Yak 50 / nanchang cooling gills - axle rework

Radial Engine Cooling Louvers live in a world of high vibration and varying air loads.
Add resonance - and any looseness in rivets or bushings in this mechanically complex Gills assembly quickly shows up as wear.

Tension springs between the gills are there to mask this, but grab any gill without a spring attached and 1/2” of rotational play is not uncommon. If you don’t check for missing louver springs during the Daily Walk around, you may soon discover gills with irreparable damage.

One weak spot of this CJ and Yak 50 Louver design is at the lower end of the steel axle where it passes through a hole in the aluminum ring. ​On your plane, if you can grasp any Gill/Louver at its base and create perceptible movement, that hole in the Inner ring isn’t round anymore.​

The head of the axles that run down the center of the Gills have been formed from round bar into a square T shape (to stop axle rotation) - and there’s a matching slot in fishplates riveted to the exterior cowl ring.  Like spokes on a bicycle wheel, tension in these axles via two 7mm lock nuts nuts at the inner end, draws the outer ring into an even circle.

On the Yak 52, which came later, design of the gills assembly changed in these two areas.

The Yak 52 inner ring has riveted M4 anchor nuts for each of the 28 axles ( no fiddly double locknuts involved) and the top of the axle now has a 100 degree countersunk head, slotted so you can adjust tension easily with a screwdriver, (fishplates riveted to the Yak 52 outer ring have a mating countersink instead of the T slot.)

Let’s say your 50 or CJ has many Louvers that are worn in this area.

​For Yak 50 owners, one solution is to buy TYC’s fit-and-forget Factory Refurbished Gills assembly, which will have the newer 52 style components.* The prop hub must be removed, but then it’s a simple task of undoing the old brace supports & re-attaching the entire new assembly.

At $2300 USD however, the convenience and reliability of a zero-timed gills assembly comes at a price that may not be for everyone.
 
For the more cost-constrained, hands-on CJ or Yak 50 owner looking for a robust improvement to one or two sloppy louvers, here is a solution
​
This repair kit includes a plastic template that will align a larger hole precisely over the original center of any damaged axle holes in the inner ring. 

​Use the plastic template to scribe a 5/16” circle that encloses most or all of the damage. 

Carefully enlarge the damaged hole with a round file to just inside your scribed line....

...then complete the hole by passing a 5/16” diameter hand reamer through the template.  
Remove the template & insert the steel bushing provided with its 7/16” hex facing out. 

Tighten the MS21042-4 locknut provided onto threads protruding on the inner side. (Use a 5/16” wrench for this locknut).

​This new bushing for the lower end of the gill louver has an internal M4x.7 thread to receive the axle.​

​The last step is to use a piloted 100 degree countersink on the slotted fishplate riveted to the outer ring. 

Don’t forget to reinstall the original washers you found on both ends of the louver, then use a flat blade screwdriver to adjust roundness of the assembly.  

Ensure all gill louvers have axial free play and remember to reinstall all tension springs.

The critical bearing surfaces between steel and aluminum has been been increased and now the steel axle bears on a steel bushing.

Individual Yak 52 style axles and additional repair bushings/ extra locknuts are available individually on the website.

*(The Yak 50 does not have cut louvers for blast tubes to the compressor, generator etc, so the Yak 52 Factory Replacement Gills set will need these louvers replaced with the solid style when used for a Yak 50)