RC Giants
How to field-fix a bent crankshaft on a Zenoah G-38
Sal Calvagna [[email protected]]
A very happy, healthy, joyous, and peaceful holiday season to all. As 2008 comes to an end, so does the flying season for many. As fall turns into winter, a day at the field becomes a day in the shop. It's time to start working on the next kit or to make all those repairs you have been putting off.
Last month's topic was how to recognize and repair a sheared Woodruff key on a Zenoah G-38 gas engine. Using the same power plant, I'll explain how to field-repair a bent crankshaft.
A nose-over on takeoff or landing usually results in a broken propeller, especially if you are flying off of a hard surface such as macadam (asphalt) or concrete. A grass field is certainly more forgiving, but if your rpm is up, even there you can easily break a propeller or shear the Woodruff key. Ratchet this up a notch with a gruesome crash and you could even bend the crankshaft. I know I have.
In an accident where the ground comes up to smite you, it's the engine that is usually impacted first, taking the brunt of the force. Don't despair. Before you decide to throw it out, order new parts, or send it away for repair, try the following idea.
Please keep in mind that this is a simplified field fix. As such, I have not included the use of a dial indicator or similar tools.
Remove the spark plug. Turn the propeller by hand and watch the flywheel. If the flywheel wobbles slightly up and down when you rotate the propeller, there's a good chance that the crankshaft is bent. Another indicator is the flywheel scraping the magneto as it rotates 360°.
Still another way to identify a bent crankshaft is to quickly rotate the propeller by hand while you track the tip of the propeller from the side view. If the tips do not track evenly—they wobble—you most likely have a damaged crankshaft.
Once you have made the determination, remove the propeller adapter and flywheel. Don't lose the small Woodruff key as you remove the flywheel. It should be seated in the keyway located on the taper of the crankshaft.
Install the propeller adapter back on the flywheel and a propeller to the adapter. Push this whole assembly back onto the crankshaft without the Woodruff key. You are going to use this assembly to rotate the crank to locate the high point of the bent crankshaft.
As you rotate the propeller, watch the flywheel. You are looking for the high spot of the crank. When you find it, stop rotating and observe the location of the high spot. Using a suitable hammer, give the crank a good rap on the high spot. Do not hit the threads or you will damage them.
Rotate the crankshaft again to locate the new high spot. Repeat this tapping procedure as necessary, giving progressively lighter raps each time until the crankshaft runs true. You may have to loosen the propeller nut slightly to allow the crank to move.
After you have finished, reinstall the Woodruff key, the flywheel, and the propeller. Check the run-out by turning the propeller by hand. If you have access to a vise and soft jaws, you can improve the result by supporting the engine with the crankcase in the vise and applying downward pressure on the propeller while tapping the crank. Be careful not to damage the crankcase.
If the crankshaft is bent severely or if it breaks, replacement is necessary. Also, internal bearing damage or crankcase distortion may require professional repair or replacement. Good luck and safe flying.
I take this second opportunity to wish all our readers and their families a happy and healthy holiday season. Peace on Earth. See you in 2009. Now go build something.
Sources:
- Zenoah
(800) 338-4639 www.zenoah.com
- Nick Ziroli Plans
(631) 467-4765 www.ziroliplans.com
- Hammer for crankshaft fix: The Hammer Source
(877) 496-2537 http://hammersource.com
Transcribed from original scans by AI. Minor OCR errors may remain.
