Single points conversion, how to.

toglhot

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Like mechanical things, but don’t like twin points, simple, convert to a single points setup like some bikes have and most cars.

For anyone interested, here’s the steps I took, you’ll need a lathe for this conversion though, a mill would also be handy, but the milling can be done on the lathe if you don’t have a mill.

Before you start, check the round key with two slots that operate the auto advance unit: You’ll notice on one side it has a faint arrow denoting which way the auto advance rod rotates, that obviously goes on the outer where you can see it. You may need to remove the key and flip it over. On the auto advance side the rod rotates clockwise, so fix the round key so the arrow points in a clockwise direction as viewed externally. On the points side the rod rotates anti clockwise. When timing make sure you only rotate the crank and advance rod anti clockwise so the cam chain remains taught. I’ve no idea why there is a direction arrow on the auto advance key, perhaps the advance weights aren’t set exactly at 180 degrees?

I used mild steel for the cam I made and it appeared to wear quite well with no scoring. However, 304/316 may work better as stainless work hardens and can be very, very tough. Other options are bronze or cast or even carbon tool steel.

First, mount some stock in the three jaw and using a centre drill first, bore through with a 7.5mm drill, followed by an 8mm. This gives a nice tight fit on the advance rod. Then turn down the stock to around 19mm, face the end then cut off so the cam is slightly over 20mm long. You’ll probably need to cut off using a fibre disk as if using stainless it’ll be pretty hard by now. Turn the cam around, face the cut off end and stamp it. The last facing stamped will be the external face of the blank – important! Alternatively, if you have 19/20mm stock you can ream the centre hole with an 8mm reamer, but you’ll need to cut the stock to length first due to the taper at the start of the reamer. But turning down from say 25mm stock will harden the stainless.

The blank is then mounted in the mill vice with the first face up and the keyway milled to fit the pin resident in the advance rod, 2.8mm from memory. It only needs to be about 6mm deep, not all the way through. I used a 2mm endmill and widened the way until the pin was a snug fit. Just be careful with the 2mm endmill, they can snap rather easilly and the end can round off when cutting hardened stainless.

Mount the advance rod in the lathe four jaw and centre it using a dial gauge on the part of the shaft the cam sits on. This has to be absolutely spot on, extremely important as this affects the timing of ‘both’ cylinders. Next place the cam blank on the advance rod with the pin in place and tightened the nut. The concentricity of the advance rod should be checked again, using the shaft area behind the cam, to make sure concentricity hasn’t been disturbed. then turn the blank down to 18.1mm using very fine cuts so you don’t disturb the concentricity. Remember the points gap is .13mm, so concentricity with the advance rod is important. Finally, check the concentricity of the finished cam. If out by more than .1mm, bin it and start again! But if you have centred the advance rod properly the cam concentricity should mirror the concentricity of the advance rod.

A couple of don’ts!

Do not turn stainless before drilling, stainless work hardens, so if you try drilling after turning you may burn out your drill bit or the hole my be enlarged

Do not turn a points cam independent of the advance rod. Three jaw lathe chucks are not perfectly centred, and four jaws take time to centre, so, if turned independent of the advance rod, when fitted to the advance rod one side of the cam will be higher than the other. How much higher will depend on how far off centre your lathe chuck is. My three jaw was of .2mm, so the cam would have been off by .4mm. That means if you set your points at .13mm, then rotate 180 degrees, the gap will be .17mm and that will upset the timing of one cylinder.

Do not attempt to cut a keyway in the cam after filing the ramps, it is an impossibility to get it right. Trust me!

Thirdly do not remove the advance rod/cam blank from the four jaw until you need to face the external face of the cam, and make sure you stamp it so you know it is the outer face. Very important the first face turned that fits against the step on the advance rod is square to the 8mm bore.

Part two, the points backing plate.

The points plate is best made from mild steel, although stainless would work just as well.

Using 3mm sheet, cut a rough circle big enough to turn a 62mm circle from, and drill a 12mm hole in the middle. Stick a bolt through it and tighten with a nut, then mount it on the lathe. Turn the stock down to 62mm, remove the nut and bolt, mount the round plate in the three jaw and centre the 12 mm hole with a cutting tool, then drill out to 23mm. Finally fit the plate in the points bucket and make sure it is a snug fit but still rotates freely. If a sloppy fit it will affect the points gap.

Clamp the original points backing plate to the new one and drill holes for the points. 4mm for the points anchor and 3.2 for the clamping screw, then tap the 3.2mm hole with a 4mm threading tap. You’ll notice the spring on the points runs very close to the points bucket, I drilled the holes an extra 1.5mm towards the centre of the plate to give a wider gap.

After that you’ll need to file the cut outs for the points plate fixing screws. The cut outs on the plate are not 180 degrees apart, so, clamp the original plate to the new one and mark around the cut outs with a scriber, then file away. Make sure when fitted the plate still loves freely without binding on the fixing screws.

The cam ramps. This is fiddly and time consuming. Fit the cam and backing plate with points screwed in place and gap the points. Rotate 180 degrees and check the gap again. There should be very little difference, somewhere in the region of .1mm

Turn the motor over, anticlockwise, until the pointer on the rotor is just before the first F mark, attach a timing light, then with a sharp blade or scriber, Mark a line on the cam underneath the heel of the points, remove the cam and file away up to the scribed line, file a flat until the cam reads 17.5 from the flat across to the round side. Then file across the edge where the flat meets the round part, no wider than about 2mm. Then replace the cam on the advance rod and check the timing with the LED timing light. Be very careful from here on that you do not file too much. When the light comes on, you’ll need to polish the filing marks so they don’t wear the heel of the points prematurely. A nylon fibre whee is best used to polish as it will also round off the edges for smooth points action. This polishing should also advance the timing so the rotor mark is in the centre of the F marks. If not start again and make a new cam, retarding the timing before marking the cam under the points heel. But, it should work out unless you have used a bastard file.

Turn the motor over so the rotor pointer sits just before the first F mark and repeat for the other cylinder. Take care to file the same amount off as on the other side by measuring the thickness of the cam across the flats. It should measure 17mm.

Now you need to check the dwell. You may need to extend the width of the ramp flats to suit the dwell you are after, you can use this to put a slight curve in the flats to smooth the journey of the points heel. Some points aficionados prefer a long dwell, some a short. Preference seems to be a longer dwell, either way, ensure the dwell is the same for both ramps.

You’ll need a dual output coil for this conversion. This will give a wasted spark, so, no need to check the valves to see which cylinder fires, they both will.

This how to was written well after the fact, so, I may have made some errors, but you’ll find and remedy them as you go, don’t be surprised if you have to make more than one cam, I have a few with ramps too advanced as I am a little impatient at times.

Here’s some pics, I’ll try to find the pics I took of the cam filing process, no idea where I’ve stored them.
 
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