Installing a Hitec HS-81MG into an RC18T isn't all that tough, but it can be challenging.  The hard part is getting the steering linkages to work properly, allowing a full rang of motion out of the servo, without binding on the servo.  I thought of a few different ways to accomplish this task and settled on this for two reasons: simplicity and it stays true to the original design.

The first thing I had to do was get rid of that plastic drag link, because no matter how I looked at it, it was always going to hit the servo.  The most obvious material to replace it was carbon fiber, which works out well since the carbon fiber is only 1.5mm thick, compared to the stock plastic drag link which was over 3mm thick.  The thinner link will still allow it to be mounted on the rear side of the servo arm, but it won't hit the servo even when fully swung to the right or the left.

The servo saver that comes included in the AE kit doesn't offer an insert that's large enough to fit around the 24T spline on the HS-81MG.  So I scrapped the whole part.  I figured that with the metal gears and those soft spongy tires, there was enough cushion from hard shocks, so everything would hold up even without out.  I learned later that the tires actually aren't soft enough to prevent something from braking.  After about the 6th run with this setup, I accidentally ran full tilt into a spare tire on the ground and busted the steering bell crank.  Lesson learned:

Without some type of servo saver installed, something will eventually break during a hard crash.

But I still decided to go without it.  This meant that the stock servo arm also had to go.  So what was I left with?  Well , that left me with the really cool blue anodized servo arm that Hitec actually provides with some of their servos.  There are two major modifications that need to be done to this servo arm to get it to work:

Dremel off the top two holes making the servo arm shorter. Tap the two remaining holes with an 2.0mm tap.

Well I didn't have a 2.0mm tap on hand, so I went off in search of one.  Home Depot doesn't carry anything in metric, my LHS didn't have any in stock, and the Ace Hardware near me doesn't have anything smaller than a 4.0mm, but the guy said that the Ace in town does carry the smaller ones.  I passed on making another trip into town.

I thought I'd get cracking on making the servo arm shorter, since that just meant cutting it.  I hooked up my Dremel to my router table, popped on a carbide bit and took off the top two holes of the servo in just a few minutes.  That was fun!  So I thought, let's cut the new drag link out of the carbon fiber piece now.

The way I do this is with a 1:1 printout of a sketch I draw up using some CAD software.  See the images above.  I measured the original drag link, and then copied it exactly, while altering it slightly for the graphite.  Basically I just fattened up the area around the holes for more strength, since the CF tends to break and crack if you drill or cut holes too close too an edge.

With the printout in hand, I cut it out using my sweet curved Lexan scissors, and then glue the piece directly to the carbon fiber piece using a glue stick.  The glue stick works great because it holes your artwork on (even while your cutting), and comes off easily under running water once your done.

I'll get some pics of me Dremeling out these two pieces when I get a chance, but for now suffice to say that both the drag link and the servo clamp were cut out with a Dremel and my Dremel Router table.

Drill the two holes in each end of the drag link before cutting it completely out.  This prevents the CF from delaminating and cracking at the edges.

In order to prevent slop in the steering, the holes in the drag link need to be nearly exactly the same size as the screws going through them.  The bigger the holes relative to the screws, the greater the slop.  This slop however is only transferred to the left wheel.  The right wheel has only two mechanical linkage points that can induce slop - the ball stud on the servo arm and the ball stud on the caster block.  The left wheel has four mechanical linkage points that can induce slop - the screw-to-drag link on the servo arm, the screw-to-drag link on the bellcrank, and the two ball cup and ball studs.  IMO, this is a compromised design (not on my part, but the part of AE), which was made worse by simply using a plastic drag link and regular screws.  But I'm sure it was cheap, and sometimes that's the name of the game.

Unfortunately I just copied the design since it was simple, and I didn't have much hardware on hand to mess around with, however the ideal approach given this setup is to use an aluminum insert (like a shoulder washer) on the drag link and a pair of kingpins that run into both the servo arms.  This way you can keep the I.D. of the inserts and the O.D. of the kingpins within +/- .001 of each other and therefore reduce a lot of initial slop in the left wheel as well as prevent wear over time and thus keeping the slop in check for many runs to come.  I've got some parts in mind that I'm going to buy once I get a chance and will make a few more CF drag links which implement the insert and kingpin concept.

But otherwise with the new drag link cut and the holes drilled and the servo arm tapped, the installation goes together exactly as described in the instruction manual.  If everything goes together as it should, the new drag link should not rub on the shaft or the upper deck.  And, I have to say that after using this setup for a few weeks now, it is completely awesome.  The HS-81MG has more than enough torque to keep the wheels in check and there is very little slop in the steering.  This setup is also super reliable and requires almost no messing with once it's done.  I love it.

MOVING ON TO THE SERVO CLAMP

The next part to my HS-81MG installation was getting the servo to stay stuck to the chassis.  Since the servo ears had to be cut to get it to fit into the narrow tub of the chassis, I couldn't use the conventional approach.  So I found some 0.25" x 7/16" standoffs and dremeled a servo clamp out of CF.  See the above .pdf file.  I think you can get these standoffs at Ace Hardware, but they may only have them in 1/2" lengths, and since the servo is just slightly less than 1/2" high, you may need to put something between the servo and the clamp so that when the screws are tightened down, it actually tightens down on the servo. 

Installing the servo is fairly easy at this point, I put the servo in with the clamp on top and marked two spots for the holes in the bottom of the chassis so that the servo was sitting exactly where is should for the drag link to sit perfectly horizontal like it should and then drilled the holes in the chassis for a 4-40 screw.  Once everything is screwed down, the servo is in there for good.  I turned on my radio and centered up the servo and set the end points and I was off driving.  Nothing better than a rock-solid steering, except for when you hit something (like a rock) and break your steering.  I broke the bell crank running into a spare tire on the ground in my backyard, but I was running it hard, I was almost trying to break something, to just see what kind of abuse it could take.  Well it can take some abuse, but running full speed into stuff still isn't recommended.  I fixed the bell crank and I'm off an running again and haven't had any problems since.

So if you're looking for a good servo and method to install it, and you've got the tools handy, this is a great method for getting an HS-81MG into your RC18T.