I have lost count of the times the hex Arctic clutch has been badmouthed in my presence. Mostly I just grin and momentarily reflect on numerous past wins due in large part to Arctic hex clutch usage. To put this in perspective let’s take a brief look at some notable racing applications.
The clutch originally was similar in design but drove on three pins instead of the later hex bushing. These oldies featured three slider pins that transferred power to the sheaves and ran on a circular bushing. A spline held the two halves together and was removable. Imagine a quick change Rhodie or Micro Belmont in 1969!
As good as it was to be able to swap outer units as conditions changed, it was not to be. Arctic decided that it was expensive to manufacture and some of the wilder EXT 650’s could break these slider pins. The solution was to reduce costs by driving on a hex bushing and solve the slider pin problem by simply eliminating them. Unfortunately that led to bushing failures and had a big hand in the bad rep to follow. Evidence of the old pins is displayed in the three small holes on a hex clutch cover.
I remember them as a superb clutch for the times and highly sought after for serious racing. Keep in mind that Ski-doo and Skiroule were using older dog and notch clutches. Some used Salisbury and other simple devices but head and shoulders above was the Arctic hex.
Early on the Thunder Jet used Arctic hex clutches as eventually did Merc. Yamaha had a kit update for their racers but it was still the old weight system. Most racers of Yamaha that I met had switched over to the hex offered on Thunder Jet. The GPX had a dreadful carb set up and a bit of an obsolete clutch. Most if not all Yamahas’ raced on carbs off Sno- Jet and clutches off Arctic.
The beauty of it was that arctic had a common tapered clutch that would quickly update the Yamaha engines to mods of high regard. There were lots of springs and ramps available.
Aaen and others were quick to market springs, ramps and rollers that could match any shift desired. Need more out of the hole? Just throw a couple of spacers under a green (El Tigre) spring and you are off. Need even more and you could drop in a set of steep ramps and maybe a smaller roller weight and bang, you were right on the money.
Harkin back to the fact that lots of people were still on three dog and notch units and you see the advantages of Arctic hex.
So much for history, now let’s take an honest look at what was really wrong with the hex clutch. Firstly it drives off a bushing that is prone to wear. When this wear is sufficient to misalign the inner and outer sheaves, the weight arms come into contact with the ramp fixtures. The original weight arms wouldn’t stand a lot of wear and would break one or both sides of the arm assembly. This is compounded by a very whimpy fulcrum pin locating the arms.
When this small pin wears it allows the weights to flop back and forth sideways, and that wears the arms against the ramp anchor even worse. If it is allowed to go on for even a short while the performance and dependability are doomed. Our racing bunch of the early seventies had a couple of tricks for the competitors. I will detail how you too can turn a boat anchor into a terror, if you pay attention to a couple of factors.
The bushings will wear past being useful in a year of racing. First thing is to order up a set of good quality bushings available from Roetin or others. Be prepared to press in a new set for each year raced but the cost is small and gains are large. Do this very carefully to get exact alignment between the sheaves. When you press them in they tend to be quite tight on the shaft. Sanding is not a good idea.
Work the unit back and forth to free the travel but don’t worry about a bit of stiffness. This will instantly go away the first test run you make. Tight is good, too tight is bad and loose means you have already given away some performance. I know guys that made a steel hex insert for the bushings and inserted them prior to pressing into battery. That plus a bit of muscle power seemed to produce results better by far than sanding the bushing surface. Do what you must but try for a nice tight slide action and perfect sheave alignment.
Next victum is the weight arms and skinny little fulcrum pin. You may feel at times that there are no weight arms that are not already worn. True, but hunt on and you will come up with a good non-worn assembly sooner or later. My experience is that about one clutch in four will be ok. Most are driven to death in a sloppy assembly but some are quite all right. Take the spider assembly that is best and clip off one side of the fulcrum pin. Drive this pin out with a punch freeing the arms from the spider.
We have fabricated a clamp jig that holds the arms and spider in an upright position. Drill both arm and spider to fit a comet 102 weight pin. Use good lock nuts and install the arms on the spider using the comet pin in place of the small original.
Before replacing the rollers and pins and weights, weigh the assembly and record. Adding or subtracting weight increases or decreases shift force.
The last step is truing up the clutch faces. Most old hex’s have a bit of metal worn in some locations along the sheave face. We use a crank tapered end, cut off to fit in our lathe and mount the drive sheave. Set your angle cross feed and remove only a whisker of metal. You just want it flat not lightened.
The driven sheave half is a bit of a challenge. Mount the sheave holding the top of the three ramp mounts. Be sure to clamp the unit lightly and on center. Repeat the whisker cut. Depending on your skill (finish) you may or not need to use emery cloth to smooth up small lines. It must be smooth or it will eat belts.
Run the clutch clean and dry. The metal on metal pin and arm bearing surface does well with dry graphite. You must keep these clutches absolutely clean of belt dust and grit. Regular service every race is essential to smooth shifts.
Rule of thumb tuning simply goes like this. Flat ramps hold a shift back (rpm up) sloped ramps allow early shift (rpm down). Larger rollers and more weight shifts harder (rpm down) less weight reduces shift force(rpm up) engagement can be raised or lowered by more or less spring tension or by spacers under the spring seat. You can space just this side of coil bind.
Another method of engagement altering is in a notch cut where the roller first touches the ramp. Not much of a round notch gives quite a bunch of RPM. If you do this you may need a tighter secondary to avoid hole shot shift out. I can only assume at this point that your rollers and pins are in top shape.
This clutch makes a deadly drag race unit, easily tuned and foolproof in design. Keep it clean once you get dialed up and you can drag race for years on one clutch. For oval racers try a tighter secondary and as little engagement preload as you can get. Less spring gives better backshift for cornering. A well set up clutch will gain about 200 RPM in the corners due to backshift. Most singles use a light spring but our Merc 340 and Tigre 400 love the green El Tigre spring.
This technical advice is strictly my opinion so use at your own discretion. We have had great results in vintage racing using this simple format.
Gord Healy #100
Check out more of Gords great submissions here.