Lower Unit Drain Plugs

colbysmith

colbysmith

Well-known member
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C Dory Year
2009
C Dory Model
25 Cruiser
Hull Identification Number
UWH25301J809
Vessel Name
C-Traveler
Sick and tired of lower unit drain plugs that strip out on the screw head. All are common screwdriver driven style. I'd like to find some that are allen wrench driven. Anyone have a lead on who makes such? I'm looking for three that fit in the Mercury 4 stroke 115hp (3/8 x 16 I believe), and two for the Tohatsu/Nissan 6hp Kicker (M8 x 1.15 or 1.25, can't remember which it is). Thanks in advance. Colby
 
This may not help you: but be sure that the screwdriver fits the drain plug slot. I have never had one strip out. I use bits in a socket. As I recollect mine are 3/8" drive. You can get bits in either slot, or phillips head which fit properly. With the socket/driver, you can keep good pressure on the plug head/driver bit, as you back it out.
 
I was just thinking about the same thing. I saw that my lower unit had a couple drops come out while parked for a week in hot weather (hadn't lost a drop all winter in the garage). I looked at the lower screw and it was mangled, so I thought that I would get a new one, and a new gasket, when replacing the oil. My "go to" place is usually Tacoma Screw Products because they are close, but Fastenal generally has stuff like this. I could pull the screw, measure it and get the thread pitch, but I usually just walk in with the old part. If you don't want to pull the screw first, and you have the fill attachment for the engine, you could just measure that and go shopping.

Yamaha actually lists the screw (in a flat head) for $5 and the gasket for $4. I'm guessing I can get both at Tacoma Screw (in a stainless Allen head) for $3.

Mark
 
ahhh, the misery of drain screws!
I would second thataway's principle that pressure is key. My tool choice is a little different, but accomplishes the same goals of pressure applied in a straight line to the slot.
I use a large [that is, LARGE] screwdriver. One which fills the slot. The screwdrivers I use have a hex bolster/fitting on the shank. A square shank serves the same purpose. On my hex bolster I apply a ratcheting box wrench, [a fixed box or open end wrench can serve] to the hex bolster. Then, carefully applying pressure in a straight line with the longitudinal axis of the screw, I can apply torque through the end wrench while maintaining alignment with the screw.
For me, this is a workaround for becoming older and arthritic, but I have found that the technique works well for most any screw, both slotted and Phillips, and I rather wish I had adopted this method years ago. My latest old/arthritis workaround is using a repurposed patient lift to get outboards on and off transoms.........
As a more direct answer to your question, I know that BRP/OMC offers hex recess drain screws, and I love the Fastenal approach.
 
Perhaps McMaster Carr?
 
Any chance that you are over tightening? Isn't the
Torque spec 35 ft lbs
 
I use a large screwdriver (don't know number size) where the width of the blade is almost as much as the screw head slot, and the thickness of the blade has very little play when in the screw head slot. Keep the screwdriver perpendicular when tightening. Use a new O-ring every time.

If you want to use a ratchet, get a bit that fits the screw head as above but beware of over tightening. My Honda 90 service manual says only 4.8 ft-lb torque, which is about what you get when tightening "snug" with the large screwdriver. If you are unsure, get a small torque wrench.

I have never heard of one of these screws coming loose. Anybody?
 
I had the same vexation. You can't get it tight enough not to leak without slipping out of the slotted head (why slotted? :amgry).

I stopped in the local Yamaha mechanic (where I buy parts sometimes) and asked them how the heck they do it? He advised what island andy describes above. I followed this procedure and, Voila! I was able to get the screw MUCH tighter (which I had to or it would tend to seep).

1) Get very large screwdriver with tip that fits the screw really well and has a square shank (I had to go to a large Ace Hardware).

2) Fit this into the slotted head, apply pressure to keep it there.

3) Grab the shank of the screwdriver with a spanner that fits the flat and only turn with that, while keeping up the pressure on the screwdriver head. You can get it a lot tighter. And no slipping.

A note is that I did have a fresh O-ring on the screw and the slot was fine, but even so, I could not get it properly tight without the recommended method. I also bought a new screw-with-o-ring at the Yamaha place just so I'd have it on hand (but with the recommended technique didn't need it -- just used the original screw with the new o-ring I was already using.

But Colby, I agree. Would be great if it had a Robertson drive or something else similar (maybe it can't for some reason?).
 
For everyone telling me not to strip it out. Thanks! :roll: But as long as I've owned boats with outboards, I've had problems with those slot heads!!! Over time they do strip out. Yeah, I may be overtightening the suckers. But since it's underwater and I don't want to chance having one come lose on a vibrating piece of machinery, I like to make sure they are tight. Then getting them out again is always an issue. Yeah, I have several screw drivers, all of various size. But seems none of them are a perfect fit. Plus, that slot in the head of the screw is fairly shallow. Without exactly the right angle, blade just pops off, causing more damage to the screw head. Sounds like some others know exactly what I'm talking about. So anyway back to my question. If anyone knows of a part number or brand that has the allenhead, I'd like to know. Also, are the plugs stainless or aluminum? Since the screws tend to strip out in that slot, I have a hard time believing they are Stainless...especially going into aluminum units. I see Amazon sells some advertised for Evinrude/Johnson, but the description doesn't say the size of screw.
 
For those who have aluminum in the lower end casing, just pointing out that aluminum and stainless don't mix. The aluminum will become corroded.

Damn near ruined a sailboat mast that way. Don't ruin your motors lower casing.

Boris
 
I just pulled the lower screw out to change my oil. Two reasons I'm going to keep the slot head screw. First, an Allen head screw would set proud of the casing. Not a big deal, but the thin head on the original is flush. The second reason is that the original has a magnet. There was just a trace of silver powder on it, so it's doing it's job.

The original Yamaha screw is stainless, as are probably all. I don't know if there would be a problem unless you don't have good anodes. If you think that a SS strips easily, try an aluminum screw. It goes in but you get to drill it out. Even aluminum screws in aluminum have this problem. I had to drill out several when I rebuilt my 1933 Evinrude.

There is a special product (I forget the name) that is used for screws going into aluminum. It's a non-conductive Teflon paste. I used it on my aluminum trailer wheels that have SS lug nuts. I suppose it would work for a SS screw into an aluminum lower unit, but I've never heard of that screw being a problem.

Mark
 
I like Klein tools and their screw drivers. The right sized screw driver depends on the blade thickness and width. Using something that sort of fits usually is the cause of the problem. I think a decent screw driver that will fit correctly and hold up for years you might not find the right one at Kmart, Wal-Mart, ACE hardware or True Value. Get some new screws and head to Grainger or a decent tool house and find a quality and correctly sized screw driver. I would bet they use that type of screw so you won't /can't mess up the lower unit casting by stripping out the screw threads.
D.D.
 
The beauty of using the socket screwdriver, is that I can set the torque properly--How do you do that with a screw driver and a set of vise grips?>???

Colby, I'll bet you are using undersized screwdriver bits! Not doing the correct torque? Maybe not changing the O ring or Crush Gasket each time.
 
I have taken a different approach to loosening and tightening the oil drain and vent plug. I went to outboard school way back in 1974 (actually I got to go to several time at the expense of the USCG).... I use a simple hammer driven impact wrench with a bit in it that fits the screws (for any brand of outboard) and have never stripped a screw nor have had a leak. The hammer driven impact wrench once was an expensive tool, but now can be purchased very cheaply from most tool suppliers (Harbor Freight has the tool with bits for less than $9, http://www.harborfreight.com/impact-scr ... 37530.html )
All of the C-Brats that get outboard maintenance training at my home have learned the value of this very simple tool.
 
I thought about using my impact wrench with a screwdriver bit, but first I just took a big honkin' screwdriver to it and it came out easy enough. Was it the right size screwdriver? The Yamaha screw is metric (8mm x 1.25mm thread pitch). It wouldn't surprise me that the slot is also metric. I'm not sure that I've ever seen a metric screwdriver. Yamaha probably sells one just for this purpo$e.

The oil change went without incident, so I'm okay for a few hundred hours.

Mark
 
In case anyone is interested, I did find a screwdriver that fit the Yamaha slot perfectly at a decent Ace Hardware. It was one of their "good" ones - the type that has a black section at the tip - and fit like a glove. Slot is still a pain though!

As for torquing, I typically do like to use a torque wrench, and had the figure to hand, but when the Yamaha tech told me how he does it, I decided to try it his way (he was one of the "been there awhile" fellows and has spent time in the past when I've had questions). As it worked out, any looser and it would not have kept from seeping (did use Yamaha screw and new O-ring); any tighter ..... I don't think it was going any tighter!

The guy did say that it's hard to get the O-rings to "clamp down" (or whatever the term is... spread or what have you). He said that when they have a parking lot of new motors (they sell new boats with Yamahas and Mercs on them), they typically have to go around and tighten them all with their method as many of them seep from the factory.

I like the idea of the impact screwdriver. Good tip.
 
I use a Impact screw driver to remove them. I don't hit it just the big handle gives enough leverage to get it loose. New crush washer and a torque wrench to the proper spec. Haven't had one leak or come loose yet. They don't get a whole lot of torque if I hand tighten them they would be to tight might be a lot harder to get them off again.
 
Going to ressurect this thread.... I finally found a screw driver that fits the heads of these outboard oil drain plugs perfectly. And now, I've got a stripped lower unit hole in my kicker! The screw is an 8mm. Not sure of the the thread yet, but looks like I'm going to need to go up one size and retap the case. Just wasn't my day today. Oil plug on the main stripped out also. Didn't really think I was overtighening that one, and I suspect it got cross threaded. I was able to buy an oversized plug with an internal smaller plug within it. I'm hoping that works well. Had a hard time getting the oversized plug to go in straight, but it seems to have pulled it self flush as I tightened it. The only other option I can think of, is to find a 14x1.25 plug, and retap the hole for a helicoil. The original plug was 14 x 1.5, however the helicoil and tap set I have is for a 1.25 thread. Other than that, I suppose I could pull the bottom cowl off and see if the block piece the oil plug fits into, is a separate unit that screws into the main lower unit.
As a side note, I asked my local shop how they fix stripped out holes for the oil plugs. This was in regard to the Main's engine oil drain. The mechanic said they don't have that issue, as they "suck" the oil out. I wonder if one can really get all the oil out this way? That drain plug is pretty low on the engine. Colby
 
I use one time use crush washers bought in bulk from SIM . I use a big screwdriver n no torque wrench. No leaks or or other damage to date
 
I have a big screwdriver with a square shaft. Makes it easy to apply the appropriate sized wrench for torquing.
 
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