The International Cessna^® 170 Association

I found ferrous metal filings in my oil screen this week, which appear to be coming from the starter drive gear (the little one that engages the crankshaft gear). The ends of the gear teeth are worn down, which seems to be normal, but also the tops of the teeth feel rough and jagged to the touch along their entire length. I caught the problem quite early, and the crankshaft gear appears to be undamaged.

The engine is an O-300-A with the stock starter motor and pull-cable. I've not noticed any unusual noises from it, and I haven't engaged it while the engine was running.

Any ideas what would cause that?

I have a spare starter clutch assembly, which I'll install, and watch the screen closely for a few hours before I breathe easier.

As a side note, I had a very early warning that this was going on, from my Oberg oil screen. It has a light on the panel that comes on when the oil is flowing too slowly through the screen. When the engine is cold, this light will normally come on at a certain RPM depending on the cold oil temperature, and extinguish when the oil temperature reaches a certain level. If the screen gets too much gunk in it, it will take a higher temperature to extinguish the light. I watch my oil temperature on a digital gauge during warmup, so I could note the increase in temperature when the light went out.

I sent an oil sample and the contents of the Oberg screen off for analysis, and that may provide some confirmation that the the starter gear is the only source of the metal. I had some cylinders that need valve grinding, so I removed those and inspected the cam and followers. They appear to be fine. I think the only possible suspect I haven't looked at is the oil pump.

John, the TCM Overhaul Manual for this engine gives details of how to properly adjust the starter clutch and it's operating lever adjustments.
If the clutch should not fully dis-engage... sufficient to completely remove itself from the crank-gear, then the ends of the starter clutch teeth may chatter against the crank gear.

The original clutch does not ride upon the crank gear as many think it does. (It does not engage by enmeshing, but by sliding fore/aft.) When the engine starts, neither do the meshed gears collide in any way.... the clutch merely overspeeds and rollers within the clutch prevent the engine from driving the starter.

(For clarity, the "ends" of the gear do not equal the "tips" of the gear teeth.... but refer to the forward face (end) of the small gear, which engages the crank gear. The "ends" of those teeth are chamfered or bevelled to facilitate engagement. It is common to find small scratches and marks in the chamfer. Any damage there would be very difficult to detect in the corresponding area of the crank gear.)

Thanks, George. I'll consult the overhaul manual about starter adjustment. (It's about time I bought one!)

John

Installation instructions and starter engagement instructions can be dnloaded free (pdf file) from:
http://www.niagaraairparts.com/pull_ica ... 0_2003.pdf

Strong and IMPORTANT warnings to prevent the sort of damage discussed in this thread are found in in the pdf file.

"It is very important that the pivot lever moves the starter clutch 7/16” BEFORE the adjusting bolt contacts the starter switch. The remaining 1/8” to 3/16” of travel will be used up in making the electrical contact of the starter switch.

It is important that the clutch actuating lever is correctly adjusted after the clutch is installed (as per
original instructions and service bulletins). Failure to do so may result in the starter motor turning the
clutch before it is engaged with the crankshaft gear. This would result in failure to start and gear damage inside the engine.
VERY IMPORTANT – Be absolutely certain that the clutch is set up to become fully engaged into
the crankshaft gear BEFORE the electrical connection is made to the starter motor."