ICS M4 Tappet plate head scratcher

[Blitz85]

Hi All, I could use some help troubleshooting a problem I'm having with a friends M4.

When the upper gearbox is in the upper receiver the tappet plate gets stuck in the back position and barely moves, just a slight wiggle really.
However when I put the upper gearbox onto the lower without the upper receiver it cycles just fine, the rear pin is in while I do this so it's definitely down all the way.
While messing around with it for hours the part on the tappet plate that the spring attaches to snapped off, so that was the end of my tinkering for the night.

They are all brand new lonex parts aside from the cylinder head which is SHS.
The issue was also happening before upgrading the parts and I have tried both a stock ICS tappet and a new SHS one.

Any and all help is much appreciated.

[ox56]

Is the lower end all stock parts or is it modified as well

[RainyEyes]

This might be relevant:

Originally Posted by pestobanana

May be SHS quality control. SHS tappet plates from more recent batches are off spec. Fin is too far forward, roughly half a mm needs to be removed. It causes the tappet plate to be pulled back past the its limit and the front buckles forward against the cylinder head so it doesn't snap. This pushes the nozzle forward slightly while it is supposed to be feeding a BB.

Just throwing a bunch of stuff out there because I don't see pics/fully understand:

You might have one of those bad QC tappet plates from SHS and you may need to find a different brand of tappet plates. With the gearbox fully assembled, is the nozzle moving back when you push it? Any possibility the tappet plate is too fat for the gearbox (width wise)? Is the nozzle getting stuck on the cylinder head that is inhibiting tappet plate movement?

If the problem persists with the upper receiver when both upper and lower gearboxes are assembled, your problem will usually lie there and it could be a problem of fitment issues with the upper receiver. See if it's getting caught on anything like the receiver or the hop up unit.

Pics will help if you're trying to recreate the problem and you're asking for advice. Demonstrating how you recreate the problem will assist in identifying what is potentially happening.

[pestobanana]

Isolate by closing the top half of the gearbox with only the tappet inside with the spring and check for movement.