M4 piston problems

[Lozart]

As some of you may know I have an old TM M4A1 that has become Triggers Tactical Broom. The guts and outers have pretty much all been changed over the last few months in a seemingly endless chase to get the thing working better than it was stock (230fps, 3 parts of f-all range). When it was finally all together I loaned it to a buddy who used it for half a day before it jammed. Despite my protestations that once it had jammed he should maybe stop pulling the trigger, several exploratory bursts of auto were forced upon the poor thing.

 

Stripped down once again it seemed to merely be a BB stuck in the hop. I cleared it, tried again and the same thing happened so I ordered a new rubber (Prommy soft) which I installed tonight. Nice. All went back together nicely and so to test firing. A few single shots came out very nicely thank you so a burst of auto was called for. THRAAAAP! THHRAAAAAAAAP! Cool. One more for luck - THRRAAAAcccgghhhhh cghghhhhhhhh cjhghhhhhhhhh. Bugger.

 

 

 

One spectacularly chewed piston.

 

So - to avoid this tale of woe in the future, what do I need to do? New piston (obvs) but what else? The gun is running a 7.4v LiPo, standard gears, original motor and a Nano AB mosfet. Do I need to worry about AoE or just get a new piston with all metal teeth? Please also bear in mind that this is the ORIGINAL TM piston, the gun was already 10+ years old when it got to me!

 

What say you AFUK massive?

[Samurai]

Setting AOE is a must. The a new piston head might be thicker so it's possible that the gear hits in between the teeth of the piston. If that happens with steel teethed piston then the gears break.

The marks on the side of the piston is something I've never seen. It looks like the piston has turned sideways.

One more thing. If you put this much grease on the piston head, it will make it's way to the hopup and screw up your accuracy.

[dex]

I would give gearbox a total clean up. Reshim The gears if they are OK, replace if not. Replace the piston and head. Set aoe with sorbo.

[Lozart]

Cheers for that guys. Samurai, yes - at one point during a particularly hamfisted reassembly the piston missed the guides and turned a bit hence the marks. Never thought about the grease getting to the hop though, good point!

 

So I'm thinking that maybe this would be a good opportunity to swap out the gears for maybe a 16:1 set to raise the RoF, sorbo pad and AoE adjustment. Would I need anything else to go down that route?

[Lozart]

Just for info - the gun is running an M95 spring, 7.4V LiPo, NanoAB mosfet, 300mm barrel, SHS piston head, SHS cylinder head and nozzle, standard TM gears, rewired with 1.5mm low resistance wiring and Deans connectors.

[dex]

So I'm thinking that maybe this would be a good opportunity to swap out the gears for maybe a 16:1 set to raise the RoF, sorbo pad and AoE adjustment. Would I need anything else to go down that route?

 

Running an M95 with high speed gears may lead to pre-engagement issues (less so with a 7.4 lipo). If thats the case, consider - lightening the piston (take the bearings out - if you have a BB spring guide, swiss cheese, etc) and/or short stroking and using a harder spring.

[Lozart]

 

Running an M95 with high speed gears may lead to pre-engagement issues (less so with a 7.4 lipo). If thats the case, consider - lightening the piston (take the bearings out - if you have a BB spring guide, swiss cheese, etc) and/or short stroking and using a harder spring.

 

Screw it then, I think I'll stick with an AoE adjustment and the standard gears for now. Thanks for the advice!

[Lozart]

Something that has been making me wonder - the Dytac receiver seems to be a bit tight fitting onto the TM gearbox. There is always a slight gap at the back where the upper meets the lower - is it likely that this is causing enough of a misalignment between the nozzle and the hop rubber to cause the BBs to get jammed? The original TM (plastic) receiver used the rear pin to lock down the upper, whereas the Dytac merely has a lip at the rear of the upper that fits under the rim of the upper.

[Samurai]

You can check that easily.

Remove the magazine. Shoot a few times to make sure no BB is in the hopup, then turn the gun upside down and shoot until the nozzle stops about halfway into the hopup. You can see it through the BB feeding part of the hopup.

Remove the battery to be safe.

Get a bright light and direct it into the hopup from the magwell. Now if you look into the barrel from the "hot end", you will see if the nozzle is in the center of the barrel or not.

[Lozart]

Cool, thanks for that, I'll give it a try when the new piston etc is in.