I was searching the boards for information on replacement springs and came up short. Thus this post. First I'm going to cover how springs are described and rated, and how that might apply for us. Then I'm going to summarize what has been posted about what replacement springs to use for different guns, and where possible I will list data for those springs and indicate what it means for relative power. Due to the number of posts involved, I'm not listing credit or links for any of the suggestions - just assume none of them were me. Search the boards if you really want to know who said what. (If you have an objection to one of the suggestions, it was probably already posted in the original threads. In some cases I quoted contradictory suggestions, because that's what I found.)

* The general method for replacing springs seems to be: take the spring out of your gun. Compare it to other springs. When you find one that looks close, but is better, use it. Not bad advice, if you have a wide selection of springs to compare it to.

* One poster suggests: use a spring 6/8 the size of the original spring, but at least 3 gauges thicker. This thickness gives you the resistance, and the shortening will give you less stress on your plastic internals.

RATING SPRINGS

The lists are useful, if you have the listed stores and the listed springs are sold with the listed labels (which they aren't always). But it would be ideal to have an online resource, not to mention some way to tell whether the spring you got is really going to make a difference, and how much.

Century Spring Corp ( www.centuryspring.com ) sells an extremely wide variety of springs, in fact too many unless we understand what the specs on their site are. Personally, I was trying to find a replacement spring for the Longshot, so that is my example. Same considerations apply for other springs. It seems like it would be useful to have a collection of spring data so you could get your replacement in advance, and not leave your gun disassembled for days while trying to find the right spring. But I digress.

The Longshot spring has the following specs:

* Outer Diameter (OD): 1 3/8" (1.375") - this is the width of the spring. Your replacement should be close. If it's too big, it won't fit in the plunger.

* Free Length: 5.5" - this is the length of the spring when it is not under pressure. Note that, when it is installed in the Longshot, the spring is always compressed, never longer than 4.75". So a spring as small as 4.75" won't rattle around.

* Rate: estimated 2.0 lbf/in - see below.

* Solid Height: 1" - this is the height of the spring, when you squash it all the way. If the solid height of your replacement is larger, the gun may not cock properly, as the spring can't compress enough.

* Wire Diameter: 1/16" (0.0625") (approximate) - this is the thickness of the wire.

* Total Coils: 13 - number of times the spring goes around.

* Material: Steel (maybe with a zinc finish?) - duh.

* Ends: Closed - an open spring is just a coil, with the ends continuing at the same angles as the rest of the spring. A closed spring bends them back against the adjacent coil, which helps even out the force the spring places on the adjacent surfaces. That will be better for our plungers. Closed and ground means the ends are also ground down so the end is even flatter. Probably it doesn't make much difference for us whether our replacement is Closed or Closed & Ground.

These are not super precise measurements (I don't have any calipers) but for our purposes they should do.

The overall strength of the spring is expressed as the Rate (or spring constant). The Rate is calculated as the ( force / deformation ) or, in simpler terms, the pounds of force required to compress the spring by one inch. I estimated it for the Longshot spring by compressing the spring with a 2 lbs weight (it compressed to 4.5" length) and a 5 lbs weight (it compressed to 3"). The hard part is compressing the spring without it springing away before you measure it. It also helps to have large lead weights lying around.

The rate (or spring constant) is:

k = F/(Lfree-Ldeformation)

where

F = force

Lfree = free length

Ldeformation = deformed length

So the first measurement gives us:

Rate = k = 2 lbs/(5.5"-4.5") = 2 lbf/inch

and the second gives us:

Rate = k = 5 lbs/(5.5"-3") = 2 lbf/inch

Normally the rate will be constant over a range of 20% to 80% of the deformation range, so calculating at different values is mostly just a check on our accuracy. We got the same value for both measurements, so we're doing good. At very small and very large deformation values the rate varies more, but we can probably ignore that.

In general a higher rate means a stronger spring. Larger diameter wire and more coils will increase the spring rate, while a larger spring diameter will reduce it. But we are more or less locked into a given spring diameter. The material will also change the rate.

When looking at springs in general,

* higher rate = higher power, but also higher risk of damage to the gun.

* others have recommended a free length 75% (that's 6/8) of the original spring, in order to reduce stress on the plunger. If all else is held equal, however, this will reduce the power.

The power of the spring in the gun is just the reverse of the above equation:

Force = Rate * (Lfree-Ldeformation)

Ldeformation here is the length of the spring when it's compressed in the cocked position in the gun. You need to look at the spring inside the gun to determine this value.

(This is an approximation, assuming the rate is constant over the range we're using, which it may not be if the spring is compressed all the way down. But it should be good enough.)

From this equation, we can see that, if both springs are the same length, doubling the rate will also double the force. It also means that reducing the length of the spring will reduce the force. If we assume the normal Longshot spring compresses to, say, 1.5" (which looks about right), and that k is still ~2 lbf/inch at that compression (likely), then

F = Rate * (free length - compressed length)

F = 2 lbf/inch * (5.5"-1.5") = 8 lbs of force

(It's worth noting that this is the force exerted by the spring, not the force exerted on the dart. The force on the dart depends on the air system and I am making no attempt to calculate that. But bear in mind that comparisons of force are really only useful between different springs for the same gun, not between different guns which will have different air systems and different barrel lengths.)

Looking at the Century Spring Corp website ( http://www.centuryspring.com/ ) I see a couple good candidate springs:

* CSC S-3002: OD 1.375" (same), free length 5.38" (slightly shorter), rate 3.3 (~1.6x), solid height 0.690" (smaller), wire diameter 0.077", total coils 8, stainless steel, closed ends, no finish. This gives us:

F = 3.3 lbf/inch * (5.38"-1.5") = 12.8 lbs of force -- about 1.6 times the stock spring

* CSC S-160: OD 1.296" (narrower), free length 4.5" (short), rate 5.4 (~2.5x), solid height 1.15" (larger - might be too big?), wire diameter 0.092" total coils 11.5, stainless steel, closed ends, no finish.

F = 5.4 lbf/inch * (4.5"-1.5") = 16.2 lbs of force -- about 2 times the stock spring

* CSC 12060: OD 1.359" (slightly narrower), free length 4.880" (shorter), rate 6.30 (~triple), solid height 1.010", wire diameter 0.092", total coils 10, spring steel, closed ends, zinc finish.

F = 6.3 lbf/inch * (4.88"-1.5") = 21.3 lbs of force -- about 2.66 times the stock spring

* CSC 10363: OD 1.375", free length 4.75", rate 6.8, solid height 1.07", wire diameter 0.097", total coils 11, spring steel, closed and ground ends, zinc finish.

F = 6.8 lbf/inch * (4.75"-1.5") = 22.1 lbs of force -- about 2.75 times the stock spring

We can also compare some springs suggested by other posters.

* The spring from the Big Bad Bow can be used in the Longshot. We only have limited data (see below) but roughly:

F = 5 lbf/inch * (5.2"-1.5") = 18.5 lbs of force -- about 2.3 times the stock spring

* Ace Hardware Spring #69 (actually by Hillman Fasteners). Some posters suggest that it should be cut down; one poster noted that it broke his gun. Measurements are approximate.

Hillman Fasteners #69: OD 1.25", free length 4.625", rate estimated ~19, solid height ? (too strong to compress by hand), wire diameter 0.125", total coils 10, closed ends.

F = 19 lbf/inch * (4.625" - 1.5") = 59 lbs of force -- about 7.4 times the stock spring

It's no surprise that this spring is too much for the gun.

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So, how powerful a spring do you *want* for a gun? For the Longshot, the Ace #69 spring exerts more than 7 times as much force as the original Longshot spring; this has been shown to be too much force (can break the gun). The BBB spring exerts about 2.3 times as much force, and there are no comments about it damaging the gun. For the Nitefinder, the two most common replacement springs exert 5-6 times as much force as the original spring; after putting one of these in a Nitefinder to test it out, I suspect this is near the breaking point and will reduce the longevity of the gun significantly, but that's just speculation. I would guess the ideal is 3-4 times as powerful as the original.

This will also be affected by other factors, of course. For example, after replacing the spring in the Nitefinder with the Handyman 9713, but making no other modifications, you can see the darts fishtailing in flight. There is too much power for this short of a barrel. The spring will be more beneficial in conjunction with a barrel replacement.

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REPLACEMENT SPRINGS

If you're just looking for those little springs for the catch/trigger, this post is pretty irrelevant, but:

* Try the spring out of a bic pen.

NITEFINDER

* Ace Hardware, spring #49 in the hardware bins.

* c 386, came in a pack of two, needed to be cut down. (Didn't specify brand, store, etc.)

* 7/8 x 4 x .080", found at Farm and Fleet, two to a pack, package was yellow at the top of the plastic bag.

* SP-9713 Handyman Springs from Home Depot.

* I put the Longshot front gun spring in my NF and left the NF spring in for a combo.

* Spring from a Tek 9, in addition to the original spring (so there are two springs in the gun).

* Spring from a Big Bad Bow (reinforcement is recommended).

Nitefinder Spring

OD: 13/16" (0.81")

Free Length: 3.25"

Rate: estimated ~2.5 lbf/in

Solid Height: 1 1/16" (1.06")

Wire Diameter: 1/16"

Total Coils: 15

Ends: Closed

F = 2.5 lbf/inch * (3.25"-1.44") = 4.5 lbs of force

Handyman Spring SP-9713 (from Home Depot)

OD: 7/8" (0.875")

Free Length: 4"

Rate: estimated ~9 lbf/in

Solid Height: 1.5" ?

Wire Diameter: 1/16"

Total Coils: 13

Ends: Closed

F = 9 lbf/inch * (4"-1.44") = 23 lbs of force

Ace Hardware Spring #49 (actually by Hillman Fasteners)

OD: 13/16" (0.81")

Free Length: 3 7/16" (3.44")

Rate: estimated ~13 lbf/in

Solid Height: 1.5" ?

Wire Diameter: 1/16"

Total Coils: 11

Ends: Closed

F = 13 lbf/inch * (3.44"-1.44") = 26 lbs of force

Big Bad Bow spring:

See below for data

F = 5 lbf/inch * (5.2"-1.44") = 18 lbs of force

BIG BAD BOW

* The BBB spring is in between 5 1/8" and 5 1/4"

* Spring 884 from Century Springs is great for BBB's, when cut down a bit. I am consistently getting 105' with it.

* Original spring is 15 lbs at 3" compression. (k = 5 lbf/in, see below.) Modder replaced it with a spring twice as powerful: Handyman Spring 9713 (specs listed under Nitefinder). It is pretty hard to cock back, though. This spring is too short, so used a PVC spacer to fill in the extra space, plus a metal washer to reinforce the BBB against the spring.

Big Bad Bow

OD: ?

Free Length: between 5 1/8" and 5 1/4"

Rate: estimated 5 lbf/in

Solid Height: ?

Wire Diameter: ?

Total Coils: ?

Ends: Closed

Century Springs CS-884

OD: 1"

Free Length: 8"

Rate: 5.60

Solid Height: 2.41"

Wire Diameter: 0.091"

Total Coils: 25.5

Ends: Closed

LONGSHOT

* A 4-3/4" section from an AR-15 action spring. You can buy them for $2-$3 from any gun shop or for $4.50 from http://www.bravocomp.....ne action.htm

* You need a spring with roughly a .080" wire diameter that can fit over a 3/4" rod. Ideal length is around 4-3/4" with upper limit of 4-7/8".

* You need a cut down #69 (at Ace Hardware), or a stretched and tempered ,... 52 I think?. the 69 will work whole, but you will have to re -enforce the trigger plate pretty well.

* Don't use ace hardware spring #69 (I think). That beast of a spring broke my longshot.

* Spring from a Big Bad Bow.

* Nitefinder, Firefly, ACE #49 Springs, and Handyman Springs are generally used in addition to the stock Longshot Spring, or in addition to some other replacement spring.

Longshot primary spring

OD: 1 3/8" (1.375")

Free Length: 5.5" (but compressed in the gun to a max of 4.75")

Rate: estimated 2.0 lbf/in

Solid Height: 1"

Wire Diameter: 1/16" (0.0625") (approximate)

Total Coils: 13

Ends: Closed

F = 2 lbf/inch * (5.5"-1.5") = 8 lbs of force

(Replacement candidates: CSC S-3002, S-160, 12060, 10363)

The Big Bad Bow spring (based on data above) will give roughly F = 5 lbf/inch * (5.2"-1.5") = 18.5 lbs of force.

Ace Hardware Spring #69 (actually by Hillman Fasteners)

OD: 1 1/4" (1.25")

Free Length: 4 5/8" (4.625")

Rate: estimated ~19 lbf/in

Solid Height: ? I can't compress it that much by hand!

Wire Diameter: 1/8"

Total Coils: 10

Ends: Closed

In the Longshot, F = 19 lbf/inch * (4.625" - 1.5") = 59 lbs of force <--- no wonder this breaks some people's guns.

Longshot sidearm

OD: 5/8" (0.625")

Free Length: 2 5/8" (2.625")

Rate: estimated 2.5 lbf/in (VERY approximate)

Solid Height: 0.5"

Wire Diameter: 1/32" (0.0313") (approximate)

Total Coils: 9

Ends: Closed

(Replacement candidates: CSC 71739, 71875, 71887)

MAVERICK

* Spring from a Dart Tag 10.

* The maverick uses a very small flimsy piece of plastic to pull back the plunger. It is what the silver colored rod pushes back on. If the spring is too powerful, the plastic will simply snap off, making the maverick useless.

Maverick primary spring

OD: 3/4" (0.75")

Free Length: 2.5"

Rate: estimated 4.4 lbf/in (VERY approximate)

Solid Height: 3/8" (0.375")

Wire Diameter: 1/32" (0.0313") (approximate)

Total Coils: 7

Ends: Closed

CROSSBOW

* The spring from the Defender T3's Arrow Shooter.

**Edited by jhd, 20 July 2007 - 09:48 AM.**