The International Cessna^® 170 Association

Just went through the expensive experience of having my Bendix mags overhauled after 500 hrs. New coils, condensers, points, one internal gear, one new impulse coupling (other one good for another 250 hrs), seals, bearings, one 1/2 of the mag case, & one used mag drive gear. Teeth were damaged (from use) to a point they needed to be replaced. Got a used one that is OK @ 30% of a new one. New one is supposedly $1100 (OUCH!). Had lots of fun timing them, but finally succeeded in doing so with ample help of my hangar neighbor & fellow 170 member Now a question to all you mechanically inclined types----At 1600/1700 rpms one mag drops 100 rpm or maybe a bit more, then comes back up by 25 rpm. Is this normal? Or do I have a problem? Other mag drops about 75 rpm. About 75 rpm difference in the mag drop between the two mags.
On a related subject, what are you using as a guide regarding spark plug replacement? I have 500 hours on my plugs (replaced at overhaul). At December '05 annual my AI said they looked OK & pressure tested OK. Just wonder when I need to plan replacement. What is the current recommended plug for the C-O300B? Best source?

Thanks in advance. I always appreciate the data this forum offers on vrs subjects.

500 hoursis the replacement time suggested by Champion.

Hi,

Champion offers a spark plug wear gauge for sale. When the plug is properly gapped, if the wear gauge fits over the electrodes, the plug is considered worn out. This has always occured around the 500 hour mark for us.

The mag drop you have is exactly the same as my O-300A powered 172. It is within all of the established limits so I would say it's good.

Hope this helps,

Mark

Frank, you just missed a great sale on spark plugs over at Arlington (Van Bortel). Unison's UREM40E's sold for halfprice. Call 'em and tell 'em you're tired of waiting for their notification that they're finally in stock and was just checking on the ones you orderd last October. Maybe they'll be nice to you.

Thanks to all who responded to my inquiry. George, I'll give your suggestion a try and see what happens.

Pokey, if your plugs are not too eroded (football-shaped center electrode) and the plane starts & runs OK, I wouldn't sweat replacing them. That 500 hour figure from Champion is a suggestion, just like engine TBO, it's not written in stone.
Even if the electrodes look good, if you develop some starting / running problems, the resistors inside the plugs ( that's what the "R" in the sparkplug number stands for) may be failing- even if the plugs check out OK in a pressurized tester.
At over $200 for a set, I wouldn't be too eager to replace the plugs until they prove troublesome.

Eric