Round impulse jewel on Bunn Special?

[JohnC]

Hi all,

I am working with an Illinois Bunn Special Model 9. When I opened the watch I found the impulse jewel missing, but found it during disassembly. The roller is a 2-piece double roller, with the part # of the impulse roller being 651 (catalog number 6237).

The confusion I have is that the jewel hole is round. Not oval, but perfectly circular. You can see an image of the part here:

https://pocketwatchdatabase.com/search/result/illinois/4006134/parts/catalog/651/Bunn Special

However, the jewel I have is D-shaped. I have never heard of perfectly circular roller jewels. It feels like this jewel must be a later replacement, where the guy was unable to find the correct jewel.

I refit the jewel into the hole, but obviously I am worried about the longevity of this fix given the different shape of hole and jewel.

My question is, does the watch really take a D-shaped roller in a round hole? Or is there a round jewel out there that I should try to find?

Thanks in advance.

[Randy55]

Hi John

I found this from an old NAWCC forum response;

Sometimes the manufacturers drilled a round hole even if the roller table was intended to hold a D shaped jewel. It was done because it is less costly to drill a round hole than to drill a hole then ream to a D shape, and it serves the same purpose. The D shaped roller supplies a more powerful impulse than the round jewel will, so I would advise using a D shaped jewel.

 

you can use a D style roller jewel in place of the oval roller jewel, if the supply house have oval jewels that will be a new one on me. Been at the bench for 52 years and even though your watch had a oval and most likely factory there is no problem using the D style as long as the slot opening on the tail of the pallet fork has the correct size and you have properly shellacked it in place.

 

Hope this helps

 

Randy

Edited May 20, 2022 by Randy55

[JohnC]

@Randy55that's really helpful, thanks so much. The D-shaped jewel I have is indeed a perfect fit in the fork notch. I've got it shellacked in now, so we'll see if it will hold. So far so good. Thanks again!

[JohnR725]

11 hours ago, JohnC said:

I have never heard of perfectly circular roller jewels.

From the beginning of time roller jewel's did Come in a variety shapes including round. Usually all the weird shapes were in the earlier versions of watches. Typically on a lot of single roller watches the hole was drilled round and you're still stuck trying to get a half Round jewel into the hole kind of a pain it's much nicer if the whole is properly shaped.

[JohnC]

@JohnR725eeah a total pain for sure. I got there but had to use more shellac then I think I would like. We will see if it holds..

Edited May 20, 2022 by JohnC

[Nucejoe]

 We had a relevent  discussion showing how to adjust the jewel as its suppose to be concentric with the staff, otherwise it will mess up the lift angle, wont it?