saiga .223 firing pin dimensions/spring specifications. (modifying ak7

[tastyratz 0]

so I am building a custom ak74 in .223 and I really would like a sprung firing pin. It seems like something simple to do with a few minutes on a lathe... I just need specifications to work with.

 

Does anyone have any idea on spring dimensions? uncompressed length...od...id... wire etc?

 

what about firing pin dimensions?

 

I have a funny feeling I can pick up a $5 spring from mcmaster carr, drill out the ak74 bolt, and turn down the firing pin a little to accomodate. 10 minutes on a lathe and 5 bucks? sounds like a solution... and I am sure this could open up replacements to saiga owners as well.

 

 

If I could get my hands on a saiga pin for a few hours I could easily do it but sadly, I can not.

[mancat 2,368]

My stance on this is that you never hear of many problems from any of the other free-floating designs chambered in .223/5.56, including the AR/M16/M4 series, Mini-14, and others. Some people have claimed that the AK has a stronger force placed against the primer while chambering a fresh round, which can cause it to pierce the primer and slamfire. I don't really understand this theory, since it would also mean that any other brass-cased, soft-primered cartridge used in the AK action would have problems with slamfires, yet you rarely hear of such a thing.

 

5.45 and .223 bolts disassembled

 

Firing pins have identical shank width and total length. As you can see, the .223 pin has a T-shaped spring retainer at its end. The spring is retained against this by a small collar ring, which I assume is there to prevent the end of the spring from being mushroomed against the flat retainer, and to prevent the pin from flexing in the enlarged spring retaining channel when struck by the hammer.

 

Since the 5.45 pin doesn't have any extra meat here to machine down, you would have to add material to the pin here and then shape it back down to provide the rearmost pin retaining surface. The pin is hardened steel so you would have to account for this if you weld onto it. I can't say any more than that because I'm not a welder.

 

The tail of the bolt has an enlarged channel to house the spring, and to act as the forward pin retainer. The bolt channel is then a standard internal diameter ahead of this larger part of the pin channel. You would need to enlarge this short length of the bolt tail channel in the 5.45 bolt tail to accommodate and retain the spring.. Obviously this isn't a super-precise machining job, but the enlarged channel needs to end in a 90-degree step-down to properly retain the spring without damaging it.

 

Spring length: 33.9mm

Spring diameter: 5.1mm

Spring channel depth (inside bolt tail): 27mm

Spring channel diameter (inside bolt tail): 5.25mm

Spring retaining collar diameter: 5.2mm

Firing pin spring retainer width (T-shaped end of the firing pin): 4.9mm

 

Have fun.

 

Edited February 26, 2013 by mancat

[tastyratz 0]

Excellent, thank you so very much!!!

I posted this as a longshot I am very glad you replied.

 

I did not even know the collar existed from other photos.

Good to know the T is actually wider than the pin OD in general at the end. I do not believe the rearward force will be very great there, with only a couple lb of spring tension I should be able to cut the collar but just press fit it in place on the firing pin (easily seeing no movement).

 

 

2 challenges: the square seat base obviously as an end mill that can cut that depth just... doesn't exist on the shelf. A close OD to the hole size should hopefully keep the spring in line there. It might be unrealistic to expect a square seat although I could grind a flat faced drill.

 

The second part to that challenge: determining an appropriate spring rate.

I fudged a spring calculator and came up with a guess of 0.335N/mm or 1.9lb/in

 

If I switch to an american sizing I can get one that has a .218 (5.53mm) OD and 1.375in (35mm) length all the way from a .76 to a 7.58lb spring rate

http://www.mcmaster.com/#compression-springs/=lowwd6 (only $6.88 for a 12 pack)

I can pick 9657K71 for a spring rate of 1.18lb/in or 9657K74 for a spring rate of 3.62lb/in

 

 

The closest metric I can find on Mcmaster is 94125K428

http://www.mcmaster.com/#cadinlnord/94125k428/=lovttr

Metric Compression Spring Music Wire, 30 mm Overall, 5.5 mm OD, .50 mm steel music Wire and a 1.8lb/in rate

$10 for a 5 pack.

 

That sounds like that might be the ticket provided it would fit around the firing pin OD

[mancat 2,368]

the spring doesn't have much force to it. it only needs to be enough to pull the pin back into the bolt.