Loki 76/8000 Assembly - O-rings Get Stuck in Snap Ring Groove

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mccordmw

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I must be doing something wrong here. I'm assembling my Loki 76/8000 M1969 motor. I'm putting together the forward closure with the smoke element. I'm following these instructions:

2. Rub a thin layer of grease on the inside of the bulkhead, each end of the case and over all o-rings. Temporarily place the o-rings on a clean surface.
3. Place the small 1.25” OD by 3/32” thick o-ring onto the tracking smoke grain. Apply a light film of grease to the back side of the smoke grain.
4. Loosen the head bolt at the top end of the bulkhead. Push the smoke grain into the smoke well, being careful as the o-ring is compressed into the well. Push
the grain all the way in and then tighten the head bolt down. An eye bolt may be used on the bulkhead in place of the head bolt. Transfer the small o-ring from
the head bolt to the eye bolt and tighten with a jam nut.
5. Install small snap ring in the smoke well to retain the smoke element.

I put a thin layer of SuperLube grease on the inside of the bulkhead, on the O-ring, and on the end of the smoke element (not entirely sure why that's needed). As I slide the smoke element in, the o-ring in the smoke element groove pops enough into the snap ring groove that the whole thing gets stuck. I couldn't insert the smoke element any further in that about 1 cm.

After much wrestling with a strap wrench, I managed to pop out the smoke element. The o-ring was damaged. Fortunately, I have a big bag of the exact same 3/32” thick x 1.25” OD o-rings, so I greased up another one to try again. Same result! I tried pushing harder to get the o-ring to pop past the snap ring groove, but instead, it kind of smashed out of the smoke element slot and wedged in between the smoke element and the bulkhead. It's now insanely tight.

I couldn't go backward any more, so I might as well go forward. Using all my weight, I managed to get the smoke element all the way inserted. That smoke element is going nowhere! I'm hoping I can chisel it out after it burns.

I'm very hesitant to move on with inserting the bulkhead and especially the graphite nozzle into the case since I'm afraid those will have the same problems with the o-rings getting caught up in the snap ring grooves. I can force the bulkhead back out if there are issues, but with the force I used on the smoke element, I'm sure I'd chip or crack the nozzle.

Am I not using enough grease? The instructions don't say to slather it on. Maybe I'm inserting too slowly? Should I pop it in quickly to avoid the o-ring from settling in?

I've assembled a lot of smaller Loki motors and never had this issue.

???
 
I can't speak to this motor, but on my EX motors, I slather the grease on pretty heavily for the o rings on the bulkhead and nozzle. Even with that, sometimes it takes some really hard pushing to get it in.

Looking forward to seeing this motor fly at MWP!
 
I'm not sure I understand the question, but the 3" forward bulkhead I use doesn't need any grease or o-rings. Slide the smoke grain in and snap ring in place - seems you have a different style bulkhead. I would email Scott, he usually calls or emails back promptly.
 
I've only built one Loki M, but I was also worried about getting the remains of the smoke grain out after the flight. It ended up not being a problem. The leftovers come out without much hassle.

The forward closure and nozzle hung up a little on the o-rings, but not nearly as much. Grease them well, and they pop in place without damage. Removing them was similar to the smoke grain, pulled out easier than assembling after the flight.
 
Just like when you install the bulk head and nozzle, fill the snap ring groove with grease. This should help get the o-ring past the grove.


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Only have a second to reply, so very briefly:

Fill the snap ring groove full(flush) with grease prior to inserting anything. If the grooves are filled with grease, they don't cut or stop the rings.
 
I would email Scott, he usually calls or emails back promptly.
Calls get the quickest response, but sometimes you get lucky and someone drops a weblink to a forum question in my inbox.
For those reading allong who are lost, please refer to figure 1 on the instructions. https://www.lokiresearch.com/images/Documents/76mm%20M1969%20Spitfire.pdf

First off, I’m pretty sure this is true with most any motor assembly, but if something using o-rings is so hard to do that you have to force it, don’t. Stop what you are doing and please ask questions first, not after the fact.

Second, since the o-ring is now damaged, you MUST use the supplied head bolt with it's o-ring to guarantee that the bulkhead is sealed. The head bolt and o-ring will hold the motor pressure by themselves. If you have removed it and re-installed an eye bolt, it must have a flared shoulder and you must put the o-ring below it. Please call me if you have ANY questions on this AT ALL. If you have an alternative harness attachment point, please use it and put the head bolt with o-ring back in place. BTW, the head bolt is more of a back-up safety feature to the smoke grains o-ring. One will work without the other, but one of the two must be used with an intact, sealed o-ring.

There are 2 different styles on Loki 76mm bulkheads. Both styles use a snap ring to keep the smoke grain in place.

1) the old style plugged bulkhead with knurling at the top of the bulkhead. This style is just now being reintroduced. It does not require the use of an o-ring on the smoke grain and it will not work if you try to use the o-ring. The smoke grain cavity is completely sealed to the motor chamber.

2) the threaded style that was designed for head end ignition that TMT poo pooed the certification of. It never developed from there so I reverted back to the knurled design which just started shipping last week. This threaded design has a small hole for the ignition wires through the top of the bulkhead and thus the smoke grain requires an o-ring to seal the pressure from passing though the hole at the top. There is a 3/8” bolt with an o-ring (like the 54’s thumb screw) which must be cracked open to allow the air on the back side of the o-ring to escape before it can be pushed in.

The diameter of the o-ring groove on the smoke grain should be around 1.079 +/-.005”. These are cut personally by myself and each one gets measured before it comes out of the lathe chuck. I will admit that I tend to lean to the high side on the o-ring compression range, but I have only ever had one other customer who has contacted me about an issue with this specific problem. He had 3 reloads with 3 smoke grains and so I suggested he try the other smoke grains instead. He said he had the same problem with all 3. I contacted the local dealer to see if he could help him in person at the next launch.

This was part of the email he sent me a few weeks later. “Later when I brought it to a local launch it went together like a well fitting glove.”

I honestly never found out what the cause of his issue was. I only know that it was some kind of user error. I don’t know if this is the case here or not, but I will try to provide you with a few tips for next time, since now it is stuck and there is no way to measure the smoke grains o-ring groove to check the tolerance, or to do anything else really.

1) When inserting the smoke grain with o-ring installed, push it in with one quick motion all the way past the snap ring groove. Do not stop and rest when the o-ring hits the snap ring groove. O-rings roll back somewhat when they are pushed in and if you stop at a snap ring groove and realease the pressure, they will roll back strait again. When you reapply force, they are able to push into the snap ring groove more easily that they can compress back into the o-ring groove.

2) When installing or removing the smoke grain, use a twisting motion while pushing it in or pulling it out.

3) Push the smoke grain in as strait as you can. If you push it is sideways with uneven force, it can cause one side of the o-ring to force into the edge of the snap ring groove, causing it to catch and get stuck. This is where a twisting motion will help.

4) Slightly off topic, but use a light coat of grease on the back side of the smoke grain, especially when using the plugged/knurled bulkhead. If too much is used, there is nowhere for excess grease to go without having to force it up the sides of the smoke grain. This will be extremely difficult to do since there is an extremely small amount of space around the sides. If you have the threaded bulkhead, any excess grease would just push out the small hole in the top. The grease is there to help aid in clean up as it help keeps the burned soot off the inside surface of the bulkhead

Overall, the use of solid phenolic smoke grain tubes is highly reliable as the smoke grain is one complete solid piece.
Hopefully after your motor is burned, the phenolic should give a bit more after it's heated up and not put so much pressure on the o-ring, allowing you to remove it more easily than it went in. Give me a call when you do if you have any trouble. You could always send it to me and I can get it out if need be. I wish I could be of more help, but at this point, there is nothing that can be done. :-( I do expect to see some really great photos from this launch though. ;-)
 
Thanks, Scott. I'm going into it assuming user error since I couldn't find anyone else discussing this via Google searches. The likely error is that I stopped pushing once the o-ring ran into the snap ring groove which allowed it to settle in there and lock. I already have the bolt tightened down as you described, so I'm not too worried about containing the pressure. I also have my kevlar harness epoxied to the motor mount. I'm not attached to the case.

So, for installing the bulkhead and nozzle, I'll make sure to use adequate grease in the grooves and insert the parts in one continuous push with no stopping.
 
I don't think anyone mentioned here....
The 2 sides of the grooves closest to the center do not hold any pressure, it's the 2 outer edges the hold the pressure of the motor. Therefore you can carefully de-bur the corners of the 2 inner edges. They are the ones that cut o-rings as the bulkhead or nozzle is inserted. Doing that AND greasing the components will ensure to not cut the o-rings.

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Did you get it figured out, Mark?

Not yet. Been busy in the evenings with parent-teacher conferences for our kids. I should have time tonight to finish assembly of the motor. Had to go pick up some high temp grease first. I'm using Amalie (68326-93) Blue Grade-2 High Temperature Grease.
 
Filling the snap ring grooves with round wire will also help in getting o-rings past grooves. Just find the right gauge wire to fill the groove, I typically use copper wire.

Edward
 
Looks like the snap ring grooves in the case have a slight bevel on the backside? Anyway, the o-rings on the nozzle and the bulkhead slide into the case with hardly a hitch. I didn't need to fill the grooves with anything.

Thanks all.
 
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