Discussion in 'Rifle Country' started by Colt107, Nov 28, 2021.
In the automotive world, mismatched materials, say steel bolt in aluminum block, will cause corrosion so anti-seize must be used.
He didn’t want to go into detail because there are no details which substantiate the asinine claim.
That being said, I've switched entirely to ion bond/DLC Coated Titanium Gas blocks for my builds. V Seven is about the only source for them nowadays since Battle Arms discontinued their very excellent Titanium Gas Block. They always had super snug tolerances...
Theoretically, steel will expand more than Titanium as the barrel heats up leading to a better seal but my reasoning is actually the weight savings up front. It's about an oz & a half but it still makes it handle just a bit nicer.. YMMV.
Minor changes in Temp Co (Temperature coefficient of expansion) show up in larger objects at higher temperatures, or in smaller objects that may have a significant temperature difference.
It is what makes a typical mechanical thermostat work.
Two different metals (often copper and steel) are bonded together, often than coiled to create a larger bond area and increase the area.
The older classic design used a small glass vial with a pair of contacts at one end and nothing at the other, and a tiny drop of mercury.
The movement of the tiny brass-steel mounting coil made the drop of mercury be at one end connecting the contacts, or the other end with no contacts.
This is the reason behind the thermostat requiring a level mountain position on the surface.
Modern solid state electronic thermostats do not have this weakness.
They typically us a diode junction.
The leakage across the junction is very well defined as a function of temperature.
@brickeyee - as an engineer myself, thermal expansion coefficients are not lost on me. However, as an AR builder with literally hundreds of AR’s under my belt, between carbon and stainless steel, aluminum, and titanium blocks, I’ve never witnessed gas leaking as the block heated faster than the barrel. Why? Dunno. Doesn’t really matter - hundreds of data points prove the case.
I'm not sure if the finish matters much
Let's look at this and see if we can learn something . . . .
At 1 rounds every 6 seconds (60 rounds per minute, or RPM), for 15 minutes the barrel under the gas block gets to 350 F, and 15 minutes is long enough for the block to be in equilibrium with the barrel. So, with the following block materials you will have the following fits (assuming the standard 0.7495" diam barrel and 0.7500" diameter block):
- 7075 aluminum: + 0.0019" (4x room temperature clearance)
- Ti6Al4V titanium: + 0.0001" (1/5 TR clearance)
- 304 stainless steel: + 0.0010" (2x RT gap)
- 1137 mild steel: + 0.0001" (same as Ti, this is the material the FSB is made from)
- 41XX steel: + 0.0005 (original clearance)
At 1 round every second (60 RPM) for two magazines, the barrel gets to 800 F, the gas block will not get to equilibrium with the barrel so will be slightly cooler. These are the predicted clearances for the same block material:
- 7075 aluminum: + 0.0036" (7x RT gap)
- Ti6Al4V titanium: - 0.0010" (you now have 0.001" crush on the barrel, that is a significant constriction)
- 304 stainless steel: + 0.0015" (3x RT gap)
- 1137 mild steel: + 0.0005" (original clearance)
- 41XX steel: +0.0005 (original clearance)
Throwing out aluminum, because it has erosion issues, you can see the best choices are 1137 (the original material chosen) and 41XX alloy steel as these maintain some clearance, while staying within the original 0.0005" gap.
lysanderxiii well done and many thanks.
I no longer have access to many of my detailed ME design material.
Working over wide temperature ranges at tight tolerances has always been a bit of a nightmare.
I wonder why third degree burns don’t happen at every 3 gun or PRS match? 350F surface temperature is pretty damned hard to achieve.
It doesn't have to make sense. LGS momo can't answer your question, because he doesn't know why. Nobody does.
If you use a smooth mirror finished melonite barrel, with a melonite gas block, it's a major pita keeping the bolts and gas block tight. Use for a SPR only if both are visible outside the rail and you want a consistent black appearance. If the rail covers the gas block, use a phosphate block. (Could just be cheap aftermarket parts and tolerance stacking. But I've had plenty of good luck with cheapo builds.)
The rough finish is like a lock washer on a screw bolt, maybe. ???
Round count is not high enough at most 3 gun matches. I think the most I have ever shot in a single stage was just over 70 rds the handguard was unpleasantly hot.
Data point of one so..., using a thermal camera I measured gas block/muzzle temps while firing 30rd magazines of 556 fired as fast as you can easily do with a semi-auto. It was a fairly low res thermal camera (first gen Seek Thermal that plugs into you phone) Gas block to ~165 F after the first magazine, ~230 F after the second, ~340 F after the third, ~460 F after the fourth, 550 F after the fifth and peg the thermal camera at its hottest measurable temp of 626 F before then end of the sixth. ~63 F ambient temperature.
I bet a 308 gas block would climb even faster.
So...gas port erosion is caused by expansion/contraction? He’s never found an eroded steel gas block in 11,000 samples?
I’d like to see the corresponding barrel that gas block was fitted to. Expendable parts are expendable.
Yup. Ditch the gas block with the barrel when it’s burned out, and that kind of thing is never a concern. Which, in itself, means barrels shouldn’t be employed past their useful service life if you expect peak performance from your rifle either. If a guy puts 10,000 rounds through a barrel at 40¢ per, eating up $4,000 in ammo cost, you really can’t complain when even a $300 barrel and a $100 gas block is also consumed...
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