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Discussion Starter · #22 ·
Ok. I got some chrono data.
Started with a 7.4v 1500mah 60c rc brick.
Velocities were very consistent and all shots sounded the same.
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Full auto burst
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Then moved on to the pack I use the most.
11.1v Turnigy nano tech 1200mah 25-50c crane pack.

Out of 36 shots, I had 2 that were in the 250 range. Those 2 shots sounded weak and kind of springy.
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Full auto burst had a few low shots
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Whatcha guys think? I'm hoping it's pme, as I think that'll be easy for me to fix.
 

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Not seeing drops on 7.4v narrows it down, PME is a strong contender but usually it's more consistent in auto than you're seeing... Might be because its marginal? Piston rack will tell the tale.

How's your nozzle look/feel btw? Got good spring tension, feel smooth, no/little wobble, centering OK, etc?
 

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Discussion Starter · #24 ·
Not seeing drops on 7.4v narrows it down, PME is a strong contender but usually it's more consistent in auto than you're seeing... Might be because its marginal? Piston rack will tell the tale.

How's your nozzle look/feel btw? Got good spring tension, feel smooth, no/little wobble, centering OK, etc?
I haven't had a chance to crack into it yet. Most likely not until tomorrow evening. I will check and report back for sure.
 

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Discussion Starter · #25 ·
I still haven't had time to crack into the UTR45 yet. Turnout on my Black Friday sale was a lot bigger than I expected. I'll be sewing slings when I'm not at my 9-5 for the next 8-9 weeks straight!

If I can squeeze in an extra hour or 2, I hoping to be able to do it sooner, but it is looking grim.
But I am still thinking it is some mild PME.

I'm also going to take a close look at the piston rails while I'm in there.
When I built this up, the piston was tight in the rails.
It it so intermittent, that I suspect that is not the issue though.

I was also thinking to take another close look at the tappet plate guides in the gearbox.
That was also a little tight when I put it together, but I very lightly sanded/polished the tappet plate and cleared away flashing from the GB when I built it as well.

I'll report back once I've gotten into it.
 

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Discussion Starter · #26 ·
So I squeezed in a little time late last night to crack open the UTR45.

First thing I found was one of the lonex bearings I installed had failed. It's a good thing I opened it up without fielding it again.
Bottom/Left bearing for the spur gear was missing a ball.

The other thing is there were signs of very light contact with the last tooth on the piston rack. Probably VERY mild PME.
I'm not sure why I didn't remove that last tooth when I built it. It was my first time removing teeth from a rack.

You can see in the pic a shiny spot, but I think that may have been from my dremel tool when I was grinding teeth off. But there was a faint contact mark from the sector tooth as well. Sorry my pics kind of suck.
You could see there were light contact marks in the grease for sure.

I am thinking about changing the bearings for some bushings, but for now I had an extra set of the same bearings on hand and am just going to replace the 1 bad bearing. I just don't have time to change to bushings right now, as I would have to completely redo the shimming.

I did double check the shimming with the new bearing and everything seems good, so I'm going to roll with that for right now, as I am planning on playing this upcoming Sunday. I'll see how everything works from there.

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I did some more swiss cheesing to further lighten the piston and replaced the piston bearing with a spacer.
Piston with POM head and all associated hardware weighs 22 grams now.
I will reweigh it after I remove that last tooth in a little bit.
 

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Spur is a typical bearing failure point...did you lubricate the bearings with a thick grease? Bearing failures are more common with 6mm and 7mm, 8mm is supposed to handle loads better, interesting that happened to you.
 

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Discussion Starter · #28 ·
Spur is a typical bearing failure point...did you lubricate the bearings with a thick grease? Bearing failures are more common with 6mm and 7mm, 8mm is supposed to handle loads better, interesting that happened to you.
Good to know. Should I use a thick grease?
They were lubed from the factory with a white/clear grease, so I didn't touch them when I installed them.
 

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Could it be possible you're mistaking the spacers between the ball bearings as grease? And yes, you want a thick grease, something like super lube synthetic. Thinner lubricants like silicone oil will get flung out when the bearings spin.
 

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Discussion Starter · #30 ·
Could it be possible you're mistaking the spacers between the ball bearings as grease? And yes, you want a thick grease, something like super lube synthetic. Thinner lubricants like silicone oil will get flung out when the bearings spin.
OK, good to know. There was definitely grease in them.
I did repack them with super lube synthetic last night, but haven't put it back together yet. I will double check they are all properly packed before assembly today.
 

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What you think these things are made to a standard and QC'd?
 

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Yes, overall good...but not perfect
 

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Discussion Starter · #37 ·
OK, so I think all is good now. Got it all put back together and did some FPS tests.
I got the piston weight down with some more swiss cheesing and removed the last unneeded tooth.
Replaced the bad bearing and made sure they were all packed with some super lube grease.

50 round test.
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Full auto test

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Piston on scale
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Also decided to do a quick stipple job on the grip. It was seriously lacking traction.
BTW, Taiwan Gun just released UTR45 magazine adapters for MP5 cyma mags.
A little modification and I was able to get this angel customs drum to work. I needed to modify the back of the adapter to allow entry into the BB fill port.
The adapters are great. Good fit to the mags and not too expensive.

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