gasrc
Posts: 231
Joined: 7/24/2002
From: Houston, TX, USA
Status: offline
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This is an in depth look at a STS 28 or D28M.
Disclaimer: I’m not an engine expert. I know only a little about engine modification. I’m in no way affiliated with STS, Racernine, or any of their sponsored. My opinions are simply my opinions. Linear measurements (inches) were taken with the best of my abilities with Mitutoyo Absolute digital calipers. Angles (degrees) are visual guesses.
I have this engine in my Revo and absolutely love it. It’s a pure beast that starts easily, runs cool, and doesn’t flame out. I’m running it on a cheapo Merlin Plug (P/N MGP2003 ?), and 20% nitro fuel with a castor blend. I love the fact that it is a rotostart (shaft-start, officially), so I bought 2 more to install in my other trucks. I haven’t had the first one for very long, so I can’t speak as to its longevity. All pics are from a new, never-fueled, engine.
First thing to tick me off about this engine is that I can’t for the life of me figure out how they got that conrod and piston in there! I sure as gee-willickers can’t remove it! There is very little space between the conrod and the piston at the wrist pin, so once the piston touches the wall of the case, the rod doesn’t travel on the pin much further. Using borderline non-recommended force, I can only get the rod half way off the crank, which is not even close to getting it off. I gave up after several tries.
THE CARB:
Smooth. Very smooth. It is toleranced very well such that there is nearly not ability to wiggle the slide in the mid range and an inconsiderable amount at WOT. In the picture below, you can see the carb has a round tip needle. What you can not see is that the taper of the air induction bore is not conical, but spherical.
The carb has no insert. It also doesn’t have much of a rounded entry. It’s practically flat. I plan to fix that.
The boot seems very protective. I pulled it back in this pic, to show the sizeable lip used to keep dirt out. The boot appears and feels heavy-duty, but flexible enough not to make your servo work against it much.
The exposed carb casing is composite, but it is mostly steel and aluminum. The part that mounts in the engine case is thick wall steel tube with a conical tapered composite liner. The widest area of the inlet of the carb measures .507, the widest at the exit measures .442 where the liner ends. I used a drill bit to ascertain that the center ID is really close to .360. From tuning one of these engines already, I can say that the low-end needle comes from the factory quite far from where it will be on after you feel you are done with the first tune. This one appears to be around 2 full turns out from tuned after breakin. The highend is about 1.5 turns out form that spot. I use the heat-cycle method, so I was turning the needles from the start. The carb is sealed in the engine case via an o-ring that sets in a groove in the case as you will be able to see in a later pic when I talk about the crank.
THE BACKPLATE:
I have fallen in love with shaft starting my bashers. There’s no going back for me. This pic shows the outside of the shaft start and backplate. Note that the socket is located directly left instead of top left. This means Savage users (like me) will have to do a little cutt’n on the truck to make it accessible.
This pic shows the shaft start gears. Both are steel. The bushing appears to be bronze.
Here is the unique inside. The plate is o-ring sealed. What you can not see (because I didn’t take a picture) is the work behind the one-way bearing. This system seems better sealed than any pull-start engine I’ve ever opened.
THE CRANK
As you can see it is turbo ported. I would like to get in there to smooth the sharp edges and contour the start of the turbo port. I find the round end of the trailing corner of the counterweight interesting. The best I could measure without removing the crank, is .405 for the bore.
The induction side could benefit from a sharp leading edge and contour of the throat. I don’t know how much room there is for adding intake duration as it is already about 215 degrees. Of course I can’t do anything any way …..because I can’t get the conrod off the crank! Unrelated, what you can’t see in this pic is a tiny whole drilled in the case above the crank to lubricate the front bearing.
THE EXHAUST
Well from the inside, there is some nice looking finish work done to the case at the exhaust port.
From the rear, it’s not as impressive with the sleeve installed, not that there isn’t plenty of port area as-is. By this pic, you can see room to add exhaust port open duration. ….…..(continued with next pic)
….and maybe there is. There is about 165 degrees of it as-is. The right black mark represents the start of the piston exposing the bore. The left mark is where the port is closed.
THE PISTON & SLEEVE
It has three oiling rings around the top. The skirt is polished. The top is typical. The skirt has already has a cutaway at the boost port, but I see that a little more may be beneficial. I just need to make sure that a little more is not detrimental. I won’t do it nearly as wide as the factory cutaway. I won’t do anything if I can get that connecting rod off.
The con rod seems strong and contoured well. A bit could be done around the bottom bushing, but I don’t see much positive return from trying to modify it any further. That doesn’t mean that I won’t do it though, if I can get the….well, you know the story. The bottom bushing has a much heavier wall than I am use to seeing.
THE SLEEVE
This thing kinda scares me. Have you ever seen a sleeve this thin? The wall thinkness is .0345. There’s almost no room to do any porting to the outside of the sleeve. With the wall being so thin, there isn’t as much need to do so as heavier wall sleeves with not efficiency porting. There’s enough room for an experienced person to work with, but I would highly discourage a first-timer or anyone with shaky hands to try it. The bore is highly polished.
Here is a pic of the head shims. One measured .0045 and the other, .0125. The head clearance with shims installed is .016.
Well that’s all I have. I hope this proves to be informative. So far I really like the way the engine performs. I’m impressed with how tight things go together, but I hate that con rod situation. Only method I can see that they could have used to assemble the engine, was to install the wrist pin last by inserting it thru the exhaust port, which of course means that if there is a click-in retainer on the other side of the piston, you can never remove the wrist pin. If someone can tell me how to remove this thing, I would greatly appreciate it. I have disassembled at least 20 engines and have never had this problem.
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STS 28 a Deep Look (56k murder) - 
