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The article relates to the AJS / Matchless Lucas K1F, K2F, KVF, N1 magneto or an MO1 or MN2 magdyno. Removal, disassembly , disconnecting the capacitor and re-assembly with an EasyCap.

This task is in principle quite straight forward, however and it's a big however; the inner race of the drive end bearing is extremely difficult to remove.

You will need a lathe and a 4-jaw chuck to make the correct tool. There is NO ALTERNATIVE!

So you have no spark or a weak spark?

On the assumption that you've checked that the contact breaker opens and the HT brush, then you probably have an issue with either the capacitor or the winding. The capacitor is fixable at home, but the winding is best left to someone with all the equipment and costs about £120. You have a very good chance that the capacitor is dead.

There are some articles around about removing the capacitor but nothing really that I could find explaining why and simple description with pictures; so...

In a nutshell, the original capacitor is fitted in parallel with the armature, so if you fit an external one like an EasyCap, you will have two capacitors in parallel with the armature. One good and one bad. This will almost certainly alter the total capacitance and thus the timing and spark strength, although you will probably see a good spark and think all is well.

I'm going to use an EasyCap because whilst I could make my own with foil, a HV capacitor and some insulator, the people who make EasyCap have sensibly priced it at a level where it's just not worth the time. Basically you need to isolate / sever the ground on the old capacitor and leave the positive as is.

Removal from engine

Remove the drive chain cover with the AJS logo on and you will see the chain and sprockets. You need to remove the top sprocket, so undo the bolt and find a three leg gear puller. You will probably have to grind the feet down on the puller if you are using a standard 2 inch one, to make sure the feet don't touch the case. The gear is held on only by a Morse taper, bit sticks pretty firmly.

The magneto is mounted on a plate and then bolted to the engine with three different sized bolts. These can easily be removed but as you do so, spacer tubes and washers will fall out. There are various things on British bikes that are poor design compared to German ones of the same period and this is one of them. They can be awkward to get back in.

To remove the advance and retard cable you need to take out the points and the big copper 'C' clip. You can then unwind the cable.

Removing the armature

Flip over the magneto and remove the 4 bolts holding on the mounting plate noting which way around it was fitted. Now remove the single, "odd one out" protection screw underneath. This is VITAL else you may break the bobbin inside.

Undo the bolts and screw holding the contact breaker holder on. On my Lucas N1 there where was 1 nut, 1 cap holder swing nut and 1 security screw. The security screw head was made to only be able to screw in and not out, so drill and 2-3mm hole in it and insert a small tap extractor. Once you have this screw out, cut a slot in the end for future use.

You can withdraw the armature. It will be stiff as the magnets inside should be strong. Make sure you pull it straight and try not to scratch it. The magnets on the N1 do NOT need a keeper, but other magnetos may do. Read the manual by clicking on the Lucas Magneto Repair Manual image.

Removing the bearing and bobbin

You need to remove the bearing to get at two long screws behind the bobbin. Now I know you will try with a gear puller anyway, just as I did, but don't waste your time. This inner race was pressed on with something that meant business and you WON'T GET IT OFF. The issue is that you can't get a good grip on the bearing and can't get in behind it. You will damage the surface if you try too much.

You need to make a special puller which will take you about 3-5 hours! Basically it's a two-part square apx 4cm across with at 20mm hole in the middle. The hole though has a protruding ring that fits into the bearing concave. This is why it takes so long. You will need a lathe with a four jaw chuck or a milling machine with a 360 degree rotating table. Either way it's a time consuming pain for a one off job. Use Blue Tack to push onto the existing bearing to get an idea of the concave profile you'll need.

Once you get it off (and it only just comes off even with the right tool!) you can pull off the bobbing by hand if you apply equal pressure with many fingers. Be careful not to bend the HT spike that goes from the windings to the armature. Once off, keep the shims safe and undo the two long brass screws. The armature will pull apart at last.

Removing the old capacitor

Once you've spent a year getting the armature apart mark the live side with a red marker and the end of the armature so that you know how to put it back later. The positive side is the side with the insulator and connects to the capacitor where it flows through and out the centre hole to the contact breaker long screw.

The negative or ground is connected to the armature case and the capacitor. The idea is to have it ONLY CONNECT TO THE ARMATURE CASE The picture below shows the factory paths and the new path in blue marked 'C'.

Now is a good time to test the old capacitor to make sure there is no short between live and ground.
Also test the positive is still connect to the centre contact breaker screw hole on the back.

Note that you can see the cut off old ground connector, leaving a nice brass pin. Make sure this is DOES NOT TOUCH THE CASE WHEN RE-INSTALLED


If you've got this far you're more than capable of putting it back together, but the three main things to watch out for are:
1) That the armature is true, so ideally spin it on a lathe.
2) That the bobbing spike makes contact when putting the bobbin back on carefully.
3) Ensure that the Ground Screw is refitted underneath!! else your mag may arc through the windings and burn out. Do this LAST!! after installing tha armature.

Once assembled, spin up on a cordless drill with the HT lead connected and you should see a lovely blue spark; mine was good even at around 6mm.

Conclusion: A trickier problem to solve than some, but well worth the effort and trying before sending your magneto off for a rewind. The EasyCap capacitor seems a good product and well made and I'd recommend you try one.Think of it more as a tool, rather than a permenant solution. Remember, you can install an EasyCap and if you get a spark back you can then embark on this repair to see if it fixes your magneto. Do that first, before you take the magneto apart.

Once you've put it back together, Check the LOW RPM sparking by spinning by hand else you won't be able to start your bike with the kick start! It should blue spark when spun by hand.


Good luck.

Any questions? Comments? feel free to ask.

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Steve Hawkins   25/03/2019 at 16:01:12
Hi, Do you have a recommendation for someone who can rewind a K1F armature please.
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Peter Giraudo   24/06/2018 at 19:23:54
Excellent article, really important information.
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Trevor Harkin   25/08/2017 at 23:13:39
Hello Julian,did Robert get back to you to let you know if he was able to just get rid of the link plate so as not to have to dismantle the armature,can you contact him to find out as i am about to undergo the same procedure.Regards Trevor.
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Richard Johnson   07/04/2017 at 09:41:04
Hello Julian, thanks for your reply. It looks like I have the same problem with the Comet mag. It was reconditioned 10 years ago so time to get it checked again. Richard
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Richard Johnson   05/04/2017 at 13:31:09
Hello, I have a Vincent Comet with a Lucas K1F mag. On tick over, the engine maintains a good steady tick over but sometimes the engine spits back and rotates backwards and stops. I have checked all the main issues, timing, carburation, auto advance etc. It appears the mag is producing an additional spark, randomly, on the induction stroke. Any thoughts before I remove the mag for an inspection / rebuild.
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 - julian   06/04/2017 at 07:26:12
Hello, Ive not had this problem on this kind of mag but I have had the exact same issue on a big two stroke. It would give a big heavy knock when ridinf or at tickiver ad if it randomly fires way to advanced...I checked everything 10 times like you did and it turned out to be the stator windings...insulation beeakdown.

Vogelsang Peter   04/02/2017 at 09:48:35
Hello, when I have a dynamo with minus earth, do I have problems with the magneto with plus earth? greetings from Swizerland
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 - Julian   04/02/2017 at 10:17:04
Hello, Hmm I think that would depend on what else you have on the equipment or bike - for instance if you have a bike with battery and electronics etc it would be positive or negative earth through the frame and engine so reversing will not work. If you have no battery or equipment I think it will probably work off the top of my head. Regards

sergio   23/01/2017 at 22:35:10
Hello I need to contact you to buy modern capacitor for magneto lucas the same as seen on your website ajs 500 cc year 1947, with new modern capacitor does not need to open the magneto I am sergio mansilla from argentina thanks
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 - Julian   25/01/2017 at 20:51:32
Hello, you can find one here:

Joe cowell   10/08/2016 at 19:47:25
Yes, It is just possible on a Lucas NI to isolate the original capacitor without taking the armature apart. The grounded side of the capacitor comes through on a tag which is connected to the ground and joins onto the coil primary. Once you have ground or cut through the tag to disconnect the capacitor, you will almost certainly find that the capacitor is still electrically to ground (check with an ohm meter) This is because there is very little space between where the tag comes through and the brass armature end. It is just possible to slide a very thin piece if insulating material between the tag and the brass, or to push the tab away and run some silicone down between the tab and the brass armature end. Brightspark recommend taking the armature apart on an N1 magneto which is no doubt the surest method of isolating the old capacitor.
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Robert   09/06/2016 at 14:01:34
Hello, thank you for the excellent write-up on the N1 magneto! I have a "weak" sparking Lucas N1 on my 1953 Matchless G80s and intend to make use of the information you have so kindly shared :) The bike was de-registered 1960 and I doubt the magneto has ever been serviced beyond oiling the lubricating wick. I have verified that the contact breaker gap is according to spec and the HT brush looks fine. In addition, the primary coil measures 0.6 Ohm and secondary (HV) ~6kOhm. Which (surprisingly!) seems to be reasonably OK. Therefore, I am assuming my capacitor is shot. I have a couple of EasyCaps on order to see if sparking improves with on fitted (seems a cheap enough test). But. Since I lack a lathe or any other obvious means of making a special puller I hope avoid the whole Condensectomy operation and simply disconnect the original cap by grinding off the ground link plate "in-situ" so to speak. I got the idea from your "Before" picture (thank you!). Do you think this is in any way feasible?
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 - Julian   09/06/2016 at 14:37:49
Hi, thanks. I spent a long time creating this article out of frustration so I thought Id save someone the weeks of pain!. Im glad you found it useful. If you can get at the capacitor to disconnect it, your plan may work. Remember though it MUST spark when you flick it even gently with your hands else you will never be able to kick over the bike fast enough to start it. Good luck
 - Trevor   24/08/2017 at 20:59:48
Hello Robert,i also have a 53 matchless g80s with weak spark like yours,can you tell me if your idea of grinding off the link plate worked as i will embark on the same mission,also have easy cap on order.Regards Trevor
 - Trevor   24/08/2017 at 21:00:14
Hello Robert,i also have a 53 matchless g80s with weak spark like yours,can you tell me if your idea of grinding off the link plate worked as i will embark on the same mission,also have easy cap on order.Regards Trevor

Julian   27/03/2016 at 21:20:35
Hi, Ive not fiddled with a K2F but from what I can see that are pretty much the same in that provided a spark is being generated then the oly real difference is the dual distribution and dual pickup. All I can suggest is that you concentrate on the items that make it dual, so points (check the insulation beneath the points on the duff side for a short), pickup, lead and plug. I cant thingk of anything inside the mag windings and cap that may cause this if that helps.
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Brian Bullard   22/03/2016 at 12:34:00
Good day, i wonder if you could offer me some advise. I suddenly experienced that i am only getting one spark from my K2F Magneto on my Pre Unit Triumph T110 motorcycle (1955) Now i checked by swapping the leads and still only one side of the mag is sending a current in the HT leads. What can i do to fix this and any tips to look for . I would really appreciate any advise. Kind Regards Brian Cape Town South Africa
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 - Julian   27/03/2016 at 21:20:54
Hi, Ive not fiddled with a K2F but from what I can see that are pretty much the same in that provided a spark is being generated then the oly real difference is the dual distribution and dual pickup. All I can suggest is that you concentrate on the items that make it dual, so points (check the insulation beneath the points on the duff side for a short), pickup, lead and plug. I cant thingk of anything inside the mag windings and cap that may cause this if that helps.

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