The Tortured Primary: 911 Response Cover The Tortured Primary: 911 Response Cover
While the snowmobile OEMs have continued to add horsepower, torque and displacement there continues to be a void in one very important area –... The Tortured Primary: 911 Response Cover

While the snowmobile OEMs have continued to add horsepower, torque and displacement there continues to be a void in one very important area – the drive clutch. With perhaps the exception of the latest drive clutches from Ski-Doo, the rest of our sleds are coming with drive clutches that are of the same basic design as what they had 20 years ago. Some have even used lighter weight and weaker clutches to save money. We recently inspected a drive clutch from a brand new 2017 model that had accidentally been dropped on the floor, and it was SHOCKING to see how thin the sheave was where a chunk had broken away.

We now have engines producing 200 horsepower in stock form and up towards 400 horsepower when modified, all with pretty much the same clutches they have had all along going back to when their machines had 50 to 75 horsepower. We are (trying) to run this 200 to 400 horsepower through a 10 or 12 pound clutch made from cast aluminum. These clutches are HIGHLY overstressed units. Then consider the operating conditions. Extreme hot to extreme cold, heat and humidity, heavy pulling, constantly changing loads and engine rpms.

There is a great deal of twisting, flexing and distortion that takes place in a drive clutch. This is how you get cracked (or broken) clutch towers & spyders and accelerated wear in general. The OEM’s have done slow motion photography on the loading and unloading of drive clutches and have made the comment to each other as to whether or not they should even be in the dyno room while this is going on. Now add to this the torsional vibration from the constant firing of the engine as well as harmonics working up and down the crankshaft. The drive clutch acts as a sync or mass kit for the harmonics.

When you look at the whole picture and really add it up, it’s amazing the drive clutch lasts as long as it does. Most old timers can tell you about clutches they have seen blow up, they often reference a hand grenade and for good reason. We’re talking damage to your machine and your personal safety.

Now that we have identified the problem, what can be done about it? Installing a complete aftermarket billet clutch is one option, but will usually run you over $1,500 to do so. Another option is to beef up the stock clutch with a 911 Response cover. Offered for the primary clutches from Arctic Cat, Arctic Cat Team, Polaris and Yamaha, the 911 Response cover is a machined cover assembly made of high grade billet aluminum, not cast like the OEM piece, which is about 5 times the strength of your stock clutch cover. This cover ties all of the towers (in the movable sheave) together into a rigid assembly to limit the twisting and flexing.

911 Response cover

One of the best features is how the 911 cover places a large melonite sleeve over the main shaft in your drive clutch to increase the diameter and the length where necessary. Sliding up and down this over-sized shaft is a large Oilite bronze bushing installed into the billet cover. This does several things – it increases the bearing surface by 250%, but more importantly it keeps everything right on the centerline of the crank and holds it in place. As the drive clutch is opening and closing this large bushing sliding on the larger diameter sleeve helps keep the entire clutch, from stationary sheave to moveable sheave to towers, in much better alignment with less flexing and less twisting. This translates to better power transfer, less binding and greater durability of the clutch components.

Another nice feature of the 911 Response cover is the front belt adjuster that comes with it. This makes it easy to get your belt side clearance down to .010 to .015 like it should be. The 911 covers for the Arctic Cat and Polaris also have three large vents machined into the cover for increased air flow and cooling.

With the 911 Response cover installed you now have a heavy duty, high performance clutch that will work better and last longer. What does all of this technology cost? $249.95 available from Thunder Products, Inc. Call them at 320-597-2700 or visit

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