Fit a magneto oil seal

'Trying to fit a new oil seal can be frustrating, fighting against the magnet’s pull. Here’s how a bit of lathe work can help'

It’s not common but does happen: duff new plugs, and wiring diagrams

It’s not common but does happen: duff new plugs, and wiring diagrams

Working with Ferret on the Project Honda this month, I mentioned the curse of faulty-from-new spark plugs. It’s rare to buy a brand new dud, but it happens sometimes and the rarity means you’re all the more likely to get caught out by it. Ferret added that one of his recent rewires hadn’t gone according to plan for a similar reason. The bike was fitted with a Villiers 1H 225cc engine. “It’s the first one of these I’d ever wired, but it was simple enough and I followed the diagram. It started and ran fine, but as soon as you put the lights on the engine cut. We all make mistakes, but after checking it all, I couldn’t find anywhere I’d gone wrong. It all matched the diagram. But when I thought about it, I realised that was the problem – the diagram in the factory manual was wrong. I made a small alteration and it worked perfectly.” Mistakes in manuals and faulty new components are a dirty trick on the amateur restorer. Because Ferret is a confident electrician, he knows if there’s a problem and it isn’t just his silly mistake, something is not as it should be. His experience tells him he has done his part right and teaches him where to look for mistakes in the work of others. It’s much more of a problem to the rest of us, whose confidence is based upon fitting new parts and following manuals to the letter. But blind faith has always been a source of trouble.

 

Oily sparks

Robin Frith tells me that despite a new seal, oil is getting into the magneto of his 1955 Triumph Tiger 100. The magneto bolts to the back of the timing chest on the Triumph engine and a special seal on the spindle keeps the internals dry – or should do. Magnetos can often cope with running in a bit of oil, but it’s not good and ultimately misfiring is likely to occur. I can think of a few possibilities here. First, if the engine is breathing heavily, due to blow-by from worn piston rings or cylinder bore, it’s possible the increased crankcase pressure will overcome the seal, forcing oil into the magneto. Allied symptoms would include smoking from the exhausts and breather outlet and leaks from the engine. If the engine is healthy, there must be a problem with the seal itself. It’s not an easy thing to fit properly (see ‘how to’ panel on opposite page). The tabs on the paper bearing insulating washer are easily torn off, leaving the bearing (and seal) a loose fit. The test is to remove the timing cover and check there’s no up and down movement on the magneto drive gear. If that’s OK, remove the drive gear and make sure the seal is fitted the right way round with the spring facing outwards, that the seal lip is intact and that the spring hasn’t broken. There are two sizes of seal, the later type has a larger outside diameter than the earlier one, and if the small one has mistakenly been fitted to a large hole it could float about and cause problems. Also you can get a wear ring on the magneto spindle, where the seal runs, which can loosen its grip. I think the magneto will need to come apart.

Torn tabs on the paper bearing insulating washer could be to blame, or...

Torn tabs on the paper bearing insulating washer could be to blame, or...

select a Sprocket

Maths was never my subject at school, but that was before I’d worked out there were more practical uses than calculating how long it takes how many men to dig a hole. Take gearing, for instance; Bob Chambers owns a Royal Enfield Super Meteor and has now fitted a spare 700cc Enfield engine into a BSA A10 chassis to make a café racer special. He asks what combination of sprockets would gear the special something like his other bike. British bikes generally use chains for both primary (engine to gearbox) and final (gearbox to rear wheel) drive. The overall gear ratio is the relationship between the four sprockets and is calculated by dividing the two rears by the two fronts and multiplying the results. A standard 700 Constellation uses a 29-tooth engine sprocket, 56 on the clutch, 20 on the gearbox and 46 on the rear wheel. 56 / 29 x 46 / 20 = 4.44 to 1. Bob’s Super Meteor works out at 4.47 to 1. At the moment the special has a 30-tooth engine, 56 clutch, 19 gearbox and 47 wheel. This works out at 4.87, which is a bit low. 32t engine sprockets are available and this would raise the gearing to 4.56, just slightly lower than standard. Alternatively, a 20- tooth gearbox sprocket would raise it a bit less to 4.39 to 1.

Rotor magnetism

Graeme Mackay emails from Aberdeen to ask if I could suggest anywhere he could get the generator rotor of his Z1300 re-magnetised. Old horseshoe magnets fitted to vintage bike magnetos were made from a soft iron that was easily magnetised, but it also lost magnetism easily. Magneto reconditioners usually have equipment for refreshing the magnetism in these and later magnetos, but over the years magnet material has improved to create (relatively) ‘permanent’ magnets. These lose far less magnetism in the course of their lives – but this magnetism is also harder to restore, so much so that I know of no one able to do it. So I thought I’d consult Marcus Rex of Rex’s Speedshop (01580 880768) who specialise in the manufacture of replacement CDI units. Marcus told me that since the magnets in the Z1300 will have been charged before the rotor was assembled, he doubts it is possible to re-magnetise the rotor complete. The good news is it’s unlikely to need it. Marcus explained that the magnets are ‘rare earth’ type, a super-strength variety developed in the 1970s. “Rare earth rotors never really lose magnetism unless they’ve been in a fire or something,” said Marcus. “The problem is more likely to be excess clearance between stator and rotor. We see a lot of problems with this, often when aftermarket stators, made to slightly different tolerances, are used. “The clearance is critical in many cases, but it can be tricky to find the correct figure – it’s seldom in the manual and more often found in service bulletins when problems have arisen, post production. It should certainly be less than 1mm,some are 0.4mm. On types
where the coils are wound onto pole shoes sometimes the shoe position can be adjusted; but with the Z13’s spider-type core no adjustment is possible, so replacement genuine parts may be the only answer.”

Wheely helpful

Delighted to hear from Tony Plumb saying he found my feature showing James May how to rebuild his Honda 90 wheels inspiring. He says “I built two very true wheels” following it. Oh well, that’s Tony and James I can strike off my possible wheelbuild customer list...

 

How To

fit a magneto oil seal

Trying to fit a new oil seal can be frustrating, fighting against the magnet’s pull. Here’s how a bit of lathe work can help

1. The small bore of the oil seal prevents using your finger to hold the seal, paper cup and bearing in place against the pull of the body’s strong magnet long enough to press it home.

1. The small bore of the oil seal prevents using your finger to hold the seal, paper cup and bearing in place against the pull of the body’s strong magnet long enough to press it home.

2. The answer is to turn up this mandrel from plastic. Make a radiused step to hold the bearing, and a projection that, gripped by the seal, will hold everything together as an assembly

2. The answer is to turn up this mandrel from plastic. Make a radiused step to hold the bearing, and a projection that, gripped by the seal, will hold everything together as an assembly

3. Make the outer diameter a sliding fit inside the body. Then the mandrel will guide the bearing straight in, allowing you to finally push it home in a press or using a drift.

3. Make the outer diameter a sliding fit inside the body. Then the mandrel will guide the bearing straight in, allowing you to finally push it home in a press or using a drift.

4. Frayed tempers avoided, and the mandrel has seated the bearing squarely enough to avoid any damage or misalignment of the paper cup. Perfect!

4. Frayed tempers avoided, and the mandrel has seated the bearing squarely enough to avoid any damage or misalignment of the paper cup. Perfect!

In