Balanced pin stacks

[Lelandwelds]

Lots of posts and other places mention the desirability of equal length for the complete pin stack. Why is that a good thing?

[GWiens2001]

Done properly, it reduces wear on the lock and key. It also can balance spring tension.

If you have a long key pin and a long driver pin, the spring is compressed a great deal, especially when the key is inserted. Combine a short key pin and short driver pin, and the stack may be too short for the plug, allowing the spring get damaged when picking or if someone tries a key that is cut too low at that position.

There are no doubt other reasons, but I am just a hobby lock enthusiast.

Gordon

[gumptrick]

When I was taught to re-key residential locks as part of my first job I was instructed to always balance the pin stacks. Our basic procedure was to put the new key in the core, fit the keypins, then remove the keypins and place them on a pinning tray. We'd then choose the driver pins based on the length of the keypins so that all the stacks were the same (give or take a couple thousandths) and then re-assemble the lock. I was never taught why this was done, only that it was standard procedure.

Looking back at it, I can see that having balanced stacks would keep equal pressure on all the springs while the key is out of the lock. However when the key is inserted into the lock then the loading on the springs would be uneven.

Gordon is right that there are certain situations you want to avoid. If you have two short pins then the stack could be short enough that it doesn't get much or any pressure from the spring. And likewise if you have two long pins then you could end up with excess pressure on a spring--or in an extreme case you might not even be able to insert the key into the lock. But these conditions can be avoided by making sure that the pin heights fit between a minimum and maximum range--actually balancing the stacks isn't required to avoid those issues.

Perhaps an actual locksmith can give us some more insight here; my knowledge is limited to what I was taught about rekeying & my hobby experience.

[cledry]

What you both said is correct. This is especially important when master keying the locks. I say especially because when assembling the plug into the core it is unlikely either key will be in the plug so you could potentially end up not being able to insert the key. A workaround if this occurs is to cut the deepest cuts of the change key and master key onto a blank and remove the peaks, sometimes just filing the peaks between cuts will let you insert the key to take it apart.

So if your MK was 348562 and your change key was 526346 you would cut 548566 and it should insert and get the plug back out so that you can correct the issue.

[peterwn]

Done properly, it reduces wear on the lock and key. It also can balance spring tension.

If you have a long key pin and a long driver pin, the spring is compressed a great deal, especially when the key is inserted. Combine a short key pin and short driver pin, and the stack may be too short for the plug, allowing the spring get damaged when picking or if someone tries a key that is cut too low at that position.

There are no doubt other reasons, but I am just a hobby lock enthusiast.

Gordon

Local locksmiths in NZ years ago had to make do with 5 pin cylinders for masterkeying (or customers had to order manufacturer masterkeyed systems from overseas usually UK Yale or Union). A trick was to use a masterkey with deep cuts. The plug would be removed using a follower from a new stock cylinder, wafers inserted as needed and plug replaced. Some drivers would then be too long for the pin plus wafer lengths so the key would be stiff to insert and springs crushed.

[demux]

What you both said is correct. This is especially important when master keying the locks. I say especially because when assembling the plug into the core it is unlikely either key will be in the plug so you could potentially end up not being able to insert the key. A workaround if this occurs is to cut the deepest cuts of the change key and master key onto a blank and remove the peaks, sometimes just filing the peaks between cuts will let you insert the key to take it apart.

This technique also works well if you happen to slip while assembling the lock and the core turns and a couple pins drop in... Only needed to use it once for this, but it was a lot quicker than picking/shimming.

[tpark]

On GMS and Schlage Mortise locks, here's the difference in spring compression between balance and unbalanced stacks
Max balanced Spring Space: 396 On cut 0
Min balanced Spring Space: 155 On cut 9
Max unbalanced Spring Space: 445 On cut 0
Min unbalanced Spring Space: 134 On cut 9

Unbalanced driver size I used was 186, resting height (No key) was 164, I used 340 as the height for the uncut key.

So the balanced stacks are much better for limiting the range that the springs have to compress. I think that balancing the stacks is worthwhile for lock longevity, as the extra compression could cause the springs to fail early. If there's interest, I'll post the code I used to figure this out, otherwise you can take my word for it.

What's interesting is that with SFIC cores, correctly assembled cores have precisely balanced stacks for all chambers.

[Lelandwelds]

On the part about lock longevity, i think I'm hearing it's not just the overall max/min length for any of the pin stacks with the key fully inserted. It is the key being inserted and the stack moving for each cut down the whole length of the key. Dynamic not static.

Seems really basic and obvious to me now. Thanks.