Careful there, Brad! The NAPA seal might be good for "ground test" purposes...
I truly don't mean to seem so disagreeable... I just usually need to keep my airplane operational until I have the part "in hand", then I can remove the leaking seal and install the correct one. I don't usually have the luxury of time, to down the airplane or bother with removal, take it to NAPA, let the auto parts guy "guess" which autopart might be comparable, ...then go back home and hope the auto part will perform better than a genuine aircraft part that was failed and leaking. (An unlikely scenario at best.)
Usually the reason most folks are endeared to auto parts that can mimic aircraft parts is their pleasure at a $aving$ that is meaningful. I don't agree with that when there is a legality/traceability problem AND there's no real savings!
There are certainly comparable materials in automotive/marine/aviation common useage. Some examples might be bulk gasket materials made of cork, or fibre... Or gasket sealing compound such as Permatex (which although the packaging states "Aviation Permatex"...actually carries no other indication that it meets any aviation grading system whatsoever.)
These are the sort of things that involve a smidgeon of good ol' common sense. I'd never worry about using an automotive Shrader valve in an inner tube on my Cessna 170 .... but I'd certainly never do it on a high performance aircraft, or on a high-pressure application such as an oleo nitrogen-fitting.
But the
standard I try to make and support is the one that, even tho' we might not like the price.... aviation parts are better manufacturing quality, better identification, and better storage (which keeps them in better condition for installation and recordkeeping) than the local autoparts store ever thought of.
I can be easiliy criticized for violating my own public high-standards in my private world of ownership of N146YS. Anyone who knows me and my airplane knows it has an automotive back-up lightbulb in it's aft nav light fixture. (It's a GE-5606.) Not because it's cheaper, because it's not. It costs about the same as the Cessna-specified GE-93 (which also happens to be automotive.) The reason I violate my own rule about this matter is because the 5606 bulb puts out more light due to it's higher candlepower. (But it is well below the amperage rating of the wiring and CB in that circuit.)
I also use automotive (well, marine actually) grease in my wheel bearings....rather than aviation Mil-spec Aeroshell 7, etc. ... because the boat-trailer wheel bearing grease is designed for a more challenging purpose, and the Cessna Service manual allows "any high-quality, brand-name" grease for the purpose. I'm absolutely positive that the green, waterproof marine wheelbearing grease that has kept my boat trailer wheels smoothly running since 1986 without a re-packing or cleaning, despite those wheels/bearings being subjected regularly to being plunged below the water and then run at highway speeds for hundreds of miles without a single failure,... makes that grease quite serviceable for Cessna 170 aviation purposes.
I'm at risk for being intransigent on this subject of recommending aircraft parts for aircraft, while publicly admitting that I violate that rule myself from time to time in certain situations. I like to believe those particular situations are especially non-threatening to safety.
However, running the risk of publicly endorsing the use of automotive parts to another person, who might subsequently run the risk of dumping all his/her engine oil out the generator during flight (at night? ifr? over the lake?) in unknown conditions.... makes me pause and think about whether saving only a buck or two merits the risk of offering what is technically illegal and unsound advice to a fellow member who comes here seeking the
best advice he can find...not necessarily the most expedient or that which puts a devious smile on his lips merely because it saves him two or three dollars by purchasing it from an unconventional source.
Philosophically...I endorse the use of only aviation products on airplanes.
After dragging this thing out for 6 weeks or so I put the dye in the oil and used a black light and guess what-----the leak isn't at the alternator flange after all 
Thought I would pull the tach drive housing off without draining the oil (again!), but NO, alternator wouldn't let the tach drive housing go back far enough to come off. (OK George, you can rag on me for having an alternator--I probably deserve it.) Drained the oil one more time and loosened the 3 bolts on the alternator enough to let the tach housing come off. Checked it close (this time) and, you guessed it------the housing by the bolt hole was cracked. I should have seen it sooner, but didn't. So now I am awaiting a loner tach housing from member Steve Grimsley, who is overnighting me one. With a bit of good luck (I think maybe I'm due a little 
) I hope to get Ole Pokey buttoned up Tuesday evening so Paul Wood and I can leave early Wed for Kelowna. Thanks to Steve I didn't have to order the needed part from a vendor. Yes the 170 assn has come to my aid multiple times. Those who don't feel they get anything from being a member better not try to convince me of that 
Steve is sending me a new garlock seal to put in the tach drive housing. Anyone know the 1-2-3 steps to remove the old garlock seal and install the new one 
I'll probably just fly another 15 hours to oil change and then do the work. I don't think the gen oil seal is known for catastrophic failure. I guess I'll call Niagra Air Parts...or the place down in KY that did the generator overhaul for the seals. Will be interesting to see the condition and orientation of the seal when I get it out. When I put lipped seals in, I always check three times before taking the block of wood and mallet in hand as it is easy indeed to get them in backwards of you rush things too much 
