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TDC Mod – how to improve your hop-up?


What is TDC Mod?

TDC Mod or Top Dead Center is a solution to change the way the hop-up system of our airsoft gun works. In articles on the Gunfire blog, I have already touched on the topic, so this time I will only briefly describe how this part of our toy works. To make our BBs fly further and more stable, we want to give them a rotational motion. We achieve this by the action of the rubber on the projectiles fired. We change the strength of this impact through the chain of the spacer, arm and some knob or adjustment levers. There are a lot of elements here that work together. I can see at least two reasons for such a high degree of complexity of the discussed system. On the one hand, we have a great need to standardize hop-up chambers, which need at least a few common parts. On the other hand, due to the fact that airsoft guns have a very diverse construction, their creators have a difficult task to place the required parts in the receivers. The essence of the TDC Mod is a certain simplification of the hop-up and, above all, a change in the direction of the forces acting on the spacer's arm.

airsoft gun modifications

By introducing the modification, which is the subject of this article, we give ourselves the opportunity to more precisely adjust the pressure of the spacer on the hop-up bucking. As the name suggests, we do this by inserting an adjustment element directly above the spacer and the patch pressing the BBs. Sometimes, to achieve this effect, we have to interfere in the construction of the replica itself, but more on that in a moment. This modification is popular among airsoft snipers. In fact, in VSR-based replicas, making this upgrade is quite simple due to the multitude of materials with the description of the required activities. In addition, there are several ready-made kits for these constructions on the market. However, it will not work without drilling, which should be done quite precisely. But there is nothing to fear. If I can do it, I'm sure the average person can do it too.

How to do this conversion?

In my opinion, if we use a fairly standard airsoft gun, there is no reason not to try one of the ready-made sets for this modification. Airsoft sniper rifles built on the basis of the VSR system or MK 23 pistols can enjoy a wide range of such solutions. The thing is, we still have to have some way to put pressure on our spacer and stabilize that force. Most often we do it with a fine-thread screw placed in a housing that rests above the window of our barrel's spacer. In my case, unfortunately, the sniper replica's hop-up chamber was non-standard, so I had to improvise a bit. Ready-made sets for popular airsoft guns even contain elements that help us determine the drilling location.

workshop carried out on black mat specna arms
airsoft workshop

The first step is to determine where we need to make the hole. In my case, the first step was to determine the centre of the barrel. Then I measured how far from its end the spacer arm is. Usual measuring tools such as a ruler and a calliper proved to be helpful here. If you do it at home and, like me, you have a non-standard replica, you need to find a place where the drilled hole is perfectly above the replica's spacer. In my case, it was also necessary to remove the elements responsible for adjusting the pressure of the hop-up bucking. They got in the way and blocked access to the spacer arm from above. As you can see, you need some creativity to adapt your chamber to work with the described modification. The diagram below should help you visualize what effect we want to achieve. If our hop-up chamber complies with the VSR standard, ready-made conversion kits are often equipped with guides to help determine the location of the hole.

tdc mod diagram

Drilling the right hole is, in my opinion, the most difficult part of the whole process. Especially when we do not have access to a well-equipped workshop. In my case, it was helpful to make a small hole, thanks to which I could check whether I had marked the place for the modification correctly. It turned out that I must have made a mistake and then, enlarging the hole to the target size, I also tried to move it. It is comforting that after drilling in the hole, we can see the spacer arm. To be sure, we can then mark on it, e.g., with a marker, where the hole is made. It should be in the middle of the arm directly above the spacer.

tdc mod knob

The last stage of the described modification is the installation of the pressure element. Most often, a machine screw is used for it. In my case, it is M4 purchased in an online store with accessories for 3d printers. Some TDC mod kits have ready-made knobs, so if we use this solution, we don't have to look for it on our own. We have a choice of several methods of embedding the described part. Probably the cheapest of them is the use of epoxy mass. We create a kind of saddle to embed our clamp. I used elements of one of the ready-made sets, which had a specially prepared element made by 3D printing. After assembling everything together with the inner barrel, the hop-up bucking and the spacer, it's time for a test. We should easily see how the extension of the element pressing the BB increases with the tightening of our screw. It's easy to see by looking through the barrel against the light.

Is it worth doing a TDC Mod?

TDC Mod is often mentioned as a basic mod. Especially in the case of sniper airsoft rifles. It helps our hop-up system to work more efficiently. Standard adjustment systems based on a small lever on the side of the barrel are very imprecise. Their additional disadvantage is that they can lose their setting over time. This means that once adjusted, the hop-up must be controlled and corrected during the game. For an airsoft sniper this can be a big problem. Often in this style of play we have only one chance to hit the enemy and a missed shot can reveal our position, exposing us to enemy fire.

golden chamber hop up

Even if we decide to buy an aftermarket chamber for our replica, they rarely have an easily accessible adjustment system. It is usually done by tightening a small screw after removing the magazine. Such adjustment may be more precise than the factory one and has less tendency to lose settings, but most often it requires us to remove the magazine every time we want to adjust the downforce. TDC mod solves both of these problems. In addition, it gives hope to people whose airsoft guns are equipped with non-standard HU chambers. Therefore, in my opinion, it is a modification worth attention. Admittedly, it requires focus and precision to perform it correctly. In addition, when we do not rely on a ready-made set, the level of difficulty increases, but the satisfaction and improvement of performance when we succeed are worth it. If you are interested in how the SA-S02 airsoft gun works after making the TDC mod and other modifications, check out the necessary other articles on the Gunfire blog describing this construction and my Instagram account.

Author: Boreq

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