This morning I had a rather pleasant drive in to Woodhall Spa to meet a chap that had advertised a Lucas K2F magneto for sale on Gumtree. Mine was kaput, I could have had it rewound for about 120 quid but as this one was in working order and just 75 quid, I thought it was a bit of a bargain. The sun was shining an it's a nice drive over there, Woodhall itself is a really pleasant little place, so much nicer than the dump that Boston has become in recent years. Anyway, I met Roger outside the Golf hotel, he suggested that would be much easier to find than his house, I got there about 30 seconds before him, so that was splendid.
he showed my the mag working, I paid him his asking price, we had a bit of a chat and I headed home to fit the mag. It was quite straightforward although timing it up was not the easiest of tasks as I didn't have a timing disc. The book says max advance should be 36 degrees before top dead centre, as my head is a twin spark one it needs to be about 8 degrees less than that at around 28 degrees. You can use a dial test indicator and maths to work out what the height of the piston should be at that setting but when I tried to set up my dial test indicator I couldn't doo it well enough to get a consistent reading. after a bit of head scratching I had a eureka moment, I remembered my inclination gauge that I bought when I was building the chassis for my hot rod. I knew it was accurate and that it gives a clear digital read out so I used the age old formula 1 approved method of taping it to the crank extension with some electrical insulation tape. I used the dti to find tdc and then reset the inclinometer to zero, I then turned the engine backwards until I got to 28 degrees. This was the setting where I need the points to just start opening on the magneto. I forgot to say, it's important to have set the points gap accurately before doing any of this.
The best way of finding the moment at which the points start to open is with a cigarette paper, which are just a few thousandths of an inch thick. You turn the mag until the points are open, put the paper between the points and then turn the mag until they close, trapping the paper in place. Now you can feel when the points just start to open as it will be possible to pull the paper out without tearing it. It obviously requires a delicate touch, alternatively you can use a DVM on the ohms setting - the choice is yours. I am within half a degree, my motto has always been near enough is good enough so that will do me. No doubt some experimentation will be needed anyway.
One thing worth mentioning is that when I came to take the old magneto off I had a nightmare getting at the bottom bolt and ended up having to modify a spanner to do the job. You can buy a thing called a long nut or extension nut that extends the bottom nut far enough so that you can undo it from the points end of the magneto using any old socket or spanner of the right size. They can be bought for about a fiver and save a whole ton of hassle.