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Pro54 K780 Assembly Concern

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Pallen

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I have a K780 Blue Streak to assemble and fly for this weekend and I have an issue I haven't encountered before.

The forward bulkhead of the motor is so loose in the liner, that the small, aft o-ring won't even engage the inside diameter of the liner.

I'm wondering if epoxying the bulkhead in place would be an option?

All the other fits seem fine (forward and aft bulkhead o-rings engage the motor case, the liner slides in the case fine).

Thanks for any advice.
 

tfish

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I think there are two different liner thickness for the 54mm motors. And there is a different FC's to fit inside each of the liners. I wonder if you got a FC for a thick liner and the K780 uses the thin liner?

Tony
 

Pallen

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I think there are two different liner thickness for the 54mm motors. And there is a different FC's to fit inside each of the liners. I wonder if you got a FC for a thick liner and the K780 uses the thin liner?

Tony
Interesting. The liner ID measures about 1.870" and the forward and aft bulkhead OD both measure 1.865" (seems reasonable). It's like maybe the o-ring or the o-ring gland itself on the forward bulkhead is on the small side. I haven't taken the o-ring out to do any further measurements though. I'll see what the dealer has to say about it.

Don’t fly it. Contact your dealer.
I certainly wouldn't fly it as-is. I've contacted the dealer. Thank you.
 

tfish

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I just measured a couple..one is 1.856 ( like you have) and another one is 1.768 (I think it's for a K300)

So, that's not your problem..
'
When you hear back from your dealer/CTI let us know what the issue is/was.

Tony
 

rfjustin

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I'm going to bet CTI will advise you to glue the forward closure to the liner. Running a sizable bead of 5 minute epoxy inside the liner, just on top of the top grain. Pure speculation on my part though... let us know what they advise please.
 

mikec

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The forward bulkhead of the motor is so loose in the liner, that the small, aft o-ring won't even engage the inside diameter of the liner.
Which O-ring is that? I've flown a lot of Pro54 and I never recall seeing an O-ring in the position you describe (maybe I just wasn't paying attention.) Can you post a photo?
 

Pallen

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Which O-ring is that? I've flown a lot of Pro54 and I never recall seeing an O-ring in the position you describe (maybe I just wasn't paying attention.) Can you post a photo?
Sure. Photo attached. It's the smaller o-ring. It sits inside of the forward end of the liner. It's part of a Pro54 that's pre-assembled. There's technically no need to disassemble a Pro54 to this point, but it literally fell apart when I removed it from the pink bag, which was a bit of a red flag. Plus, I like to check the grains anyway to ensure that there's nothing visually anomalous.

In this case, it seems like the o-ring barely protrudes from the o-ring gland. I don't have another one to compare it to, unfortunately.
 

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mikec

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Sure. Photo attached. It's the smaller o-ring. It sits inside of the forward end of the liner.
Interesting. Some Pro54 forward closures don't have a ring or a groove in that location; e.g. J240. I wonder which ones do?
 

Nytrunner

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My K750 (6gr) didnt have that O-ring, but was also a bit loose. I was using epoxy (30 min) on it anyway to avoid forward closure burnthrough, and had no flight issues
 

mikec

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I wonder if adding this O-ring is a new development by CTI -- there have long been issues with leakage at the forward closure-liner joint in Pro54. I flew an L990 last year that I'm 90% sure didn't have this O-ring.

If it's loose, though, it seems like they're still working out the bugs.
 

timbucktoo

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I wonder if adding this O-ring is a new development by CTI -- there have long been issues with leakage at the forward closure-liner joint in Pro54.
No, it's been around for a while. You see it mostly on 5-6 grain 54mm motors.
 

plugger

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No, it's been around for a while. You see it mostly on 5-6 grain 54mm motors.
It's been around for a while but it's definitely a change. The L935 I flew last month had the same forward closure (2 o-rings, 3 grooves) whereas historically CTI 54mm forward closures in 6GXL motors had two grooves and 1 o-ring. The smaller o-ring and groove does seem to be a recent change. I don't know for certain but I suspect the L935 I flew was manufactured in 2016, I ordered it mid 2015.
 

timbucktoo

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I’ve got a 6 grain K590 from 2014 with 3 grooves & 2 orings so it’s not that recent.
 

plugger

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I’ve got a 6 grain K590 from 2014 with 3 grooves & 2 orings so it’s not that recent.
And I've got a P54-FC (Pro54 Threaded Closure) I ordered from Wildman back in June of 2015 and it had 2 grooves and 1 o-ring.
 

Pallen

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I have a K780 Blue Streak to assemble and fly for this weekend and I have an issue I haven't encountered before.

The forward bulkhead of the motor is so loose in the liner, that the small, aft o-ring won't even engage the inside diameter of the liner.

I'm wondering if epoxying the bulkhead in place would be an option?

All the other fits seem fine (forward and aft bulkhead o-rings engage the motor case, the liner slides in the case fine).

Thanks for any advice.
Hey guys. I figured I owed you guys an update since I finally have one. I called CTI on this issue and they suggested just using a liberal amount of grease on the suspect area. I did this and flew it yesterday with no issues. The picture shows how I greased up the area, prior to pushing the forward bulkhead in with the liner. More details and videos on the can be found here:
http://allenp3.blogspot.com/2019/07/competitor-4-flight-7-k780-blue-streak.html
 

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Nytrunner

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Surprisingly aggressive? Lol, It's got a 200lb kick, and weighs less than that M you put through your rocket.
 

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