Why screws with high/low alternating threads?

[ranger58sb]

Decided I should replace some screws that have slightly buggered-up Phillips heads... while doing routine maintenance on the head (replacing the joker valve).

Turns out the existing/original screws have high/low alternating threads... so eventually I learned they're 18-8 series #8 self-tapping high/low thread screws with a blunt tip.

WHY???

And why would a manufacturer not use some standard off-the-shelf screws?

What benefit do high/low alternating threads provide? If any...

Is there some sort of starting procedure so the replacement screws thread into the existing holes the same as the originals? Or does it matter? And if it doesn't matter, why have high/low threads in the first place?

When I replaced this joker valve in the past, I noted the screws actually could be replaced then... but had to get on with the project at the time, so I just logged the screws for replacement "next time" (which would be now). I didn't do anything special to start the original screws then, not noticing the thread difference at the time...

-Chris

 

[CaptSteve53]

Basically Applications are for use in plastic, nylon, wood or other low-density materials. Thread design reduces driving torques, enhances resistance to thread stripping, improves pullout strength and lessens risk of cracking the work piece.

They are normally made from Stainless: 410 Martensitic Stainless Steel and not from 18/8 so not really the best for marine use, they are basically self tapping

I would try to get a marine grade self tapper if possible 304 min but 316 best how ever think it will be a job to find


Cheers Steve

 

[CaptSteve53]

Forgot to mention they use the Martensitic grade stainless as they are heat treated and then annealed to end up around Rockwell C hardness of around 45-50 (How hard is that?) not much under a poor quality drill bit

Cheers Steve

 

[FF]

Self tapping screws save a few seconds in Manufacturing.

 

[HopCar]

Basically Applications are for use in plastic, nylon, wood or other low-density materials. Thread design reduces driving torques, enhances resistance to thread stripping, improves pullout strength and lessens risk of cracking the work piece.

They are normally made from Stainless: 410 Martensitic Stainless Steel and not from 18/8 so not really the best for marine use, they are basically self tapping

I would try to get a marine grade self tapper if possible 304 min but 316 best how ever think it will be a job to find


Cheers Steve

Thanks Steve. I had wondered about that as well.

 

[diver dave]

Sounds like a "self-THREADING" or "thread FORMING" screw for plastics. I would NOT change the style of screw at all. You don't want to recut whats left of the plastic. I vaguely recall this same screw on my electric head joker valve body.
The best engineering solution is a SS machine screw insert and then use a machine screw. BUT, in your case, look at Mcmaster carr or similar or go to the head vendor who will ship you 3 for likely free.

ps; self-tapping (not to be confused with sheet metal screw) is a machine screw with the lead threads designed to work as a tap to make the threads. It makes swarf as it goes in new material. In a self-threading screw, the entire length is cutting threads in a soft material by deformation (cold flow).

 

[ranger58sb]

Thanks, guys. Useful to know there's a bonafide reason, not just somebody messing with my head.

Yep, it's for attaching discharge assembly (with joker valve inside) to toilet base. Yes, it's plastic to plastic. Yep, it's an electric head.

I added the word "self" -- the manufacturer's rep actually just said "tapping".

And I got some. Manufacturer's part number, set of three screws. Not free, but not all that expensive. The trick was finding a source who didn't charge $15+ for shipping, and the $2.38 (or whatever) part set. (So I got two sets.)

So how do the tall threads know to engage the deeper cuts that the high threads from the previous/original screws made in the plastic?

-Chris

 

[diver dave]

Shoot, now ur asking hard questions.
I'd say the new screw threads will pretty easily align themselves with the old cut threads. Hopefully, the thread pitch is the same. It will be obvious as you start it in.

 

[jleonard]

Oh well this "thread" is unfortunately about 2 hours too late for me. I just replaced the joker valve in one of my heads and yes I dropped one screw into never never land.
I had to replace one of the oem screws with a standard pan head screw as I didn't have anything else onboard.
I was very careful with it and it seemed to tighten fine.

 

[alormaria]

To get a screw or bolt to align with the existing threads I find that if I turn the screw/bolt counter-clockwise (backwards) until I feel a "pop". The screw/bolt has now just fallen into the start of of the existing thread. Now start turning clockwise to engage the existing threads.

 

[ranger58sb]

To get a screw or bolt to align with the existing threads I find that if I turn the screw/bolt counter-clockwise (backwards) until I feel a "pop". The screw/bolt has now just fallen into the start of of the existing thread. Now start turning clockwise to engage the existing threads.

Ah. Got it, I think... thanks, Al.

Yes, Dave, the threads are (look to be) the same pitch, just different heights, alternating.

-Chris

 

[ranger58sb]

I had to replace one of the oem screws with a standard pan head screw as I didn't have anything else onboard.
I was very careful with it and it seemed to tighten fine.

FWIW, I also asked the manufacturer's rep about the blunt "point" on the original screws, and whether it'd be OK to use normal same-length pointed screws. Looking at the engineering drawing, he said he thought not, in case the point went in too deep, and then perhaps punctured (?) something. OTOH, he may have just been providing the more cautious version of their guidance.

And he seemed more cautious about that then the idea of using standard same-height threads over the whole length of the screws... at the same time cautioning to not over-torque.

Now that I see it's largely about the plastic, I think I get that now.

-Chris

 

[C lectric]

To get a screw or bolt to align with the existing threads I find that if I turn the screw/bolt counter-clockwise (backwards) until I feel a "pop". The screw/bolt has now just fallen into the start of of the existing thread. Now start turning clockwise to engage the existing threads.

I do the same with many fasteners. Reduces chances of cutting a new thread, of monkeying the existing thread and generally goofing up the existing old part.

 

[ranger58sb]

Can't yet tell how the fix is going. Somehow a hard clog turned up about 3' down the discharge hose... so once connected, the system wouldn't properly flush. Thought at first we'd just acquired a clog in the vent while we were gone for a week (found recent wasp mud daubs on the boat in other places) but the vent lines seemed to be clear and even with the pump-out port open, no flushee.

Went back to the head end, and found a hard stop in the hose. Even got a plumber's snake stuck in it for a while. Can't tell what it is with the inspection camera yet. Can't quite imagine what the cause of all that is; open the system, leave it for a week, change the joker valve, hard clog? No clue...

Anyway, more work today.

To the point about re-starting the screws, though... can't tell whether I could feel the "pop" or not. Maybe. If so, was more of a faint impression than a positive recognition.

-Chris

 

[koliver]

Chris:
re the funny looking screws; I just replaced my Joker and the pump tower before it on a manual head. Same screws as yours, in both places, so 6 in total. The new pump came with 5 new screws (?) so I had to re-use 1.

re your hard clog; Same here. Plumber's snake went a couple of feet down the hose, then stopped. I undid the other end of the hose (with a bucket under the end.....but no lasting mess there. Then the snake went up the hose to the same spot. Out came the hose. straightening the hose out on the dock, revealed that the inside diameter of the hose had shrunk from the original 1.5" to a new average of ~1/2". Beating the hose against the side of the dock broke up the scale (wouldn't do this at the dock if it was other stuff in the hose, but scale is harmless) and after just a few minutes work I could see all the way through the hose to verify that it was again like new. Re-assembly, then what is essentially a new system from the pump handle to the holding tank.

As these things always come in clusters, the other head began misbehaving, while on overboard discharge, but not while on the HT. Over the side with a large screwdriver, I found the discharge thru hull blocked with barnacles. They were plentiful all the way up the pipe to the 90° elbow. Explained why the valve on that thru hull was so hard to move!
While under the boat, I jammed the screwdriver into every opening. Found the inlet to the salt water pump also had some growth. It now works much better too.

Keep on it! There is always more work to do, but there are always results.

 

[alormaria]

To the point about re-starting the screws, though... can't tell whether I could feel the "pop" or not. Maybe. If so, was more of a faint impression than a positive recognition.
-Chris

Yes, it's a bit like safe cracking.

 

[ranger58sb]

re your hard clog; Same here. Plumber's snake went a couple of feet down the hose, then stopped. I undid the other end of the hose (with a bucket under the end.....but no lasting mess there. Then the snake went up the hose to the same spot. Out came the hose. straightening the hose out on the dock, revealed that the inside diameter of the hose had shrunk from the original 1.5" to a new average of ~1/2". Beating the hose against the side of the dock broke up the scale (wouldn't do this at the dock if it was other stuff in the hose, but scale is harmless) and after just a few minutes work I could see all the way through the hose to verify that it was again like new. Re-assembly, then what is essentially a new system from the pump handle to the holding tank.

Thanks, that's useful info. My first discharge in line is 1" diameter for about 3' and there it's connected to a coupler (I think that's the right word), probably hose-to-hose, and the next hose in line looks like 1-1/2" diameter from what I can see of it. The first hose runs straight downward, then bends about 90-degrees to a short mostly-horizontal run to the coupler. I think the snake is hanging on the coupler... haven't had any luck getting the snake past that yet, even though I can get the inspection camera through there. (Never used a snake before, so it can be operator error...)

In the meantime, I've maybe made a small bit of progress, without yet disconnecting the discharge hose at the other (holding tank) end... which of course would be a major (MAJOR!) effort.

I put a shop-vac on the discharge hose at the head end, sucked out a bit of the standing "treatment" stuff I had in there, and also it got out some scale. Don't think it's a huge amount of scale, not enough in my mind to account for the clog... but now I at least have sewer odor, oddly a good sign I think, whereas before no odor at all even thought the system was standing open for more than a week.

Next, I opened the pump-out port on deck and then gingerly put an air compressor on the discharge hose at the head end. No explosion!

I think I need to be in both places at the same time, to tell if I'm getting good airflow through the system. I could just reconnect the discharge hose and try a flush or two, but wifey may be able to come down later to help...

Yes, it's a bit like safe cracking.

-Chris