The International Cessna^® 170 Association

My O-300 has an oil leak on the cylinder through stud. My A&P though it was coming from the o'ring on the base of the cylinder, so pulled out the cylinder replaced the o'ring but the leak still there on the same stud. The overhaul manual does not mention an o'ring on this stud. How could the oil be leaking there? Just splashing action wouldn't case such a leak. The engine is loosing a quart every 5 hours. Not bad just messy. Is there a way to replace the stud without splitting the case?

It may not really be a "through-stud". If it is, there is a small O-ring installed on that stud prior to case assembly. You cannot replace it without splitting the case.
But it may be only a regular stud. (You don't say which cyl, or which bolt.)
If it's a regular stud, it can be removed with a collet and epoxied back into the case.
Or, you can simply remove the nut, clean the stud with electronics cleaner, put a tight-fitting O-ring on the stud, apply a bead of Permatex gasket sealer (Non Hardening is best) and torque the nut back down. I've seen several successfully sealed with this technique.

How were you able to determine the leak was through the stud? I've been chasing an elusive leak for a long time. Maybe this is what I should be looking for!

One of the most difficult to find leaks I ever dealt with was the left, forward engine mount studs. I simply had a troublesome time discovering that (and was afraid it resembled a cracked case), but by cleaning the engine, dusting it with flour and running it around the patch once it was revealed to be one of those studs. The stud goes through the sidewall and into the sump, and oil can follow the threads to the outside.
On C-145 engines this can also happen at cylinder studs. O-300 cases were blind drilled, and the majority of cyl studs are in a dead-end threaded hole. (One of the differences between C-145's and the later O-300's.)

Okay, now this sounds even more interesting. When it was leaking for the left Fwd engine mount, where was the oil presenting itself? Simply dripping, or was it somehow finding itself leaking down the mid-left cowling of the engine? We have looked and looked at pushrod tubes, o-rings, all to no avail. This sounds like a great lead for me.

It was all over the lower left side of the engine, streaking from the front of the engine (as if a prop seal) and all over the inside of the cowling and firewall. That's why it took cleaning the engine, powdering it with flour, and making a very short flight once around the patch to find it. Oil leaks get blown around in funny ways and can be very elusive.

Thanks for that explanation. I think we're going to redo all the pushrod tubes with REALGASKET and then see how many leaks we've killed. I'm certain the one leak from the front left will still be there though. I'm hoping you've given me the direction I needon this one.