MT Prop Rebuild
Here's Craig Woolston's tips on getting an MT prop rebuilt without the hassle of sending it to Germany.
Word Document Version
I bought a pair of very long (11 inch) needle nose pliers from Harbor Freight. I filed the points down to make them into large circlip pliers. So rather than tapered tips I made them into round tips which would fit the blade circlips. I also removed the serrated edges to reduce the scratching on the prop and hub while using them.
I was very careful during disassembly to make sure that I marked every thing and kept all blade one items together, blade two items together, etc.
You might not need to remove the spinner backing plate but I did to reduce chances of scratching it. After removing all of the safety wire the bolts are easier to remove.
The double jam nut is removed with a simple box wrench. It was not very tight at all. Remember to take a picture just like this or count the number of threads showing so you can put it back in the same place.
You’ll notice that there is actually a hole in the hub cover plate. It turns out this is aligned with blade number one as shown in the next picture.
After removing the double jam nut I slowly worked a putty knife around the perimeter of the cover and the hub assembly. It’s just sealed with RTV. I tapped it gently with a hammer to drive it between the hub and the cover mostly at the blade marking areas since the hub was thickest there. You can see in the drawing and in the picture below the cover sits on a raised surface so don’t try to wedge the knife in too far, you’ll just scar the inner surface.
The giant return springs are trying to pop the cover off anyway so once the glue starts to let loose it will pop right off. Once you get the cover off the next thing you will see is in the picture below.
Here’s where you need to really pay attention and start putting blade specific parts into marked bags. You’ll notice in the next photo the white and black plastic blocks. They are marked with serial numbers and should be returned back into their original spots on re-assembly.
After all the blocks are removed, the next photo shows how the assembly looks after you rotate all the blades approximately 180 degrees. That way the pin sticking out of each blade lines up with the slot in the triangle shaped piston so that whole assembly can be lifted out of the hub.
Now you can use the circlip pliers to expand the circlips and move them out of their retaining ring spot and outboard on the blade. Look in the last picture you will see the circlip and the gasket out on to the blade root. The next photo shows the only unique thing about the ball bearing assembly. You’ll notice two of the red spacers are back to back. The rest are spacer, ball bearing, spacer, etc. Therefore there are 25 ball bearings and 27 spacers per blade. I put them all in a blade marked specific bag. Once the circlip is on the blade root, the blade easily slides into the hub where split race and ball bearings will fall into the hub. I was told not to use anything magnetic to retrieve them out since the motion of the blades is so little that if the ball bearings became magnetic it would not wear/rotate correctly. I also did one blade at a time, so that there was no confusion. So I didn’t release any of the other blade circlips until I had that blade completely out of the hub and all of it’s ball bearings collect into the right bag.
I cleaned everything up with mineral spirits and green scotch-brite pads just before reassembly. I used Aeroshell #6 grease when re-assembling everything.
The re-assembly was basically the reverse of this disassembly. I drill holes in a piece of ¾” plywood so the hub would mount in it. I drilled enough holes so that you can rotate the hub to the three blade positions. I made the pies of wood approximately a foot long so that I could clamp it into a bench vice. I clamped into the vice so that it came off horizontal such that the hub was now approximately 8” from the vice handle. This allowed the hub to be mounted to the board so that one blade would stick down and the other two off a their respective angles. See the sketch below.
First you place the correct circlip on the correct numbered blade for the position facing down. Then put on the circlip and plastic shim. I then lubed up the split race and the blade itself and stuck the race on the blade. The grease was enough for it to “stick” to the blade and not fall off during the next step. Insert the blade into the hub. I positioned the blade in the hub so that that o-ring was inside the hub. Then I placed a small stool under the blade tip so it would not fall out. It will basically hang there by itself from the o-ring but you don’t want it to fall out and damage the tip. This position should be just right for the next step.
Basically you want it at a level you can insert the ball bearings and spacers and they will fall down into the groove and not roll back out. I had a partner for the next step, I would get my hands lubed up with grease and then he would hand be a ball bearing and a spacer one at a time. I would stick the spacer to the ball bearing and then insert the two of them together into the hub. Remember that two different sets of spacers go back to back to each other as shown in the picture above. They go in the assembly 180 degrees from each other. I would put them in just after the first ball bearing, then keep inserting ball bearings (12 of them) on till it was pushed around the back side, then insert the next set of back to back spacers.
Once all the ball bearings and spacers are in you slowly tug on the blade until it drops back into the correct position. Then you can put the circlip back into it’s groove. At this point the blade can’t come out of the hub or fall back in, so you can un-mount it from the board, rotate to the next position and repeat.
Now, with all the blades in you rotate them so they are shown in the picture above. Once rotated into the correct position the triangle piece should slip down into place. Here’s where I had a little bit of trouble that can save you later. Once the triangle piece is in, take the cap screws and make sure that you can get them started in all three holes simultaneously. Apparently, the piston that these screw into can rotate a little bit if you happen to push it in and out like I did when I was cleaning it. If they don’t line up, push the piston back down, take the triangle spacer back out and then screw the cap scres back in, then while you tap the piston back in use a small board to apply pressure to the shaft of the cap screw so that it rotates the direction you need it to so that it lines up with the spacer holes. Once you are assured you can get the screws started with the spacer in the correct position your ready from the next step.
Put the triangle spacer in position and then rotate the blades 90 degrees. Place the correct black spacer on the pin sticking out of the blade and then rotate the last 90 degrees to put it into place. Insert the correct white spacer into it’s position. The writing goes in facing the center. Now, you can bolt the spring shaft back into place using the cap screws so that it looks like the next picture.
Insert the spring assembly onto the shaft and then prepare the hub cover with either RTV or Form-A-Gasket. Remember the small hole in the cover, that goes on pointed at the number 1 blade. Place the first large nut back onto the spring shaft and tighten it until you get the correct number of threads showing from when you took it apart. Don’t go any farther as the nut will not bottom out on the cover Next, run the second nut down on the first and “jam nut” them together.. Now you are ready to put the spinner bulkhead back on. Here’s one last place that got me. There is a number 1 stamped in the cover, place it toward blade one but make sure that the screw holes for the spinner are centered on each blade. I was off one hole, had safety wired everything and when I went to put the spinner back on the screws wouldn’t line up, so don’t let that get you. After all the screws are on you can safety wire them as shown in picture one.
After that, I put the prop on the plane, cowled everything up. I then ran the prop up to 1700 rpm, cycled it three times like the manual said. Next I went out on the runway, ran it up to 2200 and then move the throttle in and out a little to make sure it would track 2200. That all checked out fine, so I pour it in and took off, while monitoring RPM. Mine showed 30 RPM under the 2700 redline. Probably because I tightened the jam nut combination a little more than it was before. So after flying for 20 minutes making sure I could get various prop settings I came back in and shutdown. Adjusted the governor a tad and she’s flying great.