1963 75hp Lower Unit Level and Fill Plugs

[yorab]

My parts manual clearly shows the same part for both top and bottom plugs: "307551: Plug and Washer". These are parts #37 and #75 in the exploded view.

​I had previously bought NOS plugs and I recently looked at my stock of them. It seems that I have 2 sizes of plugs. The threads seem to be the same but the diameter of the heads are different. There is also a dimple in the end of one of them (to differentiate it?).

​It seems that the small-head-diameter plug fits into both holes, but that the large-head-diameter plug will only fit into the upper (Level) hole. There also seems to be nylon washers that have different ODs. One is black and one is white. During a recent trip to a local marina, the guy told me that I shouldn't be using the nylon washers but I should instead be using the composite seal washers in the pic below.

​Does anybody know the story behind the plugs and seals and which ones are proper?

 

[racerone]

??---Use smaller plugs and the white washers.-----Been done for 50 years or more

 

[yorab]

Anybody know the part numbers for the smaller plug and white washers? Also the larger plug and black washer so that I know what to avoid?

 

[hardwater fisherman]

If you look on that same diagram now #75 is omitted. #37 is part number 0307551. Note these are allen head screws with washer included. IBOATS has some part number 18-4255. I did not see a listing for just the washer but I think it is 0311598. And probably there is a part number for a bulk pack but I dont know it.

 

[yorab]

I found another thread on this topic: http://forums.iboats.com/forum/engi...307551-slotted-head-drain-fill-plugs-obsolete

​I wonder if 18-4255 is the smaller-diameter head like in my first post. If it's the larger diameter with the dimpled end, then it won't fit into my bottom drain hole.

 

[racerone]

Can you machine / file down the bigger plug so it will fit ?

 

[yorab]

Yeah racerone, I could do that. I think that I go ahead and order one of the plug/washer combos (18-4255) and see if it is the smaller diameter. If so, then I'll just get a few of those and use them in the top and bottom holes. If not, then I'll file down the 4 or so large diameter plugs that I have and keep them in my stock. I'll post back here once I get the item and can compare.

In the meantime, I was just wondering how it came about that OMC has these two oh-so-slightly different plugs and also why the holes that accept the plug head differ between the top and bottom on my lower unit.

 

[yorab]

It looks like the original screw/seal combo (307551) has been replaced with 18-4255. I bought one from iBoats to test it out. These are the allen head screws, which I like. However, the diameter of the head is too large to fit into the bottom fill hole. It does fit into the upper level hole. The sealing seems to be different under the heads of the newer screws in terms of the area that contacts and crushes the nylon seal. I assume that the newer sealing design is an improvement over the old one? ​These also come with the white nylon seal washer and a black seal that I don't believe is nylon.

​I think that I'll shave off a bit off of the diameter of the newer screw head like racerone suggested and I'll try to use the new ones.

​The new allen-head style and seals:







​A comparison of the older, small-diameter head with the new allen-head style:



​And the problem that I face with the old-style screw shown first and the larger diameter screw shown last:

 

[racerone]

The motor is 54 years old ?-----Those grooves in the newer screws may be for an o-ring type seal.-----Many folks favour the nylon washers still.