TA/B/C Front brake twin leading shoe (TLS) conversion

Issue 55 featured an article by Steve Priston about a TLS conversion he did on his TC, using a backplate from a Morris Marina 10cwt van as a pattern for altering his TC backplates. The article raised not a whisper, so Steve, having done the job a second time (this time for Trad Harrison), thought he would explain in more detail the various processes to encourage other brave souls who might be contemplating the task.

Over to Steve………

The job can be broken down into the following tasks:

  • Stripping the original back plates of the unwanted parts
  • Modifying the back plates (some ‘surgery’ required)
  • Making pieces to fill the holes in the back plates
  • Welding to fill the holes (akin to ‘sewing the patient up after surgery’)
  • Cleaning-up after welding & straightening, due to the distortion
  • Using a template for the first holes to be drilled into the backplates.
  • Preparing the wheel cylinders.
  • Modifying the brake shoes
  • The revised return spring plates, along with those little triangular guide plates, including the last bit of welding.
  • Ensuring that the return springs are able to let the shoes move adequately.
  • Positioning the adjuster peg/pins on the shoes
  • Final assembly

Stripping

Modifying

Cutting out the ‘bulge’ at the bottom of the backplate to enable the lower brake shoes to fit.

Making pieces to fill the holes

These pieces to fill the gap where the bulge has been cut out and to be welded in, along with the circular pieces.

Cleaning-up after welding & straightening, due to the distortion.

I didn’t take any pictures of the actual straightening but this is how it was done. Firstly, I found a small circular steel blanking flange, which when it had 2 opposite edges cut back, then sat nicely into the recess, in the centre of the backplate, along with a second unmodified flange of a similar size.

Then I found another much larger steel blanking flange, to act as a flat base plate, under the hydraulic press, larger in diameter than the backplate; but any heavy enough plate, larger than the backplate will do.

Now the Work’s press, has a pair of parallel supports so with the ‘bananafied’ plate the right way up, to ;de-banana’ it, the cutdown little flange, was put into the recess, for the press to push against and the plate was very carefully adjusted, probably only showing a 2 or 3 tons of pressure so even a small press in someone’s shed or garage would do it.

The next stage, was to lay the large flange onto the parallels, with the plain small flange under the centre offset portion of the backplate, then the cutdown flange was dropped into the recess, with the press then used, simply as a clamp on top.

So out with a steel rule, to measure the variations between the rim and the plate below, time to realise that the offset should have been carefully measured before welding but I did have 2 original plates so, “be warned”, measure the original offset of the backplate before getting it welded, if you want to be really careful!

You can however, when measuring what is before you, determine what you should be aiming for, as a gap measurement, then with that naughty number 2 Thor mallet, that you frighten those spinners off with, instead of using the next size down, use it to adjust the rim of the back plate, remembering that you want the area between the rim and the centre recess, to be parallel with the flat plate below, as well as getting the gap between the rim even, with the lower plate.

I didn’t require anything else to get it flat again but you will need to revisit the press, after stage 2 of welding.

Using a template for the first holes to be drilled into the backplates.

Imade a cardboard template, combining the centre hole, with the four 3/8″ diameter mounting hole positions, of the MG TC backplate, along with the required geometry, from the Morris Marina 10cwt van backplate, that I had previously used, but needed to somehow easily create a drilling jig from it.

The jig needed to be made by the person in possession of a single sheet of A4 paper, with the hole centres printed on it, this I have managed to do, if you look at the sheet of paper, marked in pen and highlighter.

I wanted a readily obtainable material, that was easy to work with, for the drilling jig, not initially realising the advantages of my first choice, which was clear perspex, something that can be purchased, cut to A4 size, in various thicknesses.

So, I obtained some 6mm thick clear perspex sheet, cutting it into two A4 pieces but as I wanted to create a “master” template, I asked my friend, with a nice milling machine in his home workshop, to align a 2.5mm drill, with each of the marked hole centres on the paper template, drilling through both paper then perspex below.

What Mike, my welder friend and very clever model engineer said, was that he thought simply using a centre punch, through the paper, indenting the perspex below, would probably be good enough, to then pick-up the hole positions OK for what was intended.

But what he produced for me with his mill, also gave me a working template/drilling jig, hence the 2 sheets of perspex, the reason for choosing only 2.5mm, as a drill size was because with a 0.5mm propelling pencil, it could be used as an accurate stencil, producing just a dot on the paper, also drilling such a small hole through both perspex and 10 gauge mild steel, is very easy.

The first thing to do with the perspex jig, was to open up the 4 mounting holes to 3/8″, by first using a centre drill, then a twist drill, as shown.

Then using these first holes, the jig was attached to the backplate as shown, with the perspex being spaced away from the backplate by a single plain washer, at each hole, in an effort to keep the plastic as flat as possible.

One thing I must now point out, is that the remaining holes in the jig/template, must stay at only 2.5mm and that the first upward face of the jig to be used, must be marked as “SIDE 1“, as must the plate that it is used on, when orientated this way.

The other side must be marked as “SIDE 2”, to avoid any confusion because the layout of the twin leading shoe brake, has to be “handed”, for it to work and by simply turning over the jig, you obtain the opposite hand so clear markings must be made, to avoid any mistakes.

I only used the very basic drill shown (see next but one pic) because I could lift it outside for pictures, then with a piece of board attached to it, the plates fitted OK. The most important things to consider when using the jig, is to make sure that the drill is perpendicular to the job, that the hole in the plastic is lubricated, that the job is not clamped to the drill table and that before the drill is under power, that it is free to move vertically in the hole of the jig, to preserve the accuracy of the hole for later.

As can be seen from the pictures, the use of clear perspex has another great advantage, i.e. that you can see exactly what is going on below.

There are two important things to note about the jig; first none of the pictures of it show the positions for the adjuster holes because I had to work those out later and the second, most important thing to mention, is that there are two hole positions on the jig, closest to the centre, are for holes that cannot be drilled at this stage, for the return springs, the plates for which are not yet attached, hence the two pieces of yellow tape over them.

At this stage, the two hold down pin holes are drilled, the two pairs of wheel cylinder fixing holes, the holes which form the ends of the slot for the elongated wheel cylinder port boss and now on the updated jig, the two holes for the brake shoe adjusters.

This drilling process is then repeated on the second backplate, only after the jig has been attached the opposite way up, as previously explained.

When both plates have had this batch of 2.5mm holes drilled, then both plates can systematically have their hole sizes increased, as required.

So, that is the very important bit of the process started, getting the geometry right.

The drilling table.

Ed’s note: This article is continued further on in this issue.

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