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TOPIC: DT Monoshock Rebuild

DT Monoshock Rebuild 20 Feb 2019 18:38 #1

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Once the shock is removed from the bike the first step is to back off the spring preload as far as it will go. There are two styles of preload adjuster, the threaded type and the cam type

On the example shock with the threaded type, remove the small Phillips screw and then rotate the adjuster "down" as far as it can go... you're trying to remove the pressure on the spring. The pic below shows the screw removed and the preload adjuster ready to be "loosened".


If you have a later model with the "cam type" preload adjuster, there is no screw to remove. You simply rotate the sleeve until you reach the least preload. I found that rotating in the direction of the "H" the cam gets to the hardest preload position and then one more click takes it to full soft which is easy to notice.

Second step: You'll notice there is a cotter pin holding the upper/front shock eye to the shaft. You might be tempted to try and remove it... don't. Not yet. It will probably be impossible to get out at this point anyway.

Before removing the cotter pin, you need to loosen the shock eye by slightly turning the upper spring seat away from the shock eye. You want to be careful not to shear the cotter pin during this step.

In the pic below, I will push down on the wrench in the foreground to loosen the spring seat ever so slightly from the shock eye.


Step three: Now that the shock eye is loose, straighten and remove the cotter pin. Notice the small gap between the spring seat and the shock eye.


Step four: Unscrew the shock eye and then the spring seat. The spring is under almost no pressure so you should be able to do this by hand.


Done!

Footnote: In step two, you might turn the spring seat a little too far and it will jam on the shaft. If this happens, take two 10mm x 1.25 nuts and double nut them on the shaft to hold it so you can un-jam the spring seat.
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

DT Monoshock Rebuild 20 Feb 2019 18:54 #2

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The DT shocks are sealed and do not have a way to recharge the nitrogen once the following steps are taken. Follow the steps below at your own risk and with the understanding that the shock will be ruined unless you install a gas valve or find a suitable (YZ) shock body with a gas valve that you can use.

After removing the spring, spacers, and bump stop, the next step is to relieve the gas pressure by drilling a small hole in the bottom of the shock. Where you drill the hole is where the Schrader fill valve will be installed so I drill about where Yamaha installed the YZ fill valves.

CAUTION: The gas is under high pressure! Wear eye and face protection, gloves, and all the safety gear you have. Better yet, just find a good shock or send yours to a pro for rebuilding. ;)


I'll admit it's a little scary drilling the hole... on this one the pressure came out with a loud HISS. And luckily the internal gas piston was good so no oil came out. (What a mess that makes! It can happen, so be prepared!)

CAUTION: After drilling the base to relieve the pressure, the shaft should no longer be under gas pressure and the shaft should compress easily and stay compressed. If not, DO NOT PROCEED with disassemby! The pressure must be completely removed before taking the shock apart. A bad separator piston could keep the main body of the shock pressurized. Letting the shock sit for a few days might let the pressure escape. Do not attempt to diassemble the shock if any pressure remains in the shock body as the result could be serious injury or death! If you are not sure, consult a professional or better yet, have a professional do the job.

Now to get to the shaft seal, you need to remove the preload adjuster. Both types used on the DT are lightly pressed into the shock body. (It's a good idea to take pictures of the position of the adjuster and/or mark where it is located "rotationally" so you can tap it back into the same orientation when installing later) The pic below shows me lightly tapping the preload adjuster with a hammer and a brass punch to remove it:


Work your way around the shock and tap the adjuster out evenly. But watch where you tap on the threaded type to avoid this: (Notice the adjuster on the left has a piece missing) :Ugh

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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DT Monoshock Rebuild 20 Feb 2019 19:07 #3

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b]CAUTION: After drilling the base to relieve the pressure, the shaft should no longer be under gas pressure and the shaft should compress easily and stay compressed. If not, DO NOT PROCEED with disassemby! The pressure must be completely removed before taking the shock apart.[/b] A bad separator piston could keep the main body of the shock pressurized. Letting the shock sit for a few days might let the pressure escape. Do not attempt to diassemble the shock if any pressure remains in the shock body as the result could be serious injury or death! If you are not sure, consult a professional or better yet, have a professional do the job.


Now we need to remove the seal head. There is a wire snap ring holding it in place. You can see the ring if you look closely at the next pic.


To remove the wire retaining ring, we'll need to push down the seal head slightly. CAUTION: Do not attempt to remove this retaining ring unless all pressure has been removed from the shock! Serious injury or death could result!

I was unable to press down on the retainer, hold it, and use a pick to remove the ring... so I had to use a piece of PVC and my $30,000 press to hold the seal retainer out of the way:


Removing the wire ring is the hardest part! Be careful to avoid scratching or gouging the bore.

With the retaining ring removed, it's time to pull the seal retainer. I HATE this type of shock because invariably the retainer sticks and then *POW* suddenly comes loose and oil goes everywhere!

That said, many years ago when I was tuning my Fox shocks, I came up with a method to pull the seal head that works pretty well:


What I do is put the weight from my slider hammer onto the shock shaft. Then a washer (in this case I used a bump stop) and a nut. Then I gently tap the shaft and seal head out of the shock body. I've never had a "disaster" using this method.

CAUTION: Once the seal head parts come out STOP!!! On these shocks there is a second snap ring that will prevent the piston and shaft from coming out!

Here's a pic of the parts of the seal head:



It's obvious at this point a new seal head will need to be purchased or made. This "stack of parts" could probably only be sourced from Yamaha. And now the second snap ring makes sense. Installing a "modern" one piece seal head will make the shock much easier to work on and eliminate the need for the lower snap ring.



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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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DT Monoshock Rebuild 20 Feb 2019 19:16 #4

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OK, so once the lower retaining ring is removed, the shaft and piston comes right out. A couple of pics:


The last part to remove is the gas piston that separates the nitrogen and oil.

A rag over the end of the shock to catch the piston and a very slight blast of low pressure air in the hole drilled to release the pressure and it pops out. CAUTION: It only takes a couple of psi to turn the piston into a deadly projectile. Hold a heavy rag over the shock to catch the piston. Use extreme caution and do not aim the open end of the shock body at anything you care about!


Now it's time to clean everything up, inspect, and put it back together.



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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

DT Monoshock Rebuild 20 Feb 2019 19:33 #5

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Some notes about these shocks.

First off, there is no "seal" of any kind around the preload adjuster so any water that gets in there will simply sit and rust the ID of the shock body and possibly damage the shaft as well.

Luckily, the shock sits inside a frame tube which should prevent most water from getting into this vulnerable area. Just remember that if you spray water around the shock when washing the bike, it would be a good idea to remove the shock and drain any water that may have settled under the preload adjuster.

Next is the fact that the shock shaft bushing is on the "outside" or "dry" side of the shaft seal. (This means the bushing is not lubed.) And there is no dust seal. So if the stock seal starts to seep oil, dust and dirt will stick to the shaft and form a nice "grinding paste" that can destroy the shaft like the one in the pic below.



Fortunately, these flaws are corrected when replacing the seal head... and as long as the shaft is still good, the new seal head should outlast the factory one... it's just a better design.

Last are the shock eye bushings... particularly the shock eye that attaches to the swingarm. Do yourself and your shock a favor and keep this bushing well-greased and freely pivoting. (sometimes this pin will be completely rusted in place on a neglected bike.) Friction of this pin puts a "bind" on the shock shaft and will wear out the piston and shaft bushing prematurely. On the YZ's of the era, I believe Yamaha recommended removing and greasing this pin every race.

If you are shopping for a used shock or deciding if your existing shock is a good candidate for a rebuild, these are some of the things to keep in mind... just remember that most of the damage caused by water ingress or grit in the shaft bushing cannot be seen unless the preload adjuster is removed from the shock.

I've taken apart many of these shocks and while the free length is different between models, the shaft length and shock body** is the same on the DT monoshocks, the early YZ100 mono, and the IT125. The length is simply changed by different internal spacers. The YZ body has the advantage of having a fill valve built in (needle type) which means you don't need to source and install a Schrader valve.

**The earlier 250/400 body has a solid tube welded to the body for the lower shock mount while the smaller and later bikes have a rubber-type lower mount. Same size pin so interchangeable.

I like the YZ piston because it has a piston ring to help the seal. On to the assembly!
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

DT Monoshock Rebuild 20 Feb 2019 19:54 #6

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I'm using a YZ100 shock body as it is identical to the DT175 shock body but already has a nitrogen charging port built in.

The following picture shows that Yamaha simply threaded their fitting right into the end of the shock so if I didn't have a YZ body with the valve, I would simply drill, tap, and install a Schrader or RaceTech nitrogen fill valve into the end cap ala Yamaha.

Here's a pic showing the inside of the Yamaha nitrogen charging valve where it comes through the shock end cap looking down inside the shock body (You can also see the top and bottom snap ring grooves) This might help locating where to drill the hole if you're installing a Schrader valve.


So after everything is clean, I like to cover the inside of the shock with a light coat of shock oil. I stand the shock body on end for a few minutes to let the excess drain out.

Next, install the nitrogen piston. The end with the o-ring closest goes in first. (My piston o-ring was in excellent condition so I reused it. I'm sure a replacement could be found if needed)

I insert a hypodermic needle into the fill valve (With a Schrader valve, remove the valve core) to release the trapped air so I can push down the piston. How far is the question! I could not find any specs so I had to come up with my own based on inspection of shocks and calculations of required volume, etc. The measurement I used is 7" from the top edge of the shock body. Use this spec at your own risk!


Once the piston is set, I remove the hypodermic needle to keep the piston (more or less) in place for the rest of the operations. (With a Schrader valve, install the valve core)

The next photo shows the shock shaft ready to install. I'm using a RaceTech seal head that fits these old shocks PERFECTLY! Part number is SYSH 461413. This part also fits modern Sachs shocks used on several bikes so there might be other sources.


Next is the messy part. Filling the shock with oil.

But first we need to select an oil!

I found some old Yamaha literature that stated the proper oil weight was SAE 10 in the monoshock. This was from the earlier YZ's with the big bladder-type reservoir. I can can only guess that they used something similar in the later models?

Actually, it really doesn't matter. Oils have changed so much and "SAE 10" is not the proper scale to measure suspension oils with.

I think some manufacturers use the "SAE" scale because we are so used to these motor oil ratings. Just keep in mind that when it comes to suspension oils, one companies 2 weight is actually "thicker" than another's 5 weight!

Which brings us to another dilemma... modern shocks tend to use thin oils.

Again there is a TON of information out there on oils and you can pick whatever you like. An important spec is the viscosity index (VI)... higher is better.

After researching what I could find on Yamaha "SAE 10" specs, I decided to use Torco RFF 20 as a good modern alternative.

So now I fill the shock body with oil, insert the piston, bleed the air, install the snap ring to hold the seal head and charge with 213 psi of nitrogen (per my Yamaha DT175 service manual)

(Sorry, no pics... MESSY job and it seems everyone has a different idea of the "best" way to do this... plenty of instruction available on the Internet.)

One caution: Make sure the snap ring is in perfect condition and is located in the groove correctly before pressurizing the shock. Serious injury or death could result if that snap ring pops free when pressurizing!

Now you can tap the preload adjuster back into place (index it so the right side is up before tapping into place) and install the spring and retainer... it's just the reverse of the disassembly process... and use a new cotter pin.

Done!

P.S. Don't forget to install a new bump stop on the shaft before installing the spring!!
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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DT Monoshock Rebuild 20 Feb 2019 20:09 #7

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Advanced info... shaft piston removal and valving shims:

The shock I'm built in these pictures is a "hybrid" made from the parts of a couple of pretty beat up cheap junkers from ebay. As a rule, I don't recommend removing the piston from the shaft but in my case I needed to.

First thing you'll notice is that the shock shaft threads are heavily staked on the end. You'll need to grind that off.

The second problem is holding the shaft to remove the nut without marring the shaft. You can buy tools for this but I take a block of wood, drill a hole a tiny bit smaller than the shaft, and then cut the block in half so I can sandwich the shaft in a vise using the blocks. A little rosin added to the grooves in the blocks and no slip or marring.

Here is what you'll find:


The third and fourth washers from the left are the rebound shims. The fifth washers from the left are the compression shims.

These shims are different between the YZ100 and the DT250/400 shocks I took apart.

The shafts are the same. The YZ piston has the advantage of having a piston ring but otherwise looks the same. The other "thick" washers also seem identical.

Here is how the first washers go on the shaft:


The rebound shim is actually two shims spot welded together. One has a slightly smaller OD than the other. This is important to know because the compression shims need to fit over this "step" as shown in the following picture:



It's hard to see, but the rebound shim is actually on top of the compression shims. The compression shims seal on the piston.

For serious work, I would probably find a piston assembly from a more modern shock as I doubt much is available to "tune" the stock valving. (RaceTech makes one)




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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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