2016 - HPI Nitro MT2
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I missed the boat on the Losi Ten-T truggy by about a year, and by the time I was ready to buy something that big all that were left were beaten-up Ten-Ts that were obviously being sold because the owners didn't care about them anymore. Not being inclined to jump into a money pit like that, I passed. I ended up getting an HPI Bullet ST instead, and it's a great truck, especially with the Losi 3.4 engine I shoehorned into it. However, since nitro engines are ringless, it's not really practical to run them in a wide range of temperature conditions (by which I mean 20°F - 100°F) -- people who are really into tinkering with carburetors and glowplugs can kinda-sorta make it work (more speculation about this later), but that's too much fiddling for me. (amazingly enough, yes, even I have my limits when it comes to tinkering. I like to actually finish things and move on, not fiddle endlessly with the same thing.) I'm really not interested in completely retuning my engines from summer to winter, so when the weather gets cold, the nitro vehicles go on the shelf and stay there until spring.
And yet, I still wanted to run nitro in the winter. The obvious solution was to buy a nitro truck (a car wouldn't really handle snow, salt, and sand very well) and use it just for winter duty, so I could break-in the engine in cold weather and leave it tuned for near-freezing temperatures. But which one to get? I already had a Bullet, I suppose I could've gotten another one, but I already had another one that I'd built as a semi-loaner for use at my girlfriend's parents' farm, and I didn't want three of the same truck. But the Losi Ten-T is an even bigger risk now than it used to be, and the Kyoshi DBX is too long and not enough ground clearance, and the HPI Firestorm is RWD -- not so great for potentially icy conditions -- and Traxxas' 4WD nitro trucks are all enormous and having one T-Maxx is big enough, thank you very much. So I dug around to see what other models might be available, maybe something discontinued but with better parts-support than the Ten-T.
The HPI MT2 looked promising -- it was 4WD, a reasonable size, shared lots of parts with the RS4, it could use the same engine as my RS4 too, and I could find spare parts fairly easily. If only I could find the truck itself in good condition. So I kept an eye on eBay for a couple months, and eventually I found this for $150:
The radio was ancient, and while the engine was well-maintained it was also caked with dirt and old oil, but that's all that really needed to be addressed immediately. The receiver got replaced with a Spektrum SR201 so it could talk to my DX4S, and I replaced the engine with a brand-new HPI T3.0 with Teflon clutch shoes, the same engine and clutch that I use in my RS4, and figured out which clutch bell would fit in the available space since the T3.0 uses a fixed-position motor-mount. Then I started having fun with it.
However, since I intended this to be a cold-weather truck and I got it in December when it was record-breaking warm where I live, I only got a few days I could run it. So, of course, the upgrade bug started to bite, and by the time I was done I ended up with this:
I ended up buying a bunch of new-old-stock parts from Hong Kong, Australia, and Norway of all places. The diffs got rebuilt with hardened-steel gears, the dogbones got replaced with nickel-plated HPI CVDs, the crappy fiberglass brake disc got replaced with a GPM steel disc and HPI R40 brake pads, and the center axles got replaced with GPM universal-axles (pretty much all of this is the same as what I did to my RS4 -- hooray for platform-sharing!). The wheels and tires got replaced with Pro-Line Dirt Works tires stuffed with Pro-Line dual-density open-cell foams mounted on Pro-Line FaultLine 2.2 SLW-compatible wheels with Vanquish .350" SLW hubs. The shock towers got replaced with GPM aluminum bits because they're purple and look cool, the shocks were used as-is until I was able to score a set of HPI aluminum shocks, and the suspension pins got replaced with a HPI stainless pin kit that also came with a rear brace to keep the rear toe-block from being damaged by cartwheels. And lastly, all of the screws and nuts were replaced with stainless steel -- even the bazillion screws on the SLW beadlock wheels. This is going to be running on salted roads, after all.
So, having done all that, what I intended to be a cheap winter nitro basher is no longer cheap, but it sure works well.
When rebuilding the diffs, I filled them with the same silicone grease I used in the RS4, T-Maxx, and ECX Ruckus diffs, because the diffs on those vehicles don't have good-enough seals to retain silicone oil. I also put "diff tuning springs" in both the front and rear diffs for a little extra limited-slip in addition to that provided by the silicone grease. It performs well off-road as well as doing "speed" runs ("speed" is a relative term since traction is limited in the winter), with a front swaybar giving the chassis a bit of understeer bias to make it more predictable when cornering.
Since I started using this truck, the Dirt Works tires turned out to have not enough traction in near-freezing and sub-freezing weather, so they got replaced with Gladiators instead. If the Gladiators don't work well enough either, I guess I can try Calibers, which are made of softer rubber, but hopefully the Gladiators will be good enough because I'm sick of buying tires now.
The original gearing worked okay, but I had to really rev the engine to get it to go anywhere fast, so I recently changed the gearing. The 12t clutch bell was replaced with 13t, and the 52t HPI spur gear was replaced with a 50t GPM spur gear. (it should've been 51t to keep the gear mesh the same with the fixed-position motor-mount, but the GPM gears mesh slightly differently so I had to drop the spur an extra tooth.) This will allow the MT2 to go 13% faster, or to more directly address my concerns, it will let the engine run 13% slower at the same speed. This will be helpful because the engine is still breaking-in and I don't want to rev the crap out of it all the time, and also even with a hot glowplug there's only so rich I can tune the carb before the engine just won't start in cold weather. Being able to run the engine slower should reduce the temperature spikes I've been seeing after high-RPM runs -- it doesn't really seem to care about workload, because the air it's breathing is so cold, but when that little piston is scrubbing away at 30,000rpm even sub-freezing air doesn't keep the engine from heating up.
Needless to say, I haven't needed to put a sock or a mitten over the heatsink to get the engine to warm up and run strong, which is nice -- and frankly I think that's because the engine (especially the lower part of the piston and cylinder liner, which aren't heated directly by combustion and have to rely on heat conducted down from the cylinder-head) was broken-in in cold weather, so it doesn't suffer from excessive leakdown past the piston in cold weather. If you've ever overheated a nitro engine and worn-down the sides of the piston too much, you know what I'm talking about -- you have to overheat the engine every time after that just to get it to seal-up and run strong. This is kinda the same thing, but caused by thermal contraction due to cold weather, instead of excessive wear due to overheating. Having an engine broken-in for cold weather does an end-run around that problem, and it's working nicely thus far.
The only thing that will kinda suck is when spring comes and I have to put it away. But then I'll get to run my Bullet, T-Maxx, and RS4 again.
Survey says...yes, the tire chains work:
Somehow I managed to not get a picture of the exhaust shield I put on the right-rear shock.
This is an idea I first tried on my T-Maxx, after discovering RPM's shock guards and putting them on my Telluride trail-trucks. I realized they could also be used to keep nitro exhaust from spraying all over the shock shafts and getting sucked past the seals into the shock oil, where nitric acid and half-burned castor oil definitely do not belong. I was so pleased with it on my T-Maxx that I asked RPM to make them for HPI vehicles too. They politely declined, citing low sales numbers, but I realized I could combine the guards made for smaller-diameter Associated springs with the special square-profile lower eyelets made for Traxxas shocks, and put that on my HPI nitro vehicles. The spring isn't quite the right diameter, but a layer of heatshrink tubing over the bottom coil of the spring took care of that. Also, the rear shocks on the MT2 are a bit too long for the guard to protect the entire length of the shock shaft, but under normal circumstances the droop on the rear shocks means the unprotected part of the shock shaft is inside the shock body anyway.
As it turns out, only my MT2 needed this mod. The shocks on my RS4s are fully-shrouded by the body shells, and somewhat more surprisingly, the shocks on my Bullets are also fully-shrouded by the body shells -- my T-Maxx and my MT2 are the only nitro vehicles I own with the right-rear shock directly in the path of the exhaust. But in any case, it's all good -- the shock shafts are protected from corrosive exhaust and should last a good deal longer because of it.
After a month of waiting for "one last part" to arrive from Australia (where old RC parts go to hide from me...temporarily), I finally finished giving my MT2 it's 2-year overhaul.
The overhaul consisted of removing the major assemblies from the chassis plate, washing the major assemblies and chassis plate, checking the chassis plate for tweak (there was none!), regreasing the diffs inside and out, replacing all the original bearings with Boca ceramic bearings, and putting everything back together. There wasn't anything besides the bearings that was actually worn-out, but I upgraded a few stock parts as long as I had the truck apart:
I replaced the stock diff outdrives with HD hardened outdrives, which are harder than hell to find anymore -- but the stock ones hold up very well because they're beefier than the RS4's outdrives, so if I can't find another HD set when these wear out, the stock ones will be acceptable.
The MST servo-saver I was using before had a design flaw that turned out to be more problematic than I anticipated, so I replaced it with a servo-saver from the Associated MGT 8.0. I didn't need the servo-saver to operate at full stiffness because the MT2's wheels are so much smaller than the MGT's, so I removed the outermost of the three springs. As a bonus, this servo-saver is available in a version that fits 24-spline servos, so I was able to switch to my preferred truck servo (Hitec HS-7955TG) instead of the low-profile car servo I was using because it has a 25-spline hub.
Oh, and you may have noticed the shiny pan-head screws holding the steering servo in-place. Those are stainless-steel screws from the Traxxas Spartan boat, so I don't have to use smaller screws with big flat washers in order to have stainless-steel screws holding my servos in-place. The only downside is you have to buy the entire screw kit for the Spartan to get four of these pan-head screws, but if you don't already have a pile of stainless screws then you can add the rest of the screw kit to your stash.
I haven't had any trouble with the caster blocks breaking on any of my RS4-based vehicles, but when I found out I could get caster blocks that have ball-bearing pivots, I decided those were a worthwhile upgrade -- I could have the pivots shimmed nice and snug to minimize play without worrying about them binding and messing up the steering. I now have these on all of my RS4s and Tamiya DF-03Ra as well.
After two winters, the original body I got with this truck (which was already used) was pretty well trashed, especially in the rear where the body has a tendency to snag on the ground if the truck lands a jump nose-up. I'll have to figure out a solution for that problem, but in the meantime it got a brand-new body that I'd had in my spare-bodies box for the past couple years. This one has been sprayed on the inside with Spaz Stix nitro-protectant acrylic coating to keep exhaust fumes from slowly dissolving the paint. It's also reinforced with Traxxas body-mount washers stuck to the undersides of the body-mount holes with 3M foam tape to prevent punch-through failure if the truck lands a jump upside-down, as well as foil tape to protect the body from the heat of the exhaust header, and felt to protect it from abrasion where the exhaust pipe touches the body. It's a lot of prep work, but I've found it helps the bodies on my nitro trucks hold-up far better than they would otherwise.
Of course, while I was doing all this work, the outside of the engine got a thorough scrubbing and the carburetor got taken apart and flushed to remove any debris (like cat fur) that might be obstructing fuel flow. However, when I took off the cylinder-head to check the internals, I saw this, smiled, took a picture, and put it back together. I've never seen a happier-looking engine, and all this one needed was a few drops of after-run oil on the moving parts to keep the residual castor oil from gumming-up while it sits on the shelf for the next few months.
Still one of my favorite nitros. It wouldn't work well in the summer because the grass is so thick (seriously, it makes my T-Maxx bog down), but it's perfect for running in the winter when the grass is flat.
It's amazing what a difference a single new outdrive can make.
The rear outdrive on this truck's center gearbox is made of hardened steel, but after a couple winters it was worn-down anyway, because the brake disc rides on that outdrive so it can't be oiled like all the rest. This is unfortunate because that outdrive also has the higher workload of the two, since I drive this truck mostly off-road so it's almost always accelerating against drag from the grass -- work which is primarily done by the rear half of the drivetrain.
I know for a fact the lack of lubrication is what caused the outdrive to wear-out, because the other end of the rear-center dogbone shaft also fits into an outdrive, and that one is oiled and has almost no wear on it. (Side Note: This is why my answer to "should I lube my CVDs/dogbones?" is always "YES!" Dirt may stick to the oil, but now it's oily dirt, so it's still not as abrasive as dry steel against dry steel.)
My Winter MT2 had been running slower lately, which I attributed to engine wear, because it's on its third winter using the original piston and sleeve, and it's had a couple failed crankshaft bearings which could've allowed the crankshaft to widen the crankcase bore underneath the carburetor and compromise the airtight seal in that area. But I figured I'd limp it through this winter before doing a major rebuild on the engine (though I did replace the bearings, obviously), so I just replaced the rear outdrive on the center gearbox and called it good-enough. While I was at it, I decided to try a little Tamiya Anti-Wear grease in that outdrive, because it's super sticky and I was hoping it wouldn't spray all over the brake disc. (I was wrong, but it's possible I just used too much grease -- only a microscopic film of lubrication is necessary to reduce friction, after all.)
Now the truck runs so well I've had to enrich the fuel mixture to keep the engine from over-revving and temperature-spiking. It wasn't the replacement crankshaft bearings that did the trick, because I replaced those weeks ago and the truck ran about the same. What made the difference was the new outdrive and its fresh smooth surfaces, with no divots to catch the dogbone pins and cause extra drag on the drivetrain.
I like simple fixes.
I put new wheels on my MT2. After two and a half winters of running, the plastic beadlock wheels were losing their grip on the tires, culminating in a blowout where the rear-right tire came completely off the rim and went rolling down the lawn. I didn't want to go all-out and switch to aluminum wheels for a tighter grip on the tires, because that would just be way too much rotating mass for a go-fast vehicle, so since I was sure I wouldn't want to experiment with any other tires, I pulled the existing tires off, washed them, and glued them onto a set of HPI Split 8 wheels from the Crawler King. I suppose they're too heavy for racing, but I'm not racing, so durability is my main concern -- and these wheels are nice and chunky.
While I had the tires off, I discovered that the foams had started to degrade from trapped water, so I punched holes in the tires and decided to try to rehabilitate the foams if I could. I was pretty sure that heat would be necessary, and I discovered that steaming the foams in the microwave was a very good way to heat the foams evenly. I just got the foams wet and squeezed them out (which I was going to do anyway to remove dead foam granules and any dirt embedded in them), and then I stuck them in the microwave for 1 minute apiece. The microwave heated-up the remaining water inside the foam and gave it a nice thorough steam bath, which puffed-up the foams to their original size and shape quite nicely.
I want to take a moment to celebrate my MT2, which has survived 5 winters now. :) It's not my only 5-year-old RC, but it's my only nitro RC that's had to deal with the extra difficulty of running in sub-freezing conditions.
Yes, those are the same tires I've been running for years. Apparently dead grass doesn't wear-out tires very fast, though the pins are finally starting to show their age, and the foams are slowly crumbling to dust. Most of the other major components are the same as they were after the initial build, too, with the exception of the upgraded titanium turnbuckles and aluminum hubs to handle the higher forces exerted on them by the stadium-truck tires. (the hubs are the same as used on the RS4, which was an...optimistic design decision by HPI.)
It hasn't been all roses, I admit. The cold temperatures caused the spur gears to strip over-and-over until I finally wised-up and bought a Robinson Racing hardened-steel spur gear, and a clutch bell to match. It has an irritating tendency to break axles (even the aftermarket MIP axles, which I thought would be stronger!) because the CVD joints are too small; I will probably resolve that by changing back to plain old dogbone axles, though I haven't done that quite yet. And for a long time I fought with its suspension, because it doesn't have quite enough suspension travel in factory configuration, causing it to smack its butt on the ground and then front-flip in a most displeasing way when it runs over a sharp bump such as a tree root while accelerating. (and it's always accelerating when driving off-road, because it's always fighting drag caused by running through grass.) I eventually sorted that out by remounting the rear shocks to the suspension arms instead of the hinge-pins, thus moving them inboard, and then drilling mounting holes in the front suspension arms for the front shocks to get the same amount of suspension travel in the front too.
The truck handles real off-road conditions (i.e. not a clay track, but actual grass with hidden divots and tree-roots) much, much better now. Of course, the longer suspension articulation makes the situation with the axles somewhat worse, but it also makes it somewhat better, because the truck is less likely to cartwheel all over the place and bend one of its axles so much that it breaks.
But the thing I'm really pleased with is, the truck has run for 5 winters with the same piston and sleeve, and they still look great:
The engine hasn't been completely maintenance-free for the past 5 winters; I had to replace the O-rings in the carburetor once (maybe twice, I don't remember anymore), and I replaced the crankshaft bearings with stainless/ceramic bearings after the front bearing rusted from exposure to road salt a few years ago, but honestly I consider that normal maintenance for severe operating conditions. I have no idea what caused the piston and sleeve to turn that lovely gold color shown in one of my previous posts from a few years ago, but I've never seen it since; must've been a change in the oil premixed into the fuel I'm using, or something. But regardless of what color the parts may be, the piston has no significant scoring or scuffing along the sides, the upper part of the sleeve is polished mirror-smooth, and the piston and sleeve still have enough pinch to push the sleeve out the top of the crankcase after 5 years! But that's what tuning to achieve a consistent operating temperature will do: the piston and sleeve always heat up and expand the same amount, so their fit is always the same.
I did notice a little polish-mark on the crankshaft counterweight where the bottom of the piston skirt is starting to rub against the counterweight, though. I checked the connecting rod and it's got some small-end play, so I'll need to replace the conrod, and possibly the wrist-pin too, because I'm not sure which of the two parts is out-of-tolerance. I gotta say I never expected the conrod to wear-out before the piston would, especially on a cheap RTR engine. It's mildly disappointing that the conrod needs replacing at all, because the bronze big-end bushing is still in great shape, no slop at all. Maybe I'll drill an oil port into the top of the new connecting rod to see if improving oil supply to the small-end bushing will make it last longer. Even if it doesn't, though, it's hard to complain about 5 years of service from something that is fundamentally a toy.
Here's to another 5 winters of numb fingers. :)
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