Kit review 7cc Double-cylinder 4-stroke Air-cooled Nitro Engine by Semto

[James H]

7cc Double-cylinder 4-stroke Air-cooled Nitro Engine (and starter kit) by Semto

Available from EngineDIY for $298.98

 

 

 

A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

 

Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum (negative pressure) in the cylinder through its downward motion.

Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.

Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.

Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust port.

 

Four-stroke engines are the most common internal combustion engine design for motorised land transport, being used in automobiles, trucks, diesel trains, light aircraft and motorcycles. The major alternative design is the two-stroke cycle.

From Wikipedia

 

The kit

First, some real important information about the engine. 

 

• Engine Type: Nitro Internal Combustion Engine Model
• Displacement: 7 (3.5*2)cc
• Cylinder: Inline Double-cylinder
• Stroke: Four Stroke
• Cyilinder Diameter: 16.6mm
• Stroke: 17.00mm
• Speed: 4000-16000rpm
• Power: 0.6ps (0.59 horsepower)
• Cooling Mode: Air Cooling
• Lubrication Mode: Mixed Oil Lubrication
• Starting Mode: Electric
• Ignition Mode: Ignition Modules (Not Included)
• Type of Electric Plug: F-type Four-stroke Electric Plug (Not Included)
• Starting Voltage: 7.4V 2S Li battery (Not Included)
• Fuel Type: 20-25% Nitro Fuel

 

For what is essentially a real engine, in terms of this being one that requires fuel for internal combustion, the box is quite diminutive but weighs almost a kilo. Inside the box, the first thing you'll see is the instruction manual. This is a nice little booklet that is mostly quite easy to follow, with a few caveats that can easily be cleared up with a few views of the videos on the EngineDIY website. The illustrations are in colour, clearly drawn and also illustrate where lubricant is needed. What perhaps isn't as clear are the use of the very useful extra 'tools' included in the kit, that aid the insertion of the pistons and to seat bearings. I'll explain that soon. The instructions also include a map of each of the FOUR parts trays and where to find the specific parts for each assembly, both in the trays and in the compartmental box of screws, circlips, etc.

 

As you can see, the kit does supply hex keys and also a spanner. However, I took the opportunity to use my new Wera tool sets that I'd seen in other excellent videos of some of the EngineDIY builds. 

 

The first point of assembly is the crankshaft. This part is supplied in a plastic sleeve which protected crankshaft's mineral oil film. The part is also beautifully machined, as you can see, and comes with a few parts pre-attached to one end. The additional parts shown have to be attached in a very specific way, as does everything else in a kit which you will run with nitro fuel. 

 

 

The next task is to insert the pistons into the crankcase. The piston rings are supplied in the piston sleeves and you need to very carefully remove these from there. The rings are then slipped onto the pistons and the inside of the sleeve lubricated with a little grease. The rings are metal and need to be compressed against the pistons before insertion to the sleeve. For this, a metal ring is supplied which has a slightly conical internal profile. You pop the piston into the top of this part, sat atop the crankcase, and then gently push the piston downwards through the ring until it's fully inserted within the piston sleeve. The ring forces the piston ring to close up enough for the piston to be inserted. It's simple and it works exceptionally well. Note that the pistons have a brass saddle that needs to be removed before you add the pistons. It also has an elongated indent which you can clearly see on the right hand side of the brass area. It's important that those are matched back up later.

 

 

The assembled crankshaft is now fitted to the crankcase, and the crankshaft connected to pistons. The sump is also now fitted, and a gasket used to seal the sump to the crankcase.

 

More to come....

 

 

 

 

[James H]

The sump is now securely fitted to the crankcase.

 

 

 

The next task is to assemble the camshaft. This model kit has a single unit. The assembly for this weren't too clear in the manual, so I resorted to watching the videos on the kit website, and it did appear to fit together the way I thought it should. 

 

 

This is now inserted into the cylinder head block. Again, this is beautifully machined and comes with the four valves pre-installed. Some care was taken here to make sure the seals were properly seated at either side of the camshaft. A metal tube is supplied so you can tap the bearings fully home without damaging the bearings and camshaft. 

 

 

Using a gasket, the cylinder head block is attached to the crankcase.

 

 

The rocker arms are now assembled, and along with the valve heads, fitted to the cylinder head. 

 

 

Before the carburettor can be fitted, this manifold and sealing gaskets are first installed. The manifold has been reworked from earlier versions and is different to the one shown in the manual. However, you can see it installed in its correct orientation.

 

 

The next parts to be installed are this sliding adjuster for the drive belt tensioning wheel, and another drive wheel. The adjuster is left slack for the time being as the drive belt will be quite tight when installed.

 

 

More to come....

[James H]

This engine is air cooled, and watching some videos, this is actually quite effective. Before the fan can be installed, it needs to be assembled. This is nice and simple, and like all bolts on this kit, I use some thread-lock to ensure nothing shakes to pieces when it eventually runs.

 

 

The fan is now fitted, along with the drive belt. With the belt installed, the adjuster is then tensioned fully and tightened. 

 

 

The exhaust ports are now fitted to the cylinder head, with gaskets to seal them.

 

As you will have seen from the engine description, this engine is electrically started, and therefore needs a starter motor. This racy-looking unit is not assembled to the crankcase. The mounting for this is different from the one in the manual, but looks far easier to fit, and is very obvious. With the starter motor fitted, the second drive belt is slipped over the flywheel and onto a drive wheel on the starter motor. The latter is then tightened to it's shaft and the starter motor tightened to the mounting, ensuring the belt is tight.

 

 

The last part of assembly is the cylinder head lid. Another gasket is used to seal this to the cylinder head. 

 

 

 

 

This engine needs to glow plugs which aren't supplied with the standalone engine. EngineDIY sent me the starter kit too, which supplies these plus, so these were now fitted to the engine. To secure these, you need a glow plug spanner, or a tool like I used here. The spanner in the base engine kit isn't suitable for this. 

 

At this point, the engine is 'almost' ready to operate. What I need to do now is to find something that the engine can be securely fastened to whilst running. I am in the process of sorting this out. The sump does have threaded holes for the builder to utilise, so there's no problems there. In this photo, you can see the pink fuel tubing, the 100ml nitro fuel tank, and also the ignition set. You will of course need a battery to kickstart this too, and I need to source one of these. The engine also operates on 20 - 25% nitro fuel. I opted to buy the 20% fuel, simply for some engine longevity, plus I've not fired up a glow engine for almost 40yrs at this point!

 

 

 

As soon as I'm in a position to, I'll create a video to show my attempts at setting up this sweet little engine.

 

Conclusion

Pending running this engine, I have to say that the few hours I've spent in my workshop have been very enjoyable and this kit has fit together without any issue whatsoever. I would recommend using some thread-lock for the various bolts, making sure you don't get it anywhere you shouldn't. The instructions did pose a couple of questions for me, but those were resolved. In all, a wonderful little kit and for a working internal combustion engine, not too expensive either. the starter kit is most definitely recommended if you've never built an IC model kit before. In fact, this particular kit would be an ideal introduction into this genre.

 

My sincere thanks to EngineDIY for sending this kit out for review on Model Ship World. To buy this gorgeous little kit directly, click the link at the top of this article. For a discount, you can use the code JAMESHATCH.

 

[Kevin]

wonder why the glow plugs are not supplied

[James H]

4 minutes ago, Kevin said:

wonder why the glow plugs are not supplied

Those are things that model engine enthusiasts would normally have. This is why the starter kit is a good extra to have.