Yes, I know a single-cylinder engine can never be fully balanced without a counter-rotating balance shaft, and nobody will ever build a little nitro engine with that level of complexity. And yes, the procedure you described is the procedure I used, except I set the crankshaft on the edge of a glass table with the counterweight hanging off the edge. Glass tables are the flattest surfaces most people own. Because the HPI 15FE engine has a bronze conrod, I didn't need to remove any weight from the counterweight, I actually needed to remove weight opposite the counterweight, which I accomplished by lightening the piston, conrod, and crankarm.

I don't think Awesome Scoops™ on the crankshaft really do anything to improve mixing or airflow, but I do notice engines that have Awesome Scoops™ added at the factory always have them positioned opposite the cutout for the rotary intake valve. So I think they're just cosmetically-appealing methods of balancing the crankshaft to account for the cutout for the rotary intake valve. Anyway, that doesn't apply to a 4-stroke engine.

So...where is the excess weight hiding in the FS-40 engine? The procedure you described is a good starting point, but it doesn't account for side-to-side imbalances, such as the conrod traveling downward on one side of the engine while the counterweight is traveling upward on the other side of the engine.

We all know you aren't going to be chopping up that FS40SCX.

Not haphazardly, no. But if I can determine with a reasonable certainty where it would be beneficial to remove rotating mass, then I might do it. Anyway, it's a worthwhile discussion to have in a thread dedicated to the discussion of how to improve these engines.

Why not grab a cheap fs40 to play with? Whatever works on that one will work on the 40sc.

To be completely honest, I don't have enough spare time nowadays to spend hours during the day taking an engine apart, modifying it, putting it back together, testing it, and taking it apart again to modify it further. Partly that's because I have a toddler now, and partly it's because I'm one of those people whose workload increased due to the current national emergency. I was hoping someone else might've already done that work and figured out what modifications are beneficial. I can look at the internals in a 2-stroke engine and at least roughly estimate what modifications are likely to be beneficial, but I don't have that expertise with OS 4-strokes. (also 2-strokes are much faster to disassemble for modifications.)

However, since I have pieces from a FS-40 that I previously bought to get rebuild parts for my engine, I dug them out and weighed the piston, wrist pin, and conrod. They weigh 12.4 grams together. The wrist pin and conrod are 6.3 grams, which is so close to 50% that it makes no difference. When I hang the wrist pin and conrod from the crankshaft, the wrist pin and conrod drop to the bottom and the counterweight rises to the top. So it appears this engine is similar to my 15FE in that the counterweight is lighter than it needs to be.

The weight isn't 'hiding' anywhere, simply a fact of life with a counterbalanced single cylinder using a flat plane crankshaft. Multi cylinder engines with a flat plane crank also have more vibration than cross plane cranks for the same reason. As for 'side-to-side' imbalances, it's the same the 'up-down' imbalance. There is no other force acting on the the rotating assembly other than centrifugal. There is a very small side load as the rod transitions at TDC and BDC but, it is quickly gone as the crank rapidly accelerates in the opposite vertical direction. There have been many experiments in offsetting the cylinder in relation to the crank centerline to reduce the side load at transition. There have only been minimal improvements and, with your FS-40 it would require a custom crankcase, which is out of the question.

Reducing weight overall in both the crank and piston/rod will help due to the lower inertia. If your assembly is already near 50-51%, you could try taking weight out of all the parts and likely see an improvement in not only vibration but acceleration and peak rpm.

This discussion is fun, I haven't been able to talk engine design in quite a while. All the focus is on electric, which is expected but, it's still nice to turn fuel into noise sometimes.

Iv been thinking about doing weight reduction/balancing on my .91. Haven't had the time and maybe the motivation to do so yet though. But when I do I plan to start with piston/rod weight reduction. Then go to crankshaft as im looking for more rpm with vibration dampening. Ill be doing the razor blade setup that was talked about above. Really want 20k+ rpm out of this beast!

"Where is the excess weight hiding" was a figure of speech.

Understood. I like your avatar. I too am a fan of whirling doritos of death. I've built many of the O.S. version.

Guys been reading through the thread. Lots of great info. Trying to consolidate important stuff in on post. Missing lots let me know what and will add.

FS30 To car spec
Cam OSM45762010
Valve Spring x 2 OSM45760210
Intake Manifold OSM45269400
Carb Retainer OSM21481700

Other options
Complete OS26 Head OSM44104020
Compete Valve Assy x 2 OSM45760020

FS26 to 30 size
Liner OSM43003100
Piston OSM43003200
Ring OSM43003400
Pin OSM43006000
Gasket OSM22714100

FS40 To car spec
Cam OSM45262050
Valve Spring OSM45260250
Cylinder Head OSM45204150
Exhaust Nut OSM45169200

Other options
Compete Head OSM45204050



Flywheel
Heavy steel flywheel recommended for low idle.
Option 1
OS Drive Washer OSM 45208010
Standard Flywheel
Associated Clutch Nut 2312 (Drilled and tapped to ¼-28
Option 2
Get Crankshaft ground by machinist to 7mm https://www.rctech.net/forum/showpos...postcount=1891
Then use standard collet like Associated part 7618 and standard flywheel
Option 3:
Get crankshaft ground to 7mm by machinist, then use BuKu 34mm brass flywheel and BuKu BBK shoes with standard springs.

Option 3:
Get crankshaft ground to 7mm by machinist, then use BuKu 34mm brass flywheel and BuKu BBK shoes with standard springs.

Speaking of BuKu BBK shoes: I've been having trouble with my Revo's clutch slipping and overheating while driving through grass with 2-gram "heavy" ABK shoes, so I ordered a set of 4-gram BBK shoes to get more grip. I was surprised to find that Dave also sent me a set of 5-gram extra-heavy BBK shoes! He guessed that I was working on something unusual and needed the heaviest clutch shoes around. Now I just have to decide which set of shoes to try first; obviously the 5-gram extra-heavy BBK shoes are the best bet, but I wonder if they're so heavy that I might encounter spring-tuning issues. The 4-gram heavy BBK shoes are still 2x the mass of the "heavy" ABK shoes I was using previously, so they ought to work pretty well too. Then again, I'm never going to run a heavier vehicle with this unusual engine setup, so maybe I should just go straight to the extra-heavy shoes and save the slightly lighter ones for a future project. Choices, choices...

He sent me the extra heavy ones also, as he knew the project. Truth is, I haven't tried them since the tungsten weighted aluminum shoes have been so successful. So I'm really interested in your results.

The extra-heavy BBK shoes DEFINITELY have more grip. The first time I blipped the throttle, the truck stood up on its rear bumper, and then fell over upside down. This is the first time I've had to adjust a BuKu clutch to make the shoes engage at different RPMs for smoother engagement.

Alas, even the extra-heavy BBK shoes can't withstand the BONE-CRUSHING POWERRRRR of the FS-40S-CX. While the shoes no longer slip constantly, they do start slipping as soon as the transmission shifts into second gear. When the transmission shifts, the engine RPMs drop and the centrifugal force on the clutch shoes decreases, and at that lower RPM they just can't transmit the engine's full torque without the extra leverage that first gear offers. I'm just going to have to reduce the pinion/spur gear ratio and accept the lower top speed; I suppose it's not too much of a loss since I rarely drive the truck on pavement anyway.

- - -

Oh, and the first time I ran the truck with the extra-heavy shoes, about halfway through the run the engine stopped and wouldn't start again. I didn't have time to figure out why, but when I went back and inspected the truck later that night, I discovered the flywheel was spinning freely on the crankshaft. The extra-heavy shoes were resisting slip so effectively that the next-weakest joint started slipping instead -- the crankshaft spun inside the flywheel collet and ground it up. I spent 15 minutes cleaning brass shavings off the engine and clutch before reassembling it with a new collet, plus a few drops of Red Loctite between the collet and crankshaft to hopefully prevent it from slipping again.

On a related note, is there a special tool used for holding a flywheel while tightening the nut that holds it in-place? For the past few years I've used a pair of parallel-jaw pliers to brace the pivot pins on the front of the flywheel while tightening the nut, but every now and then I wonder whether this improvised method is someday going to bend the pins on one of my flywheels.

Yeah, there are special tools. I've used channel lock pliers for 18 years.
Cool update! Not sure why you would have slip on those monster shoes though. My setup has more power, pushing an 11lb truggy against a tall ratio to hit 46mph, and it doesn't slip. Maybe your shoes need to wear in?

Wouldn't that flywheel breaking free issue be the reason your truck isnt doing well in second gear? Did you rerun it after the red loctite? Or does engine bog down really bad once it shifts and rpm never recovers? Mine did that with the .30 in the heavy truggy. Was just to much gear, clutch never slipped and only had 2.6g weight on them.

Your shoes shouldn't be slipping unless those springs are just to much. Or those composite shoes don't work that well. There is talk on other forums here about not using those shoes in heavy truggy only buggy. I have some to try also just holding off for now.

I guess I don't know for sure that the flywheel didn't slip a little bit before it finally let go completely, but I assume the D-cut in the crankshaft acted like a machining bit as soon as it was able to turn at all and it reamed-out the collet pretty fast. I don't see any rub marks on the crankshaft that would suggest the collet was slipping for a long time before it failed.

The truck bogs down only in 2nd gear. But also I'm driving it in grass that only gets mowed every 3 weeks or so, and that causes an enormous amount of extra drag. There's no problem at all when I run it on dirt or pavement.

I've thought about whether solid brass or stainless-steel shoes would work better, but I have no machining capability so I'm at the mercy of what the market offers. The BuKu clutch is the closest I can get to a perfectly designed package, because it has a heavy brass flywheel, heavy brass shoes, and adjustable springs available in multiple strengths. (I'm using the lowest-strength springs so the clutch will drag for the least amount of time as the engine revs-up.)

I haven't had a chance to drive it again. Partly I've been busy with other things, and partly I'm waiting for a 20t clutch bell to arrive in the mail. I'm going to drop the gear ratio down to 20/38 from the 22/36 I was using previously; if that is sufficient to fix the problem, then I can start investigating ways to regain the lost speed by modifying the engine for higher RPMs instead of relying on high gearing.

Sounds like a good plan dropping the ratio. I think your setup will be happier topping out around 28-30mph or so. The 40sc is a car engine and revs well, yes, but it's still just a 40. And you truck weighs what, 12lb? Imagine it in a 6-7 lb stadium truck.

"Just" a .40, LOL. It's double the displacement of any of my other engines (or more!), including the Losi 3.4 engine in my T-Maxx, which is only 290g lighter than my Revo. Sadly, the parasitic losses from driving a 4-stroke valvetrain appear to sap a significant amount of the power it could make if it didn't have that extra hardware. (or maybe it can't breathe well enough.) I suppose that's why 4-strokes never caught-on for ground vehicles.

Haha. Yeah... "just" a .40 sounds weird. But you're right about the losses. When you go up some in displacement though, the power comes on pretty good. My 62 is a beast... and speedworks even has a .91 build. I haven't seen that one in person yet, but its gotta be insane.

With a bigger engine I'd have even more trouble with the clutch slipping while driving in grass, so that's a non-starter.

I'm contemplating moving my FS-40S-CX into one of my HPI Bullets. The Revo is a nice truck, but I think it needs a big-block 2-stroke to really move. The FS-40S-CX could really haul ass in a smaller truck like the Bullet, but on the other hand, the Bullet has a 75cc fuel tank and the FS-40S-CX annihilates fuel. The FS-26S-CX might be a better fit for the Bullet, but it might lack the necessary power to make that truck run well. The HPI T3.0 engine in my other Bullet is (supposedly) rated at 2.2hp @ 31000rpm, and it still requires lowest possible gearing for that engine to move the truck through grass that hasn't been mowed in a week. I'm completely confident the FS-40S-CX has enough power to drag the Bullet through grass at a fun speed -- hell, it has enough torque to pop a standing wheelie in the Revo -- but the fuel tank would run dry in a few minutes; whereas the FS-26S-CX may not have enough power to do that job.

I suppose this is why 4-strokes never caught on for surface RCs. And yet, I really do want to have a 4-stroke in my lineup, for variety's sake if nothing else.

Give it a shot. My 62 was in my bullet for a bit. It required making an extended chassis to fit. Pics somewhere in this thread.. anywho, put a 2 speed trans in it. There's plenty that will fit within the bullet's trans mounts. I tried with single speed and I wasn't satisfied with the performance at all. 4 stroke requires two or three speeds, in my opinion.

How much of an extended chassis? The WR8 chassis is the same as the Bullet chassis except that the WR8 chassis is 30mm longer in the rear. Why a rally car needs a longer chassis than a monster truck, I have no idea, but nonetheless the WR8 chassis is a good option if you need a Bullet with 30mm more length behind the center gearbox.

The HB RGT8 has a 2-speed transmission that should fit the Bullet's center gearbox housing with no modifications at all, but...I dunno, 2-speed transmissions are not really compatible with driving off-road unless they're fully-enclosed and there's a separate input gear with a slipper clutch attached. That's why I decided to use the Revo in the first place, because it has a 2-speed transmission (with reverse!) that's actually built for off-road use. Most 2-speed transmissions will be damaged by driveline shock if the vehicle goes off a jump.

When you used your FS-62 in your Bullet, before you switched to a 2-speed transmission, what single-speed gear ratio did you use?

Good info in that wr8 chassis. I made my own, exactly the same as original except a couple inches extended in the rear, and I extended the drive shaft.
You have a point that most monster trucks with 2 speeds traditionally have a slipper as well. Truth is, on a lot of those trucks guys just crank the slipper down tight anyways. I'm thinking back to my Tmaxx, Savage, MGT days. You shouldn't land a jump on-throttle anyways. I don't have a slipper setup on my current losi truggy/fs62 build and it's fine.
The bullet might actually benefit from a slipper, since its 1/10 scale driveline is so weak. I'd try it anyways.
To clarify, I never went to the 2 speed on the bullet. I ended up putting it back to stock, then went through a few 2 strokers on it. But while it WAS 4 stroke, I geared it for 30mph top speed at 20k rpm which had "okay" acceleration.. and then tried a taller ratio that would be good for closer to 40mph and it just sucked the off-the-line torque right out of it. Also, a taller ratio from a stand still murdered clutch shoes. I didn't keep the 4 stroke in there long.
With a 2 speed in the truggy, I get way more speed compared to the original 2 stroke (46mph vs like 32) as well as having a faster acceleration off the line. The trans has a 4 tooth spread between gears.

30mph on pavement is what my stock-engined Bullet can do now. I have a second Bullet with a larger engine that can get closer to 45mph while also popping rolling wheelies, but I drive it so rarely that I decided to put it in storage until I come up with a better idea for it. Even the 30mph Bullet gets a little squirrelly when the tires stretch at top speed; with the 45mph Bullet I have to constantly be on-guard against it deciding to spontaneously wheelie at 40mph and then lose control, flipping ass-over-teakettle until it hits something. Eventually my touring cars took on the role of satisfying my desire to go fast on pavement, and my my monster trucks got the job of going semi-fast in grass.

Yeah, I know most people just lock their slipper clutches rather than fine-tuning them to do their intended job correctly. Traxxas doesn't even install slipper clutches on their new models of monster trucks anymore; they just have a rubber torsion-damper inside the spur gear. I'm apparently a weird monster truck owner, though that comes as no great surprise.

I know what you mean about the Bullet getting hard to control. The engine I've finally settled on is an O.S. .18CVRX. It's not as insane as the losi 3.4 or traxxas 3.3 but still much faster than the stock g3.0. With the other engines, it was pretty much guaranteed to crash and break every time I took it out. I will say, the slipper in my bullet IS adjusted just to the point of not slipping under normal acceleration. With it's driveline being so fragile, I don't lock it down.

I'm pretty happy with the stock engine, now that I have a big orange heatsink to help it avoid overheating. I was able to lean-out the fuel mixture significantly once I found a bigger heatsink to put on that engine.

Anyway, when you had your FS-62 installed in your Bullet, how did you have the air filter mounted? I was eyeballing the fitment tonight and it looks like the rear shock tower would get in the way of the air filter, unless I come up with some ridiculously convoluted piping to position the air filter remotely.

a short adapter with the carb sticking straight back and an MGT 8.0 air filter which has a 90° turn in it. I had just enough room with the lengthened chassis.
The chassis was lengthened to where the wheelbase would fit an SUV style body like a jeep or something, which would give more room around the shock tower. However, I took the engine off before getting a body for it.

Gotcha. Still not sure if I'm going to go ahead with moving the FS-40S-CX engine to the Bullet, but I'm trying to figure out if it's possible without cutting everything to pieces, so I can at least make an informed decision.

I don't think I'd attempt it with the stock layout. It's just so crowded. If it were a forward or side facing carb, maybe. Like an HP, or a Webra.

Working on another thumper build. O.S. .91. I made this collet and flywheel for it on the lathe today. The flywheel is not finished. It needs to be 1/4" shorter, and have the 5 (not just 3) clutch pins added. Before that though, I'm going to bench test the engine with the flywheel as-is to see what reliable idle RPM I can get from it. That's over 280 grams of flyweight right there. Once I know it will idle low and stable, I will finish the machining on the flywheel, and make a clutch pilot shaft.

https://i.ibb.co/kcxWkbr/20200815-195930.jpg
https://i.ibb.co/Yy2kxbm/20200815-223756.jpg
https://i.ibb.co/bF85NGL/20200815-224831.jpg
https://i.ibb.co/ZKQVQhH/20200815-195923.jpg
private picture upload

Why a half-pound flywheel? That seems excessive. Small appliance engines have lighter flywheels than that.

Well like I said, I'm not done machining. It's still going to be heavy though when it's done. I want a super low, reliable idle. This engine won't idle on a flywheel we normally consider pretty heavy. Also, it's belt start (like boats) and 5 shoe clutch using a CEN bell. Pulling all the stops with this one.

Quick idle RPM test. I got as low as 2500-3000 stable off camera, but only with the glow ignitor on. Anywho, I was only hoping for sub 4k, so it's good. Not sure if you can tell on camera, but the engine bearings are absolutely shot. In person the bearings screaming are about as loud as the exhaust. Like TV static turned all the way up.

+ YouTube Video

Belt start? I didn't know that was done. Interesting.

In my experience, worn bearings have a distinct REEEEOWR sound that doesn't really have a word to describe it -- the closest I can think of is the sound of a ricocheting bullet flying past your head (which happened to me once), except without the Doppler shift because the bearing isn't moving. A hissing sound like static sounds more like the bearing is extremely dirty/rusty or under-lubricated. Though it could also be that the engine vibrations are complicating the sound to the point that it sounds like static to human ears anyway.

What are you going to use this engine in, that the performance penalty of such a heavy flywheel wouldn't be a problem?

Yep, this engine needs to be able to go into monster trucks, which means no bump start. So I went with belt. As for the bearings, you're dead on. They're completely rusted out, which I knew before running it. I had to run it to get the wrist pin free from the con rod, its all seized in there.
This engine was a basket case off ebay.
As for performance penalty.. obviously physics dictate that there must be *some*... but it's clearly not measurable based on how the engine still reaches peak RPM instantly when free revved.
Think of this: When you pick your RC truck up and rev the engine with the wheels off the ground, once the clutch is engaged the entire drive line all the way out to the wheels is effectively one big "flywheel". Adding rotational mass at the flywheel versus at the tires shouldn't be any different. Accelerating mass to a velocity requires the same energy whether its directly on the engine, inside the engine, or placed after a series of gears. I think it's fine.

Kinetic energy(joules) = 1/2 mass(grams) * velocity(meters/second) ^ 2. (I admit I don't know how to modify the formula for rotating mass, but the fundamental relationship of the mass and velocity doesn't change.) Squaring the velocity means a heavy flywheel spinning at full engine speed does have more of an effect than simply multiplying the extra mass by the drive ratio and then attaching it to the wheels instead. That's why car tuning enthusiasts love to install lightweight flywheels and say that it makes the car feel hundreds of pounds lighter. But your point is taken, the engine almost certainly has enough torque that the effect is minuscule, and wouldn't matter for bashing anyway.

My understanding was that the inertia doesn't change for a given part. Say I attached a 200g weight to the flywheel, and then swapped it for a 200g weight to a wheel. (Assuming the diameter of said weight doesn't change)
The total inertia doesn't change. What does change, is that the flywheel would place a larger torque on the engine than the wheel would. But, the wheel would apply its lower torque over a longer period of time, conserving the total energy (minus friction blah blah). I might be wrong, I'm honestly not too sure at this point. But yes, the engine doesn't seem to even notice that slug of metal hanging off the end of it.

Momentum, inertia, and kinetic energy are very easily confused concepts and I admit I don't have a good understanding of them myself. (I passed physics with calculus in college, but that was 18 years ago.) I just googled it for a quick refresher, and inertia represents the amount of energy required to produce a specific change in an object's velocity -- e.g. how much energy would be required to accelerate it by 1m/s. Inertia is essentially a unit of measure, whereas kinetic energy is the total energy that an object has due to its mass and velocity. So, an object's kinetic energy also represents the amount of potential energy that was required to accelerate it to its current velocity, and the amount of energy the object is able to impart to another object through torque, impact, or friction. Over a large span of acceleration, that squaring exponent ends up making the object's velocity far more significant than the object's mass. As a really simple example, it requires 2x as much energy to accelerate a 100g weight to 200m/s than it does to accelerate a 200g weight to 100m/s. That's why a piece of insulation foam (moving at hundreds of miles an hour) was able to shatter the carbon tiles on the wing of the Space Shuttle back in 2003.

Don't ask me what momentum is though; I still don't understand how it's a useful measurement as compared to kinetic energy.