Slide-to-Frame fit: How tight is too tight?

I am working on fitting a JEM carbon steel slide to a 100% JEM stainless steel frame and it’s going well. The slide will go onto the frame by hand with very little force, but it will not slide freely when the frame is tipped front to back.

Some builders shoot for a .0002" side-to-side fit - I’m not sure I can even reliably measure that small!

But I’m thinking about stopping with the fitting at this point and building the gun out for test firing. If the slide ‘wears in’ during testing, I’m good to go. If it does not, I can stone a tiny bit more off the frame rails and the bottom of the slide (the places that there is the slightest contact now).

How bad an idea is that?

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Wow, those aren’t Colt standards :grin:

Any thoughts on what temperature changes could mean to those tolerances?

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I don’t know. I suppose if it’s a carbon slide and a carbon frame the expansion rates might be similar enough not to bind.

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What type of stainless is the the frame made of? If it is 416R, then you can usually run a tighter fit than other alloys. Like Robert mentioned, keep in mind the difference in thermal expansion rates between two dissimilar metals. You can check the Machinist Handbook and do the math on that if it’s a concern.

Personally, I go for a slightly tighter fit than what I want after all the fitting is done. Wear in usually opens up your tolerances a bit. I go for 0.001" total indicated run-out in flat, square, and parallel after the break-in. Which means that I do .0004"-.0006" (that’s .0002"-.0003" per side) side-to-side fit.

Keep in mind that barrel bushing to slide and barrel to barrel bushing fit is far more important for accuracy.

Slide to frame fit is a personal choice and should reflect what you plan to use the gun for.

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I’m not 100% sure, but I believe it to be 416.

When I do its test firing, I’ll be sure to watch for temperature related binding. But I’m guessing I’ll be OK.

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416 or 416R? They are two different alloys.

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By the time you are done with the build you will probably think it should have been tighter.
The sharp edges contribute a lot to resistance with a fresh fit and go away quickly. SS gauls a little and when that clears up things loosen also. I shoot for light rubber mallet fit when they come off the mill and have never had one that I thought “damn, should have started looser”.
Saw an STI fitting video where the smith was doing what could only be described as a “heavy” rubber mallet lapping compound fit. He was pounding on that bitch.

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Barrels. Don’t think frames or slides.

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I want to be clear about this… if you use proper machining techniques, there is absolutely no need for lapping or a rubber mallet.

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Agreed

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I stand corrected! Maybe that’s why I burn up cutters!

Knew I was going to stir up debate with that one. Just saying when first cut, before the edges are de-burred, there is some friction. I don’t personally use lapping compound, but before I take the frame off the jig, it can take a little tapping with a rubber mallet to move the slide. As soon as I de-burr, nice and smooth. The mill is cutting sharper and more square a corner then the matching grooves in the slides. Easier to take material off then put it back on,
Here is a video of the STI guy big mallet and lapping compound. Warning, graphic content.

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Sounded sweet when he was finished

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I used to lap the frame and slide together. And have gone away from it since.

It is one thing when you have contact between two parts along the entire length of travel immediately at the start. It is another where you slowly wear in the starting surfaces and continue to wear them in as you approach the end of travel.

It is also funny that in the video the guy shakes the slide when it is moved back on the frame - toward the end of travel, rather than toward the beginning, where it will be the loosest since the front section of the frame rails continued to be lapped by the entirety of the slide rails.

Parallelism is key here. If you had a fixture that firmly held the slide and frame without allowing twist or tilt of either part, you would have a better result. Mallet and hand pressing down on the slide will not control the direction of the travel enough to control which areas get lapped. As the slide starts to move side to side or up and down with each mallet blow you will be opening up the mating surfaces more and more.

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I ended up hitting the frame rails (outside) and the slide rail (bottom) as indicated by the magic marker one more time. And that was all it needed - when you’re close, you’re really close.

Now the slide moves forward and back of its own weight when I tilt the frame.

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Fusion and 1911 Builders just say 416 Stainless.
We did a 1911 Builders frame cut pretty easy.
I have gotten other SS frames off Gun Broken that just said “stainless” that wiped out a couple of my HSS cutters, had to go to solid carbide.
Wonder if that’s the difference?

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