Help: Can't unscrew stem from crown!

[butler182]

Hey Guys, I'm trying to do what I thought would be a simple operation. Namely, cut down the winding stem from a 116710 SH3186 movement about 1mm for better fitment into the movement (stem is a tad too long and won't allow the crown to fully screw flush to the case) . As such, I'm trying to unscrew the stem from the crown so I can file off some material from the top of the stem. However, the stem doesn't seem to want to budge, and I'm afraid to force it for fear of breaking the stem. I tried heating up the stem/crown to 300 Celsius with a precision heat gun, but it didn't seem to help matters. The stem/crown assembly is currently sitting in a pin vise and I'm grabbing the knurling of the crown by hand and trying to turn CCW. Is there a better technique? Any suggestions on how best to unscrew without breaking the stem would be much appreciated!

 

[tripdog]

You heated that to 300°C ? If that didn't work soak the whole thing in acetone for 30 minutes.

It may seem obvious but if you are holding the pin vice in your left hand then you should be turning the crown towards you with your right hand.

 

[butler182]

You heated that to 300°C ? If that didn't work soak the whole thing in acetone for 30 minutes.

It may seem obvious but if you are holding the pin vice in your left hand then you should be turning the crown towards you with your right hand.

Yup to 300°C. I have a programmable heat gun that goes up to 480°C. FYI: SS doesn't melt until ~1500°C. The acetone idea is good one...I'll give it a try next. I'm guessing they must have cemented the damn thing in there...so maybe the acetone will do the trick!

 

[butler182]

Soaking in acetone did the trick tripdog. Thanks for the tip. After soaking in acetone, I reheated the crown/top of stem back to 300°C, then blew some compressed air on the bottom of the stem (ie cold air to compress the stem), quickly put it back in the pin vise and the crown unscrewed with ease! Not sure if the acetone or cold air had more effect, but certainly combining these 2 extra steps worked great.

 

[fatarms]

I'm missing the function of heating metal when trying to unscrew the stem? Heating causes metal to expand, I'd think you'd want to lower the entire temperature to cause the metal to constrict or shrink? Then again, I'm no mechanic or engineer, so there may be a perfectly logical reason I'm unaware of?

 

[butler182]

I'm missing the function of heating metal when trying to unscrew the stem? Heating causes metal to expand, I'd think you'd want to lower the entire temperature to cause the metal to constrict or shrink? Then again, I'm no mechanic or engineer, so there may be a perfectly logical reason I'm unaware of?

Yes, you are correct. Heating does expand the metal. The benefit of heating initially is to loosen the bond of any loctite, cement or oxide build-up. In my case, the heat alone did not work. So adding the acetone bath and then creating a temperature differential between the stem (by blowing cold compressed air on it) and the crown (by blowing hot air on it) allowed the crown to expand from the heat and the stem to contract from the cold....and then winner winner chicken dinner, the crown unscrewed (easily)!

 

[fatarms]

Yes, you are correct. Heating does expand the metal. The benefit of heating initially is to loosen the bond of any loctite, cement or oxide build-up. In my case, the heat alone did not work. So adding the acetone bath and then creating a temperature differential between the stem (by blowing cold compressed air on it) and the crown (by blowing hot air on it) allowed the crown to expand from the heat and the stem to contract from the cold....and then winner winner chicken dinner, the crown unscrewed (easily)!

Makes total sense, thanks for the explanation.