Umarex Notos CRK Review

[Firewalker]

NICE! Thanks Paul! 

Yes, the oring is for a compressed fit for the end cap, its not a seal as the hole in the cap lets lout air freely. The cap itself is tabbed but is a loos fit inside the housing, not even close to a snap fit. The tabs are for assembly so the pins find the holes without guidance, push the cap in place, drive the pins, done!

With the first generation of inserts in, the cap was used to hold it all inside the housing, with the 8" version, the pins alone hold the insert in and the cap sit this dance out.

[Rat Sniper (AKA: PaulT58)]

Paul, the problem you are seeing may be an illusion:

Based on the images posted by David, including the one below, that ring pops out easily.  My designs all assume the ring is popped out, and that the insert fits into the casing directly without either the ring or the encap with its air stripper. 

See part "B" in the image below.  That is a separate part.  Dave did bore away plastic at the shoulder deeper into the casing, that the conical end of the stock insert bears against.  You do not have to do that.

The central bore in part "B" is smaller than the casing ID deeper in.  So I am not going to attempt a design that fits through that smaller hole.  If you removed your stock red insert, you would have to remove that ring part anyway.  Try it...

It is up to you to decide what you will or won't do, but I think you are mistaken about the degree of to which removing that part constitutes a permanent modification.    I suggest that you ask David (Firewalker) about this directly, as he has taken the part out.

Yea, I see the error in my assumption!  Funny thing, I don't have a Part B!  Not at the end of the housing anyway.  After pulling the end cap and the insert I see those tabs are just for alignment purposes and have nothing to do with the pins or retention within the housing!  Early morning brain fog and lack of sleep is taking it's toll! 

[subscriber]

Paul,

As long as the average ID of the casing is near 24 mm, we are in business.   Would you please measure that in a few places around the casing to see.

Thanks

[Rat Sniper (AKA: PaulT58)]

It’s not perfectly round!  Across the tabs it 24.39mm, 90 degrees from that point is 24.18mm, and at 45 degrees from the tabs it’s 24.11mm!  Got to love exact manufacturing specs, but then again, this is not Space X! 

[subscriber]

The insert should go in, although the casing ID may taper, getting smaller as you go in, as an artifact of injection molding.  Then sand the insert smaller until it goes in all the way to the shoulder.  Without making it sloppy.  If that happens, wrap some tape around it.  Or @@@ some narrow tape strips.

[Rat Sniper (AKA: PaulT58)]

The insert should go in, although the casing ID may taper, getting smaller as you go in, as an artifact of injection molding.  Then sand the insert smaller until it goes in all the way to the shoulder.  Without making it sloppy.  If that happens, wrap some tape around it.  Or @@@ some narrow tape strips.

Will do!  Thanks!  It should arrive on Monday, so I'll post the results on fitment and performance.  Thanks again for all the R&D you have put into this project! 

[Firewalker]

Part B is the shroud alignment insert and isn't related to the housing or barrel, it fits between the receiver and the shroud tube.

[miksatx]

@50% 6h12m short snout RS

[Firewalker]

It’s not perfectly round!  Across the tabs it 24.39mm, 90 degrees from that point is 24.18mm, and at 45 degrees from the tabs it’s 24.11mm!  Got to love exact manufacturing specs, but then again, this is not Space X! 

Also, runout (wobble) was present as the mount to the barrel was not concentric with the bore.

I bored the housing to fix this and to open the space for a full depth insert, making it easier to clean the insert when the time comes.

[subscriber]

Folks,

Bluesman (Dino) took detailed measurements of his Notos.  They vary enough from Dave and Paul's that I recommend you measure your own Notos before deciding which print file to try. 

Dino's pin spacing is real close to 16 mm, so he should try the print files from here:   https://www.gatewaytoairguns.org/GTA/index.php?topic=206735.msg156435698#msg156435698

Dino measured the casing ID at several depths and clocking positions.   Just as Dave and Paul observed, Dino's casing ID is not very round, but surprisingly it is neither parallel nor a straight taper.  The average ID at the front, 50 and 95 mm depths are 24.5; 24.3 and 24.3 mm, with quite a bit of variation "around the clock".  My CAD models have the OD at 24.2, so it might contact at the out of round bits, but is smaller than the average values.  Will have to ponder this one...

[Bluesman]

For reference this is what I measured on my Notos. It occurred to me that maybe the universal pin hole shape would be a kind of oval (picture two offset 2.0mm circles) to allow variation in pin CTC spacing. Precision dimensions especially for inside holes are hard to 3D print accurately so maybe in the end it doesn't matter so much.... not sure.

Another thought, maybe the OD of the insert slightly smaller (e.g. 23.9mm) with a o-ring groove near each end to locate the device within the outer casing. Hopefully this description makes sense, I can't think of a good way to put it into words 

Pin to pin, CTC 16.14mm
Front face to pin c/l  3.64mm
Pin diameter  2.0mm  (non-knurled part)

Axis        At front   50mm from front   95mm from front
0-180   24.39   24.28                24.32
90-270   24.63   24.39                24.30
45-225   24.52   24.21                24.32
135-315   24.50   24.30                24.24

0-180 = vertical

Dino

[TorqueMaster]

This all great info -- the lack of precision manufacturing of the casing is disturbing, and will affect barrel alignment with any baffles.  So a bigger bore size may be needed... for starters.

I found a teardown elsewhere, where they removed the casing.  Not pretty.  It *appears* the casing is molded onto the barrel.  The end of the barrel has circular grooves, and these grooves have snap-rings installed, then the casing is molded on, the snap rings embedded into the casing plastic during the process.  Casing not reusable after destructive removal.

The barrel size OD is reported at 11.95mm-ish.  If a baffle assembly were to (and able to) reach down to the barrel itself...

[subscriber]

So, I took Bluesman's suggestions and have oval pin holes that span 16 to 16.5 mm on centers.  The holes are sized 2.2 mm by 2.45 mm across the pin hole diameter, so they break through into the air space just a little. 

If you need to chase the pin holes with a drill, having dust fall into the insert is not a great idea, so I added a little material to shroud the pin holes.  See images below.

I also added ribs to both insert ODs that follow Bluesman's casing average dimension, above.  The ribs should help tighten the insert fit.  In the case of a too tight casing, the ribs should help you file or sand down a particular section of a rib that is making hard contact.

The STL print files are attached below.

[subscriber]

If I had the pin hole position and spacing right from the start, I would have designed the cones to stay clear away from intersecting the airspace.  But I am not messing with the cone layout now.  Don't have time for that...

[Bluesman]

If I had the pin hole position and spacing right from the start, I would have designed the cones to stay clear away from intersecting the airspace.  But I am not messing with the cone layout now.  Don't have time for that...

You're willing to leave 0.1dB on the table? 

[subscriber]

Dino, because you requested it:

A version of both inserts that has O-ring grooves, sized for this 22 x 1 mm O-ring, or similar: www.mcmaster.com/9262K621/
The groove depths are slightly different, so the O-ring effective ODs should approximate 24.3 and 24.5 mm respectively.  There should be a little room for the o-rings to compress into the grooves because they should print slightly longer than 1 mm.

I placed the O-rings such that they are clear of the key cuts and pin holes near the "canister" shoulder; and opposite the baffle roots where the wall of the insert is thicker.

The pin hole size and placements are the same as the above "universal" version.

STL print files are attached below.

[Bluesman]

Peter, that's a work of art!

[Rat Sniper (AKA: PaulT58)]

Very nice!  I like the idea of the two o-rings for centering it in the housing!

[subscriber]

Peter, that's a work of art!

You helped by providing objective information. 

[subscriber]

I like the idea of the two o-rings for centering it in the housing!

Changing O-rings to get the ideal fit is less work than sanding down ribs.  I would use the softest O-ring available.

My only concern and it is not major, is that the insert walls are unsupported between O-rings.  Now, for this low power application, and the ample primary expansion volume between muzzle and insert, I seriously doubt that the insert walls will fracture from over pressure.  If they did, you would not be aware of it, until you removed the insert.  And, unless such a fracture caused an air stripper cone to wonder off-center, there would likely be no consequence.

Grease the O-rings to ease insertion and extraction, should it take more than a little force to install.  Try it without the O-rings first.  Due to the ovality of the casing, there may be some interference already that will require additional force - or sanding/filing.

Let me know what improvements are required or would be nice, and I will address them when I get back:  Take two inserts, and call me in a month