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I have a new to me 2015 Softtail Slim 103 with 4700 miles.
When I purchased the bike and rode it home and parked it in the garage. I looked it over and was making a list of service items I wanted to take care of.
On the second night after being parked I found drops of oil under the bike. After further inspection I discovered the oil leak was coming from the transmission drain bolt and had run down the side of the left shock. I was already planning on the trans fluid change which I did a few days later. The old O-ring was replaced with a new one. The drain plug was tightened to proper torque spec.
The oil leak stopped but then started again about a week later after a few casual rides.
Any ides on what else to look for or other ways to fix the leak??
Thanks a lot for any help you can suggest.
I have a new to me 2015 Softtail Slim 103 with 4700 miles.
When I purchased the bike and rode it home and parked it in the garage. I looked it over and was making a list of service items I wanted to take care of.
On the second night after being parked I found drops of oil under the bike. After further inspection I discovered the oil leak was coming from the transmission drain bolt and had run down the side of the left shock. I was already planning on the trans fluid change which I did a few days later. The old O-ring was replaced with a new one. The drain plug was tightened to proper torque spec.
The oil leak stopped but then started again about a week later after a few casual rides.
Any ides on what else to look for or other ways to fix the leak??
Thanks a lot for any help you can suggest.
Assuming you are using the correct size o-ring, put a little oil on the o-ring before install, and properly tighten the the plug, I have a couple ideas you could check;
Can you use a light and a mirror and get a good look at the perimeter of the trans plug? You want to make sure the o-ring didn't get pinched when the plug was tightened. You'd see it popping out from under the bolt head.
Next time you remove the plug, again use a light and mirror and look at the face of the shoulder in the trans case that the o-ring seats in when the plug is tightened. On my then new 2003 Heritage, I had a couple leaky trans plugs. When I removed it to inspect the plug, I found a burr on that shoulder that was cutting and partially shredding the o-ring during trans plug install. Proabably a rare occurence but something to look for.
Are you positive the trans drain plug is the leak..?? I ask because, I believe it was earlier Twin Cams in the 88" days, but the bottom two bolts in the transmission door, or the transmission side cover could be loose and cause a small leak. If you had a similar leak, it would run down the side, then across the bottom of the transmission case when put on the jiffy stand. The leak would eventually drip down onto the shocks.
Anyways, these possibilities just popped into my head when I read your post. Worth checking them out....
Good luck and let us know what you eventually find....
Last edited by hattitude; Sep 19, 2025 at 09:39 PM.
hattitude,
Thanks for the recommendations. I will certainly check the bolt head and seat area to make sure they are smooth and free from any burrs. I used new O-ring on the drain plug and checked with a mirror to make sure it wasn't pinched when installed. I will re-torque the bolt and if needed, drain and recheck everything as you suggested.
Thanks for the insight!!
I've removed the plug and drained the transmission. Took a scotchbrite pad and cleaned the case surface around the drain and the entire drain bolt head surface. Tried using oil and then grease to lube the o-ring and installed the drain plug and each time the o-ring deformed and bulged out of place in the same exact location. I then removed the o-ring and installed the drain plug. Using a dental pick checked the clearance between the bolt head and the trans case surface. I think I discovered the problem as one half of the bolt was making full contact with the case surface and the other half had a very noticeable gap. So at this point I assume someone before me cross-threaded the bolt so it no longer sets flush against the entire case surface. Certainly NOT what I wanted to find.
My question is it possible to drill and re-tap the trans case and/or install a heli-coil to correct the drain plug alignment on this Softtail transmission?
I've removed the plug and drained the transmission. Took a scotchbrite pad and cleaned the case surface around the drain and the entire drain bolt head surface. Tried using oil and then grease to lube the o-ring and installed the drain plug and each time the o-ring deformed and bulged out of place in the same exact location. I then removed the o-ring and installed the drain plug. Using a dental pick checked the clearance between the bolt head and the trans case surface. I think I discovered the problem as one half of the bolt was making full contact with the case surface and the other half had a very noticeable gap. So at this point I assume someone before me cross-threaded the bolt so it no longer sets flush against the entire case surface. Certainly NOT what I wanted to find. My question is it possible to drill and re-tap the trans case and/or install a heli-coil to correct the drain plug alignment on this Softtail transmission?
Bummed out right now!
If I understand your explanation, it sounds like that is a possibility...
However, it will be hard to do, upside down, with limited room, and between the shocks.
I would drop the shocks and use a scissor jack to make more space and gain better access. Then.... take your time.
It sounds like the threads are too far gone for a thread chaser or tap... probably will just pull the threads out trying to clean them up.. but doesn't hurt to check and maybe give it a try. I would use a Timesert instead of a Helicoil, or dill/tap for an oversized plug.
Good luck, and please let us know how it all works out...
Last edited by hattitude; Sep 27, 2025 at 05:50 PM.
Before getting to crazy on this - While doable, maybe other options far easier could fix the issue.
Try what is called a Stato-seal. It is a seal that is a metal outer ring [ washer ] with rubber as the center for the seal. In my Aeorspace work, that is what they were called. I have others use different names for the same seal design.
I can post a picture of a Stato-seal if you would like.
It is highly unlikely that the drain bolt is the issue, but have you threaded another normal bolt into the drain hole and checked for a gap? Maybe it's a defective drain bolt??
You say that you can properly torque the bolt. I would think that the threads are not damaged if you can torque the bolt properly.
I would try the sealing washer as mentioned if there is still a gap with the test bolt. Maybe even a silicone gasket maker or the like to help seal the head of the bolt?
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oil leak problem
