Ever had a hop unit that doesn't fit?

[Gunboat Diplomat]

Been tinkering with my recent boneyard Specna One series M4 (still unsure of the exact model, but the externals are very nice, think it's an E&C OEM).

It came with the old style metal 3-gear hop unit.  When I tried to fit my preferred ZCI rotary plastic hop, it sits a tiny bit further back in the upper when fully seated into the outer barrel, making it impossible to close up the lower (albeit only by a couple of mm, but no amount of shoving will allow them to close).

I've measured both hops with my verniers and overall outer dimensions are the same, just with differences in the shape and position of various protrusions.  

Hard to make out from the attached image but the outer barrel has quite a lot of profiling going on, presumably to ensure a secure fit but possibly the source of my issues.  I don't want to take a Dremel to it, I'd rather find something that just fits.

I suspect the answer to this is just 'airsoft tolerances innit' so I'll have to play the parts roulette to find something that fits.  Maybe I should try the newer Specna blue rotary hop, better chance they have engineered it to fit all their receivers?

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[Fatboy40]

Yep, been there.

However, it was with the holy grail to some, the Retro Arms AR15 CNC hop unit. I bought two of them and the first shock was their internal finishing which had burs here and there, but easily sorted. They were ever so slightly too long though in four AEG's I tried them in, with the upper never going back on properly (both metal and polymer receivers), other than in a G&G Firehawk.

The Maxx Model M4A-PRO though, hated by Reddit, fits perfectly in every AEG I put one in, but had to change the nozzle in all of them to get the best out of them.

 

[DerDer]

Had some unpleasant experiences with some hop up units in the past. Lonex AK hop unit that required filing and cutting to fit together. Required modification to assemble it and attach it to the barrel. Did fit in a Galil that was quite picky with hop chambers. 

Then there was a LCT HK33 which I tried three aftermarket hop units. One wouldn't fit at all. The second fit the AEG but after assembly wouldn't allow a magazine to lock in. The third did but just wouldn't feed.  One of the units was a Retro arms and the other a G&G, going off memory. Eventually just put the stock unit back in and sold it at a heavy loss to a local at a site. 

 

[Adolf Hamster]

This is the issue with giving cookie cutter replacement part lists.

A given product might be great, but if it aint a good fit for the gun it aint a good fit.

And it doesnt matter how premium it is, for as much as a fusion engine costs the internet is littered with "how to align your nozzle" guides when the reciever doesnt quite match up.

 

[Gunboat Diplomat]

The Maxx Model M4A-PRO though, hated by Reddit, fits perfectly in every AEG I put one in

Interesting, I'm planning to HPA this one and I know the Maxx units are hit and miss when it comes to nozzle alignment.

 

[Skara]

Retro Arms AR15 CNC hop unit

Same, I still have it (old version, silver with red wheel) and it wouldn't fit any of the AEGs I had at the time...

Be careful when picking hop units, due to tolerances and shit you may need a different nozzle or to make a few spacers.