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Masking Helicoil Threads in Aluminum before Plating


An ongoing discussion from 2006 through 2015 . . .


Q. We are a machine shop with a lot of helicoil holes into aluminum 6061-T6.
We have cad plated, nickel plated, gold plated parts with these holes.
We have some problems with plated parts coming back with plating in the helicoil threaded holes.

Does anyone know of special plugs for the dimensions of these helicoil tapped holes?
I can't find them anywhere.
Sizes we need are typically: 4-40 STI, 2-56 STI, 0-80 STI, 6-32 STI, 8-32 STI, 10-32 STI, .25-20 STI

Nick Harkeem
- Manchester, New Hampshire, USA

simultaneous 2006

I do anodizing, not plating, the plugs I buy are made from silicone. For blind holes, I'm able to cut the tapered plug with a razor blade to pretty much any diameter that I need. Shercon, and numerous other companies offer these plugs, and also special ones for STI holes, which insure that the very first thread gets masked.

Marc Green
Marc Green
anodizer - Boise, Idaho


With gold and cad, you only have to reduce the amount of plate in the threaded holes. With nickel, it needs to be better. Thru holes are relatively easily done with silicon pull plugs which are reusable several times. Holes with bottoms are tougher. Common neoprene plugs work well most of the time, but are more time consuming to put in than pull plugs. In the past, we used vinyl tubing, cut a tapered lead in on one end and then would pull 20 or so parts onto the tubing. Needs a tight fit. Then we would come back and cut them into individual sections with good scissors or a razor knife. Sometimes the customer would not tap the hole till after plate. Distilled water and alcohol is a decent cutting fluid that does not stain or can be rinsed off easily.

James Watts
- Navarre, Florida

March 10, 2015

Q. My apologies to beat a dead horse, I am not sure how old this thread is, but I was hoping for some help.

So I am running into the same situation, my plater always seems to get plating material on the first few threads on blind STI holes, enough material that warrants a good chase. I searched the new Shercon site (Caplugs) for specialty STI (Screw Thread Insert) tap plugs but no luck.

Typical products include 6061-T6, with various plating, from anodizing to nickel. Hole sizes vary from #2-56, #4-40 to #6-32.

This is very cumbersome as I am having to constantly go back and chase threads. Is there any plug out there that can save me the time of chasing threads? Could anyone be so kind as to provide a web link?

Thank you in advance!

Troy Chase
- Los Angeles, California, USA

Ed. note: We've added the dates to the original postings, Troy. Thanks for reviving the thread. Dragons live forever, though not so little boys.

March 10, 2015

A. Shercon does make a plug that seals the chamfer/first thread on a threaded hole, works quite well. Here's the link:


I've used them with success in anodizing. With a helicoil already installed, my concern would be how much life one would get out of these plugs, the end of the helicoil may tend to grab/tear the plug. They are relatively inexpensive though, when compared to the labor of chasing threads, so even if you only get a couple of uses out of them, you're still probably ahead. It's a shame that you are having to do this research, instead of your plater.

Marc Green
Marc Green
anodizer - Boise, Idaho

March 12, 2015

A. I re-read your question this morning,and noted that you were talking about just the STI threads, nothing with helicoils already in them (I misread that). The plugs I linked should last quite sometime, with no tearing issues.

Marc Green
Marc Green
anodizer - Boise, Idaho

March 13, 2015

thumbs up signMarc, Thank you for your response, I appreciate the link. I will see if my plater can use these plugs, unfortunately, it looks like these plugs start at 1/4-20, my tapped holes are on the smaller side, 0-80 to 6-32 max.

And no shame, I'm just doing what I can to make sure we all succeed!

Thanks again for your reply.

Troy Chase [returning]
- Los Angeles, California, USA

March 14, 2015

Gotcha, I didn't delve deep into the size of what they had available. The ones I use are made for the 1/4-20 sized holes (works fantastic for a 10-32 STI). If your volume of work warrants it, Shercon will also custom make plugs of almost any size, so it might not hurt to give them a call to discuss your situation. They do make another plug that might solve your issue, but for the life of me, I can't locate it on their site. This plug is tapered, but has a bunch of "ridges" that also help mask the first threads. Unfortunately, it's a neoprene plug, so the life expectancy isn't as long as the silicone plugs, but they are good for several uses. I'll have to look at my catalog when I get back to work on Tues, I know they are in there.

Your plater also needs to insure that they are actually twisting in the plugs as far as they will go, almost to the point of breaking them off, instead of just pushing, or twisting just a couple of times. Those ultrabake plugs (silicone) work well for this, so make sure your plater isnt using the neoprene plugs.

One thing you, as a manufacturer, can do to help this issue, is to minimize/eliminate any chamfer/bevel you may have at the top of the threaded hole. I anodize 1000's of parts with STI threads, and the only issue/complaint I've had is with parts that have a large chamfer, that was resolved by using the plugs that I linked in the first response.

Marc Green
Marc Green
anodizer - Boise, Idaho

March 20, 2015

thumbs up signMarc,

Thank you for your replies. I will continue to look into a better plug to suit my needs. I think I already have minimal to no chamfer at the leading edge of the threads, I will keep an eye out for that on future designs.

Contacting shercon is a good idea, I may have to do that.

Thanks again!

Troy Chase [returning]
- Los Angeles, California

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