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View Full Version : Crankshaft idler gear backlash



Russell Peitz
01-11-2014, 09:48 PM
Hey Everyone,

Hoping someone can shed some light or provide insight to a problem I think I may have.

I have a One Design DR-107 with a Lycoming O-360. Recently I noticed a "slop" in the rotation of the propeller when turning by hand, wasn't much but definitely more than other aircraft. This airplane is fairly new to me so I can't tell if it's been this way all along or not.

Further investigation shows that the slop I was feeling is backlash in what appears to be the crankshaft idler gear to camshaft gear. I'm seeing about 2 degrees of rotation on the crank before the camshaft starts to move, once under load and rotating there is no slop or backlash. Stop the rotation and rotate in the opposite direction again the crank will rotate about 2 degrees then the camshaft starts turning.

I checked the mag timing and the slop/backlash doesn't appear to be in those gears, I get the points to open and a very slight rotation (less than 1/2 degree) and the points close, go back the other way (1/2 degree) and the points open again, so I don't see an issue there. Can't pull a mag and look at those gears without removing the engine, they sit right up against the firewall.

I don't have a vacuum pump so I opened that access and can see the crankshaft gear. I don't see any major issues, some slight discoloration is some gears over others, but for a 16 year old 300 hour engine that had some sit time, it looks ok, I think.

I have only done two oil analysis since I have owned it. This first one came back with high iron 141ppm, however that was on oil that had been there for quite some time, and through a period of inactivity and some starts with months in-between flying, so I expected somewhat high levels of something. The second sample came back still high but much lower than the first at 68ppm. Although that is the first sample since installing an oil filter. I'll be cutting that open tomorrow and checking the media.

My question,

Does the amount of backlash in those gears seem excessive? I'll also pour through the Lycoming manual tomorrow to see if it gives any guidance.

My current line of thinking is to change the oil/filter, fly 10 to 15 hours and look at the backlash, oil analysis and filter again. However that will put me in the middle of aerobatic season, so if I have a problem I might like to tackle it now instead of during great flying months.

Any thoughts, tips, suggestions are appreciated.

Thanks,

Russell

Matt Gonitzke
01-12-2014, 08:47 AM
Does the O-360 have the idler gears mounted on stub shafts? I cannot comment on whether or not that is too much backlash, but I'll share a story that may be relevant. When I was getting my Private I had to fly this clapped-out 152 while the plane I normally flew was getting its annual done. This one started hard and always seemed down on power. We'd complain about it every day when we brought it back, and then they'd clean the spark plugs or retime the mags or something every night. When I came back a year later to rent it again, I found out that the mechanic had finally discovered the problem when doing some other maintenance when the prop could be turned a fairly significant amount without the cam moving. They took it apart and found out the studs holding one idler gear shaft onto the accessory case had broken off, so the gear, and consequently the cam and mag timing were floating around all over the place. Not sure if that is what is going on with yours, but after that experience I now know what is possible.:eek: