to fix the drill bit with only a very short length of the shank actually in the chuck.  This means that the drill bits are very flexible and the end is prone to wander (of course compounded by the difficulty of punching the centre of the screw in the first place).

Drilling depth restricted by the clash of the drill chuck and the hub...

The end result was that the pilot holes on both screws did not really start off on centre so by the time I started opening the holes up with 3mm and 4mm bits it was clear that the holes were going very awry.  The other issue is of course that with the hub being aluminium the drill bits found it much easier to slip off the hard steel surface of the screw and chew their way through the softer aluminium.

Opening up initial pilot holes...

By the time I got to the 5mm bit, I realised I needed a Plan B, as the result was that I was drilling out about 3/4 of the area of the screw and the rest of the hole was through the aluminium hub.

Plan B was to use a Helicoil kit (thanks to that well known on-line retailer and their next day delivery service).  Helicoil inserts are a threaded metal insert used to repair stripped threads; the stripped hole is drilled oversize and tapped for the metal insert which is then inserted into the tapped hole to reinstate the original thread.

My logic for this approach was that by drilling the hole slightly oversize for the Helicoil that this would allow me to remove all the remaining portion of the screw thread from my misaligned drilling.

Helicoil Kit - includes drill, tap, insertion tools and a selection of inserts

The hole in the hub should be an M6x1.0 thread so the first step was to drill out the hole using the supplied drill bit for the corresponding insert.

Prior to drilling - misaligned attempt to drill out screw is clearly visible!

After drilling - operation didn't quite clear out all of the remaining screw

Next step is to tap the holes for the Helicoil inserts using the tap provided for the M6 inserts.

Tapping holes for inserts.  During...

...and after

Next stage is to insert the Helicoil.  The inserts have a small tang on the bottom of them which is inserted into the slot on the appropriate sized insertion tool.  This is then used to twist the insert into the threaded hole until the insert is approximately half a turn below the surface.  Care needs to be taken to allow the insert to thread itself into the hole without applying too much downward pressure.  My first attempt unwound the bottom of the insert... Fortunately, it was not too far into the hole and I managed to extract it without too much excitement.

M6 threaded insert in place on the insertion tool

The final step is to use the punch supplied with the kit and a sharp tap with a hammer to break the tang off the bottom of the insert.  

One repaired hole!

Replacement screw fits perfectly!

In the end, it was not as traumatic a job as I was expecting it to be (although equally, it was not a simple as I would have hoped...).  Both holes are now repaired albeit both are slightly off from where they were originally.  Hopefully, the brake backing plate will still line up - if worst comes to worst I may have to slightly oversize the holes in the backing plate.

Right, time for a cup of tea to celebrate.

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.

Rear Axle Strip Down - Part 9 - Hub Cleaning

In amongst getting various parts powder-coated, I have been progressing with cleaning up the rear hubs.

These are an aluminium casting and, after many years of service on the donor vehicle, were suffering from some heavy oxidation, particularly around the area where the steel brake back-plate sits, as well as having a good coating of general road grime.

The first step was to give the hubs a good soaking in a bucket of boiling water and some detergent - I used a couple of dishwasher tablets.  Vigorous brushing with a stiff nylon brush shifted some of the grime.  

One grimy rear hub ready for a bath...

...after a long hot soak and a bit of brushing

Note it is not recommended to use steel or brass wire brushes on aluminium as particles of the wire can embed themselves into the soft surface of the aluminium which can lead to future rust staining in the case of the steel brushes and severe corrosion in the case of the brass brushes especially if exposed to road salt.

After initial cleaning - note heavy oxidation around brake back-plate mounting flange

Next on the list of beauty treatments for the hubs was a session with the sandblaster.  Actually quite a lot of sessions.  Over many weekends...

The sandblasting gave reasonable but variable results.  Some areas cleaned up quite nicely - generally the flat areas.  The blasting removed the white aluminium oxide corrosion around the back-plate flange.  However, under the oxidisation the surface was quite pitted and, despite trying different media, the sandblasting did not remove all the of the oxidisation.  The resulting finish still had quite a lot of black oxidised aluminium in amongst the shinier areas.  This was the same case for some of the edges and recesses within the casting.

Hub after sandblasting - some areas of ingrained oxidisation remain...

The rear side of the hub - it was tricky to get sandblaster nozzle
 into the bottoms of all the recesses

Oxidisation still present in areas of pitted aluminium

The next treatment involved a liberal dousing with Frost Ali-Clean and a vigorous brushing with an old toothbrush.

Time for another scrubbing...

I have to say I was quite disappointed with the Ali-Clean.  It is supposed to be used on unpolished aluminium castings to clean and etch the surface.  Having applied the product as per the instructions and rinsed off with water, I'm not sure I noticed any difference in the finish.  The aluminium surface was no brighter and the areas of deep black oxidisation still remained.

This left me resorting to the electric drill and a brass wire brush (totally ignoring my own recommendation above....).

Recommendations?....more like guidelines...

This treatment did improve the finish slightly - but still did not remove all the areas of black ingrained oxidisation.  I even tried a further treatment of Ali-Clean (and I was still unimpressed...).

A before and after comparison - front side....

...and rear side.

When you look at the before and after photos, the cleaned hub does look significantly better than the 'before' version.  But I thought there was still room for a lot of improvement.  

Given that to get to this stage had probably taken me about 5 or 6 weekends and I had only 'cleaned' one hub, I came to the conclusion I needed a Plan B.

Plan B involved a trip to see the professionals, in this case, a local sandblaster in Braughing, RFI Screening.  They offered to vapour blast both hubs for the princely sum of £20 each.  In hindsight, I should have just taken this approach in the first place.  The results from RFI are amazing and I could have saved myself a lot of effort.

Hubs - straight back from the sandblasters...

...vapour blasting leaves a clean satin finish...

...even recesses on the rear are nice and clean.

Strippers and Dubious Chemicals

My last few blog posts have dealt with a few "issues" that I have encountered, the outcome of which is that I need to redo the powder coating on a number of parts.

One of the main reasons for powder coating the parts in the first place is that the coating is very durable.  In other words, it's a bugger to remove.  I had a couple of attempts with the sandblaster and various sanding discs and wire wheels; these would eventually remove the majority of the coating but would take a heck of a long time. 

If I lived in the US there would appear to be a number of chemical products available to strip powder coat.  However, none of them are available in the UK.

The internet suggested that, after giving the surface a good abrading to remove the top surface of the coating, paint stripper could be used to remove the remaining powder coat.  Well, normal Nitromors paint stripper had zero effect.

Further research suggested that to be effective, the paint stripper needed to contain a particular chemical ingredient, namely Dichloromethane (DCM).  This was a common solvent many years ago and used to be widely used in paint strippers.  But, probably for good reason, in the modern health and safety-conscious era, it is has been removed from most products.

It took some tracking down but I did manage to find some industrial strength paint stripper containing DCM on the well-known internet auction site and purchased 5 litres.

Innocuous packaging, limited H&S warnings (and no data sheet supplied with the product...)

I need to recoat the rear wishbone spacer tubes and the rear wishbone tie.  I gave these parts a quick session with the angle grinder/flappy wheel combo (and in the case of the wishbone tie, some wire brush treatment to get into the recesses) to scratch up the surface of the powder coat finish.  These were them brushed liberally with the professional strength paint stripper and left to soak for a few minutes.

Health and Safety Tip - DCM paint stripper is potent stuff.  Even outside in well-ventilated conditions, I could smell the solvent content!  Make sure you wear gloves, long sleeves and eye protection.  I got some small splashes on my skin when brushing the stripper on and it starts to burn any exposed skin immediately.  Also, work quickly; this stuff burns even through gloves!  After just a couple of minutes of holding the parts while applying the stripper my fingers were starting to tingle!

The potency does mean that the product is extremely effective.  It only took a few minutes for the surface of the abraded powder coat to start wrinkling and using a wire brush I was able to remove the majority of the original coating.

Less than 5 minutes for paint stripper to do its stuff...

After a wire brushing and rinse off with water

Wishbone tie after flappy wheel/ wire brush treatment

After a few minutes of soaking in paint stripper...

...the majority of old coating could be removed with a wire brush

After rinsing and drying

These parts were then re-prepped for powder coating.  I gave the spacer tubes a thorough going over with the grinder and sanding discs to try and remove as much of the surface pitting as I could.  These and the wishbone tie then had another session in the sandblasting tent before an application of Afterblast.

Wishbone tie after sandblasting...

...and after etching with Afterblast

The wishbone tie was then recoated in Satin Black; this time around I made sure to get a good coating of powder all over the part prior to curing.

Ready for coating (again...)

After curing - better than the first time!

For the wishbone spacer tubes, I tried another approach for the application of the candy red topcoat (another technique sourced from internet research...).  The High Chrome coat was applied and cured as before. This time though, rather than allowing the parts to cool and reheat them prior to applying the candy coat, I tried a technique called 'hot flocking'.  This technique involved applying the second coat to the parts as soon as they were removed from the oven after curing the first coat.  Due to the parts being at or just below curing temperature, the second coat of the powder started to cure as soon as it is applied to the part.  This also meant that the powder adhered far more readily to the part and did not get blown off as easily.  The parts were then cured in the oven as normal for 10 minutes.

Extensive sanding to remove surface pitting

High Chrome coating applied and cured as usual...

...before red candy is 'hot flocked' to parts straight out of the oven

After 10 minutes of curing in the oven

No blemishes and no patchy coating...

Nice result!

The results this time were far better.  The spacer tubes were blemish-free and the consistency of the red candy coat was much more even with no patchy bits. The colour is amazing in the sunlight!

So now I am back on track and I can get on with finishing off cleaning up the rear hubs!