New exhaust pipe and extra support added. Hopefully this one lasts more than a single drive; these flex pipes are not cheap. (as if anything in this hobby is cheap...)

http://i.imgur.com/V7aKJna.jpg

...aaaaand...nope. The second flexible exhaust pipe broke too, at the same spot right next to the endpiece. Dammit.

The engine runs well, at least. Popped a wheelie at one point. Peak temperature next to the exhaust valve was 385°F; I'm unsure if that's too hot for a ringed 4-stroke engine. I burned my fingertip on the valve cover when I had to restart the engine after a rollover, but the nerve endings in that fingertip are apparently dead because it doesn't hurt. The inside of the combustion chamber looks fine despite the high temperature.

Man that sucks, what the game plan for new exhaust? You should temp the engine while running, with fan mine can hit 230-300deg.

I'm going to try piecing together a rigid exhaust from available angled exhaust headers, and use a shorter flex piece for the last part of the exhaust. I think what killed the first two flex pieces was a combination of the intense heat right next to the exhaust valve plus the bend that was required in the exact same spot. The instructions warn against bending the flex pipe right next to the exhaust valve for that very reason. I thought the first breakage was just a fluke, but two breakages in the same spot is a pattern. A couple pieces of rigid pipe should absorb most of the stress from the hot exhaust and cool it down a bit before it reaches the shorter flex pipe. Now I just need the damn parts to arrive.

If this doesn't work I will have no choice but to bend and thread some rigid pipe, and I'm not looking forward to that. The required tools could easily cost more than multiple pre-bent exhaust headers from OS.

From what I see in the pictures above, that bend doesn't look bad at all. So id say if its still breaking abort that idea all together. Dies for threads can be had cheap on eBay. Along with bending device, its the turning down the pipe that will be the issue. Unless you get a mini lathe "highly recommend" you'll need to contact a machine shop. Think about lathe this way, you can make custom flywheel, clutch shoes, exhaust pipe, mufflers, and the list goes on.

OS recommends using a rigid exhaust header upstream from the flex pipe for larger engines, so the flex pipe doesn't overheat and crack. The FS-40S isn't one of the engines they listed, but maybe because it's the car/truck version of the engine it runs hotter than the airplane version.

I found some 8mm ID / 11mm OD brass tubing on Amazon, so it shouldn't be necessary to lathe the pipe to make it fit; the outer diameter of the threaded section of the exhaust pipe is about 10.8mm, so a M11 threading die shouldn't struggle to remove the tiny bit of extra diameter while simultaneously cutting the threads. With the brass tubing, a M11 threading die, and a pipe-bending tool, I should be able to make a single-piece rigid exhaust if I have to. The only thing that would suck about that solution would be needing to remove the engine every time I need to unscrew the exhaust pipe, because there's no room to unscrew a bent exhaust pipe from the engine without the pipe whacking against the chassis.

Oh wow, some updates on the ol' thumper thread, sweet!
That flex pipe stuff is notorious for cracking, even when used as intended. Brass tubing will likely crack as well, as it is soft and work hardens. Your best best is to do what I've started doing, and get some .020" wall stainless tubing and bend it to shape. Then flare the end to fit the O.S. nut.
Second order of business is, you need a fan no way around it. Both speedworks and I use 40 x 28mm. They are bad to the bone. On a small 800mah 2s lipo, the fan runs all day and puts out so much air that not only does the engine stay cool (sub 300 even after long runs in grass) it cools very fast at idle down to 120 or so if I just let it idle for a minute. Without a fan, we found, you have to run so rich that you truly dont see peak rpm.
Also regarding rpm, if your clutch isnt engaging soon off idle, youre probably slipping pretty hard and will melt down the shoes. But thats an easy fix by adding weight to the shoes, and running stiff springs. (Stiff, not soft.)
Im looking forward to more updates! My projects have been on hold due to other high priority life crap. But soon I'll be back to the 4 stroke life.
edit:
Oh another thing regarding the cams. If you look back in this thread somewhere, you'll see where I measured out several cams including both the .26 and .40 car cams. Theres more to it than advancing the profile over. Theres differences in lift, duration, and overlap.

I'm running BuKu heavy Aramid shoes with normal springs, with the spring tension tuned to engage just above idle. (why would anyone tune a clutch to engage higher than necessary? that's just asking for trouble.) I've seen Aramid shoes overheat a clutch bell until it was glowing a dull red and the shoes themselves were fine, so I'm not concerned about melting anything, but obviously the Aramid shoes do fade pretty badly when they get hot, hence why they slipped enough to overheat the clutch bell in the first place. That was on a T-Maxx, though, and the T-Maxx has an embarrassingly undersized clutch. I'm running a proper 1/8-scale clutch in my 4-stroke Revo.

I know there's more to cam tuning than just timing advance, but timing advance is the only adjustment that can be made without precision machining tools.

How did you mount your cooling fan to withstand the bumping and jarring that a RC truck experiences?

Why would it matter if brass tubing hardens from heat-cycling? The exhaust pipe doesn't move.

Well normally with the 4 strokes guys first try a normal clutch. They find out it wont work because they engage at 10k+ rpm. Then they try lighter shoes.. that sorta works to get the engagement point lower, but they still have the issue of low shoe weight not producing enough friction to clamp the shoes to the bell, and the shoes burn up.
Thats why I said heavier shoes, and stiffer springs. To get engagement right off idle (regular springs would drag at idle speeds with heavy shoes) and once engaged, the clutch actually grabs.

However, with you using a Buku setup, none of this applies... so onto the next point.
As for the cam timing, Id be interested in seeing what you can come up with.

The fan is just mounted on a little bracket hanging off of the transmission.. for the revo, Im sure you can find a good spot. The only vibration mitigation youll need is a dab of glue on the wires coming out of the fan. They will vibrate and the solder will crack. Thats how one fan of mine failed.

As for the exhaust not moving, of course it does. Tiny little vibrations, and of course at specific RPMs that exhaust pipe will resonate and move quite a bit. Id put money on a brass exhaust pipe failing. But feel free to test it out.

True, the exhaust does resonate at certain engine speeds. Hopefully the wire hangers I'm using to brace it in the middle will minimize that effect.

The BuKu clutch really is an ideal setup for this engine. I can use the adjustment screws to preload the clutch springs as much as necessary to keep the heavy shoes from engaging too early, which means I can use normal (or even soft) springs to ensure the shoes engage fully as quickly as possible once the engine revs-up to the engagement RPM. Stiff springs would keep the heavy shoes from engaging too early, but also prevent the shoes from fully engaging as quickly as possible, because stiff springs require a higher amount of force per each degree of bending. With stiff springs there's a wider range of RPMs where the clutch shoes are only partially engaged.

I'd appreciate a picture of the fan bracket you're using. Is it wire, sheetmetal, machined billet, or something else? I'm just trying to figure out how much effort I'll need to put into making a fan bracket that won't break, rather than figuring it out through trial-and-error.

I have a BUKU brass flywheel with their brass composite shoes that they made custom for me in their "heavy" size, so they're massive. However, the regular ol' aluminum shoes with tungsten weights pressed in, and .8mm springs have worked out so well that I havent had to try my Buku yet.
and yeah here are some pics. Its literally just a piece of angle that I found in my junk drawer that I cut up to fit with a dremel.
https://i.ibb.co/j31jjJv/20200226-200656.jpg https://i.ibb.co/8XTTcph/20200226-200708.jpg https://i.ibb.co/RTCQGC4/20200226-200755.jpg

Gotcha, thanks. There's a surprising lack of accessible hardpoints on the Revo chassis, but I can probably get away with sticking the fan to the top of the servo box just in front of the engine. Maybe I can scrape up a couple spare servo-mount brackets to use as hardpoints for the fan to attach to.

What are the specs on the fan? Voltage, amperage, RPMs?

- - -

In other news, I decided to add a one-way valve to the crankcase ventilation hose after all, because I'm still seeing a little oil spray on the chassis just behind the flywheel. The one-way valve should ensure the engine generates at least a slight vacuum in the crankcase, keeping oil inside where it belongs.

The fan is a Delta brand. 40x28mm. 12v, 0.87amp, 26cfm. Again, I dont even run it at 12v. I use a small 2s lipo. With a 3s lipo this fan will blow itself across my table top. I got mine from frozen CPU. Then got a 4 pack of the same model number off amazon. My particular model isnt even available anymore, but frozen cpu still has three other 40x28mm fans.
It must be 40x28. The 40x20s dont have the blade pitch for the high cfm.

Gotcha.

I don't want to jinx my truck by saying I've solved the exhaust problem, but my newest exhaust at least survived a single drive:

http://i.imgur.com/j5ZWvI5.jpg

It needs an extra support hanger, so I'll have to make one before I run it again. Also I need to connect the cooling fan to the battery; currently it's fitted to the chassis but the wires are loose inside the receiver box.

You’ll have to get a vid of the truck in action!

Make that two drives. I took it out again for half a tank to tune the shift-point on the transmission. Not sure I have it just-right yet, but at least it shifts now.

No more driving until I get the exhaust properly braced and the cooling fan hooked-up.

The fan is connected and the exhaust is properly braced now. All I can do now is drive it some more and see if anything else breaks.

On a totally unrelated note, does anyone know of a good way to cover a large hole in a Lexan body in a way that will reinforce the hole and not just snap off (and take the body paint with it) the first time the body suffers a hard hit?

Hopefully you have more luck with the squirrel cage style blower fans than I did. None I tried had enough air flow to cool the engine, and the vibration killed two of them.
As for filling a hole, I use window screen and shoe goo. Once set up its even stronger than the base material. Ive only ever used one layer of screen, too.. I imagine with two layers it would be bulletproof.

Window screen, huh? I never would've thought of that.

Blower fans are more efficient than radial fans. They *should* produce more airflow than a radial fan spinning at the same RPM. Anyway, we'll find out how it works. The one I got has dual ball bearings, so hopefully it's reasonably sturdy.

EDIT: Also, I should mention that I bought a 5V fan instead of a 12V fan, so it's very energetic running on a 6V receiver pack.

ANOTHER EDIT: 3 drives now without breaking the exhaust. I wonder when I can safely assume the exhaust will be reliable for the long-term.

Alright, time for a video!

Trying to hold a smartphone while also using both hands to drive a RC car and trying to make sure the camera has the RC car in-frame while also watching where the RC car is going reminds me that I really ought to buy a GoPro someday.

Took the truck for another drive today. I think the engine's nuts have finally dropped, because it's suddenly running much richer than it would like and I'm able to lean the fuel mixture without instantly overheating it.

On a related note, where is the best place to read the temperature of these engines? The glowplug doesn't seem to get particularly hot, which doesn't surprise me since it's a 4-stroke and it's only combusting every other crank rotation. I've been checking the temperature of the exhaust header because it's the hottest part I can find, but it's probably also not the most relevant temperature reading.

Ive always temped the glow plug. After long high speed runs I like to see about 300 on my .62. Ive got gallons and gallons through that engine.
Checking near the exhaust port is misleadingly hot.
edit: Just to clarify, I dont tune to a temp. I just make sure Im anywhere between cold and 320f (at the highest) and Im good.

Get somebody to hold the phone! Wife, girlfriend, kid, friend, neighbor,etc....! Just giving you a hard time 😎! A video would be cool though!

Good to hear new exhaust setup is working out. And very interested to hear what kinda temps your getting. Iv always temped mine on side of head-sleeveish area, not the exhaust side.

Got a part number for the fan your using?

Sorry, no I don't. You'll have to make do with an Amazon link instead. ;)

I'm not sure the glowplug temperature is an accurate overall temperature indicator for a 4-stroke. The glowplug isn't exposed to nearly-continuous combustion like it is in a 2-stroke. The exhaust valve is generally the hottest component in any 4-stroke engine, but there's no way to check its temperature directly. I know the exhaust header isn't the best place to check (obviously the rest of my engine isn't 375°F like the exhaust header is), but I haven't been able to figure out an easily-accessible location that makes more sense.

Anyway, if you're seeing 300°F at the glowplug and your engine isn't suffering premature wear, then I'm doing just fine, because my glowplug is nearly 100°F below that. Granted, it's still chilly where I live, so we'll see what the temperature looks like when the weather gets hot. I can't tune my engine much leaner than it is right now, though; if I do, it will lean-out during full-throttle runs. Or at least it did before; recently it suddenly started liking its fuel mixture to be much leaner than it was before, so...I dunno.

On an unrelated note, my homemade PCV system is finally keeping oil leakage through the front crank bearing under control, since I added the one-way valve to ensure the engine actually pumps pressure out through the PCV tube into the carburetor. I haven't had to wipe-off any oil from the chassis or the back of the flywheel in the past couple runs. Just to be sure, I opened the back of the crankcase and took a peek inside, and yes, there is a good amount of oil pooled at the bottom of the crankcase, so the lack of oil leaking out the front of the engine isn't the result of oil starvation in the crankcase.

Cool thanks, maybe ill give that a try on another build

The exhaust is still holding together. This gives me hope.

I speed-tested the truck today. 30mph on pavement. Doesn't sound impressive, but my master spreadsheet tells me this equates to an engine speed of 19605rpm, quite close to the theoretical maximum of 21000rpm. I might be able to lean-out the engine a little more, but it's probably not worth the risk of premature wear; I'll just have to figure out how to get higher gearing if I want higher speed. Currently the truck is running a 19t Mod 1 clutch bell originally intended for the HPI Savage; I'm not aware of any higher-geared clutch bells I could be using.

I think you need to lean it out. You're not going to hurt the engine. When it's too lean, you'll know. These 4 strokes like to just "flame out" under load when too lean, unlike the 2 strokes which can hesitate and change their sound profile significantly.
You should be getting a LOT more RPM out of that engine. I got 20,500 on my .62 airplane engine in my Losi truggy.
There is a 20t HPI bell you can use. There are also up to 23t bells on ebay... same typical 1/8 scale dimensions.

This engine was doing the normal "hollowing-out" sound that my other engines do when they get too lean, but that was during the first couple runs while the ring was still seating, so maybe break-in was having some effect that I don't know how to account for because this is my first ringed 4-stroke engine. Or maybe the "hollowing-out" sound when running lean is related more to the increased timing overlap that car engines require compared to airplane engines, rather than the power cycle the engine uses.

If it's possible to get a "lot" more RPMs out of the engine, why is it rated at 21,000rpm by OS? My understanding was OS doesn't bullshit their specs the way cheap engine manufacturers do. Also, since this is my first ringed 4-stroke engine, it's a good idea for me to ease into running it hard, rather than shooting the moon and frying the engine. Parts are not easy to come by; to get a spare piston/ring/sleeve set, I had to buy a whole spare engine.

Anyway, if I'm going to run this engine long-term, it's going to need some unique driving characteristic aside from sounding like a weedwhacker. The easiest unique characteristic for it to achieve is to be a high-torque/low-rpm engine, unlike my 2-strokes which must rev high by necessity. I found a 22t clutch bell on eBay this evening; that should get me to 35mph on pavement, which will match my Losi-3.4-powered T-Maxx. Any extra speed I can get above 35mph will just be icing on the cake.

For what it's worth, the rpm rating on my 62 is 11,000 rpm. It's intended for airplanes, but still!
At any rate, does the engine have the grunt to pull wheelies on the Tmaxx?

Revo, not T-Maxx. It can pop wheelies, but just barely. I didn't realize this until I weighed the truck a couple days ago, but it's a 5kg/11lb truck, almost a pound heavier than my T-Maxx. I tightened the clutch springs slightly and that gave it much more punch when accelerating, even though the adjustment I made to the clutch springs was only 1/12 turn on each of the adjuster screws. I guess I had the clutch engagement set slightly too low. I'm also leaning-out the engine a tiny bit at a time (1/12 turn on the carb dial), so I can see how the engine responds over the entire length of a run before making another adjustment. The most recent adjustment caused the exhaust temperature to spike from 360°F to over 400°F, which leads me to believe I'm very close to the ideal fuel mixture if not slightly past it.

Good to hear the exhaust is now working on the revo. Wasn't the revo the truck dpbuggy had? He's the guy from years back that got me thinking about these thumpers. Anyways like timjs said these thumpers will just flame out when to lean. I can say that over the years iv started to tune to that point almost all the time now. And iv never seen any ill effects to piston or sleeve. Now 2 stroke would be a different story for sure.

I'll keep that in mind. I'm just really hesitant to risk oil starvation or severe overheating in an engine as rare and expensive as this.

I hear ya, id be thinking the same.

O and have you thought about going the route of converting a plane engine? That way you can get this thumper thing dialed in before messing with the .40.

Considering this engine cost me 700 freaking dollars, you better believe I considered converting a FS-40S airplane engine instead, but finding a pullstart crankshaft and backplate is impossible nowadays. I gave up on the conversion approach after several months of searching.

Speaking of expensive things, I bought a "cheap" FLIR camera (I've wanted one for years) and took a picture of the FS-40S-CX after running for a few minutes:

http://i.imgur.com/slzA4rB.jpg

This confirms what I suspected about the glowplug not being the best place to check the engine temperature. While the glowplug is directly exposed to the combustion chamber, it's also right next to the intake plenum, so the glowplug is constantly shedding heat into the fresh air/fuel mixture coming in from the ice-cold carburetor. We all agree the exhaust header is also not a good place to check the engine temperature, but it looks like the best place to check is on the side of the crankcase near the camshaft chamber.

Additional notes: The reason the cylinder head as a whole looks cooler than the rest of the engine is actually because it's bare aluminum (i.e. highly reflective), instead of flat-black coated like the other parts on my engine. I assume it's actually the same temperature as upper edge of the crankcase. And the engine mount adapter plate isn't actually as hot as it looks, it's just reflecting heat emitted from the underside of the crankcase.

Cool image! I'd be curious if a handheld infrared temp gauge would verify the results and give actual numbers?
I'm probably going to stick with the glow plug because it's easy to access, consistent, and I know (for my engine at least) 300-320f at the plug is perfectly fine as evidenced by the amount of runtime I have on the engine. Temp takes a back seat to sound and performance when tuning anyways. However, I also understand your desire to keep the engine from getting potentially damaged.
Wouldn't you be able to use standard FS40 parts for the 40SC? I mean like piston, rod, sleeve, valves, ect..