Accurizing and Dwell Time

Pr589

Pr589

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Has anyone experimented with the dwell time of their M1A/M305 gas system and if so what did you find?

I was checking the dwell time on my Norc and while the recommended dwell is on the order of 0.005" to 0.010" mine looks to be 0.12" or more...:sok2 On top of that, the Op Rod was clearly coming in contact with the gas cylinder.

Even with these sub-optimal issues, I still was shooting between 0.9" and 2.0" last weekend but seeing more flyers and stringing than I'd like (hoping that fixing the Op Rod collision with the gas cylinder will help). So with a somewhat decently shooting gun, is it worth taking a whole 1/8" off my gas plug to improve my dwell time?

p.s. for folks not familiar with dwell time, see the link below:

http://forums.thecmp.org/archive/index.php/t-28025.html
 
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Hmm.. I wonder if adjustable gas plug would apply to lengthen the dwell time.
Obviously the gas is mostly bled out and only apply enough pressure to the piston to complete the cycle.
If it is somewhat in the ball park in delaying the oprod travel rearward, I think it would be an easier to apply test.
 
Instead of removing material Off the plug, experiment by shimming the plug, I've made aluminum shims on the lathe to increase dwell, but don't go too far, you don't want the oprod making hard contact on the bolt roller when in battery.
You could just crudely cut some shim stock too to test, then remove that much off the plug.
 
why would you alter the plug ?
the adjustments I've made on rifles that needed it were to the piston tail or in some cases the internal volume of the plug itself.
I've never adjusted plug body overall length to solve cylinder/oprod contact.
or maybe I'm misunderstanding the post.
I've built 18.5" rifles with major short stroking problems, despite correct gas port size and alignment and all new top end parts.
the solution was to slightly increase internal plug volume so the pressure built up just a bit more before the push begins.
dwell timing adjustments is getting into the advanced side of building/tuning these rifles but is well worth reading up on as it's often not discussed.

something to remember though.... Chinese pistons, plugs and cylinders vary greatly in dimensions and alterations to Chinese parts using data collected using USGI spec or equivalent parts may not be compatable when taking tools to the gas system to alter dwell time. voodoo magic this stuff can be :D

now if I'm readin the op's post correctly...... the oprod is pushing the piston in far enough to allow the oprod to hit the cylinder itself?
adjusting the plug or piston won't solve that.
putting the cylinder in a mill and shaving some material from the tail end to give you some piston protrusion would fix that no? if there is sufficient material there to do so.

edit to add: just read the link you posted, that's a good article by Tony Ben, I had not see that one before.
Art Luppino did a similar article/discussion years ago. was trying to find a link to it but no dice.
 
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Good suggestions re. shimming the plug, chopping the gas piston or increasing internal plug volume.

John172 - any suggestions for readily available shims e.g. standard sized washers? If not I will have to find material to make a shim.

45ACP - I bubba'd my gas cylinder (have some spares) with a file to reduce the length to the point where there is no longer contact between the Op. Rod and gas cylinder. Did not have to take much off and was careful to keep it square.
Regarding shaving the piston tail - since the tail of the gas piston comes into contact with the Op. Rod, I understand that there is some risk that taking material off the tail of the piston will expose non-heat treated material with the potential for premature wear. My main worry is if I get this working for me, how often am I going to have to find a new piston.

For me, changing the internal plug volume would be harder to play with than the other alternatives. I think I'm going to try shimming since I can tweak this fairly easily.

45ACP - What improvement (if any) have you found by altering dwell time?
 
Pr589 said:
Good suggestions re. shimming the plug, chopping the gas piston or increasing internal plug volume.

John172 - any suggestions for readily available shims e.g. standard sized washers? If not I will have to find material to make a shim.

45ACP - I bubba'd my gas cylinder (have some spares) with a file to reduce the length to the point where there is no longer contact between the Op. Rod and gas cylinder. Did not have to take much off and was careful to keep it square.
Regarding shaving the piston tail - since the tail of the gas piston comes into contact with the Op. Rod, I understand that there is some risk that taking material off the tail of the piston will expose non-heat treated material with the potential for premature wear. My main worry is if I get this working for me, how often am I going to have to find a new piston.

For me, changing the internal plug volume would be harder to play with than the other alternatives. I think I'm going to try shimming since I can tweak this fairly easily.

45ACP - What improvement (if any) have you found by altering dwell time?
Click to expand...


I'll be honest, when building these rifles, I set dwell time at the 10thou , whenever possible, as a matter of practice.... it's just another step in the process.
where I have made purposefull adjustments to pistons , I have not experienced or heard customers complain about pistons wearing or galling. it's such a minor amount of material generally. Also for altering internal plug volume, I've had to do this mainly with Krieger 18.5" criterion 18.5" and a handful of norinco rifles. in each case, the rifle was short stroking or exhibiting timing issues. increasing plug volume a hair, enable more gas to build pressure in the now larger cavity, prior to the pistons push on the oprod. in essence, cycling the action just a wee bit harder.

as far as taking an "accurate" rifle and monkeying with dwell time to squeeze out tighter groups..... or too try and negate stringing..... I will admit, I have not gone down this path very often and only with my own rifles. I don't think I've done enough statistical recording to say if it was of any benefit.
I have tried to apply some of the dwell timing adjustments , much like tony ben describes, with Chinese rifles...... but dimensional unsimilarities like cylinder, plug and piston volume, diameter ect start to wreak havoc when trying to apply "known US spec values" to adjusting or measuring dwell with a Chinese oprod/cylinder assembly. an interesting comparison is to take a Chinese piston, plug and cylinder, plug them for leaks and fill them with water. measure the amount in each one and do the same for the USGI spec parts and compare notes......

my thoughts on shimming the gas plug..... you will create a situation where the piston now travels further into the cylinder , be very careful you do not shim so much that the piston tail catches or hangs up on the cylinder tail "D" opening..... this will be catastrophic trust me
I have never had the need to shim the gas plug , not a method I've heard of personally

would love to hear tactical teacher and/or tony ben's thoughts on this as well
 
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Thanks for digging that up! I wrote that when I was in M14forum purgatory. Nearly all the info I gathered was from Art Luppino but he didn't want to have his name officially attached to the document. He's kind of funny that way. He did proof read it before I posted it and I made adjustments are necessary.

Tony.
 
DaveMachine said:
http://dlaskarms.com/collections/m14-m305-parts/products/m14-adjustable-gas-lock

Would this achieve the same result? Thoughts?
Click to expand...

no, not really at least not in my understanding of how it functions.
it's a well made unit but I don't think it affords adjustability of the actual gas flow/profile
I've installed and used it on many rifles tho ;)
seems to allow for getting the 6 oclock tight position of the gas lock while at same time allowing the sight base to time correctly with the barrel
it didn't do away with the need to shim ( or roll swage) the gas band barrel boss
 
PR.... Could what you are trying to achieve be addressed at the gas lock?
I've found "grossly" thick chinese gas locks over the years. Reducing to GI thickness would allow deeper seating of plug and this would also apply same to piston. Only thing to watch would be maintaining piston to port alignment.

Thoughts?
I love the technical side of what makes these buggers tick
I'd love to know the cure all to stringing in some rifles too hehehe
 
Thomas - I am the paduan in these m14 tricks. I was reading an article from Art or Tony on dwell time and got curious about my own rifle. On examination I was surprised at how much contact there was between the gas piston and the op. rod.

In my case, the contact occurs when the gas plug is still 1/8 of an inch from being screwed in fully. So I'm thinking that if anything I'd want a thicker gas lock so that the plug did not push the piston in as far. As I understand it, by adjusting the dwell time I'm reducing the contact between the piston and op. rod, trying to delay the action just a bit - though I'll be damned if I understand the physics of how this would improve accuracy...bullet long gone before action cycles.

As the article said, adjusting dwell may improve accuracy, or cause it to deteriorate. So I was curious what others had found.

P.s. Not sure if fixing my op. rod-to-cylinder-contact issue improved my rifle's accuracy but on the weekend I had amazing first shot accuracy, followed by some stringing. In any event, if fixing a big issue like the op. rod colliding with the cylinder did not produce a night and day difference - what effect would something as subtle as dwell time have?

Pps. This is a new unitized gas cylinder that I was hoping would show big improvements over the old shimmed gas system. Can't say I saw big improvements but maybe the accuracy node has shifted. It's all black magic as far as I'm concerned...but I need to do something with my cash.
 
Pr589 said:
Thomas - I am the paduan in these m14 tricks. I was reading an article from Art or Tony on dwell time and got curious about my own rifle. On examination I was surprised at how much contact there was between the gas piston and the op. rod.

In my case, the contact occurs when the gas plug is still 1/8 of an inch from being screwed in fully. So I'm thinking that if anything I'd want a thicker gas lock so that the plug did not push the piston in as far. As I understand it, by adjusting the dwell time I'm reducing the contact between the piston and op. rod, trying to delay the action just a bit - though I'll be damned if I understand the physics of how this would improve accuracy...bullet long gone before action cycles.

As the article said, adjusting dwell may improve accuracy, or cause it to deteriorate. So I was curious what others had found.

P.s. Not sure if fixing my op. rod-to-cylinder-contact issue improved my rifle's accuracy but on the weekend I had amazing first shot accuracy, followed by some stringing. In any event, if fixing a big issue like the op. rod colliding with the cylinder did not produce a night and day difference - what effect would something as subtle as dwell time have?

Pps. This is a new unitized gas cylinder that I was hoping would show big improvements over the old shimmed gas system. Can't say I saw big improvements but maybe the accuracy node has shifted. It's all black magic as far as I'm concerned...but I need to do something with my cash.
Click to expand...

:D
I can relate to being a little fascinated by certain aspects of accurizing these rifles.
I have a loose understanding of dwell time.
It was first introduced to me as piston dwell and of course, like yourself... I had to know.
I'm still learning :D
Doing google searches on the topic really doesn't reveal a whole lot either.
Voodoo magic it is
 
What type of ammo do you use?

I think it's Sadlak that sells the piston with the groove cut into top portion of the piston running through the gas port hole for the purpose of equalizing the gas pressure for smoother more consistent action. Which in turn is supposed to increase the dwell time.

But also says only to use with NM high power ammo and not regular commercial or surplus ball ammo

EDIT: got the info from treelinem14 part description, which also says people have had no problems with surplus. Ymmv
 
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TonyBen said:
I'll put that in the queue for another video. Much better to visualize with pictures or video.

Tony.
Click to expand...

I thought about trying that LOL
I will leave it to you, I suck at videography and such. my attempts at even photo essaying the tale of two rifles builds is pretty dismal HAHA
 
Originally Posted by jonh172
you don't want the oprod making hard contact on the bolt roller when in battery

gnmontey said:
How does one ascertain if this is happening?

M
Click to expand...

With your action locked up in battery, loosen your gas plug, your oprod should move ever so slightly forward.
You can also use a very fine pin punch or something similar to try to roll the roller while in battery, if either method won't move the respective part, then you may have an issue.
 
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