Lucas magneto cam ring

[Tazmantic]

Hi all need some help with my Lucas ms1 mag (for my 33/12) this is the correct mag/dyno for the bike but the guy removed it and fitted (badly) a different mag because the correct mag would not spark when retarded. I’ve had a look tonight and I believe it’s because the wrong cam ring is fitted how do I find out which is the correct ring?

Cheers

[Groily]

Several things here to think about.

The replacement magneto was presumably another Lucas magdynamo, possibly a later MO1 with a face cam arrangement? A perfectly good straight swap arrangement in fact, but a few years newer than the machine as they came out in the late '30s. Much more common, look almost the same, and are a bit easier to sort out when in need of TLC.

The MS1 will be either clockwise or anti-clockwise drive. I'm guessing anti-clockwise drive but don't know. The cam rings are differently-set in their housing according to rotation but the basic thing is the same for both except for the notches ground in for the advance and retard cable and for the 'stop pin' that limits the arc of movement of the A/R. The actual ramps on the cam are the same, so the same part works both ways as long as it is correctly positioned.

The only way to tell whether what you have is right, or correctly set up for optimal internal timing of the magneto, is to see whether the points, correctly gapped to 12 thou, JUST start to open, JUST after the 'flip point' of the armature, with the mag at full advance. If the points open too early there will be no spark; if late, the spark will be weakened and get worse as you retard further. There is a big difference is spark performance between full advance and full retard on any manual advance magneto, so the full advance setting needs to be right. (This is why the later automatic advance units found on some machines are 'good things' even if not very well made - they ensure that the internal timing remains the same retarded and advanced, thus offering the same strength spark at all speeds - thereby making things easier to start.)

Edit: First sign of a breakdown of windings etc is often failure to work retarded - when the spark is weakest.

The contact breaker should be the standard brass Lucas backplate, same as, or near as dammit, on any other ring cam Lucas mag, like K1F or K2F, or any number of 4 cyl car magnetos. If it's something else (eg BTH, Bosch or whatever) the internal timing may be off because the integral keyway that locates the backplate on the armature shaft may not be / probably won't be in the same place relative to the points as the original.
The contact breaker units are handed so you have to have the right one for the rotation - the fibre heel that opens the points must trail, not lead, as the thing goes round. The wrong one for the rotation will cause incorrect internal timing, and will not appreciate being driven the wrong way.

In the event that your cam ring is set for the opposite rotation to the desired one, it is almost always possible to grind additional notches to suit - but it has to be done very carefully as you only get one go. Obtaining a new one for these isn't easy, but Dave Lindsley Magnetos may be able to help. The MS and MN series cam ring is deeper than the typical later single cylinder ring found on eg K1F, N1C or R - but can be made to fit with a spacer machined up to ensure correct positioning in its housing if there's no other option.

Ideally, on a single, you want a NEGATIVE spark at the plug. Changing rotation can reverse the spark polarity as you are using the opposite flip point of the armature. It isn't critical it's negative (ie spark goes from hotter centre electrode to cooler earth on plug) but if you can see which way it jumps you'll know which you have. (On a twin, after all, you get one neg and one pos whether you like it or not.) Reversal requires reversal of the magnetism of the magneto or mods to the contact breaker position on the armature to get the points opening using the other 'flip point'. A simple test with a graphite pencil can often tell you what you have got. See https://www.themagnetoguys.co.uk/spark-polarity

Dimensions of all rings and contact breakers are the same, bar the depth of the earlier rings being greater than the later ones.

Hope this helps.

[Tazmantic]

Wow thanks for all the info, I had seen some of this info as I did reverse a twin mag (which to be fair was quit easy) some time back and have reversed a ksa1 though that’s not right yet ??? But you have answered my question saying the cams are now hard to find I was just hoping as it was a standard mag (not modified) I could find the correct cam as the one that’s fitted has been modified with extra slots ground into it.

Cheers

I’ve just thought it’s an anti clockwise mag but it has a slack advance is this correct for this mag or does it have the wrong end cap which could cause a problem I expect

[Groily]

Anti-clockwise with slack advance means the cable would enter the housing to the left of the points. The housings were made with entry on either side and used according to manufacturer's choice for slack or tight advance and direction of rotation, along with considerations of cable run (obstructions etc if doing it one way or the other).
For me, slack advance is preferable because the default position if the lever slips at the 'bars is then full advance. But a lot of machines used tight advance and there's nothing wrong with that.
The fact that there are extra slots in the ring probably indicates it has been used with both sorts of end housing (one extra notch, for the cable), or on mags that have rotated both ways (an extra big notch for the A/R movement plus one for the cable probably).
As long as it can go in so that the points JUST open blah blah as per previous comment, it ought to be OK. But if it's really messed up and impossible to set up, then another cam ring may be the only answer.
At least with a single you don't have to be swapping sliprings the way you would have had to on the twin, so that's one job and one expense saved!

[Tazmantic]

So if your not all bored with this by now I had a play today as I’ve not really had a look at the MS1 there was a bit of a spark, so I removed the points and the carbon brush was missing ??? (I’m sure I didn’t drop it out) anyway replaced that and there was an ok spark just out of interest I span the mag clockwise and there was a cracking spark and that was with what I believe is the correct points for anti clockwise.

I removed the cam and checked it against a cam I have for my KSA1 (another anti clockwise mag) and the cut out was in slightly a different place in relation to the opening cam so I tried this cam (it doesn’t fit properly as it’s too thick) and low and behold it had a cracking spark in the anti clockwise rotation (and not good to nothing clockwise) so I am presuming that it has the wrong cam? I’ve attached pics or the cam and points on the cam are black marks the mark at the bottom is where the cutout is the mark at the top is where I believe it needs to be.

Cheers

[Tazmantic]

I’ve just found this pic of the same mag MS1-4 on the internet and that has a different end plate so I’m now thinking mine is incorrect

[Tazmantic]

So am I being really stupid, I removed the carbon brush from the points earlier just to try as I was sure I hadn’t dropped it and it still sparked without it…. (I only tried as there does not appear to be a slip ring in the housing) so do these mags not need this brush??

Also I’ve had a premature senior moment just realised it sparks fully retarded if I use the gap that’s been ground into the cam ring but not if advanced at all so it’s looking like who ever did it put the groove in the wrong place.

[Groily]

More things to think about.

First, if I read the pic correctly, the end housing you have has the entry hole for the AR mechanism on the upper LEFT when fitted on the mag. It looks standard and correct for a slack advance cable on an anti-clockwise drive mag. The normal AR bits there are a lid, attached by a 2BA screw to one side, an adjuster for the cable, a plunger with a lug on it about half way down and fenestrated where it's been machined part-round to nestle against the cam ring, and a spring that goes inside the plunger. At the bottom, a split washer to retain the cable nipple, and a threaded bung to close the hole. That is what's normal. In the pic of the other one, there's a manual pull-me push-me to operate the advance. Which isn't really what you probably want as you can't It's a 'mod' anyway, as you can see the hole where the 2BA screw should hold a lid on.

The contact breaker you show is for an anti-clockwise drive mag (the points go clockwise looking from that end). No good using it on a clockwise one, it's mechanically unsuited and the integral keyway on the male taper is in the wrong place for optimal internal timing.

Some cb units have the ONLY earth brush for the mag on the rear face. On some mags the brush on the cb is an auxiliary one and can be done without. Many cb units don't have provision for said brush. Your mag may have a main earth brush under a large brass-headed screw into the body or base. If it does have one, then the one on the points is auxiliary; if it doesn't you need the one on the cb unit or there's no return for the HT current - unless the bearing insulators aren't doing their job to isolate the armature from the body (although if the drive train is all ferrous then the armature will often be earthed (badly) that way when the things is on the bike). On the bench though, you need an earth brush or you'll very probably get shocks from the HT when playing with it if you touch the wrong thing.

The cam ring may or may not be OK to go with some attention. The only way to figure out what is needed in terms of position of notches for A and R is as per my first reply. You can remove the eccentric stop pin on most of these - if there's a dimpled thingamy on the end housing, lift it with a sharp blade and then unscrew the eccentric slot-head item under it. Replace with a 1/4 BSF bolt and position the camring where you think it should be for full ADVANCE. Tighten bolt to hold ring in place provisionally. Try the mag, the correct direction, and at high as well as low speed. (Magnetic waves 'bend' with speed so what happens at 2000rpm of the mag, say, isn't exactly what happens when you give it a flick of the wrist. You can lose sparks at revs that looked good with a simple flick.) Move the ring this way and that until there are no sparks - ie you have over-advanced the internal timing. Then go back the way you came until the points JUST open JUST after the flip point of the mag as I described before. Mark it - it's where the ring needs to be at full advance. Nowhere else will do.

Then look at the notch that is there and consider whether you can enlarge what's there or or make a new notch (that won't run into the one that is there); and also see if the advance and retard plunger notch would be anywhere near sensible for correct operation. It likely won't be. In which case, turn the cam ring half a turn so that the points are opening on the other flip point of the armature, and go through the same process to get to the best position and mark it again. If things are looking good, this will then necessitate grinding a brand new notch for the stop pin which will be close to - but offset from - 180° from the original. As I said before, you get one go at this. Then a new notch for the AR plunger will be needed. The rings are hardened, so it's a grind, not a saw or drill job. Great care is needed to avoid an irredeemable foul-up. If you get it right, you have new notches that accommodate the stop pin (when replaced) and the AR plunger.

Doing all this on the other flip point you'll probably reverse the polarity of the spark. See my previous on that too. It's not absolutely critical but it is material to the best possible functioning of the magneto.

Not sure about the relative dimensions and profile of the cam ring on a KSA compared to an MS1, but the same basic design, possibly with a difference in depth, and in notching. Points closed for about a quarter of the rotation (typically 80° I think), open for the rest, is right. Which is what you'll also find on later single cyl rings, and indeed on ANY magneto cam ring from any manufacturer - the ideal dwell needed to charge the HT coil was figured out a zillion years ago by some very clever people, many of them German.

[Tazmantic]

Thanks again for all the info if I can’t sort it out from that I need to give up. I’ve added a few more pics as the end cap has the plunger type A/R not the sort you describe as is on my ksa1 would plunger be correct for 1933? (I believe that’s when it was made) Also has a number on it “M3431-A”. Also the cam is thinner than the ksa1 the metal end cap has a plastic ring under it and the stop pin end is not in the centre of the screw is this for fine adjustment?

Thanks so much again

[Groily]

A pleasure . . . if it gets us anywhere!
The stop pin you have is correct. The eccentric nose is indeed for fine adjustment, set at the factory initially. It provides a few degrees of adjustment. (Maybe enough to get your spark back at full advance if it was set fully advanced and you reset it to max retard?? Clutching at straws . . )

The end housing seems to have the more common sort of AR fittings found on MO1 - ie hole threaded 1/2 Cycle, big hollow screw-in tube-thing & plunger and spring. It's probably the right housing for the mag though, as it's for a cam ring not a face cam like MO1. Not sure what the last year of the MS range and first year of MO1 was, but 'late '30s' covers it. The dynamos changed a bit at much the same time, from the first E3 type with alloy end plates each held on with three screws to the sort with two long through-screws, which came in various versions over the years.
The tin end cap doesn't look wrong but the plastic spacer could have been used to push up against a cam ring that had a bit too little depth. The ring needs to be held by light pressure into the housing - don't want it fretting, or even coming out if the lid isn't pushing up against the outside of it.

I think you have enough bits there to get to happy - but you need to figure out where the ring should be set and see if you can get it there, with working advance and retard. You may need a spacer to compensate for the cam ring's lack of depth if it really isn't the original one, but as long as the heel of the opening point is fully on the inside of the ring, that should not be a problem and it will work OK.

In the past, I and many others have done all sorts to get these things to work properly. To the extent of drilling and tapping a new hole for the stop pin if needs must (the alloy is thick enough even off the extra-thick bit where the original pin is set), re-notching rings on the opposite side as previously described, and so on.
Another good option I didn't mention yesterday is to use an axially-disposed 'headless' screw of about 6BA set into face of the end housing as far 'out' towards the inside wall as possible, and dispense with the pin that goes through from the outside. That wheeze preserves the notch you have for the arc of movement (but not for the cable plunger). Such a system was used in fact by Lucas themselves on many early K1 / 2F series magnetos - where they used TWO axial screws a few mm apart to retain 'fixed cam ring' mags with ATDs on the drive end, and just one screw for manual ones, so that they could use the same cam ring with a big notch for both types. On reflection, that might be your best route out of the problem if your cam ring won't play using the 'normal' methods. Just need a suitable screw and tap - any tiny size will do. (Or even a press fit pin in a tiny hole, but that's a wee bit riskier.)

There's usually, almost always, a way, even if it's not 'perfect'.
The number one thing is to find the right place for the ring at full advance, and then work back from there to obtain adequate retard (we seldom need as much as Lucas offered, which could exceed 40° on the crank very typically) and correct operation of the plunger for the AR cable. Get those things right, and a lid that holds the ring inwards by the clip, and you're home and dry. New advance and retard cables come with the gubbins except for the plunger, and the plungers can be had from various sources (or made if facilities available). There are 2 lengths - so you just have to be careful on that.

My fingers are firmly crossed for you!