Press fit dimensions for low-torque application

I picked up a few IHMSA rear sights for the T/C Contender. The stock elevator stem has a crude black finish that is not hard. Its already
wearing away in the dovetail.
I did some Super High Intensity Tinkering, and found out that you can open the base dovetail and the elevator stem of a Williams FoolProof fits right in.... However, there is a problem, as the IHMSA sight (possibly T/C made investment casting) uses a 6-40 thread, and the Williams FP uses a 6-48 thread. Result- the elevator screw makes about three turns, then everything stacks up.
I picked up some stainless steel hypodermic tubing, .148 OD and .110 ID to thread internally. Drill out the existing thread, then press in the threaded tubing. Waa-laa, correct thread.
Sight base is steel (suprise!) Web thickness around existing internal thread is @.200"
The tubing comes out as .1475 OD on my Starret. The load will be light, just the elevator stem. What should I drill the existing thread out with (diameter) and follow up with a reamer (diameter)?
I dimly remember a drill should be about .005" under the diameter of the reamer.
What should the final diameter be for a press fit?
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On 07/14/2011 12:19 PM, Louis Ohland wrote:

Grain-o-salt: I ain't no professional machinist.
I don't think you should go too far undersized, if at all, and I think you should use adhesive to hold things together.
Why? Because if you make things undersized enough to really hold that tubing in there, it's thin wall enough to squeeze the ID, and jam things up.
Without adhesives you'd be wanting to thread the outside, and do a helicoil kind of thing, or use a good thick wall so you don't squeeze the tube. But if you're really intending this to be a light torque application, then why not just glue?
I'd go with Loctite, unless you're concerned about temperature in which case I'd go with Epoxy (something sophisticated, like JB-weld).
--

Tim Wescott
Wescott Design Services
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Drill undersized. Ream to size. Sounds like what the OP said he was going to do.

Loctite sleeve and bushing locker is perfect although it might be too strong for this application. I know it works on press in sleeves on engine coolant systems just fine. That used to be Loctite Green.

Me too.

Loctite sleeve and bushing works upto 200 degrees on coolant systems on engines. I tend to prefer to drill and pipe tap, but that's just me.

J-B weld is some awesome stuff. I've used it to hold hydraulics together with some mechanical assistance. Most of your clear epoxies and especially your 1-5 minute varieties will soften up at higher temps too. However for this application a needle tip dab of 5 minute epoxy might be good too.
JMNSHO
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How much relief for Loctite? Depending on how much space that Loctite can fill, I might be able to drill the elevator threads out and slip in the liner without reaming.
The best is the enemy of the good. Use a 6-48 tap in a collet chuck to align the sight, change out to a drill, clean up the threaded hole, change back to the 6-48 tap, thread the liner onto the tap, goop up the liner, then lower the liner on the tap into the hole. Unchuck the tap and let it sit in the liner until the Loctite firms up. After the Loctite sets, simply unscrew the tap.
I think I can use an end mill and plunge cut for the gib lock bushing, that should tighten up the dovetail, just like on the Williams FoolProof.
On 7/14/2011 15:39, Bob La Londe wrote:

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wrote:

I use JB Weld on my Super 14 Contenders. Holds up well enough for holding in dovetail sights
Gunner
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