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Tap .400 x 28 TPI

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John Timmins

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Does such a tap exist ? I am working on a 1946 Mercury Outboard motor, a KD-3 that is a 3.2 HP single cylinder model. Near the propeller on the gearcase is a brass removable plug to fill the lower unit with lubricant. The fill plug unscrews using a flat blade screwdriver. Here's the problem..... The screwdriver slot was so mangled, it will not hold a screwdriver, Even an impact screwdriver didn't help. Next I drilled a hole in it and put an EZ - Out and the hole yet I couldn't get the plug to budge. I have heated the part in an oven to expand the aluminum gearcase several times also. I've tried using bees wax and every trick I know without luck to get that plug to unscrew. I believe I will have to use a Dremel tool very carefully to get that plug out.

OK, finally here's my question. I have a new plug from a long time Mercury dealer. The plug is .400inch wide with straight threads that are 28 TPI (threads per inch). I have a tap and die set, but there isn't anything like a tap needed for my outboard part to chase the threads. Is there a tap out there that is .400 wide and 28 TPI ?
 
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metlmunchr

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The Cobbler said:
1/8 pipe taps are very common & often come it tap & die sets .
just don't go too deep with it when tapping or your plug will go too deep & not seal
Click to expand...
He doesn't need a taper pipe tap. He needs a straight thread tap. 1/8-27 NPS. The plug would use an O-ring for sealing. When he gets the one out that's in there now he'll likely find some hammer fisted ***** in the past has used loctite or some similar compound to seal rather than using an o-ring. FWIW, the major diameter of a 1/8 NPS thread is .390" to .397"
 
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AdAstra

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Got a thread gauge or did you eyeball with a ruler?

If it’s 28 TPI, then use a 1/8” BSPP.

If it’s 27 TPI, use 1/8” NPS.

The BSPP will also have 55deg included thread angle vs NPS’s 60, but that’s harder to see.
 
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BarrelRoll

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PoorUB

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BarrelRoll said:
Isn't 27 TPI NP and 28 TPI BP? That link you posted is showing 27 TPI for NP, the OP is claiming 28 TPI not 27. Op are you 100% sure it's 28 TIP and not 27?

If you try hard enough you can usually jam an NP into a BP and visa versa.

Here's my 1/8-28 BSPP tap, .384" OD is what I'm getting. A brass plug might mash the .015"
Click to expand...

I am just going on what the OP says and perhaps he missed the thread pitch, plus thinking that 1/8"x27 straight would have been more popular in the USA than a British thread, but who knows?

Edit, 1/8"x27 I need to proof read my posts!!
 
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RoninB4

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Propriety tooling is possible, even for something manufactured in 1946, but I'd really make a point of using a thread gauge (or even the 1/8-27 NPS tap) to identify/compare under magnification. You got one shot at getting this correct, any mistake will mean tapping new threads. It does seem unlikely that a proprietary thread would be used on something as ordinary as a fill plug when off the shelf would do. It's not like it's a component that the OEM wants to capture the sales on for a 50 cent part the way manufacturers do now.
 
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Walkers

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Get the current plug out first, then see if you need a tap and if it is the same as the plug you have. Then, if you still have a problem get in touch with RMACK here on the board. The man is the Ace of outboard motors.
 
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John Timmins

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Sorry I haven't been commenting since I have been away. I discussed this with Antique Ourboard Motor Club Inc (AOMCI) guy who is the National Race Historian. He was in his machine shop when I phoned him. He said the same plug was in some Evinrudes. I got out my thread guage and it is 27 threads per inch and not 28 as I originally thought.
As someone else posted above, It was probably installed with some kind of hard setting Permatex instead of an o-ring.

I am quite familiar with pipe fittings, tapered and straight. I was in the merchant marine as an engineer before I retired. On one ship I had NPT, Bsp, Jis, and Whitworth pipe thread to cuss through. I will try to post pictures of my project.

Thanks to all the replies. I believe I should order the 1/4 - 27 NPS tap.
 
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