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Grommet Plungerhead Mini-writeup


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#1 Lt Stefan

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Posted 21 July 2010 - 09:22 PM

So after some more testing of the grommet plunger head seal I have found a few problems that I had to fix. It is a little more complicated than slapping a seal between two washers. This mini-writeup will outline how to fix all of those problems.

Problem One: When the gun is primed, the spring pushes against the washer on the back of it. Since the rubber is pretty flexible, it squishes in and tightens the seal. This causes undue friction when you fire the gun, and it makes the seal looser when the gun isn't primed.

Problem Two: Since the I.D. of the grommet is 3/4", it has a tendency to get out of line between the washers. This causes a problem for obvious reasons.


Materials:

98026A029 1
98264A125 1
90402A192 1
93505A481 1
9307K81 1
90073A009 1

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This puts the cost at $1.49 per plunger head, except most of the materials are bought in bulk so it is only cost effective if you plan on building multiple guns that utilize this plunger.

On to the writeup.


First start out with your plunger rod. Then drill a 9/64" hole in the top of it and tap it with an 8-32 bit.

Then put on your 1" O.D. washer followed by the standoff with a lock washer underneath it (not pictured) to keep the standoff secure. Make sure the washer is centered and tighten the hardware down (I used a nut driver). This puts the retaining force of the spring on the washer and doesn't affect the grommet.

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Next slip two 3/4" O.D. washers over the standoff, followed by the grommet. These washers keep the seal centered on the assembly.

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Now center the remaining 1" O.D. washer over the top, and tighten it down with a screw. The more you tighten, the better the seal.

Attempt at seal demonstration by breathing on the plunger:

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This process just made you a great sealing plungerhead for +bows and the like for less than half the price of the skirt seal. There is one catch though... If the skirt is considered to seal 100%, I would say this plungerhead seals 90-95% based on how tight you clamp it down. However there is literally no performance loss when this head is used on such high-powered guns that it was designed for.
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#2 ModSquad

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Posted 21 July 2010 - 10:10 PM

Thanks LT, I have been wondering what that seal on your L+L was and now I know. Nice write-up, short and to the point but still is very useful. Thanks.

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#3 Split

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Posted 21 July 2010 - 11:39 PM

I was going to wait and post a more comprehensive review later, but you covered some of the points, so I'll just make it short here and now.

So far, I've tried these on 6 guns - pumpbow, 3 +bows and two RTPs. Results are varying.

First off, the price point is a huge bonus for me. I bought 25 skirt seals to revert all of my (and friends and customers') guns, and just so I wouldn't run out (easier and cheaper than ordering individually, thanks to the lax return policy). That runs about $120 right there. For the equivalent amount of skirts, I got $110 back in my pocket. I am very pleased with that. Those prices include the washers for the grommets and not for the skirts, as well.

Installation of the grommets has been overall pretty easy. I've been using #8 washer ID, 1 1/4" OD metal washers, and they've been working ok for this purpose. I mainly want to switch to 1" OD, since that allows me to switch to #6 ID, keeping them from shifting much more. I haven't had the issue you're describing about them shifting side to side - the friction from the washers combined with the plunger tube keeps them in place plenty. Your method would likely make it easier to initially center though.

These seals need a hell of a lot more lube. Whereas skirts need little initial lube, these need a heaping fingertip full. Skirts also almost never need to be relubed (8 months and counting on the initial one with no new lube and moving smoothly). Two of my blasters with grommets needed to be slightly lubed after one war, so they'll more likely be about 2-3 wars per lube. Not a big issue, just something to watch out for.

The seal - the important part. Overall, it's finicky. I experimented with how much they had to be tightened, and there's not much of a pattern to what makes the best seal with it. I used calipers to measure the OD to make sure they were all the same on all of the guns, but it didn't really correlate to the performance as much as you'd think. Two of the +bows were modified to hit full compression, so they had the most energy, followed by my pumpbow, then the rtps, then the standard +bow last. Same size plunger tube from the same source, usually cut from the same large tube, yet it was an even half and half on seal quality.

The pumpbow and one of the modified +bows were the ones used at a war with the grommets, and, initially, the seal didn't seem that great. I tightened/squished it to the first point where it became too tight to move without difficulty*, then loosened it just a bit. Once fully assembled, I couldn't stop or even slow the plunger head with my hand or barrel stopper (end cap on pvc with adjustable vents) like I normally can with a skirt. Oddly though, after the war, the +bow plunger could be slowed substantially, yet the pumpbow still couldn't.

Overall, the seal has been hit or miss. I wouldn't call it 90-95%; more like 60% sometimes and 90% others, optimistically. I never got it quite to the skirt's level. That said, I'll probably still generally use them. The trade off is more work (centering, tightening, lubing, maintaining, possibly breaking them in) for a 90+% reduction in cost. If you're working on something with lower efficiency (smaller plunger tube, weaker spring) and you're trying to get as much power as possible (as opposed to, for instance, intentionally inhibited sidearms), go with a skirt. Otherwise, there's a strong argument for the grommet.


Some other notes to consider:
Skirts expand more consistently, but grommets come in many more sizes. Neither are designed to expand their OD, but if you need an extra 1/16" od from a skirt, insert (forcibly) a spacer with and OD of the skirt's ID + 1/16". It has worked really well up to about 1/8", at which point the skirt OD just starts to deform. Skirt ODs vary about 1/4" per item number, but that's not a reliable way to pick one.

The cost of polycarb and the time spent making little circular washers for the skirts wasn't taken into consideration, though it's not quite negligible.

*At this point, the friction would stop the plunger .5-1.5" short of normal travel, depending on the spring compression at those points, which varied by gun.

Yeah, this is the shortened version.

Edited by Split, 21 July 2010 - 11:44 PM.

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Teehee.

#4 Lt Stefan

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Posted 22 July 2010 - 01:16 AM

Wow, thanks for your review. It looks like I still need to do some more testing, except I don't see why you are getting such varying differences in performance between guns with the same plungertube. I would think that it wouldn't really matter, but I guess that would be wrong.

The way I see it though is +bows have way too much power already; the darts we use don't even make use of all of it. I figure lowering the overall output of the gun by a small fraction because of the slightly imperfect seal won't affect performance at all. This is especially so with slug darts, which seem to shoot 110-120' max no matter what gun is shooting them.

I will try and experiment with adjusting the seals more and hopefully have a video up soon.

The main difference between grommets and skirts it seems is the quality of the piece. The skirt seems more molded and solid, whereas grommets tend to have slight imperfections and an overall lower quality construction. For example they are not perfectly circular; when inserted into a plunger tube one of the rings on it might be pressed against the wall halfway around it and the other half barely makes contact.

It seems the actual rubber causes more friction, which is probably why the seals need way more lube. That isn't a problem for me as well, considering I pull out my +bow and clean it very frequently anyway.

Basically it all boils down to preference. If one wants to use it, go ahead. If they don't, I don't give a damn. I just outlined the best way I have figured out to make this type of seal effective. I would love to see people try to make this as efficient as the skirt seal, and I welcome any suggestions anyone has.
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#5 Whisper101

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Posted 22 July 2010 - 06:40 AM

This looks fantastic! Just one question: any particular reason why you used 8-32 threaded hardware as opposed to the 6-32 threading tools/hardware that most of us already have? Would using 6-32 present a problem if we modified your design to use it?
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#6 Lt Stefan

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Posted 22 July 2010 - 07:42 AM

Go ahead but you won't be able to find a 6-32 piece with the same length dimensions.
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#7 Split

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Posted 22 July 2010 - 10:28 AM

Go ahead but you won't be able to find a 6-32 piece with the same length dimensions.

Use an unthreaded female spacer with #6 ID, such as 92510A443 , which is only $.27 ($.23 in bulk) in combination with a longer screw.
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Teehee.

#8 Lt Stefan

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Posted 22 July 2010 - 10:31 AM

Except I need the male-female standoff to secure the back washer, otherwise that doesn't fix the "squishing" problem. If you mean have the spacer be 3/8" long (the thickness of the grommet), then you wouldn't be able to tighten it more to adjust the seal.
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#9 Split

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Posted 22 July 2010 - 11:33 AM

I see what you mean. I haven't experienced that issue, so I wasn't planning around it. Another alternative might be something such as shoulder screws, but those are more expensive. It seems like a threaded spacer would work too, it'd just be more work. If you're already using the grommet, you're probably willing to add a bit of time to save some money though.

Edited by Split, 22 July 2010 - 11:41 AM.

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Teehee.

#10 Zorns Lemma

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Posted 22 July 2010 - 11:50 AM

Make your own female/male standoffs via a female standoff and a small section of threaded rod.
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#11 Lt Stefan

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Posted 22 July 2010 - 12:30 PM

Make your own female/male standoffs via a female standoff and a small section of threaded rod.


How would I then join the small rod to the standoff? Glue? Honestly I don't see the big deal about using 8-32 hardware, it even feels more secure in the plastic than a 6-32 bolt.

I see what you mean. I haven't experienced that issue, so I wasn't planning around it. Another alternative might be something such as shoulder screws, but those are more expensive. It seems like a threaded spacer would work too, it'd just be more work. If you're already using the grommet, you're probably willing to add a bit of time to save some money though.


It would save a few cents, but it would require a lot of adjusting to get it right. The other thing I could do would be to screw a bolt into the plunger rod, then cut the head off and secure the base washer with a nut. But that would be a pain in the ass if I ever needed to take that threaded rod out.

Edited by Lt. Stefan, 22 July 2010 - 12:32 PM.

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#12 Zorns Lemma

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Posted 22 July 2010 - 01:01 PM

Perhaps if you went with a grommet that has more OD for less ID it would seal better. I just found pictures of when we used them for NF replacement internals: http://nerfhaven.com...52 and http://nerfhaven.com...78 and you could stop the plunger rod dead by sealing off the coupler. I believe Merzlin even has a video of him doing so.

Thus it is perfectly possible to get skirt-quality seal with a grommet and we just have to figure out what's different between these NFs with 1" OD polycarb tube and +boids with 1.5" OD polycarb tubes and their corresponding grommets.
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#13 Lt Stefan

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Posted 22 July 2010 - 03:18 PM

Have you been able to find grommets with a smaller I.D. on Mcmaster?

Today I made an Lpl and used the grommet in it. Although it required a lot of lube I was able to tighten it a lot and both rings sealed completely around. The area of plunger travel was clean of lube, like it would be when using a skirt seal because it pushes it to the front and back of the seal.

Also I would have been able to slow it down considerably if not for the holes in the bushing that the front bolt goes through. On my next one I will test it before adding the bushing and see if it stops.

I think we are getting this closer to being as good as the skirt seal. With the new hardware I used I was able to tighten it down and adjust it without it slipping out of alignment and the seal increased evenly all around.

With a little more testing I don't see why grommets can't be used as an equivalent alternative to the skirt.
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#14 Whisper101

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Posted 27 July 2010 - 07:09 AM

Any particular reason that part number 93505A480 wouldn't work? It's the same male/female hex standoff as you found Lt. but in 6-32 version. I'm just thinking about using the 6-32 version because it woold save about $10 on a tapping bit and wrench.
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