Not only is it ok, but if you know what you are doing, you can get superior results without using hard blows!
Here is an email I sent to a technician who was asking me about stability. We had a quick lesson and then we started talking about bending the pin.
Well, before we could start talking about bending the pin, I needed to counteract the lies he was told by well-meaning people who told him, "You should NEVER bend the pin."
Maybe you think, like this technician did, that bending the pin is piano technician blasphemy!
("Bending" is an engineering term. I use it to mean "elastic deformation". In other words, we apply a bending force only. We do not permanently bend the pin. That, of course, is wrong. 
)
Would it help if I could convince you that not only did a famous piano technician design a tuning hammer that can eliminate bending, but that he uses this very same hammer to bend the pin on purpose!
(He actually invented the hammer to separate out the twisting and bending so that he could choose which to do, and when. It is other technicians who don't understand this who spread that false information that he designed it to eliminate bending. He did not.)
Here is the email:
Great class today. Good work. Most people would have been bored a long time before the end and you got me to continue after the hour by asking a great question:
"How do you leave the NSL tension high?"
Here's some info to help break the myth in your mind that we should never bend the pin. Be warned though. Don't share this with other technicians, especially if you can't confidently explain why and when it is ok and even desirable to bend the pin, and if you are not ok if they don't believe you.
Dan Levitan not only designed a tuning hammer that can bend (he calls it tilt) a pin on purpose, but he uses this same hammer to do just that; bend the pin.
Dan Levitan's book. (Just so you know how highly respected he is.)
$167.50!!!
Here's the hammer he invented:
And here's how it works:
At first glance, you might think that Dan invented this hammer to eliminate bending. And many people think that, but they are wrong. Read what he says on his website about "tilting" (His word for bending).
...a tuner using a C-lever can choose to turn a tuning pin without tilting [bending] it at all... [or] choose to tilt the tuning pin without turning it"
Choose to tilt the tuning pin without turning it? That sounds like bending on purpose to me!
If you still don't think high level technicians bend the pin on purpose to produce superior stability, watch this video of Dan using his C-Lever to tune a grand piano and bend the pin on purpose!
Look at what he does to tune these notes in the video.
1) He turns the pin in the block. (You can hear the "click") Now the pitch is sharp.
2) Then he bends the pin towards the string, causing the pitch to drop into place, making the unison clean.
But...it's what happens after that, that makes the string stable.
That's what I show you in the Superior Stability Course.
Talk to you later,
Mark
Watch Lesson 6 and then...
