Magneto Services
We are able to fully restore your old BTH or Lucas K1F, SR series, K2F or KVF magneto or magdyno including the competition versions, just send yours to us for a full inspection and condition report using our latest IMRIE 3000 test equipment. Work is only undertaken after consultation with you the customer and a fixed price quotation (not just an estimate) is given.
If you are experiencing difficult starting from hot or cold then it is probable that the old insulation in your magneto is failing coupled with a faulty capacitor.
Most magnetos will probably need a rewind with new capacitor (cost £210) but there may be extras that need doing such as a new oil seal and bearing insulators, new slip ring, new points and new HT pickup and brushes. These are only replaced if necessary and the final cost for a single cylinder magneto is usually in the region of £350 including aquablasting to make it look like new as well as perform like new, less if the cosmetics are not required. Please contact us for more details.
Here are four BTH KD1 TT Magnetos nearly ready to be returned to their owner after reconditioning. They just need the magnet covers to be aquablasted and then refitted.
TESTING OF MAGNETOS
Testing magnetos both before and after reconditioning is an absolutely vital part of the reconditioning process and this includes both at room temperature and at 70 degrees Celsius. While the original primary and secondary windings in a magneto can appear to work well enough to start a motorcycle from cold there is often a huge drop off in performance when hot due to the old insulation breaking down.
All original (and rewound) armatures are therefore tested on our IMRIE 300 ignition tester which simulates the magneto at running speeds. This will check the strength of the spark using an industry standard three point spark gap tester. Any problems with the armature will show up as an intermittent or even failing spark.
One of the most important test of a magneto is does it perform well at very low speed i.e. when starting a motorcycle. Lucas and BTH specified that there should be a 5.5 mm (about 1/4 inch) spark produced at 120RPM. Kickstart speeds are generally in the order of 300 to 500 RPM with Velocettes being at the low end of this range. We have a special low speed motor which can apply a speed of 120 RPM down to 10 RPM.
IF THE MAGNETO DOES NOT PASS THE 5.5mm TEST AT 120RPM BOTH COLD AND HOT THEN IT WILL NOT BE SENT OUT TO THE CUSTOMER UNTIL THE FAULT (USUALLY FAILING WINDINGS / CAPACITOR) IS RECTIFIED.
After checking the slow speed spark the magneto is then bench tested for 3 hours of continuous running at speeds of between 3000RPM down to 200RPM which is the equivalent of about 200 miles of road use. (These speeds are of course the equivalent of half crankshaft speed.) This is enough to reveal any faults or adjustments that may be needed. It is then again checked for the correct size slow speed spark.
Now you may think that a 5.5mm spark is far too large a gap for the spark to cross given that a normal spark plug gap is in the order of only 20 thou. However it is important to realise that air is an excellent insulator and air/petrol mix that has been compressed to for example 9:1 compression is such that it is many times more difficult for the spark to jump. Just seeing if your magneto sparks by taking the spark plug out, resting it on the cylinder head and kicking the engine over is a very poor and unreliable test - it will not show if the spark works under compression.
This leads us on to RESERVE VOLTAGE.
The voltage produced by a magneto will increase according to the speed at which it is rotated i.e. the higher the RPM the larger the voltage at the spark plug (up to about 4000RPM). To ensure a good spark at all times the voltage produced needs to be higher than that which is required to produce a good consistent spark, in other words there needs to be what is called a reserve voltage. Accordingly all our reconditioned magnetos are checked for this reserve voltage by using an adjustable three point spark gap. For a magneto to perform really well we are able to check that it will give a consistent spark of 12mm while running at 2000RPM.
RE-MAGNETISING
It should go without saying that for any magneto to perform to maximum effect the internal magnets must be fully charged. It is no good replacing the armature, capacitor, etc. without boosting the magnets to their fully magnetised state. Therefore, any magneto that leaves the workshop will be fully re-magnetised as a matter of course. Later Lucas and BTH magnetos that have ALNICO magnets do not normally loose more than 25% of their magnetism, but the earlier BTH KC1 magnetos which have cobalt magnets can loose most of their magnetism if not used and will loose most of their magnetism just by removing the armature. The very best Lucas and BTH competition magnetos utilised ALCOMAX magnets which, like the models that use ALNICO magnets, have their own internal magnetism keeper and will only loose in the order of 5% of their magnetism the first time the armature is withdrawn.
ASSEMBLY
A magneto is not just an electrical device, it really is an electro-mechanical system which relies on careful assembly, for example the armature has to be shimmed carefully and correctly to ensure that there is no lateral or vertical end movement of the points or it will not perform correctly. This is particularly important in the case of twin cylinder motorcycles using for example a K2F or KVF magneto. Worn cam rings and points gap discrepancies can also lead to problems such as the degree timing interval being completely disrupted. We have seen K2F magnetos where if one cylinder is timed correctly before top dead centre then the other cylinder can be up to 8 degrees retarded - very damaging to the engine!!
All magnetos sent to us are checked for these faults. They can be corrected by either installing a new points cam or by having the original cam reground back to the correct specification - a much cheaper option but just as effective as replacing with a new one. There can still be problems with a worn cam end housing but we also have ways to correct this. Suffice to say that any twin cylinder magneto will not be returned in a poor timing condition. All magnetos will be repaired back to the Lucas or BTH specification thus eliminating such timing faults.
SUMMARY.
Our considered advice is that in our experience of testing used armatures, most 50 plus year old armatures that at first sight are reusable at 20 degrees C are not to be relied on when hot even if the capacitor is changed. Insulation that is 50 plus years old gradually breaks down and although it may be alright for a few years, in time it will probably fail. Therefore in order to give a full 5 year guarantee on reconditioned magnetos we do ask you to seriously consider a rewind with new capacitor rather than just a replacement of the capacitor alone. After all the magneto and armature are already dismantled so for the sake of £100 we consider it only sensible to restore the old armature with new primary and secondary windings which are insulated and vacuum impregnated with reliable modern day materials. Your magneto will then be ready for another 50 plus years of service. Please contact us if you have any additional questions.
If you are experiencing difficult starting from hot or cold then it is probable that the old insulation in your magneto is failing coupled with a faulty capacitor.
Most magnetos will probably need a rewind with new capacitor (cost £210) but there may be extras that need doing such as a new oil seal and bearing insulators, new slip ring, new points and new HT pickup and brushes. These are only replaced if necessary and the final cost for a single cylinder magneto is usually in the region of £350 including aquablasting to make it look like new as well as perform like new, less if the cosmetics are not required. Please contact us for more details.
Here are four BTH KD1 TT Magnetos nearly ready to be returned to their owner after reconditioning. They just need the magnet covers to be aquablasted and then refitted.
TESTING OF MAGNETOS
Testing magnetos both before and after reconditioning is an absolutely vital part of the reconditioning process and this includes both at room temperature and at 70 degrees Celsius. While the original primary and secondary windings in a magneto can appear to work well enough to start a motorcycle from cold there is often a huge drop off in performance when hot due to the old insulation breaking down.
All original (and rewound) armatures are therefore tested on our IMRIE 300 ignition tester which simulates the magneto at running speeds. This will check the strength of the spark using an industry standard three point spark gap tester. Any problems with the armature will show up as an intermittent or even failing spark.
One of the most important test of a magneto is does it perform well at very low speed i.e. when starting a motorcycle. Lucas and BTH specified that there should be a 5.5 mm (about 1/4 inch) spark produced at 120RPM. Kickstart speeds are generally in the order of 300 to 500 RPM with Velocettes being at the low end of this range. We have a special low speed motor which can apply a speed of 120 RPM down to 10 RPM.
IF THE MAGNETO DOES NOT PASS THE 5.5mm TEST AT 120RPM BOTH COLD AND HOT THEN IT WILL NOT BE SENT OUT TO THE CUSTOMER UNTIL THE FAULT (USUALLY FAILING WINDINGS / CAPACITOR) IS RECTIFIED.
After checking the slow speed spark the magneto is then bench tested for 3 hours of continuous running at speeds of between 3000RPM down to 200RPM which is the equivalent of about 200 miles of road use. (These speeds are of course the equivalent of half crankshaft speed.) This is enough to reveal any faults or adjustments that may be needed. It is then again checked for the correct size slow speed spark.
Now you may think that a 5.5mm spark is far too large a gap for the spark to cross given that a normal spark plug gap is in the order of only 20 thou. However it is important to realise that air is an excellent insulator and air/petrol mix that has been compressed to for example 9:1 compression is such that it is many times more difficult for the spark to jump. Just seeing if your magneto sparks by taking the spark plug out, resting it on the cylinder head and kicking the engine over is a very poor and unreliable test - it will not show if the spark works under compression.
This leads us on to RESERVE VOLTAGE.
The voltage produced by a magneto will increase according to the speed at which it is rotated i.e. the higher the RPM the larger the voltage at the spark plug (up to about 4000RPM). To ensure a good spark at all times the voltage produced needs to be higher than that which is required to produce a good consistent spark, in other words there needs to be what is called a reserve voltage. Accordingly all our reconditioned magnetos are checked for this reserve voltage by using an adjustable three point spark gap. For a magneto to perform really well we are able to check that it will give a consistent spark of 12mm while running at 2000RPM.
RE-MAGNETISING
It should go without saying that for any magneto to perform to maximum effect the internal magnets must be fully charged. It is no good replacing the armature, capacitor, etc. without boosting the magnets to their fully magnetised state. Therefore, any magneto that leaves the workshop will be fully re-magnetised as a matter of course. Later Lucas and BTH magnetos that have ALNICO magnets do not normally loose more than 25% of their magnetism, but the earlier BTH KC1 magnetos which have cobalt magnets can loose most of their magnetism if not used and will loose most of their magnetism just by removing the armature. The very best Lucas and BTH competition magnetos utilised ALCOMAX magnets which, like the models that use ALNICO magnets, have their own internal magnetism keeper and will only loose in the order of 5% of their magnetism the first time the armature is withdrawn.
ASSEMBLY
A magneto is not just an electrical device, it really is an electro-mechanical system which relies on careful assembly, for example the armature has to be shimmed carefully and correctly to ensure that there is no lateral or vertical end movement of the points or it will not perform correctly. This is particularly important in the case of twin cylinder motorcycles using for example a K2F or KVF magneto. Worn cam rings and points gap discrepancies can also lead to problems such as the degree timing interval being completely disrupted. We have seen K2F magnetos where if one cylinder is timed correctly before top dead centre then the other cylinder can be up to 8 degrees retarded - very damaging to the engine!!
All magnetos sent to us are checked for these faults. They can be corrected by either installing a new points cam or by having the original cam reground back to the correct specification - a much cheaper option but just as effective as replacing with a new one. There can still be problems with a worn cam end housing but we also have ways to correct this. Suffice to say that any twin cylinder magneto will not be returned in a poor timing condition. All magnetos will be repaired back to the Lucas or BTH specification thus eliminating such timing faults.
SUMMARY.
Our considered advice is that in our experience of testing used armatures, most 50 plus year old armatures that at first sight are reusable at 20 degrees C are not to be relied on when hot even if the capacitor is changed. Insulation that is 50 plus years old gradually breaks down and although it may be alright for a few years, in time it will probably fail. Therefore in order to give a full 5 year guarantee on reconditioned magnetos we do ask you to seriously consider a rewind with new capacitor rather than just a replacement of the capacitor alone. After all the magneto and armature are already dismantled so for the sake of £100 we consider it only sensible to restore the old armature with new primary and secondary windings which are insulated and vacuum impregnated with reliable modern day materials. Your magneto will then be ready for another 50 plus years of service. Please contact us if you have any additional questions.