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650ss 1963 Magneto Location

Submitted by andrew_heaton on Tue, 10/04/2018
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Heavy Twins

Hello All,

I am fitting a new BTH K2F magneto

http://www.bt-h.biz/k2f--lucas-replica-flange-magneto-13-p.asp

onto what I think is a largely unmolested 650ss.

The original Lucas K2FC was fitted with 3 waisted studs, the spigot of the old Lucas and new BTH is dia50.0mm and the hole in the casting a machined 52.7mm. This clearance is much greater than I expected and as the new mag has an o-ring seal, with an inside diameter of about 52.7, so unless the mag is centred, the o-ring partially hangs into the timing case.

My questions are:-

- is the hole in the casting meant to be this big for the waisted studs to allow both rotation (timing adjustment) and lateral (chain tension) adjustment? I can go this way by omitting the o-ring and using an original triangular gasket.

If this is the case,

-how do you fine adjust the ignition timing without upsetting the chain tension? Just by feel.

- I am planning on setting the fully advanced timing to 29deg BTDC from the various notes I have read, anyone else used one of these new electronic BTH units?

Another issue to resolve is that the new mag is slightly fatter in body than the Lucas and just fouls the back of the crankcase restricting the rotation when centred!

All comments welcome before I decide which way to proceed.

Cheers and thanks, Andy

Submitted by jonathan_soons on Tue, 10/04/2018

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I have not installed that particular mag but I can tell you what I know about mags in general. The hole is bigger in the timing case so that you can adjust chain tension. This is tricky because there will be a tight spot (always is). I notice the new mag has slotted mounting holes. You should call the company and ask whether you can rotate the mag after adjusting chain tension. My guess is no. If you disturb chain tension you could cause damage. On the original mag you had to set timing by installing the drive sprocket in the right position not by rotating the mag. What do the instructions from the manufacturer say about timing? What is the chain tension when the mag touches the crankcase? Too loose is good as long as you can then adjust it (by swinging away from crankcase). Too tight and you can't use that mag without modification. My experience is only from installing and timing many standard, old fashioned mags.

You probably already know most of this. There are some more knowledgeable people here who will chime in I am sure.

Submitted by david_cooper on Tue, 10/04/2018

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Further to what Jonathan says

The reason for the clearance is only to get the right tension. You should be able to do that without the body jamming on the engine.

All timing adjustment should done in the normal magneto by adjusting the position of the sprocket on the tapered shaft after the mag body is fixed in place. As Jonathan says - watch out for tight spots (and use a dummy timing cover to keep the chain tensions correct - but you probably know that).

Now I just followed your link to the timing instructions and I see that the slotted holes are used to provide the option to fine tune if you are not entirely satisfied after the sprocket has been fitted onto the taper. The O-ring looks as if it fits on whatever non-Norton case they show in their Figure 5. Maybe someone else can add something? The business of lining things up with rods and holes is the equivalent of rotating the normal armature until the contacts just open. That then is the point when the sprocket should be fixed with the crank set to full advance position.

The nasty job with the Dommie (as again you've probably found out) is to deal with that bottom bolt. There is correspondence about options in this web site conversation history.

Submitted by andrew_heaton on Thu, 12/04/2018

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Thanks for the help, all is now clear. The solution lay in looking again at Fig 5, where I now notice the instruction pictures switch from a Dominator to a different manufacturer who used gears rather than chain to drive the mag - in this case there is no need for clearance on the mag spigot and also the o-ring is nicely sealing the hole. I had turned up a nice alloy spacer screwed onto my mag that centred the o-ring, but that then eliminates any chain adjustment and also contributed to the mag just fouling the back of the crankcase.

So the o-ring and spacer is now removed, mag fitted with standard gasket, chain adjusted, timing set to 29deg - lets see what happens. The new mag now (but only just) fully rotates with the spacer removed.

On the other bikes I have fitted electronic ignition (albeit singles), I have very carefully set the timing using a dial indicator on the piston and a disc on the crankshaft and never had to adjust again between major rebuilds, so hopefully this will be same.

By the way, I use an ANL jig to support the intermediate gear spindle which works well - I couldn't bring myself to hack holes in my spare timing cover!

 


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