Windows and 6 vs 7mm

[pizzainthemorning]

So I'm wondering, is there any real difference between a windowed (where you can look in and see the spring/piston) gearbox and ones without? I was checking out WGC and they had stock TM gearboxes, both Ver. II, except one was windowless and the other windowed, saying that the windowless one was for G3's and Mp5's, windowed for M16A2 and M4's. Is this really critical? Like will any version II gearbox fit the bill (so long as it's made by a compatible brand)?

The other question is about where they list the gearbox as 6mm or 7mm for bearing size. Do you need different gears to fit with the 7mm size? or is that the point, so it's loose? What size is "standard" 6 or 7? In other words, fill me in if you please, I'm ignorant in this.

Thanks,

Alex

[MMMiles!]

6mm is standard for TM, but some other manufacturers are starting to use 7mm for certain applications. I'm not really sure what benefits there would be to a different spec, someone else can fill in that blank.

There is no difference between windowed/sealed mechboxes, other than you can't use the sealed one on a TM armalite, and occassionally the open slot can be useful for diagnosing spring problems (say, if it's stuck). Given the choice I would never get a sealed mechbox, that little window comes in very handy.

[firemachine69]

The ICS M4/16 series utilize 7mm bearings, I'm pretty sure my MP5 takes 6mm bearing. However, the ICS bearings are copper, so it should be sufficient for most upgrades to a PDI 150% (which IMO is the "classic" maximum in a spring).

[MadMax]

I think the 7mm bearing hole dia is for ball bearings. A 7mm OD allows for slightly larger rolling elements which spreads out stress. If you're really interested, look up "hertzian stress".

When the rolling ball is loaded, the contact between the ball and race (inner or outer) actually causes both race and ball to deflect/mash such that there's a small area contact patch. A bigger ball smooshes out to a larger patch to support the same load so you end up with a smaller pressure (force/area) which means lower wear/less chance of failure.

I don't think there is any benifit for bushings to have a larger OD.

[mcguyver]

If you use a "windowless" mechbox in a gun with a full or over-capacity cylinder (no holes in it), there is a good chance your FPS will go right into the toilet. This is because on the "intake" stroke of the piston, the inrush of air in to cylinder is impeded by the lack of holes in the mechbox to allow for this volume to enter the mechbox and be drawn into the cylinder.

I did an ICS MP5 with full capacity cylinder and M4 length inner barrel. With a 110% spring, the FPS was about 240. I changed the cylinder to a 4/5 M4 cylinder from a Marui and the FPS jumped back up to 335. ICS uses a "windowless" mechbox as well.

[pizzainthemorning]

Great, thanks for the info guys.

Alex

[MadMax]

Originally Posted by mcguyver

If you use a "windowless" mechbox in a gun with a full or over-capacity cylinder (no holes in it), there is a good chance your FPS will go right into the toilet. This is because on the "intake" stroke of the piston, the inrush of air in to cylinder is impeded by the lack of holes in the mechbox to allow for this volume to enter the mechbox and be drawn into the cylinder.

I did an ICS MP5 with full capacity cylinder and M4 length inner barrel. With a 110% spring, the FPS was about 240. I changed the cylinder to a 4/5 M4 cylinder from a Marui and the FPS jumped back up to 335. ICS uses a "windowless" mechbox as well.

I think you have a more major issue at hand like a sealing issue with the nozzle and hop up. You also are not properly matched in barrel and cylinder volume when you use a full cyl and a M4 barrel.

With an unported cylinder and piston head, the only inlet for air is through the centre port in the cylinder head. You don't get any air filling the cylinder from the inside of the mechbox. If you want to do some hypothesis testing, try rotating your M4 cylinder so the side port opens towards the inside of the mechbox. If you're right about mechbox pneumatic issues, you'll observe the same chrony results as your full cyl' results.

With ported piston heads, some air from the inside of the mechbox enters the cylinder, but the rearwards motion of the piston would serve to compress the air in a sealed mechbox which would push more air into the cylinder. However mechboxes are not sealed well. There are several openings which are larger than the combined area of the openings in a ported cylinder so they serve a very small pneumatic resistance compared to the pneumatic resistance that the holes in a piston head present.

On the way forward, accelerating the pellet down the barrel presents a significant restriction. Also, the size of the cylinder head port is quite small compared to the combined area of various openings in an unwindowed mechbox so even if the bb wasn't the dominant resistance, the hole in the cylinder would be.

[mcguyver]

On an ICS MP5 mechbox, the summed area of open "holes" in the mechbox would be at best 1/4 of the cross section of the cylinder. They don't give you very much to work with. The head set was a ported upgrade "silent" set from ICS.

Nearly all manufacturers I've run across who make an M4 use a full cylinder. Marui and ICS so far are the only ones I can recall that do any barrel/cylinder volume matching. G&P, G&G and the one Classic Army I worked on were full cylinders, but also they use reinforced mechboxes with the "window". They also all had ported piston heads as well vs. Marui and ICS which do not.

I would suspect that the porting will also allow for a quicker seal between the piston head o-ring and the cylinder wall as well. As pressure increases in the cylinder during compression, the o-ring will be forced outwards due to the ports in the piston head. This seals up the cylinder very quickly. I've observed this by quickly vs. slowly cycling the piston on my bench with the nozzle sealed.

[MadMax]

The open holes in a mechbox are smaller than the cross section of the cylinder, but their area is greater than the centre opening in the cylinder which ends up being the major restriction if you forget about everything after the nozzle.

I think the amount of gap around the front flange on the piston head is bigger than the combined area of piston head ports. I don't think that porting would push on an oring to make it seal much faster.

[mcguyver]

I've noticed that when I've re-lubed a new head set or on a rebuild, if there is too much lube on the head o-ring, it doesn't seem to seal quickly at all and FPS suffers greatly, sometimes a loss of 125 FPS has been measured. Removal of excess lube to allow the o-ring to spread out and increase the "effective" cylinder volume has brought these numbers back up into the expected range.

I noticed this effect last year and my thinking was along the lines of my previous post and so far nothing has led me to another conclusion.