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Old December 14, 2019, 01:10 AM   #1
Deltadart
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Filing Lyman 17A sight

I am going to install a Lyman 17A on a Thompson Center Seneca. The Lyman sight is a little larger than the original sight base. Is this just a matter of putting the sight in a vise and filing with a triangular file, check with a caliper often, anything special about how to do this?
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Old December 14, 2019, 02:54 AM   #2
HiBC
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There are folks who are amazingly talented with files.
Some folks can do what you describe. I have no idea of your skills.

I'd probably use a dovetail cutter in a Bridgeport.

A question for you. I don't know the TC Seneca well. You mention a sight base.

Does the Seneca have a dovetailed base screwed to the barrel?

If so,I suggest you get another base from TC.

A 3 corner file with one non-cutting safe side can be used to enlarge the dovetail through the base to fit you 17A without altering the 17A.
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Old December 14, 2019, 03:22 AM   #3
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You're going to have to file down the width and also the taper or it's not going to fit the dovetail. Unless you're really good with files I'd let a gunsmith do it.
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Old December 14, 2019, 08:40 AM   #4
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Just drag the sight base across a file. As you reduce the height of the base, you also reduce the width of the base. It is pretty easy process and doesn't take long to fit one.
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Old December 14, 2019, 09:03 AM   #5
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^^^^ THIS ^^^^
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Old December 14, 2019, 10:11 AM   #6
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OK,for clarity ,The male dovetail attached to the Lyman sight I will call the "sight dovetail"
The female dovetail associated with the rifle barrel I will call the "base dovetail"

The sight manufacturer has to decide how to control the tolerances/dimensions on the sights they sell to keep the customers happy. Obviously you don't want a loose fit. The difference between a loose fit and too tight,so you need too big of a hammer ,is very small. .001 in makes a difference. The sight manufacturer typically gives you a little bit of extra steel on the sight for fitting.Its normal to have to fit the sight.If your Lyman 17 and the TC Seneca have the same nominal dovetail size,such as 3/8 or 7/16,then you should not have to remove much steel.
And,in that case,the advice to flat file the underside of the dovetail attached to the Lyman sight is good advice. The angles and slight taper are preserved and all you have to do is keep the flat underside flat

Generally you want to do the filing/fitting on the replaceable part,usually the sight,rather than te gun's dovetail.

There can be exceptions. If you have decided,for example,to fit a sight with a 7/16 dovetail to a rifle with a 3/8 dovetail,recutting the dovetail in the barrel might be practical. I'd still suggest a mill and a dovetail cutter for most folks.

But,once again,if the TC rifle dovetail is cut into a separate sight base that screws to the rifle barrel,I'd get a new sight base from TC. It would be an inexpensive,replacable part. You can fit your sight dovetail to the replaceable dovetail base.,then screw the new base onto the rifle.
That way,if anything goes wrong,all you are out is the price of the TC sight base. Order another. And,if you do it this way,a 3 corner file wth a safe flat side will work to open the dovetail on the replacement sight base for the rifle.

Once again,if we are only talking a few thousandths to fit a sight with the same nominal size dovetail,the suggestion to remove stock from the flat underside of the sight dovetail is good advice
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Old December 14, 2019, 11:22 AM   #7
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Love these Senecas

Quote:
But,once again,if the TC rifle dovetail is cut into a separate sight base that screws to the rifle barrel,
I'd get a new sight base from TC.
Not so as the front sight is dovetailed directly into the barrel and no screws present, in this area …..

Good luck with that as TC is no longer supporting these little beauties or "any" of the SideLocks. In fact, TC is no longer TC as they are now Smith which is owned but somebody else. You are going to do some internet searching for any sidelock part.

Deltadart
You are referring to the front globe sight and may I ask what, if anything you have done with the rear sight???

I have had good luck working dovetails with jewels files. Slow and easy wins this race. …..

Be Safe !!!
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Last edited by Pahoo; December 14, 2019 at 11:39 AM.
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Old December 14, 2019, 01:05 PM   #8
Deltadart
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Thank you all for the rapid responses and great information. I was not very clear in my post. It is my intention not to modify the barrel of the Seneca. That way I can replace the original 3/8" blade sight to put the rifle back to original condition, should I want to sell it, do not see that happening. The rear sight is the TC Seneca tang peep sight, and barrel mounted open sight is still there. Certainly do not have a Bridgeport. So the option is working on the Lyman sight, as stated if I mess it up it is just the cost of a sight not a barrel. I think the plan by Old Stony is the way to go, seems very simple and by working slowly I should not mess up the base of the 17A.
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Old December 14, 2019, 10:12 PM   #9
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Pahoo
I did what Old Stony said with a mill file and used the Jewels files as you suggested to finish, took me over an hour to do it, I wend super slow and checked with the caliper often, but it came out really well. Looks great, tight fit, and I still have the original blade should I ever want to go back.
Thank you
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Old December 14, 2019, 10:13 PM   #10
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Old Stony
Thank you for the suggestions, it worked great. Took me a while, but I went slow and checked the sight often. Probably too often but I am very happy with the results.
Thank you again
Paul Stone
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Old December 15, 2019, 01:26 AM   #11
woodnbow
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Nice! Do it right the first time and it’s done!
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Old December 15, 2019, 08:43 AM   #12
Old Stony
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Glad it worked for you. I have fitted many sights over the years using that method. In general, when it comes to gunsmithing...if you have to remove material from something to fit a part, you should remove the material from the easiest part of the project to replace.
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Old December 15, 2019, 01:34 PM   #13
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My compliments

Deltadart
I have upgraded the sights on a number of M/L's and like you, kept the option of returning them to original. Have never upgraded a Seneca. I'm guessing that yours is a .45. I have also taken some back, to original and a real challenge as I then have to come up with original sights.. ….

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Old December 15, 2019, 04:21 PM   #14
Deltadart
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Pahoo This one is a .36 that I found several months ago. It is a nice rifle with very few handling marks. The only issue I had was removing the nipple to clean. Although in good shape it had not been removed in a very long time. No wrench would move it, broke a couple trying. Finally ended up drilling and easy out. The old one broke coming out. I ran a tap carefully and slowly to clean up the threads, no issues. Put in a new Treso, all is perfect. I had a .45 Seneca but ended up selling it. Nice rifle but like the .36 much better. Do have a .45 Patriot that I really enjoy as well. One day .36 Patriot.
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Old December 15, 2019, 04:24 PM   #15
Deltadart
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Thank you again, I am looking forward to shooting with the new sight.
That makes sense to me work on the easiest and cheapest part to replace if something goes wrong.
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