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Old 06-26-2012, 10:55 PM   #1
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Top Dead Center Mod (TDC) Guide

So we have all heard of the Top Dead Center mod, or TDC mod. It was originally one of Newbie's kits but because it is so easy to do, it has transformed into a cheaper in price D.I.Y. mod. For a long time now there has been no guide, tutorial, or any other instructions on how to do this other than people posting questions on how to do it over and over again.

I happen to have an old Well MB-10 that I haven't done this mod to yet, so I will show you how using this rifle. This particular gun doesn't take any line of upgrades, as it is not a clone of anything, but I was still able to fit my nineball bucking in there. This is just my method, as I am sure there are quite a few methods, however similar, of achieving the same result.

So why should you do this mod? I'm sure you know that a fixed hop is more consistent than an adjustable hop, because the adjustable hop can move a bit when you fire it. This will give you the consistency of a fixed hop, while being able to adjust it. Some people also have a curving issue, most common on VSR rifles though not always, that you just can't seem to fix with shims. This will keep the hopup arm straight and keep the bb going dead straight, that is unless you either have the nub uneven or the bucking mound is uneven. I still recommend you shim your hopup arm, although this is only for greater consistency and to make sure it is all kept rigid.


MiloXC3 made this video tutorial on how to do the TDC mod. The process is slightly different from mine, in that you don't tap the barrel to make the threads, instead gluing a nut over the hole to provide threads, but it gets the same result. Thank's MiloXC3 for making this great video:




Materials needed:

First thing's first; you need to get everything in order. If you have ever tried building something without a parts list, let me tell you, it's much more time consuming than it has to be.

1) Screw- This is the only mandatory part really. I used a 10-32 size screw, as was suggested to me when I first learned of this mod, but you can use whatever size you like as long as the diameter is less than the width of the hopup arm. I would have used a hex head screw but the only screws I had on hand of this size has the thumb grips.

2) Spring that fits over screw threads- This is optional, but is recommended if you want to keep the screw from turning from vibrations during a game. If you used a thin enough screw you could probably use a cut down pen spring, although a stiffer spring is recommended.

3) Washers- This is also optional. I used them for keeping the spring in place and to keep the spring away from rubbing on the outer barrel.

4) Nut- This is just if you want more threads for the screw to hold on to because the outer barrel is quite thin and might strip somewhat easily. It also helps if you accidentally bump the screw into something, it will stop the screw from deforming the threads on the outer barrel.

5) Scope- This is if you want to use a scope dial as the adjuster instead of the screw. Woogie wrote up a small guide on how use it instead of the screw for me to add to this guide. Keep in mind if you do take the scope dial off an old scope you will not damage the scope at all, you just won't be able to adjust either windage or elevation, depending on which you take off.

Tools:

1) Drill- Duh, how else are you going to drill a hole in the outer barrel?

2) Screw tap- You could probably just force the screw into the hole you made, but where will that get you if you don't have threads?

3) J-B Weld- This is optional. It helps keep the nut in place that keeps the screw in place.

4) Masking tape- You can use any tape you want really, just as long as it doesn't leave residue and you can write on it.

5) Pencil- For writing on the masking tape. You will use this for marking the exact place to drill.

6) Screw driver- Can you guess?


Let's get started:

So now that we have all the tools and materials needed to do the mod, we can get started.

The first thing you want to do is mark out where you will need to drill. We want to drill directly over the spot on the hopup arm where the nub is. First get a general idea of where is is located and place some masking tape over that. You don't have to have the tape exactly over it and it doesn't have to be neat. It will be removed later.

I took a long piece of masking tape and wrapped it around my hopup chamber, making sure to keep the edge of it directly over the nub on the arm. Because the arm is on the top and is hidden in the outer barrel, you need to know where the nub is on the bottom of the chamber where the feeding window (the window on the outer barrel where the bbs feed into from the mag) is.


You then want to put the chamber back into the outer barrel with the tape still on it and wrap tape around the outer barrel. Make sure to keep the edge of the piece of tape you just put on flush with the edge of the tape on the chamber. Make a mark along the edge of this tape on the outer barrel. This line is centered with the nub on the hopup arm. Once the mark is made you can take that tape off.


Now that you have that mark made, you need to find the exact point on the top where the nub is, not just where it is along the side if that makes sense. To do this, take yet another piece of tape and wrap it around the outer barrel, next to where the feeding window is (the window on the outer barrel that the bbs feed into). You don't have to wrap it all the way around, just far enough that it goes over the whole barrel and touches the edges of the feeding window. Make a mark on the tape at both edges of the feeding window.


Take the tape off and find the center between the two marks. For me the marks were 2 5/8 inches apart, so I needed to make a mark at 1 5/16 inches.


Place the tape back where it was on the barrel, making sure the two outer markings touch the edges of the feeding window. The center mark you made shows exactly where the hopup arm is, and in extension the nub and bucking mound. extend the mark you made on the tape onto the bottom piece of tape.


Now it's time for the fun part, the fastest part, and the most tedious part; drilling and tapping. This will go much faster than taking measurements, but you have to be careful to get the hole you drill straight. If it's not straight down, It won't affect performance but it might not look as good had it been straight. You can actually purposely make a tilted hole to fix a very bad curving issue that nothing seems to work on. I suggest either a drill press to get the best result or a hand drill with a level built in. I don't have access to a drill press so I used my hand drill with a level built into the top. you can use a normal hand drill, but make sure you keep it straight, or at the angle you want. If you mess up on drilling you can always put some j-b weld over the hole and sand/file it down to shape.

After you finish drilling, you have to tap the hole to make the threads for the screw. this is relatively easy and probably the fastest part. You need to have the same sized tap as the screw you are using. You can get them for a few dollars at home depot or online, if you don't have one already.



Before you go directly to the screw part, keep in mind you are able to use a scope dial instead. It may be a little more lumpy, but there is absolutely no possibility of the adjuster getting knocked and you having to re-adjust it. Woogie was kind enough to give me a short write up about how to do it for me to include in this guide.

Quote:
Originally Posted by woogie
To do this mod just find a scope that is either beat up, or if you are like me......one that you will never use anyway.

What I did was went to my parts bin and pulled out a scope that I have had for quite a few years. I will probably never use it again so I pulled it out on the bench and started to think about how I could use the windage/elevation knobs for my adjustment knobs.

On many second focal plane scopes ( which is what you are going to find on most peoples rifles, because a second focal plane is going to be near the $600+ range ) the adjustment knobs are screwed into the housing by fine threads. Just take the cap off of the adjuster and grab on with a pair of pliers or vice grips. Remember lefty loosey righty tighty ;)

Grab onto the adjuster housing and give it a good twist. Many scopes will have some kind of rubber grommet around the adjuster to keep water and grime out of the scope. This will usually hold onto the mount very well, so it may take some torque to get them off.

When you have the adjuster off you can see the entire working mech. On most rifles you will have to grind down a little bit of the outer threaded portion, so that you can use the mount, but not interfere with the hop unit. You will probably have to adjust the size of the "finger" that will push down on the hop up arm. Now on mine there was a very small e-clip that kept the adjuster from being screwed all the way out.

I had to take this small clip off in order to take the dial out. I was able to get the clip off and then take the dial over to my lathe and take some material off. Once I took around an 1/8" off I test fitted the finger again. I did this a few times, until I was able to fit the entire dial back into the housing. This will keep things nice and flush even when the hop up is off.

After all of your figuring is done, you should be able to mount the part up to the rifle. After you are sure that your hole is center then you should be good to go. Just get some JB weld and get to mounting the assembly to the barrel. Now it will look like your rifle has a bit of cancer, but if you are creative you should be able to come up with something to blend it in.
Time to get rid of all unnecessary parts. with the TDC mod, you won't need to use the slider, so you can put it in a safe place in case you ever need them. I circled the parts you need to keep in green, and the parts you need to put away are in red.


This is what my chamber looks like now that it doesn't have the slider parts.


Now it's time to prep the screw. I would have gotten a hex head screw but the only screw I had on hand of the size I wanted (listed in materials) was a thumb screw. I just sawed the oversize grip down with a hacksaw and rounded the edges with a file/sandpaper. It was also too long so I cut it down with the same saw. If you need to cut it down, the threads might get a little deformed so if this happens you will need to either file off the top part of it or reform the threads with a thin screwdriver or hard knife.

Chances are the screw you will be using has a bit of a sharp bottom on the edges (that is if you don't cut it down). You should file or sand off those sharp parts to make the bottom smooth, because you are probably using the stock plastic hopup arm, and you don't want to cut into it. you could also super glue something soft to the bottom like a piece of cloth or some nerf dart foam.


If you are using a nut for more durability, now is the time to glue it on. I suggest J-B weld, but any glue that works on metal will work. Just screw it to the top of the screw, screw the screw into the hole you tapped before on the barrel, move the nut to the bottom of the screw so it is just about touching the barrel but you can still rotate the screw without too much resistance, and glue the nut down to the barrel. Make sure you can keep the nut held in place and you can still turn the screw, because if you have it too far down the screw so it is touching the barrel, it might clamp the screw down so you can't turn it. You don't want it too high up the screw when you glue it or you might get some glue on the screw itself. I didn't use the nut in this setup because I'm not going to be skirmishing with this rifle, so I don't need to go through the trouble of getting extra durability.

Just one more step to go! take a washer and move it to the top of the screw. Take the spring, and depending on how stiff the spring is, you want it just longer than the screw when put on. Finally, with the second washer under the spring, screw it all into the hole you tapped earlier. Make sure the hopup chamber is already inside when you do this. Now you are done!



Testing:

Now the only thing left to do is test it! If you did this mod because you had a curving issue, make sure those bbs go straight. If you did this so you can lift heavier bbs, put those bbs in there and make them soar sky high. If you did this just for the sake of a more consistent hopup, then test your groupings.

If i missed anything, or anything needs fixing, just let me know so I can edit it!

Last edited by Masterjuggler98; 10-24-2013 at 08:57 PM.
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Old 06-26-2012, 10:58 PM   #2
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Thanks bro, I want to start tomorrow but my old lady's going into labor so.... Anyway I appreciate it and look forward to following you instructions.

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Old 06-27-2012, 12:34 AM   #3
 
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That's very nice guide, I'll do it in my CA M24.

Thanks
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Old 06-27-2012, 01:35 AM   #4
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I don't understand how this can help bbs fly straight. I mean its just a new method to apply pressure to the hop bar thingy...

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Old 06-27-2012, 01:40 AM   #5
 
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If the pressure is applied from the top, then it won't be applied sideways, so there won't be more pressure on one side or another, so the bb will fly straight.

Also, I use a grub screw and loctite/teflon tape, lower profile.
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Old 06-27-2012, 02:42 AM   #6
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Oh. Okay that makes sense lol...

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Old 06-27-2012, 03:41 AM   #7
MiloXC3   MiloXC3 is offline
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Great guide man! Should definitely be stickied!
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Check out my TDC installation video tutorial!
http://www.youtube.com/watch?v=EPCUmPNOuYg

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Old 06-27-2012, 04:39 AM   #8
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When I first heard of TDC mods I spent ages looking for an explanation and guide for it and never found a decent one.

Well done, this is very well wrote! Thank you :)
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Old 06-27-2012, 05:54 AM   #9
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You legend

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Old 06-27-2012, 07:43 AM   #10
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King it also solves the problem of right hooking caused by the adjustment arm twisting in hop up unit Also helps with fps consistency as the bb will hit the arm with same about of hop without any movement in hop up arm. No movement up or down nor sideways, which is great for accuracy.


---

- A.K.A "FD" or "Krew"
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Bar-10
EdGI 6.01mm Barrel
Polarstar Barrel Spacers
Polarstar Piston 45 Deg
Polarstar Sear Set
Polarstar Spring Guide
Laylax Spring Guide Stopper
Nineball Bucking
M150 spring
Threaded Adapter
Mods
Teflon tape Mods (Bucking & Cylinder)
Taper Cylinder Head Mod
TDC Mod With Danger Werx Hop up Arm Type A (Pics)
Spray foam in rear stock & cut foam in the front
Multicam Paint Job

Last edited by fdkrew; 09-10-2013 at 04:49 PM. Reason: Stupid Taptalk added my house address in my signature uncool.
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Old 06-27-2012, 07:54 AM   #11
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Cheers for this! I shall be doing it shortly

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Old 06-27-2012, 08:13 AM   #12
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Thanks guys. I just edited the part about adding the nut because the instructions were a little confusing. Still are, but they are better.
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Old 06-27-2012, 12:11 PM   #13
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Great guide. This needs the sticky!
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Old 06-27-2012, 03:20 PM   #14
fdkrew   fdkrew is offline
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Mods please make this a sticky under VSR section.
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Bar-10
EdGI 6.01mm Barrel
Polarstar Barrel Spacers
Polarstar Piston 45 Deg
Polarstar Sear Set
Polarstar Spring Guide
Laylax Spring Guide Stopper
Nineball Bucking
M150 spring
Threaded Adapter
Mods
Teflon tape Mods (Bucking & Cylinder)
Taper Cylinder Head Mod
TDC Mod With Danger Werx Hop up Arm Type A (Pics)
Spray foam in rear stock & cut foam in the front
Multicam Paint Job
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Old 06-27-2012, 08:52 PM   #15
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I don't know why it needs to be in the VSR section because this is not a VSR rifle and this will work on almost any gun, including guns where it doesn't have adjustable hopup in the first place.
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