I will show you the different techniques on how to repair stripped threads. You can see on this cylinder. This is one of the repair technique. Machine shops usually do this drill and tap bigger thread holes and screw in a metal insert then tap the original size threads. Another technique is to just drill and tap bigger thread.
Holes then use bigger bolts. Just like this hole. I’ll show you the oversized bolt that was installed on this cylinder. When I bought it there, you will see the threads are different on each side. One is bigger than the other at one side is the standard size M8 and on the other side, is the bigger/oversize thread.
However, this also is now stripped and needing thread repair. This is the original size, stud bolt size, M8 thread. You can see bits of brass on the threads. That is because the brass insert was also stripped, so the previous thread, repair using brass insert, is now already stripped and needing repair. These are the helicoil inserts that I bought helicoil for M8 x, 1.5 threads. This is what we will try to use to repair the threads. The things we need to install the helicoil, the correct size, helicoil thread inserts correct thread, tap for the helicoil and the correct size. Drill bit for the tap at first need to drill out the stripped threads. It is important to use the correct size drill bit after that.
We use a hand tap to cut new threads, for the helicoil thread insert to screw into aluminum is softer and easy to tap threads. I only drill and tap the length of the helicoil insert, so only the upper part of the hole, so the threads are in a blind holes, so important to remove the burrs can use compressed air to blow or a small brush to remove the burs inside the Hole after that, we can proceed to screw-in the helicoil. There is a special tool to screw-in the helicoil. However, I already have the thread tap needed for for the size of the helicoil insert, so I only bought the M8 helicoil inserts so just used a cotter pin to screw in the helicoil insert the bottom end of the helicoil has a tang. That is where the special tool attaches to, so it is just like screwing in a bolt.
It is important to screw in the helicoil flush. No part should protrude the surface, otherwise the bolt will not screw in. You can see the tang at the bottom. That is what enabled us to turn the helicoil. We need to remove that tang so that the thread will screw-in deeper.
It just need a sharp blow to remove the tang easily fall off with one sharp blow. Then you need to yank it out there. You go that small piece so I’ll just do the same to the other tapped hole you can buy the helicoil insert as a set you can buy that online. The kit includes the special tools. I will show you, aside from the cylinder head stud bolts bolt holes on the intake are also stripped two bolt holes on the intake manifold are stripped for these.
I will use the helicoil thread insert just like before. After we drill the correct hole size, we will use the hand tap to cut new threads. Take note that this tap is not for the size of the bolt that we will use, but rather to the external thread of the helicoil. The bolt size for the intake manifold is M6 x. 1.
Here is the special tool to screw in the helicoil. For the M6 x 1.0 thread I bought the complete set bought it from Lazada. You can even see the Lazada box behind it’s that simple, very easy make sure no part of the helicoil is protruding.
The helicoil thread. Repair kit comes with a punch, close up shot. So you can see clearly so if you have a stripped thread on your engine, instead of paying a shop to do the repair just buy the thread. Repair kit, maybe even cheaper. The helicoil is very easy to use.
As I said, only 2 bolt holes have stripped threads. However, I also do all the remaining threads because it does not look like they will last longer. So in all, I put helicoil inserts on 4 bolt holes aside from that. A bolt hole on the exhaust flange is already stripped notice. The other bolt hole had brass insert back to the cylinder.
Some of those that have brass inserts cannot be repaired with helicoil, because the hole is already too big. You can see the hole is too big for the helicoil. So for this case I will try different technique. The new stud bolts I bought are longer than original. You can see it is longer by about half an inch.
The plan is to drill deeper hole to tap new deeper threads for M8 thread size bolts, not for the helicoil. You will notice the depth of the original hole still have some distance away from the intake bore. So that is what I did.
Drill deeper hole and now I tap new threads size – M8 x, 1.25, the threads near the surface are already stripped.
It is not doing anything to hold the bolt, so the bottom threads are the one holding the bolt you can see. Now it is a lot deeper that before this is the other one that have a brass insert. I did not remove the damaged brass insert. I just drill past it on this side of the cylinder I can drill through hole because it will not pass through internal bore. So after I drilled a deeper hole which actually penetrate the bottom, you can see that it is a through hole.
It is now a lot deeper than the original hole since the threads on the top are already stripped. I will tap new threads beneath it through the hole. What will happen is the upper threads are damaged and the bottom new threads are the one holding the stud bolt. That is one technique. If helicoil is not an option later, when I make up the stud bolts, you will see the big difference in the length of protruding bolts.
That is because the2 stud bolts go deeper threads I’ll, attach all the stud bolts for the cylinder head of this DT 175. This is the one with deeper threads near the intake, you’ll notice. I am using 2 nuts to turn the stud bolts. This is the other one that we did deeper threads, these 2 with brass inserts. We did not do anything, and these remaining 2 are where we use helicoil I’ll enumerate again.
The different techniques first is to drive in a metal insert fill more suited for machine shops. Second, is using oversized bolt that we can do at home. Is we have the tap for bigger bolt? Third, is using helicoil
The prefered method, and if helicoil, is not an option – and there is enough material left beneath the threads, you can drill deeper holes and tap new deeper threads there. You can see the big difference in length because it penetrated deeper threads.
You can see the 2 shorter stud bolts. Even so, since the new stud bolts are longer, there is still enough threads to engage the nut there. You can see there is no problem. So if you are going for that solution, you will need a longer stud bolt.