How To Repair Stripped Threads In Plastic

Introduction: Stripped Thread Repair Using Thread Inserts

When doing mechanical repair, we will all come to that Uh-Oh moment when we either cause or discover damaged or stripped threads (whichever story makes us looks better works). Sometimes the fix will be as easy as using a thread chaser or a file. Other times will require more work to get a quality and long lasting fix. For this instructable I will be actually replacing severely damaged threads using a TIME-SERT® thread insert kit.

In this instance I discovered (honest) that the threads for some of my motorcycle cylinder head cover bolts had been almost completely stripped out (gasp). I considered other methods for replacing the threads, but in the end I decided on the TIME-SERT® M7x1.0x10mm thread insert kit because I feel it will give me a strong and lasting fix (and they're cool).

This was my very first try using these thread inserts to complete a thread repair and while there are a few steps (like 50 or 60....kidding), you can get through it with preparation and patience (and patience.....did I mention patience?). It worked out great for me the very first time (seriously).

I'm going to share my steps for completing the repair in a blind hole, but it would be quite similar for a through hole (just easier).

Step 1: Tools and Supplies

General List of Tools and Supplies:

TIME-SERT® thread insert kit *
Power drill (I used a drill press for better control)
Tap wrench (from my trusty Mastercraft Tap and Die Kit)
Vice and/or clamps (some way to hold your part while machining)
WD-40 lubricant
30W Motor Oil
Masking Tape
Something to measure hole depth (I used a cheap Vernier Caliper)
Permatex Red thread locker (optional)
Cleaning solvent (I used aerosol non-chlorinated brake cleaner)
rags
Proper protective equipment (safety first!)
Damaged part (duh)
PATIENCE!! (lol)

The tools and supplies you need will vary with your repair and situation, so just use your best judgement. The important part is to follow the instructions that come with the TIME-SERT® kit.

Step 2: Know Your Hole Parts (lol) and Thread Insert Selection

For every thread insert repair it is important to know the hole and fastener dimensions you are starting with to perform the repair correctly. This means that you will have to know the thread pitch and diameter of the bolt you will be using in the hole after the repair and in my case the hole depth. If you are replacing the original bolt with new ones, then make sure the dimensions match the original. I used my stock bolt, so I only had to worry about the hole depth for machining.

This information also helps you decide which thread insert kit you will have to have. Each kit comes with its own set of tools (including drill bits and taps) and 5 thread inserts in the size you want. You can purchase more inserts separately, but you will need the base kit to get the tools.

In the pic you will notice a note about blind holes and depth under the kit contents section. This was important for me as my holes were blind holes and drilling or machining through by accident would result in me having to replace my entire cylinder head. My camshaft bearing caps cannot be replaced separately. For blind holes the instructions note that "the insert should be 1/4" or 6mm shorter than the hole depth". Keep in mind that this is a general rule and is for pass through of the insert installation tool, but you will mark all of the kit tools for the same depth.

Using your cheap vernier caliper* or other hole depth measuring device, measure the total depth of the hole you will be machining. In this case my blind hole was just over 16mm in depth. Since I chose to re-use my stock M7x1.0* cylinder head cover bolt I needed to go with the M7x1.0x10mm insert kit. This meant that my insert would be 10mm long which would give me the 6mm of required clearance. It also meant that I applied tape to my kit drill bit, tap and insert installation tool at 16mm from the tip of each tool. By doing this I wouldn't end up accidentally punching a hole through my bearing cap or ruining one of my tools. Either would probably make me cry.

*I am assuming that anyone reading this instructable knows how to use a vernier caliper, measure bolt diameter and thread pitch. I can't teach everything :)

Step 3: Out With the Old (Drill Out Old Threads)

The new thread insert is threaded in to the hole that you will create in this step. The kit comes with the proper size drill bit.

Begin this step by inserting the kit drill bit in to the chuck of your power drill. Be sure you have remembered to mark your depth with masking tape on your bit if drilling a blind hole. If it is a through hole this will not matter.
You will next want to fashion some way of holding your part still while you are machining it. I used a scrap piece of metal strap, a nut and bolt and a C-clamp. Your repair will determine your method.
However you are drilling it is important to drill centered in the old hole and square with the surface. Also make sure you drill at a slow speed and lubricate the bit and part with your WD-40 often. Any mistake will cause a black hole to open and suck you in.....just kidding. Do your best for this part as it determines the quality of your finished repair and whether our world survives lol.

Step 4: Counter Boring (wouldn't That Be Exciting??)

The thread insert from TIME-SERT® has lip or flange on its top edge. This flange prevents the insert from being wound further down in to the hole when your bolt is installed. In this step we will counter-bore the top edge of the previously drilled hole for this flange.

Remove the drill bit from the power drill chuck and replace with the counter-bore bit.
I prefer to clean away any excess metal shavings from the drilling step. You can use your rags and aerosol cleaner for this if you like.
The counter bore bit has a tip that is sized to fit your previously drilled hole. The tip has no cutting edges and aides in centering the counter bore in the drilled hole. In my drill press I found that inserting the tip just slightly in to the hole before starting the drill press worked best for me. You will have to judge this for yourself.
Lubricate your bit and part with plenty of WD-40 both before and during the machining process.
Start your drill press at a slow speed and lower your counter bore bit slowly. You will only be removing a small amount of material, so don't lower your bit to fast.
As noted on the instruction sheet picture you should "counterbore the hole to the full depth permitted by the tool". The larger diameter of the 4 cutting teeth act as the tool stop. Do not try to counterbore more than this tool stop.

Step 5: Tap Your Hole (not With Your Finger....with the Tap Tool)

The thread insert is made with synchronized internal external threads. This allows it to be relatively thin walled and good for areas where there is not a lot of excess material for machining. In this step we will tap the threads in the hole for the external threads of the insert.

I removed my bearing cap from the drill press for this step and held it with the piece of scrap metal in my bench mounted vice. Whatever way you wish to hold your part, be sure that it is secured tightly.
Insert your tap in to your tap wrench. Your tap should have been previously marked for the correct hole depth if you are threading a blind hole.
As with drilling it is important to lubricate your tap and part. I preferred to use 30W oil for this step, but the WD-40 will work as well.
Start your tap in the hole. It is once again important to be square with the part surface.
Turn the tap clockwise until you start to feel it bind a bit and then reverse direction slowly until you hear or feel a "click" type noise. This is the sound of the chips being broken.
Continue threading until the tap is in the hole up to the masking tape mark or all the way through your "through" hole.
Slowly remove your tap from the hole by reversing out.
Inspect and clean your threaded hole with your rag and aerosol cleaner.
Take a breath......the hard part and all the black holes are behind you!

Step 6: Insert the SERT

This is the last and I think the easiest step in the process. This is where I install the actual thread insert in to the threaded hole and finish the repair.

I cleaned the part and threads thoroughly and removed any residual oil.
I then inserted the thread insert tool in to the hole without the thread insert to make sure that it was correctly marked for depth with tape.
After applying a small amount of 30W oil to the threads on the tool I threaded an insert on to the tool just a couple of threads. This helps the tool move through the insert as it locks the last few threads in place.
At this point I also chose to apply Permatex red thread locker to the external threads of the insert just to be on the safe side. That was just my choice and is not mentioned in the kit instructions.
I then threaded the tool/insert combo in to the hole. Threading continued until I started to feel resistance like a bolt getting tight. I noted this and kept turning past this point. This is when the tool is expanding the last few threads and locking the insert in place.
I turned the tool until it started to turn easier and noted that right after that I also reached the tape mark on the insertion tool. Awesome!!!
I removed the tool by reverse threading it from the hole and then cleaned up the newly installed thread insert.
Last step......I tried my stock bolt in to my repaired part.....perfect fit! Mission accomplished!

Be the First to Share

Recommendations

Anything Goes Contest 2021