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Re-Metalling Of Big Ends
by Derek Dunn

Continuation To
My battle with an unknown foe!

During a previous life I have worked many times on white metal bearings, the interminable amount of work
with marking blue and bearing scrapers I remember only too well. However I have never before poured or
re-metalled a white metal bearing.
When I dismantled the Hundested and removed the big ends, I knew that the bearings had to be replaced, the
white metal in the top halves was like crazy paving. The bottom halves were completely devoid of any white
metal.
At that point in time my knowledge of the Hundested Hot Bulb Motor was very limited and I assumed that any
bearing metal in the bottom halves had simply disappeared. The bearing journals are 6 inches diameter and 6
inches long, far from the normal found in “ordinary” boat engines.

Now: the Hundested is a 2 - stroke engine and if one thinks about it, theoretically all the forces act in the
downward direction on the crank – pin; power stroke, followed by compression.
[This is unlike the 4-stroke where, during the induction stroke, the crankshaft is pulling down the piston; the
forces acting on the bottom half during this part of the cycle].
Hence the fact, there is no white metal in the bottom half cap of this particular 2 – stroke engine. There are,
however two deep channels which retain lubrication oil. I assumed that the oil channels were for keying the
white metal to the bronze caps.

I have a friend and colleague who is a volunteer at the Scottish Steam Railway Preservation Society near
Grangemouth and he arranged for me to spend some time in their workshops and benefit from the expertise of
their man who pours all the bearings for the locomotives. I arranged to purchase 10 kg of white metal from a
firm in Huddersfield and the scene was set.

The first job was to clean out all the old bearing material. This was easily done with the oxy-acetylene torch
and the edge of a piece of scrap tin plate. The bronze bearing caps were soon stripped down to a shiny, zinc –
coloured film. At this point the two pairs were bolted tightly together and the bearing centre marked. A piece
of wood some 2 inches broad and 1 inch deep was cut roughly to size and wedged into the assembled bearing. With the use of a set of odd-leg callipers the centre of the bearing was marked in the wood and from
this centre a circle was scribed onto the bearing caps some ¼ inch greater than the inside diameter of the assembled caps. This centre would be used for lining up the bearing in the four-jaw chuck in the lathe so that
the white metal would be bored down to the finished size.
Next, the surfaces had to be tinned, another quite simple process really. The bearing caps were simply heated
to a temperature where plumber’s solder would melt. Using liquid flux and soft solder, a thin coating was
applied to all four halves. This soft solder acts as a key for the white metal.

While this cleaning and priming was going on I was sent to find suitable material to make a former. I found a
12inch length of 5¾ inch O.D. pipe and an old piece of steel plate approx. 2 feet square. The pipe was
mounted vertically on the plate by tack welding.

In order to stop the white metal sticking, the vertical tube was blackened using the acetylene from the torch,
the oxygen being turned right down. There was soot everywhere but the effect on the vertical pipe was
perfect.
Next, one of the bearing halves was offered up to the former and with the use of a couple of G-clamps was
held firmly in place.

The bearing cap was positioned correctly so as to have an equal gap all around the circumference of the pipe.
We then searched for some scrap steel which was used to form a dam across the gap between the pipe and
the horn of the bearing cap, these dams being secured with bolts through the hole for the big-end bolt. All the
possible gaps were then sealed using ordinary fire cement.
The bearing cap was warmed using the oxy-acetylene torch, this was done in order to minimise the spitting
from the poured hot metal when run into the gap between pipe and cap.
 

While all this alignment and sealing was being done the white metal was being heated in a crucible. This
crucible was capable of being picked up using a two handed clamp which also allows easy pouring of the
contents by two men.

There is a very important tip to be recorded here and that is the correct pouring temperature of the molten
metal. If we were working in a proper workshop I suppose we could use all the state of the art measuring
gizmos but we were working in a railway workshop. I did not realise that white metal can be “burned”, that is
to say, the zinc content can be oxidised and this would make the white metal useless. As the ingots were
melting, the molten contents were stirred in order to help the melting process.

[The nearest thing I have seen to this process is when we used to have a deep fat fryer for making chips. We
often used vegetable oil in block form and as the pan heated, the fat melted but there was part of the block
sitting in the pan melting at its own pace, like an iceberg in an ocean.]

Once the entire ingot had melted a long spill was made from a sheet of ordinary newspaper by folding it over
until I had a strip of rigid paper approx. 1 inch broad and the length of the page. This spill was plunged into
the molten metal and left there for about 3 - 4 seconds. When removed and if the temperature was correct for
pouring, the paper would have turned the colour of straw. Too hot, the paper would ignite.

The pouring process was now able to begin,

The heating process is a bit of a hit and a miss and the white metal tends to spit out if the bearing is too cold –
too hot and the tinning will run out from the caps. As you will see from the photographs the pouring was done
with great care. The first bearing was done with plenty of overflow at the top to allow for contraction when cooled – time for tea. The bearing had to be allowed to cool naturally, not too quickly. After about 1 hour, the
bearing was disassembled from our jig and checked for soundness of adhesion to the cap, this was done by
gentle tapping to hear if it was hollow or not. This one seemed OK. This process was repeated with the other
half. When finished, the edges had to be hand dressed in order to assemble the pairs for machining in the
lathe.

Once the halves were assembled correctly, they were mounted in the lathe and using the centre mark as
described earlier, centred correctly for the finishing process. We did not have any clearance sizes for the
bearings but using the old rule of thumb of 1 thou per inch of diameter, we assumed a final bearing clearance
of 6 thou. This meant that we also had to leave some extra white metal for scraping. The crankshaft journals
were measured and were found to be very slightly under 6 inches. The white metal was therefore machined to
6 inches plus 4 thou, leaving us 2 thou for bedding in.
The other pair was similarly made and after two full Saturdays we had ourselves two new big ends all shiny
and ready for fitting in the engine.

When in the lathe, two grooves were machined into the bottom halves to act as oil reservoirs. As mentioned
earlier, I felt that we had to put back the deep grooves and to cut them into the white metal was not a problem
at this stage when the lathe could be used. I believe that this groove stores oil, which lubricates the big ends at
start up and allows oil to run immediately into the running faces of the bearing. This engine operates on a drip
feed, total loss, lubricating system and a few seconds could elapse before oil from the lubricator reaches the
white metal.

On the next Sunday we returned to the boat and fitted the big ends to the shaft. I was surprised just how
quickly we achieved this. With the aid of some soft lead wire and a micrometer we met our target of 6 thou
clearance and when the big end bolts were tightened, the bearings turned smoothly on the crankshaft. We
carried out one last modification to the top halves; this was to cut two diagonal grooves for oil flow. The original bearings did not have this groove but I thought it might help lubrication and would cause no harm.
Time will tell and we are all looking forward to running this monster in the forthcoming summer.

The finished bearings were assembled in pairs and stowed safely in the wheelhouse waiting for us to
re-assemble the engine.


Derek Dunn.

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