SU carburettors, trumpets, aluminum, model engineering, ford zephyr mk2, mikes-models

Triple SU carburettor trumpets for Ford Zephyr.

Steve (runs a car accessory shop in Brixham (Brixham Car Accessories) as well as
mobile mechanic - looks after my Mk2 Jaguar with me) came around to show me
the throttle linkage (see one off project menu) I made for him recently. Something he
needed to enable him to fit triple carburettors to his classic Mk2 Ford Zephyr. I had
asked for some photo's when he had fitted the linkage for the website. Now that he
had everything working, he asked if I could/would make a set of trumpets for the
carbs to finish them off. Sounded like a nice project to have a go at, so I said yes.

To help me with the design I took a quick paper rubbing of the carb inlet flange
(shown on the slide show above) and thought that making a flange into which the
trumpet would be fitted was the way to go. Further, if I made a screw fitting
between the two, these could then be loctited together. To transfer the pattern made
on the paper rubbing, I thought I would prick out the edge of the pattern and then
'join the dots'.

Next when looking around for some stock I happened upon a Ľ" thick aluminum
plate suitable for the flange could be cut from. I made a couple of paper cone's to
get some sort of size and from this exercise I decided I needed at least 2 ˝"
diameter by 3" long bar. I found a large square of stock from which I could make
the three trumpets, so I seem to be well set.

To make the flange I used the milling machine and milled to the lines as near as
possible, leaving the final shaping to be done by file. The different radius were rough
milled as near as possible and I thought I would try using 'filing buttons', which are
simply different diameter bits of metal bar from my scrap box. After finding three
suitable sized diameter pieces it was over to the vice. I must admit I was pleased
with the results. I shall certainly use this method again.

The next task was to find the center points of the three holes by using a compass to
replicate the hole sizes. Once marked it was simple to centre punch the marks, by
using the optical centre punch which makes light work of this task, and is another
tool I would certainly recommend. Mine was bought from Axminster tools (see link
on the left hand side), then back to the milling machine to drill the three holes which
were found to be on the same axis. The centre hole was bored out oversized to
allow for the thickness of the trumpet walls, and ensures there will be no restriction
to air flow to the carburettor.

The next task is to decide on the thread pitch for both components and this will be
tackled in the near future.........as they say....watch this space

Wednesday 11th June 2008 - Ford SU Triple Carburetor trumpet

Today the prototype carburetor trumpet was completed (see photo gallery)
based on the available stock size and my taper turning capacity of the top slide.
No doubt there are methods for turning longer tapers and its something I will now
investigate. However the trumpet seems in proportion 'off car'.

I decided first thing howeve,r to make a fixture to hold the aluminum blank for
turning because there are three to make. I used one of my 'scrap' attempts at
making the Myford spindle adaptor, so recycling works after all!

I tried to assess the taper angle by putting a ruler from the neck to the outside of
the blank and this came to about 10° so I decided on safety and went for 7°.
Always trust the safety route say I. 7° seems to work ok but it needs to be seen
in place before a final decision is made.

The finished prototype has been given back to Steve so he can make a decision;
I just want to make sure the flange is correct, as the one in the picture I am not
happy with, regarding its final finish. However the trumpet, if suitable is ready for
polishing up, and should be one of the three, Time will tell………..

Thursday 12th June 2008 - Ford SU Triple Carburettor trumpet.

Steve returned and said the trumpet fitted and the size was ok. So now I will go
ahead and make three flanges and two trumpets, starting today.

Steve also mentioned that he had shown the prototype to some one else and
they asked whether I would make something similair for Weber carbs? So there
may be a version for another carburettor system to come.......

Anyway back to this project. Because the stock I have is not easily to size I
have some work to do before I can start. I made use of my Axminster Hacksaw
to cut the 1/4" aluminum plate for the flanges and whilst this was under way
started to turn down a large block for another trumpet. It seems such a waste
really so I phoned a local metal stockists and for enough bar of the correct size.
They wanted Ł25! So its back to turning the block.

One idea I had for centering the block easier in the 4 jaw chuck, was to find the
centre of the block by crossed lines (see gallery photo) and then centre drill.
Using the revolving centre to hold it firmly against the face of the chuck, the 4
jaws were tightend. From this point its a case of turning down, a slow job as
little is cut until the block turns round.

Once the 1/4" plate was cut to size, blue engineers marking fluid was applied to
the three pieces. I had a thought though that it should be possible to work the
three pieces as one. So I will rough out the flange shape, using the milling
machine, whilst holding the three blanks as one.

The largest time taker at the moment is getting stock to size! I am certainly using
more time because the stock is not to size. Whilst resting I may have a look on
eBay before expending too much more effort on existing stock. Metal prices
certainly are on the rise.............

However on the psoitive side I now have 1/3rd of the trumpets and a working
example for dimensions. Happy days...........

Monday 16th June 2008

Well I gave up on cutting the large block for use as the other two trumpet
blanks. One lesson learned though was by putting cutting oil on to aid the work I
had in fact made it worse. The hacksaw was working for over an hour whilst I
continued on and the oil clogged the teeth, stopping them from cutting!

I only found this out after changing the blade because I thought that it worn. The
new blade of course was the same. After cleaning it up and cleaning the block of
aluminium, it again began to cut properly, if slowly.

Because of these difficulties I decided the best course of action was to buy bar
of the correct diameter, this I did on eBay. My local stockist was more
expensive that the cheapest eBay example even after adding carriage on!

In the mean time I decided the flanges should be cut as a set. So three suitable
pieces of 1/4" plate were put together and secured by countersunk screws
temporarily.

Because of the damage caused by the 4 jaw chuck, even with protection, I
decided that this time I would bore out and cut the internal threads first. As it
happened this was a good idea as there were good square edges for the 4 jaws.

Once the first couple of passes were made boring through the three flanges using
the top slide I checked to see if it was being cut parallel. To the experienced
model engineer this would seem normal but I have only just realised that whilst
the top slide can be shown as on the 0 mark there is sufficient scope for the bore
to be out by several thou, so I now always check that it is parallel.

The screw thread cutting went well. Refering back to my laminated aide-memoir,
the set up now seems to be producing consistent results. As the depth of this
bore was 'reasonable' no taper could be accomodated and after the three were
split ALL the flanges screwed in well and to the same firmness.

Once the screw threading was completed the three flanges were moved to the
milling machine and milled to the outside of the scribe lines. Once this was done
then it was over to the bench vice and by draw filing, the final size was achieved
prior to polishing.

When the three flanges were parted the correct diameter outside holes could be
opened out, leaving only polishing to finish and they are shown in the slide show
above.........only the other two trumpets to make and for that I am waiting on the
bar stock arriving.

Tuesday 17th June 2008

The aluminium bar arrived this morning so I decided to try and make both the
trumpets at the same time, or rather complete the same tasks on each before
moving on. So after cutting two pieces approxiamately to size I turned down the
area to be threaded. After changing the gearing for the 24 tpi both sets of
threads were cut with the stock held in the 4 jaw chuck. Next the bar was centre
drilled before drilling out to 5/8th inch (the largest drill I have with a MT2 taper
for the tailstock).

So I now have two blanks for the reamaining two trumpets which are bored to
5/8th and have their threads cut and trial fitted to the flanges. Using the dial
indicator I centred the holder made to turn the first trumpet and am now ready to
make the outside taper on one of the blanks.

So when I go out for the next session it will be the fun bit of generating loads of
swarf!!!! ...........to be continued.

Wednesday 18th June 2008

Well what should have been a 'simple' job turned into making another trumpet!
As always it is just when you are near the finishing post that Mr. Nobody comes
along and puts a spanner in the works.

I 'mucked' up the last trumpet just before the finish and therefore had to make
another. I though that by making the three trumpets as a batch it would have been
less time consuming than doing one at a time. Indeed that was the case with the
flanges. But in fact this last one took a quarter of the time of the others.
One thing I have decided upon however is a long term ambition of having a gear
change box on a lathe. The change gears on the Turner (my large lathe) are
relatively easy to change but it is still a time consuming task. As I had changed
them from screw cutting the two blanks, back to a fine finish, I then had to
change them to screw cutting for the third (or is it the fourth!) trumpet then
straight back again to fine feed……

Once the last trumpet was finished I had had enough and wasn't feeling my best,
so decided to pack up before I made anymore 'mistakes'. I then spent the next
40 minutes trying to unscrew it form the work holder! At one time I was
convinced that it had 'welded' itself to the holder. Ron Chernich of the Mills
1.3cc diesel fame mentioned the phenomenon of aluminum cold welding itself
together. This thought sent shivers down my spine because if it was so not only
would I have to make yet another trumpet, but also a new work piece holder!
Not a pleasant thought when you are tired. Just as I was about to give up, after
spraying WD40 and anything else I could get my hands on, it decided to
undo……Happy days said I.

This now only leaves the polishing of the three flanges and trumpets, left to do.

To be continued……

Thursday 19th June 2008

As luck would have it I had just received a polishing kit from Axminster tools
which should complement my wire brush and cloth mop bench top machine. This
kit comprises 3 separate mops and three different grades of polish stick. I am
putting a separate page on polishing on the site (click here) so will not go into
details now, rather just to say how pleased I was with the kit.

After polishing the flanges, doing all three on one grade before changing the mop
to the next grade, I concentrated on the trumpets. To polish these items I placed
each one, in turn, on the lathe. Paper towels were placed under the chuck to
protect the lathe bed and using the variable speed of the Turner lathe started to
remove the machining marks on the inside of the trumpets, progressively finer
grades of wet and dry were used, lubricated with Mer polish.

The outsides were reasonably clear of marks so Brasso polish was applied then
removed with clean cloths, being particularly attentive to the dangers of the
revolving chuck and its outstretched jaws. And the rest as they say is history.

Steve came over and the three completed units were fitted. I took the
opportunity at this stage to smear adhesive on the threads before screwing them
home. Steve also kindly took away the swarf and is getting it converted into
some as yet unknown shape...........Its something I want to get into later on,
casting that is..........

And that concludes what has been a very interesting project for me.