Full Size Uzi: Closed Bolt Info and Pics

[Dan0341]

Here' some info and pics regarding the full size Uzi and the closed bolt conversion. If I've missed anything, or made a mistake, please let me know.

The following pics display a closed bolt setup for the full size Uzi. This closed bolt was modified from a full auto bolt (modification performed by http://uziusa.com/ ). There may be a factory made full auto closed bolt assembly for the full size Uzi, but I have never seen one. If you are familiar with the semi auto bolt design, you’ll see that the full auto closed bolt is very similar to the semi auto bolt. There are two major differences between the two designs: (1) no side rail slot in the full auto closed bolt, and (2) there is a raised lower lip on the bolt face of the full auto closed bolt. In case you’re wondering, the semi auto bolt assembly will fit into a registered receiver Uzi. But, it will function reliably in semi automatic mode only. If one tries to shoot with the semi auto bolt in fully automatic mode, the bolt will jam on the cartridge causing a failure to feed. Reason: the exposed firing pin on the semi auto bolt will interfere with the rim of the cartridge during the feeding process. The raised lower lip on the full auto bolt eliminates this interference.

The pics below will show comparisons to the full auto open bolt only. If you need pictures, or further information on the semi auto Uzi bolt, please click on Uzi Bolts.

The following pic shows a comparison of the bolt face between the open and closed bolt. You’ll see the closed bolt has the same raised lower lip as found on the open bolt. Additionally, there is a firing pin hole as opposed to the fixed firing pin.


This pic compares the bolt rails (not sure if this term is technically correct, but you get the idea). Notice that the front rails have been extended on the closed bolt (refer to red circles). This modification was necessary to keep the sear from holding the closed bolt open like that of the open bolt (This observation was made clear by MuzzleFlash). The red straight lines show where the rails had been removed. Reason: to allow for the striker assembly (on one side) and to allow the sear to protrude upwards (the opposite side).


Next, you’ll see side comparisons of the two bolts. As you can see, the closed bolt conversion includes a modification of the ejection port area of the bolt. MuzzleFlash stated that a possible reason for this additional milling may help increase reliability when accounting for the shorter bolt stroke. That way, the ejected case can clear before the bolt comes forward and strikes it in flight. The red straight lines show where a portion of the rails had been removed and the red circles show where the rails had been extended. Notice the welding and grinding marks inside the circles that were the result of the conversion.


This pic focuses on the rear of the bolt where the floating firing is inserted. The striker assembly butts up to the two small raised pins to ensure proper firing pin protrusion.


Next is the buffer, striker assembly, and recoil spring/operating rod. The buffer and striker assembly are semiautomatic parts that are found on the semi auto Uzi. The recoil spring is a full auto version without the thin, hard buffer. A semi auto recoil spring will not work “as is” with this full auto closed bolt conversion. Reason: the semi auto bolt uses a shorter recoil spring.


The following pic shows three different views of the closed bolt assembly. This is how it looks once it has been inserted into the Uzi. When the striker is “charged” or “cocked,” the front of the rail of the striker (see inside of red circle) makes contact with one side of the sear only. When the charging handle is released, the bolt will move forward and strip a round from the magazine and chamber it. So at this point, the striker is held to the rear by the sear and the bolt is forward with a cartridge in the chamber. This is why it is called a “closed bolt.” When the trigger is pulled, the sear is retracted allowing the striker assembly to move forward and the primer is ignited by the firing pin. When the selector is in full auto mode, the striker will “ride” with the bolt as long as the trigger is depressed. This causes the firing pin to protrude from the bolt face like the fixed firing pin of the open bolt. When the trigger is released, the sear engages the striker and holds it to the rear while the bolt moves forward and chambers a cartridge.

The full auto sear does not need to be modified for this conversion to work, but the closed bolt conversion will not work with the ratchet mechanism installed in the top cover. Reason: the bolt cannot be retracted far enough to disengage the ratchet pawl. An easy fix is to remove the ratchet pawl or use a semi auto topcover. But, you need to be careful to ensure that the bolt/striker assembly has been completely pulled to the rear. If it’s not, and you release the charging handle without allowing the striker to be engaged by the sear, you may have an accidental/negligent discharge.

Conclusions: I’ve found this closed bolt conversion to be extremely fast -- at least double the cyclic rate of the standard open bolt assembly. The rate of fire is much higher due to the limited travel of the closed bolt as compared to the open bolt (the open bolts travels farther into the rear of the receiver). Two reasons for this higher cyclic rate: (1) the additional length of the closed bolt buffer (buffer is 1 and 3/4 inches long), and (2) the rear portion of the striker assembly, that protrudes from the back of the closed bolt, adds at least another 1/2 inch. Taking both of these factors into account, the closed bolt has about two inches less travel than the open bolt version. This decreased bolt travel is what dramatically increases the cyclic rate. The closed bolt conversion is just as reliable as the open bolt. I don’t find myself shooting the Uzi with the closed bolt conversion nearly as much as shooting it with the standard open bolt assembly (I usually prefer a slower cyclic rate). But, when I feel the need to go fast, it’s a lot of fun. Click on the link to see an mpeg comparing rates of fire: Open Bolt vs Close Bolt

Dan

[RoverDave]

Excellent job Dan!! Excellent pictures and detailed descriptions. I'm sure a lot of members would like to have a closed bolt setup like this to try out.

I'm curious why they cut off the right side rail on the bottom of the bolt instead of cutting off the right sear pad like IMI does on the Mini. I suppose it's safer when you put the open bolt back in due to the full size bolt being heavier than the mini bolt.

Also, what's the reason for the front of the rails being built back up? What problem is caused by leaving them in their semi format?

[Dan0341]

Dave, I believe they cut the bottom right side rail so the bolt would be a "drop in" fit. I know that I'd prefer this than remove one of the ears from the sear. I can't say for sure, but I think they added the length to the front rails to give the front of the bolt a greater surface area. With the center rails cut away, there's not a whole lot of metal left on the front. Maybe they thought a greater surface would help minimize wear? I'm not really sure.

Dan

[dan d.]

Dan0341
Great job! Did the closed bolt conversion come in your uzi when you bought it or did you buy it from D&D? Do you know if D&D still makes them and what do they cost?

[RoverDave]

I asked Dan for his permission last night to add this to our library. (And he graciously said yes.) Hopefully I'll get to it this weekend. I need to spend the whole day in a class today so I can apply for my Minnesota concealed weapons permit.

[MuzzleFlash]

Originally posted by Dan0341
...I can't say for sure, but I think they added the length to the front rails to give the front of the bolt a greater surface area. ...

Dan

Those filled in cuts are probably to keep the sear from holding the bolt open? That is where the sear engages the open bolt.

As you pointed out, in FA mode, the striker rides the bolt and the firing pin acts like the fixed pin of the FA. I'm wondering why the ROF is so much higher then. I'm guessing that is because of the buffer and not anything inherent in the bolt design.

The additional milling of the bolt opening may be to accommodate the shorter bolt stroke. That way, the ejected case can clear before the bolt comes forward and strikes it in flight.

Another question. What is the difference in weight of the OB and CB/striker?

Thanks for the pictures. You did a fantastic job!!

[Dan0341]

dan d, I bought the bolt separately from D and D Sales a couple of years ago for my Vector Uzi. I'm not sure if they're still making them and can't remember the exact cost. I'd call them and ask if they are still doing the conversions.

Those filled in cuts are probably to keep the sear from holding the bolt open? That is where the sear engages the open bolt.

MuzzleFlash, you're absolutely right - the filled in cuts on the front of the bolt had to be done so as to prevent the sear from engaging the bolt (just like that of the open bolt). As soon as you mentioned that, "the light bulb went on." I can't believe I didn't see this. Thanks for pointing it out. I'll edit my above text to add this comment.

As you pointed out, in FA mode, the striker rides the bolt and the firing pin acts like the fixed pin of the FA. I'm wondering why the ROF is so much higher then. I'm guessing that is because of the buffer and not anything inherent in the bolt design.

You're correct, the rate of fire is much higher due to the length of travel of the closed bolt (as compared to the open bolt). The open bolt recoils to the rear much farther than the closed bolt. Two reasons for this: (1) the additional length of the closed bolt buffer (buffer is 1 and 3/4 inches long), and (2) the rear portion of the striker assembly, that protrudes from the back of the closed bolt, adds at least another 1/2 inch. Taking both of these factors into account, the closed bolt has about two inches less travel than the open bolt version. This decreased bolt travel is what dramatically increases the cyclic rate.

The additional milling of the bolt opening may be to accommodate the shorter bolt stroke. That way, the ejected case can clear before the bolt comes forward and strikes it in flight

This could very well be the reason. Good stuff - I'll add this to my post as a possible reason for the additional milling of the bolt.

Another question. What is the difference in weight of the OB and CB/striker?

I'll weigh the open and closed bolts this week and get those answers. I'll weigh the closed bolt with and without the striker assembly.

Thanks for the comments!
Dan

[Paul556]

D&D advertises a semi-auto bolt for $84.99 and I think that price requires an SMG bolt in exchange. The work required is less for a full auto than it is for a semi-auto, so the price should be the same or less. Call and ask. They are great guys and are very helpful.

[chrisp929]

that took some time to put this thread together dan .. thanks ... very informative ... great job ...

[amphibian]

Great job Dan!!
I've always wondered about the CB full size Uzi setup.
What about that buffer? I'm thinking the main purpose of it is to keep the striker spring from binding.
I was thinking if there would be a way to get rid of the buffer so that the bolt would have more distance to travel and then you could have a slower ROF.
What do you think?

[Dan0341]

Thanks for the compliments - glad you guys like it.

Amphibian, I agree that the long buffer helps guide the striker spring. It also prevents the guide rod, inside the striker spring, from hitting the back of the reciever. Maybe it's possible to shorten the guide rod so as to be able to shorten the buffer. But, if this is done, then there maybe a chance that the striker spring would bind as the the assembly recoils to the rear (due to lack of guidance for the spring). Unfortuantely, I don't think there is any way to remove a major portion of the buffer while avoiding binding of the spring.

Dan

[RoverDave]

Dan, I was also wondering why you have to use the longer open bolt recoil spring. Have you tried it with the shorter semi spring? Seems like that would also slow the ROF.

[Dan0341]

Dave, the semi auto bolt uses a shorter recoil spring/operating rod because the semi bolt is not milled as deep (for the spring/op rod) as the full auto bolt. If you place the semi auto recoil spring/op rod into the full auto bolt, and place it into the receiver, you'll see what I mean. The recoil spring/op rod is about two inches (give or take) short of reaching the back of the receiver. You need a spacer (lack of better word) to make up for this. Also, I noticed that the semi spring has more tension than the full auto spring when comparing the two side by side (comparing Norinco Uzi recoil spring to my Vector Uzi spring). And, I have used the semi auto spring (utilizing this metal spacer) with this closed bolt conversion and did not perceive any variation in the cyclic rate. There may have been an increased cyclic rate, due to the stronger spring, but I couldn't tell. I've thought long and hard to try and slow this conversion down, but it seems there is no easy answer.

Dan

[RoverDave]

Dan's excellent work is now part of the reference library, available here.

If anyone talks to D&D in the near future, it would be nice if you posted info confirming price and availability so not everyone has to hound then with the same questions.

[Paul556]

I shortened the buffer on my Norinco in an effort to fix the reliability. The striker assembly did end up hitting the back of the receiver to the point it was actually painfull to shoot. I never thought an Uzi would have brused my shoulder, but this one did. I didn't have any problems with the spring when I shortened the buffer. You would have to shorten the guide ad see how it goes. May still work. Hard to say. I am sure it would slow it down a little. You may have to use a spring with an internal guide like the main bolt spring for it to work.

[Dan0341]

Here's the comparison of bolt weights:

Open Bolt: 1lb 8.3oz

Closed Bolt: 1lb 5.7oz

Closed Bolt w/striker: 1lb 9.7oz

Dan

[amphibian]

Originally posted by Dan0341
Thanks for the compliments - glad you guys like it.

Amphibian, I agree that the long buffer helps guide the striker spring. It also prevents the guide rod, inside the striker spring, from hitting the back of the reciever. Maybe it's possible to shorten the guide rod so as to be able to shorten the buffer. But, if this is done, then there maybe a chance that the striker spring would bind as the the assembly recoils to the rear (due to lack of guidance for the spring). Unfortuantely, I don't think there is any way to remove a major portion of the buffer while avoiding binding of the spring.

Dan

I was looking at it again and see that you can't shorten the buffer much also because if you did, the sear could catch the front of the bolt.
I think you could slow it down by adding weight to the bolt like I did to one of my MINI bolts.
I dropped the ROF of my MINI by 100 RPM by doing that.

[Clouseau]

Has anyone found out if D+D is still offering these?
I'd really like one.

My work schedule doesn't allow me to call and email keeps bouncing back.

[Quarterbore]

I am curious if anybody can advise of a cost to do this project? I like the idea of a closed bolt at times when I can't let her rip and shooting semi from an open bolt gun is one heck of a strange thing to get used to for me...

[Paul556]

Quarterbore, Last I knew D&D was charging about $80 to do the mod for a semi bolt. It may be cheaper for an auto bolt as there is less work involved. Add $30 to $40 for an SMG bolt and then a striker assembly. Not sure how much a striker will cost, but I can't imagine it would be more than $100. Here is one for sale on aBay.

http://cgi.ebay.com/ws/eBayISAPI.dll...category=58 8

Good luck.