So my 2 cents, unless something is broken often there is zero reason to replace it unless it's a known bad part etc. So for instance you have the tappet plate on the list, unless it's broken I wouldn't replace it. The stock versions of most of these are fine unless the little groove the nozzle rides in fails then of course replace it. Bushings, I'm not sure what stock is if it's poly bushings or bearings etc. But if it's metal bushings from the start skip those as well, unless the bushings are wallowed out on the axle holes I leave them. Sometimes bushings vary in how they ride in the shell and adding aftermarket can be a headache in some shells without much reason. Again only change them if needed.
On the piston I really like the new CYMA version on brill, seems very solid in my use and includes a piston head that on the ones I've gotten gives good compression. It's not fancy but it's good and about the same price as the SHS without the piston head. I've got SHS pistons in stuff as well and they are great, but the CYMA seems every bit as good.
Cylinder, unless there is a problem this is always low on my list as well. Maybe if I'm changing barrel length etc, but generally not unless there is a problem with it. You can polish the inside of the stock part and that will help get a better air seal but seldom is it necessary. Cylinder head absolutely, I like the SHS version the most. ZCI will work in a pinch but you might have to drill out the holes that sit over the pins since they seem odd sized for most boxes.
Air nozzle almost for certain, SHS again in that in my opinion cheap and excellent.
Spring, Core if you can still get them on clandestine are great springs. You aren't going wrong with Prometheus but generally the price is harder to stomach for me. Generally I run cheaper springs without much issue, though I will say I'd put the core springs on the same level as the higher end springs I've run over the years. Only problem is a bit more FPS variability in the springs themselves little less QC. But much cheaper.
Spring guide the ZCI isn't a bad option for cheap, but generally on an M120 I don't worry too much whatever works and won't fail usually will do the job for quite a while. I ran my old CA M4 with a plastic spring guide that was stock for something like 50,000+ rounds without issues on an M120 and eventually just replaced it because I was in there for other stuff and had one laying there.
Inner barrel, ZCI aren't bad from what I've seen but again unless there is an issue with the stock barrel I'd look at polishing that up some and using it. Often in allot of guns the stock barrel isn't bad at all and I've seen aftermarket barrels that were worst. Bore quality is what is most important not diameter when it comes to accuracy. So if the bore is clean and smooth run it and don't worry about not having a super cool barrel. Also the tighter the barrel the more likely to encounter a jam because of BB size etc.
If you Sorbo your going to have to look at AOE don't eat a new piston or gears because of trying to skip it. But generally not a huge concern at m120 levels, do go ahead and radius the front corners of the gearbox while your in there, will help as well to mitigate failure there.
Flat hops aren't bad, but I don't find them much better if any than a normal good hopup. If it's performing well now I wouldn't mess with it. I do hopup mods generally when I can't lift a heavy BB or I have inconsistency, otherwise leaving it alone is often the best plan.
And with all the money saved above, pickup a lipo and charger, really lipo's are a huge help and at 7.4v your chances of burning contacts are far less. If you want to get frisky get a mosfet while your doing all of this with the money saved as well.
And understand that I'm not saying this as a "leave it stock that's best" really I'm just saying most stock parts are fine bar a few exceptions in most guns. Most times you can put a wrap of teflon tape around the cylinder head, toss a nozzle in there, new piston or stretch the oring on the old one and get a perfect air seal out of a bone stock gearbox. Then shim the hopup to the box(I have a thread about this on here) and all of a sudden you've got a very nice shooting gun for very little money that will often compete with guns guys threw allot more money into. Personally I don't really care what parts are in a gun, it's all about how it shoots. I've seen guns guys have throw money at that shoot terrible. I've seen guns that were cobbled together that everyone feared. And if you sort of know what your doing you can cobble some things and not loose reliability like the cylinder head PTFE wrap. Is it as good as an aftermarket? Not really but it's close and if it does fail down the road your out $4 for a new one. But really it's up to you that's how I'd do a build like that for my own guns.
Luke
