A Question for All Kimber CDP Owners

[CCWInstructor]

Originally Posted by moosehog

Help me understand why the 4in compact/pro requires more frequent changing than the ultra 3in model. I would have thought just the opposite??

4" has one spring. 3" has two springs.

[KimberDude]

Originally Posted by inboost

Perivouslty I posted in reference to the 4" Kimber guns that:


Turns out this is a mistake! In re-reading my notes the stock spring force for the 4" gun is 22 pounds, NOT 18 pounds (that was for a .38 super gun). This puts the springs I measured at a much lower rate than advertised even when brand new! So I called in to Wolff and spoke to a patient and helpful tech who gave me the scoop:

"Kimber installs the 22# outer (larger) spring of a Colt Officer's model in their 4 inch guns, our stock number 49022"

This is a problem because the Officer's Model is a 3.5" gun with an appropriately smaller working space. This equates to a spring that exerts a much lower force when the slide is at rest resulting in a gun that has difficulty closing on return to battery. The amount of compression is also less so the force at full recoil position is proportionately lower. Wolff designed and released their "XP" spring which delivers a 20 pound force with a substantially higher in-battery force as seen in my testing (they added five coils and stiffened the spring). My Wife's gun actually does have some trouble going back to battery and requires a gentle bump on the slide from time-to-time so I'm looking forward to testing out the new "XP" spring.

Wow inboost! Thank you for sharing. It looks like you are the man for these questions!

[IAmASensFan]

Originally Posted by KimberDude

The Kimber manual recommends to change the recoil spring every 800 rounds. I have currently put 1200 rounds through my Kimber Compact CDP II and have not yet changed the recoil spring. Realisticly, how many CDP owners here have religiously changed their recoil spring every 800 rounds? Will it harm the firearm, since I did not change it 400 rounds ago? If not, how often do you CDP owners change your recoil spring? Thanks for your guys' help in advance!

I've never changed my recoil spring.

AMS_Mike would be proud that I change my oil as required, though.

[inboost]

Originally Posted by moosehog

Help me understand why the 4in compact/pro requires more frequent changing than the ultra 3in model. I would have thought just the opposite??

Moosehog,

I too thought this funny, but the tech at Wolff I spoke with gave a very convincing answer. The spring shipped with the Kimber 4" guns is simply not the right one because it is too short even when it's new. This means that any deterioration in spring force is amplified by the fact is wasn't enough when it was new. The 3" guns have a dual spring system that has a better choice for spring lengths and rates.

Single rate springs in general are simple machines, the more you compress them the more they fight back, and it's a linear response. The equation for spring force is:

Spring Force (lb) = k * x

where:

k is the spring constant in pounds/inch
x is the amount you have compressed the spring in inches.

When you choose a spring you need to pick both it's rate and it's free length. The free length is the length of the spring as it sits on your bench. The free length to rest position length determine the 'pre-load' the spring will deliver to the gun. This is the force you feel when you grab the slide and just begin to open the action. As you draw the slide to the rear you feel more and more resistance due to the compression of the spring.

For example:

a spring with a free length of 3.5" and a spring rate of 7 pounds per inch is loaded into a Kimber Pro Cary II HD. The length when it is installed is 2.8". This means the spring is compressed 3.5 - 2.8 = 0.7 inches. The spring force is equal to the rate times the distance the spring is compressed so we see that 0.7 * 7 = 4.9 pounds. You then pull the slide back to it's stopping point you find it moved back 1.88 inches more. Now the spring has been compressed a total distance of 1.88 + 0.7 = 2.58 inches. Multiply this by our rate of 7 pounds per inch to get 2.58 * 7 = 18.0 pounds. Funny thing is Kimber calls this a 22 pound spring.

Well, it actually is a 22 pound spring if you compress it 3.1 inches so it's compressed length is now 0.4 inches. The gun this spring was originally intended for, the Colt Officer's Model, is a 3.5" gun and does just that.

When a spring is compressed many times it begins to show wear by not returning to it's original length because the coils begin to collapse. The fatigue is irreversible, so attempting to stretch the spring back out actually weakens it even more.

If the spring gets shorter over time the effect is that the force generated when it is compressed the same amount is less. This means less energy to push the slide back forward, strip a round from the mag, and articulate it into the chamber. The typical gun symptom is failure to return to battery.

[inboost]

I have yet to test the 4" gun, but yesterday I was able to give the Wolff springs a good run for their money. I went through Greg's Advanced class and put 428 rounds of 230gr FMJ through my Ultra CDP II. I had rebuilt the recoil spring system with the tools I developed (see previous posts) and used the Wolff #51818 spring set. The good news is that the gun ran flawlessly without a single functional issue. The not so good news is that the spring pressure has fallen below that of the Kimber unit it replaced which had 1500 rounds on it. A call to Wolff has resulted in a set of their 21# springs for the gun coming my way free of charge with the invitation to send my other springs in for testing on their equipment. The guys at Wolff wanted to make a point that the 3" guns we are shooting are pushing the envelope trying to absorb this much recoil in such a short distance while still maintaining reliable cycling. These are high strung little guns and extra care should be taken with one that frequents the range more than one 72 round qualification at a time. Ammunition choices along with total round count must be used to gauge the useful life of the springs.

The basics:

For the Wolff #51818 spring set

• Battery pressure = 6.3# new / 4.9# @ 400rds [down 22%]
• Full compression pressure = 17.3# new / 15.8# @ 400rds [down 8.7%]

For the Kimber springs

• Battery pressure = 5.9# new / 4.2# @ 1500rds [down 29%]
• Full compression pressure = 18.7# new / 16.2# @ 1500rds [down 13%]

If the data I have collected is any kind of real measure of spring performance, I'd say the Wolff 18# set was not quite up to spec as new and has degraded an acceptable amount into an unacceptable rating after 400 rounds. I look forward to testing the 21# set and expect that with a little luck it will be a better performer and the spring I will choose to run in my gun.

[spc]

Nice update Inboost. Sounds like Wolf is taking good care of you.

spc

[El Gato]

What a great explaination....
I have been involved in guns and shooting for over 30 years... worked as a warranty repairman for a manufacturer and have been shooting and teaching for more years than I want remember... I learn something on this site on a regular basis like almost everytime I visit... INBOOST... great Post!... you move us all a step away from ignorance...

[inboost]

I've mailed off my used Kimber & Wolff springs to Wolff for testing. Hopefully what they find either backs me up or at least gives me the ability to calibrate my rig to match their findings.

[AMS MIKE]

Originally Posted by inboost

I've mailed off my used Kimber & Wolff springs to Wolff for testing. Hopefully what they find either backs me up or at least gives me the ability to calibrate my rig to match their findings.

now THAT is cool.

[swdr]

Very nice explanation, Inboost.

[inboost]

I received my complimentary spring kit from Wolff this weekend and put it to the test today. The new spring assembly is strong and difficult to work with. I managed to bust my recoil rod when I got too aggressive with it, snapping the 6-32 threaded end clean off. Luckily our local hero bailed me out so I could continue the test.

Wolff #51821 (21#) retrofit spring set as new (Kimber is rated 18# stock)

• Pounds of force with slide at rest position = 9.6# (New Kimber = 5.9#)
• Pounds of force at full recoil position = 22.0# (New Kimber = 18.7#)

These are stout!

[DVSmith]

Originally Posted by inboost

I received my complimentary spring kit from Wolff this weekend and put it to the test today. The new spring assembly is strong and difficult to work with. I managed to bust my recoil rod when I got too aggressive with it, snapping the 6-32 threaded end clean off. Luckily our local hero bailed me out so I could continue the test.

Wolff #51821 (21#) retrofit spring set as new (Kimber is rated 18# stock)

• Pounds of force with slide at rest position = 9.6# (New Kimber = 5.9#)
• Pounds of force at full recoil position = 22.0# (New Kimber = 18.7#)

These are stout!

Will it cycle? I am really anxious to here the results.

Great posts inboost! Next, can you discuss the physics of the forces of the projectile, casing, extractor and ejector and why a 21# vs an 18# spring even matters?

Is there a doctorate thesis in there somewhere?

[inboost]

I ran the new Wolff 21# spring set (#51821) on Saturday through a 200 round break-in on my Kimber CDP II Ultra. I used my own reloads which are 230gr copper washed bullets loaded to emulate Winchester White Box ammunition (830FPS in 5" gun). In the first few magazines the last round tended to try and turn around and head back into the magazine (short-stroke on the slide). I also had a few FTRB events. After about 50-60 rounds the gun ran 100% in my strong hand, but when I moved to off-hand it managed to choke again several times. By about the 150~175 round mark I was unable to get the gun to malfunction even when I completely removed any recoil support by holding the gun as loose as I possibly could. I had two other shooters try the gun after I completed my tests to my satisfaction and they too experienced 100% functionality.

I put the recoil assembly in my test rig today and found the following:

• Overall length had shortened from 3.03" to 2.88" (fell 0.15")
• Battery pressure fell from 9.6# to 7.9# (delta of 1.7#)
• Full recoil position pressure fell from 22.0# to 20.2# (delta of 1.8#)

So keeping in mind that the factory spring rating is 18# we're still a little high on both battery and full recoil positions, but considering the performance of my reloads and the fact that many of us carry pretty hot defense loads in these gun I feel that at worst case it's a non-issue, but is probably a plus in that it may save some frame wear.

I also called up Wolff Springs and got reports back on my springs that I sent in. At first I was very surprised at the results. I was told that the 1500 round used Kimber springs which I measured to be 16.2# were to both my and their surprise 18.0# on their test rig. I was also told that the Wolff replacements that I put 400 rounds on which I measured at 15.8# were actually 19.1# by Wolff's measurement. I was told that this was unusual as they normally keep a tolerance of about +/- 4~5 ounces per spring set, putting this particular set over 12 ounces heavy of their normal maximum tolerance.

So we both agree it must be due to our measurement routines somehow differing. It was good that I sent in photos of my test rig along with my data as this prompted a quick description of the Wolff test rig. Both springs are placed on a single rod and compressed as a pair to 1.25" total height. Remember that when I am taking measurements I am using the actual recoil rod assembly as it sits in the gun. Basically Wolff tests the springs in parallel while I am testing them in series (which is how the gun runs them). I intend to build a rig to test these springs the way Wolff does to see if I can duplicate their results. The springs which I sent it are being returned to me so I can use them to calibrate my rig. Again I have to say the guys at Wolff are great and the customer serive for such a low cost item goes far beyond what I'd ever expect of any company. We have all spent hundreds if not thousands of dollar on something only to be patched through to India with no resolve at the time of the phone splintering against the wall. Way to go Wolff!!!

[AMS MIKE]

^ ^ ^ ^ you are freakin AMAZING!

Thanks for the update. Sounds like an A++++ for Wolff's service and responses!