By VJMC editor, Steve Cooper
DOING THE SUMS
An average 1970s 250 weighs around 150 kilos, and if we assume an approximate 50/50 weight distribution the front steering races are coping with a 75kg load whenever the bike is off the stands. Factor in a rider, and we’re looking at least another 40 kilos over the front end. The top race doesn’t have to take much in the way of load as it’s really controlling lateral movement and supporting the bars, the rider’s arms and upper body. So our lower race will be coping with at least 110kg, on machined and greased annular steel tracks, separated by approximately twenty ball bearings and dollop of grease.
Looking at the maths behind our model here, the numbers suggest that the balls in the lower race are collectively seeing a pressure of 360 psi. If that doesn’t make you think, nothing will. It took the Japanese a long time to realise the limits of cup and cone bearings and when you consider they were fitted to super bikes, it kinda makes you wonder if anyone really had a grasp on the physics. That said, the system works well and if maintained and adjusted they should, in theory, last the life of the bike.
During a full-on restoration it’s pretty much convention that the steering head bearings are replaced. For the cost involved (normally less than £30) it seems peevish in the extreme not to do the job properly. If the frame is to be repainted there’s going to be some blasting which will do the bearing surfaces no favours. Although it’s possible to mask and protect the existing units, in all honesty unless you know they are new it’s easier and safer to replace them. There’s only a finite number of bearing dimensions and many are reused year-on-year so check with a dealer as to availability. One final thought before we roll our sleeves up; there are a lot of extremely cheap bearings on the market so be careful. Given the amount of work and time needed to change steering head bearings there’s a very strong argument for only doing the job once. Therefore, avoid any bearings that are overtly cheap; they are likely to be poorly case hardened and may fail in very short order.
THE ECONOMY YOKE REMOVAL METHOD
With a mate helping me, we once changed the head bearings on a Yamaha XS650 without too much disassembly via some strategic planning. Using an overhead beam and some straps we the tied the freed handlebars, headlamp plus brackets and clocks just off the top yokes, secured the brake systems safely, removed the front wheel and then slid the fork legs out. This left just the yokes and steering stem in place and the bike resting with its engine and frame on blocks. This is really a dodge if (a) the bike is a big beast (b) you’re not doing a full rebuild and (c) time is of the essence. It’s also a recipe for disaster if you don’t ensure everything is secure. The saving graces here are that you don’t have to reconnect the loom, bleed brakes etc. However, get this method wrong and it will end in tears.
THE SENSIBLE YOKE REMOVAL APPROACH
To replace steering head bearings (or inspect existing ones) we need to do some significant spannering. In this blog I’m working on Oskar the MZ and although the front end is a little unusual, the basic principles still apply. For clarity I’m also using a few Yamaha examples to illustrate certain areas.
On 99% of bikes you will need to remove the entire front end of the bike. A few strategic squirts of ACF-50 or similar a few days beforehand makes the job much easier, especially in areas like the lower yoke pinch bolts that won’t have been loosened in years. In most cases the wiring loom will need to be disconnected in the headlamp shell, so take notes and pictures to make sure you know how it all goes back together; label if necessary.
Before removing the handlebars, disconnect the brake or clutch cables. On hydraulic systems, you need to figure out the best method of ensuring air doesn’t enter the system or factor in a fluid change as best practice. With the front wheel removed and the forks dropped out of the yokes we can finally get to the heart of the job.
Loosen the steering stem nut (above) and just slacken off the notched adjuster ring(s) below the top yoke (below).
Although you can use a bicycle type C spanner purpose-made, adjustable, units are infinitely more effective. If you insist on slackening the adjuster ring with a hammer and punch you probably deserve the dented tank you will undoubtedly get. The top yoke should now lift off the steering stem taper with a couple of taps from a rubber mallet. Assuming the bearings will be reused, it’d be kinda handy ensuring all the balls are safely retained. A large sheet mounted below the front end acts as a catchall should any of the balls fall out. Slacken the adjuster ring sufficiently to reveal the lower cup and remove all the balls with a magnetic probe placing them in a magnetic tray. Occasionally some bikes use different sizes and/or number of bearings top and bottom, so make notes and play safe.
Supporting the lower yoke, remove the adjuster ring and upper race collect the balls with the magnetic probe again then drop out the lower yoke on the steering stem.
INSPECTION AND REPLACEMENT
If any of the bearings look like this with dirty grease and/or rust they need replacing. Any signs of rust, pitting or corrosion means the whole lot is scrap. There’s little sense in replacing one or two races if the rest will need to be done in a year.
Look carefully for evidence of webbing or pock-marking (above) as this is evidence of corrosion. To remove the races in the headstock, we need some brute force and intelligence not ignorance. Working in a circular pattern, apply force equally to the entire circumference of the race, driving it from its housing with a drift and a large hammer punching it on its inner edge; remember, the race is a very tight fit.
The lower race is removed from the top and the upper from below. If it proves impossible to get any purchase on the races beg, steal, borrow or purchase a blind bearing puller, this should get the job sorted. The cone fitted to the lower yoke is again an interference fit and needs to be carefully manoeuvred off its mount. Something thin and tapered such as a small cold chisel, a flat blade screwdriver or an old wood chisel normally does the job perfectly. As the bottom yoke tends to see a lot of dirt and water, the cone can sometimes corrosion weld itself to the steering stem and resist all attempts to remove it. In such circumstances, a combination of penetrating oil, heat and repeated prying will normally release the culprit. If all else fails it’s possible (with extreme care) the use an angle grinder, tungsten-carbide tipped drill and/or Dremel to slice through the seized-on cone to a point where it’s sufficiently weakened and can be levered off. However, it’s critical that this is done with care and precision to avoid any damage to the yoke, steering stem or the cone’s seat. One this occasion the race has actually cracked.
INSTALLING THE NEW PARTS
As those incredibly helpful manuals used to say...replacement is the reverse of removal...yeah,right! Having washed out the housings with some degreaser and removed any corrosion with emery paper, the headstock races can be installed. As these are an interference fit we need as much help as possible and it pays dividends if you leave the races in a freezer overnight; even minor contraction makes the job easier.
With a race placed in the housing, tap it in evenly with a non-ferrous drift until it sits squarely in its housing. It’s imperative the races are flush with the base of the housings if we’re to achieve good handling, so spend time making sure the races are flush and seated properly. Comparing the edge of the new race to the top of its housing will normally give a clear indication if it’s in on the wonk. Repeat on the lower mount and we’re halfway there.
Fitting the cone to the lower yoke is slightly less tricky as access is obviously easier. In many instances it’s a real boon if you heat up the cone and somewhere between 80-1200 degrees is a reasonable temperature. Use a non-ferrous tubular drift like this alloy scaffold pole.
This particular cone needed some serious percussion to settle it down so don’t be afraid to give it some welly. Using some good quality grease I can now stick the requisite number of ball bearings in place; I prefer to use something like ACF-50, with its well documented anti-corrosion properties, as this will protect the bearings for many years. The number of balls should be specified in the workshop manual; do not be tempted to use more than detailed even if there is apparent space as overstocking will seriously compromise operation. Having pushed the lower yoke and ball bearings home, I secure the unit to the frame with some duct tape. This allows me to sort out the top end of the steering stem with more ball bearings and grease without worry about the bottom half shooting all over the floor. You only ever drop steering race balls onto a shed floor once! With the top cone in place the adjuster ring is added, nipped up so that nothing can fall out.
ADJUSTING THE STEERING RACES
Now is the time to reinstall the forks, headlamp etc. and once they are notionally in place we can adjust the steering races. This is done by nipping up the adjuster ring(s) until the front end is just feeling tight when the bars are turned and then backing off just a crack. With the front end off the ground the bars should move through their arc freely but there must be no fore-and-aft movement at the steering head relative the headstock when the forks are rocked back-and-forth. When refitting the front end, start by tightening at the handlebars and work down to the wheel spindle bouncing the bike (off the stand) via the bars to ensure everything is settled and located. Check the brakes, plug in the loom and it should all feel factory fresh. Inspect and adjust after a few miles and the races should last for ages.
PAINTED FRAMES AND TAPER ROLLER BEARINGS
There’s an expectation that any company painting, enamelling or powder coating a frame will protect threads and bearing housings. Sadly this doesn’t always happen and if the headstock has been liberally coated in gloss black or whatever new races will never fit. Paint or powder coat can normally be removed with careful application of some paint stripper and an artist’s brush. Enamel can take more effort and it may come down to a Dremel and an abrasive wheel. Better still, tell your supplier to mask everything and don’t take no for an answer.
The benefits of taper roller bearings are fairly obvious and many a classic now sport them. They still need greasing and adjusting but due to higher contact surface areas, usually offer better performance on heavier bikes. Some taper roller kits offer are generic versions that work but may alter the distance between top and bottom yokes which can leave headlamps and brackets loose and unsupported. Always buy from reputable source that can verify the fit for your specific bike.
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The information contained in this blog post is based on sources that we believe are reliable and should be understood as general information only. It is not intended to be taken as advice with respect to any specific or individual situation and cannot be relied upon as such.