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Heavy Duty Suredrive™ Coupling for ZF 301 & ZF 302 VLD/IV with Vulkan L Couplings


We consider the HD Suredrive coupling to be a FOREVER replacement coupling for the Vulkan L element couplings on ZF 301 and ZF 302 transmissions. Barring improper installation and/or salt water damage, this coupling comes with a 5 year / unlimited hours full warranty.

Product Documentation


SKU: SUREDRIVE-MOD18 Category: Tags: , , , , , , ,



Many marine transmissions rely on a floating spline coupling arrangement to transfer power from the engine flywheel to the transmission.

The relatively short spline on the transmission input shaft, coupled with the spline in the drive plate, is the weakest link in your boat’s drive system. This spline arrangement is highly intolerant to poor alignment and is very susceptible to wear from constant changes in engine output (e.g. commercial fishboats operating at low RPM), vibration and inconsistent off-on loading conditions.

The Suredrive™ is designed to eliminate this weak link by locking a female center spline hub onto your existing transmission input shaft.


Vulastik L (VL) Coupling Failures and Best Replacements

Years back, IMO and based on tons of feedback from actual customers, you could just about count on the black colored Vulastik L style couplings to fail when least expected, sometimes at low relatively hours. The color of the engine is was used on did not matter (MANS, CATS, Cummins, etc.), and as time went by, Vulkan decided to add a “Yellow-ish” one made from a silicone compound to the Vulastik L line-up.. My thoughts are the Yellow-ish one was designed for higher temperatures and/or for a more aggressive environments and would outlast the black elements.

Well looking back, did it?? No, as I’ve seen them fail too, but they do seem to last longer before they may fail…………

IMO, I feel most engine transmissions suppliers seem to feel that a few thousand hours of service is acceptable… Tell that to a single engine boat operator operating 100+ miles from shore. As for me, I go with the flow and, if I can, I use RBD type couplings where possible (they seem to never fail), but understand they do not offer the controlled “wind-up” that may or may not be needed to deal with torsional issues.

The images below were sent to us by actual customers who purchased Vulastik L couplings that had failed in the field.



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1. Does the Suredrive™ fit my transmission? What is your transmission?
Refer to Specs. or our Compatability chart. 

2. Do the rubber blocks wear out very fast? How long do they last?
It really depends on the application, variables such as extreme misalignment, continuous low rpm operations and/or improper installation. i.e. if rubbers not completely positioned into outer drive receiving pockets can all contribute to premature wear. We recommend that all rubber blocks be changed at your regular transmission service. We have one customer that has 4 Suredrive™ in 2 fishing boats (twin) and has over 28,000 hours on the original set of rubbers and he hasn’t had to change them. 

3. What is the minimum amount of splines need to mount the Suredrive™ to the input shaft?
The Suredrive™ can be mounted with as little as 20% of the splines left. 

4. What is the Suredrive™ made of? 
The inner and outer drives are cast from high quality ductile iron or quality steel. The center spline hub is machined from 4140 high nickel steel so it retains it’s shape upon removal from the inner spider drive assembly (ductile iron is too soft). Therefore the hub would be very difficult to remove from the input shaft as well as lose it’s circumference reducing capabilities upon reinstallation. 

5. The Suredrive™ seems to be heavier than the traditional drive plate that I have been using, is that O.K.?
The design of the Suredrive™ makes it heavier than traditional drive plates. The added weight helps to increase the flywheel mass which helps reduce torsional vibration and permits a much smoother operation at a lower idling rpm. 

6. Why would I spend more money on a Suredrive™ when I can buy a traditional drive plate for 1/2 the price? 
If you look at the cost of replacing a drive plate 1-2 times a year and eventually changing the transmission input shaft, (total cost can be up to $2300) gasket kit, labor for installation, safety factor, (The Suredrive™ always remains engaged) as well as the cost of down time of your vessel and compare it to the cost of a Suredrive™, I think the decision becomes clear. 

7. The Vulcan drive coupling has a better torsional vibration dampening system than the Suredrive™ why should I buy a Suredrive™?
Yes, the Vulcan drive coupling has a greater torsional absorption than the Suredrive™, but it still has the problem of premature spline wear. The only product that you will find that will correct the problem is the Suredrive™.