I have a problem with gas - Pics added to help

[runway1]

I assembled a 6.8 SPC upper, my third, with excellent quality parts...ARP barrel, Bison BCG...,etc. Except the gas block. I thought a gas block, is a gas block, Is a gas block, so I bought a steel gas block off a reputable seller on ebay. After pinning, It axially misaligned with the tube hole about ,028...way too much. Gun wouldn't cycle.

Since I found the problem, I though I'd step up the reliability.......Amazon! Bought a steel block......it misaligned radially, but not too bad. Guns still won't cycle.

What do you suggest? Shall I just just buy a name brand block, e.g. Daniel Defense, Rock River, etc. Or, should I buy a gas block and tube pair from the same supplier? Who do you like?

[FeuerFrei]

The tube doesn't go into the gas key when bolt is in battery?
The gas port/gas block has to be way off before it's not going to blow enough into the gas rube. Could be gas key leak or...
You did test for short stroke? I rd in mag and fire...carrier locks back or not etc..
Checked alignment comparing known good upper with this one?

[wpage]

Go back to the seller for relief or options...

[Munny$hot]

There should be a small gap where the gas block butts up against the barrel to accommodate for the front handguard retainer, so it may not be aligned with the barrels gas port. By hand use a small drill bit to clean out any shavings that be left in the gas port.
Also EBay and Amazon are probably the second worst place to get parts for with gun shows beating them out by a very slim margin.

[runway1]

Go back to the seller for relief or options...

Too late. I bought the low dollar products weeks before completing assy.

[runway1]

There should be a small gap where the gas block butts up against the barrel to accommodate for the front handguard retainer, so it may not be aligned with the barrels gas port. By hand use a small drill bit to clean out any shavings that be left in the gas port.
Also EBay and Amazon are probably the second worst place to get parts for with gun shows beating them out by a very slim margin.

Alignment is good between barrel port and block. Whole different story there...ARP replaced the barrel with excellent service - shipping included.

Ports are cleared, cleared and cleared again.....yeah, agreed.

What are your thoughts on replacement? The pair or just the block?

[runway1]

The tube doesn't go into the gas key when bolt is in battery?

Tube securely in key.

The gas port/gas block has to be way off before it's not going to blow enough into the gas rube. Could be gas key leak or...

Gas key leak? How do I test for that?

You did test for short stroke? I rd in mag and fire...carrier locks back or not etc..

Crap....good point...next test

Checked alignment comparing known good upper with this one?

I have proven uppers. What do you mean by this?

[runway1]

The gas port/gas block has to be way off before it's not going to blow enough into the gas rube.

I'm thinking you're right on this comment but others have said ANY misalignment WILL cause short stroke.

So, does the gas pressure/flow have a large margin or is it at the edge? I.e. alignment is critical?

[Cadre]

Looking at tittle sound like bodily function problem.

[Munny$hot]

Alignment isn't critical but the gas port hole must not be blocked.

The way you can tell if you have a gas leak under your carrier key is to look for black soot around the mating portion to the carrier.

[nate76239]

Sometimes the gas tube diameter can be a little small and gas escapes. Since you are on the second block I would consider trying another gas tube first because it cost less than buying another gas block. I linked a thread below on the second page I had some pictures of a gas leak with a gas block maybe it will help

http://www.calguns.net/calgunforum/showthread.php?t=1099425

[ar15barrels]

others have said ANY misalignment WILL cause short stroke.

False.
Look at how big the gas port diameter is compared to the gas hole in the bottom of the gas block.
The gas block typically has a 0.159" hole while the barrel has around a 0.062" to 0.093" hole.
You have to have enough misalignemt for the LARGE hole to overlap the edge of the SMALL hole before you start to reduce the gas flow.

Gas block to gas port misalignment is probably not your issue.
Weak ammo in a barrel ported for milspec pressure ammo or too heavy of a buffer is a much more common issue.

Run the short stroke test.

[Rgarbarino]

Have you tried using an adjustable gas block? On my 6.8 build I used a syrac adjustable block. It was a little spendy but I could dial it in to get perfect cycling.

[runway1]

False.
Look at how big the gas port diameter is compared to the gas hole in the bottom of the gas block.
The gas block typically has a 0.159" hole while the barrel has around a 0.062" to 0.093" hole.
You have to have enough misalignemt for the LARGE hole to overlap the edge of the SMALL hole before you start to reduce the gas flow.

Gas block to gas port misalignment is probably not your issue.
Weak ammo in a barrel ported for milspec pressure ammo or too heavy of a buffer is a much more common issue.

Run the short stroke test.

I don't think so. Chrono measured ~2650 with a 110g bullet. That's plenty for a 6.8. Same rounds run fine in my other gun.

Also, I switched lowers/uppers between a .223 Wylde I built that runs great
(son has the other 6.8 in OH). The .223 runs fine on both lowers, the 6.8 did the same thing on both lowers....so I'm thinking the buffer spring is not a factor.

[runway1]

Run the short stroke test.

What do you mean by this?