Diff DIY: https://www.s2ki.com/forums/index.php?showtopic=99995
Tranny DIY: https://www.s2ki.com/forums/index.php?showtopic=83577

i did the diff. fluid change recently using mobil1 i bought from autozone.
very easy, didnt even have to jack the car up. havent done the tranny fluid yet.

 

Hitting a wrench with a hammer while it is on a bolt head is a sure fire way to round it off. You are NOT supposed to use this method with any bolt/nut. Trust me, I learned from experience on a stubbborn axle nut. NEVER hit a wrench with a hammer while it is on the bolt/nut. That is why they invented breaker bars, to give you the leverage you need to break it free. If it is so tight that you need to hit it with a hammer, then it is over torqued. At least on a tranny or diff bolt.

Mileage... I think.... Tranny is like 30K mile change and diff is like 60K.
This may not be exact, but I also don't have my Helms manual in front of me. I know it's not very frequent, but again, depending on your driving habits/style, you may want to consider doing it sooner.

 

The DIY on this site talks about metal to metal adhesion and the need to snap the wrench rather than applying force slowly. The DIY does not suggest the use of a hammer or striking the wrench. I prefer a sharp tap to trying a "snap" by hand.

The reason the striking method is suggested isn't due to the bolt being overtorqued. There are instances of bi-metal corrosion that cause some bolts to "stick". Using a breaker bar to exert more torque will simply round off the bolt in these situations. Feel free to disagree but this method used with a high quality tool that has appropriate surface contact will safely break the fastener without rounding it off. An impact tool might be more appropriate but cannot be used in the confines of the diff. You will likely encounter this situation with any fastner on the underbody of the car that is not frequently serviced.
I must agree that this is NOT a good way to loosten a fastener that is in fact overtorqued.
Whatever you decide is at your own risk.

 

Do they reccomend not using a hammer on a wrench because it's metal on metal contact or because they don't want a sudden jolt of force to it? I often use a rubber mallet to bang on a wrench/ratchet to get bolts off. Sometimes it takes sudden impact to break rusty nuts/bolts off, I think that's how an impact gun works.

 

Your tools will probably last longer or at least look better if you refrain from smacking them with another metal object. Whether you strike a metal tool with a soft mallet, block of wood or another metal tool there is the possibility that something may break, especially if you strike it very hard. If you have to give it more than a tap the fastener is likely to be overtorqued. If a bolt has been tightened too much there is a good chance damage has already been done to the threads. The relatively soft metal that surrounds the fill and drain plugs is an especially bad place to overtorque a bolt.
I've seen stubborn engine bolts that could not be turned with a large breaker bar turn out with fingertips after the application of a light tap. No brute force involved. Same principle as an impact wrench. I've also seen exhaust bolts on several S2000s that could not be removed without shearing despite the application of any method. Fortunately the studs are easilly replaced.
You just have to make the best of the tools at your disposal, apply caution and common sense.
FWIW, we'll be using an electric impact wrench to assist in the replacement of a friend's blown S2000 motor next weekend.