My First Engine Build

[Mechanicboy]

From my own experience, it's best to make a big model engine with piston ring and these small engines that can be difficult for beginners since there is no supervisor present and teach the beginner to make a small model engine without piston ring. A larger engine has less compression loss than a smaller engine and the engine should be easy to start and run.

Beginn with a 10 ccm engine or larger and get more pleasure from running the engine.

 

[c_mario]

Thanks everyone for all the suggestions. Since I have tried everything I can think of and nothing makes any difference I can only think that my piston to cylinder liner fit is no good. I don't know what good compression should feel like. For comparison as I mentioned I have an OS Max 40 and it ran fine. Its compression felt no different to my engine when rotating by hand but it is a ringed engine. At this point I have had this apart so many times i'm starting to strip threads in the crankcase and now needing helicoils to repair. Its not fun now.
Mechanic boy, is there a 10cc engine plan you can recommend.

 

[Mechanicboy]

C_Mario,.. here is the large 4 stroke engine with piston ring who is popular for newbeginners to create own engine..
http://www.machinistblog.com/free-plan-webster-engine-works-4-cycle-gas-engine/

10 ccm glow plug engine..
http://www.hemingwaykits.com/acatalog/Jones_605___Colin_Jones.html

The Sparey 5 cc engine is not difficult to build due the engine has long stroke and large length of piston who give less leakage than a short length of the piston and short stroke engine...
http://www.hemingwaykits.com/acatalog/The_Sparey_5cc_Engine___L_W_Sparey.html

 

[f2cf1g]

If you suspect the piston seal (it should hold compression for several seconds when held at the top of stroke) and if the piston is made of cast iron you can make it fit more tightly by "cherry bombing". First expand the piston by heating with a gas torch until it is glowing bright red then plunging into oil to cool it quickly. You should find that it has grown and will no longer fit into the cylinder. Re-lap to fit, taking care not to go too far. As you lap, keep checking by cleaning the piston thoroughly then sliding into the cylinder which should be tapered towards the top. Stop lapping when the piston gets about half way up, between the top of the exhaust and top dead centre. Assemble the engine and check that it will turn over. If too tight lap a bit more and try again.

Fitting the piston to the cylinder is the most difficult and critical process in two stroke building so you should not feel that you have failed if you have to try again. I had to make four pistons the first time I rebored a diesel. This is much easier than starting again from scratch! Goo luck.

 

[c_mario]

Hi f2cf1g,
I have actually "grown" the piston 3 times now and relapped as you describe. The last time, yesterday, I did it and it expanded by .06mm and did not fit the bore.
I lapped it till it would go half way up the bore, then fitted it back into engine. It was hard to turn over but gradually became easier with lubrication and a fair few rotations.
Same result.

 

[WOB]

Check to see if your piston is round. Sounds crazy, I know. But unless you started with stress relieved cast iron blank, machining the interior to make room for the rod small end will allow the inherent stresses in a cast iron blank to be relieved and will distort the piston out of round. Lapping does no good because the thin cast iron is flexible and simply bends under lap pressure. A dead give-away is the lack of a uniform surface finish after lapping to fit the cylinder. If your piston is out of round even by a couple of tenths, toss it and make a new one from a new blank that has been heated to about 1200 deg F ( dull red) for at least 30 seconds and cooled slowly. Then you have a dead, inert material to machine and will hold shape despite being machined all over. It is the only way to get good compression with a lapped piston assuming the cylinder shape and finish is proper.

WOB

 

[WOB]

Removed duplicate post

WOB

 

[c_mario]

Hi Wob,
After lapping I measured the piston in multiple locations and the measurements where all 16.021 to 16.022 mm.
The piston finish appeared uniform after lapping, i.e. no spots duller or shiner than any other.
I believe I machined the centre first before machining the piston to initial size before lapping.
When you say tenths, do you mean tenths of an inch or tenths of a millimetre?

 

[WOB]

It appears that your piston is round. That leaves the head gasket and even the glow plug/gasket as possible leak points. I should have been more specific about tenths: I meant 1/10 of a thousandth of an inch: 0.0001" or 0.0025 mm

WOB

 

[Billitmotors]

As Aerostar said the flat on the backplate is for piston clearance first turn the crankshaft through bottom dead centre and see if you can feel if the piston is binding on the backplate or even better machine the flat on the backplate as designed . Also check that there is no leakage on any of your joints on the crankcase. the easiest way is to pour fuel on the joints one at a time and flick the engine over while watching for air bubbles, any leakage can be enough to reduce the crankcase pressure and stop the engine from continuing to run.
Also you said that when the engine does fire it sounds like a fourstroke. Do you mean it is running roughly? If so this could be because the piston skirt is just bottoming out on the backplate. You could also simply look at the backplate adjacent to the piston and see if there are any marks on it.
If you have built it as per the plan and your fits are good it really should run. My bet is still on leakage around the crankcase joints.
I hope this helps.
Rob Jenkins.

 

[c_mario]

Hi Billitmotors.
I did put the flat into the backplate as in the plans. You can hear the engine in my little video clip in this thread, sounds rich or what the guys at the model club I was in 30 years ago called four stroking. I have looked for leaks and used sealant on all surfaces. Best run is about 4 seconds.

 

[Mechanicboy]

4 second running before stop can be caused by expansion by heat then the cylinder do not keep tight around the piston. As normal when the piston/cylinder is ready to use when the piston must stop 2-3 mm before TDC, also tight and more tight until the piston stop a bit before TDC when you are pushing the piston into cylinder after you has lapped the cylinder/piston. You can't distort the fit of piston/cylinder when you are flipping the propeller with fuel into the engine. Running-in the engine will improve the tolerance between piston/cylinder who are impossible to create on tools.

 

[f2cf1g]

After you mentioned it in your last post I watched your video (not sure why I didn't see it before). The motor appears to be oscillating not fully rotating. I've not seen this on a glow before but in a diesel it is caused either by too high compression ratio or too much fuel causing too much comp. Either way, backing off the compression lever allows the motor to turn over and run. I wonder if too much comp is the problem? A bigger prop with more inertia might help starting.

 

[KenC]

Your cylinder liner needs to be a very good fit in the crankcase because if not you will get the leaks that you have already seen. The liner should need a firm push by hand to get it into the case, it should certainly not just drop in. My view is that all the comments about the needle and settings are just a red herring. Take the plug out and flick over the engine. This should give a definate pop due to crankcase compression, though not as definate as the pop you hear when the plug is in place. If there are crankcase leaks you will not have enough suction to pull fuel in, nor enough pressure to transfer it up the transfer ports into the cylinder. If the engine runs for a few seconds it is probably just using up any fuel that is in the crankcase. Oh, you are using proper glow fuel I assume!
A further point as already mentioned, is that the ledge on the top of the rear cover is so that the piston does not hit the rear cover as it approaches bottom dead centre. If you did not make that ledge and if your piston does not hit the rear cover, there are some errors in your construction of the engine. I would start off by a detailed check of all your dimensions and correct those that are not as they should be. Hope all this helps.
Ken Croft
See my own engines at
www.flickr.com/photos/kcemb/albums/
And then find the album called Model Aircraft Engines.

 

[c_mario]

Hello Ken,
Thanks for the detailed reply. At this stage I have gone backwards. I have made two cylinders and grown the cast iron piston several times in an attempt to get a better fit. I just kept getting the same result. I then decided to make a ringed piston. Cast iron Piston Cast Iron ring. Thought this might be better. Actually worse, engine has not fired since. As for the pop when flicking over the engine without plug I believe I get that but again, never seeing one that is correct I don't know what a pop should should sound like. Yes I am using Glow fuel, It was purchased recently. I made the ledge in the cover and checked my measurements also. I made new cylinder head that reduced the distance from the top of the piston to the glow plug also to I guess increase compression. I've taken a break from this now since I clearly don't have the skill to get it right. I may build another engine if I find a suitable plan and simply concentrate first on getting a good Piston to Cylinder fit.
I wonder why most plans are diesel engines. I don't even know if hobby shops here sell the fuel for them.

 

[minh-thanh]

Hi c_mario !
I think you should try with the flywheel, the same way I do in this thread:

https://www.homemodelenginemachinist.com/threads/glow-engine.30315/page-3
That's how I do it, because with the first engine I do not know how right, exactly ...
It will be easier to run.
And when it has run with the flywheel you continue to adjust the cylinder, piston ....

 

[ixb1]

Hi C-mario
I read your problems with engine starting.If you make needle valve as in plan (needle taper and fine 4mm thread) you need open at full 4 turn to run rich (if i remember corectly).
If you put about 1/2ml of fuel into carb,few times flick and connect igniter.Engine will run.Do not put fuel into cylinder,it causes violent kick back and loosing prop.

 

[KenC]

I didn't know you put up a video, but I just warched it. I would say that the engine has too much fuel. It is flooded and does not clear the excess fuel. There is of course still the possibility of an error in your build somewhere. I say this because if a normal engine has the fuel shut off and runs just on fuel sitting in the case, it will lean out and run fast as the fuel is used up. It will not splutter to a stop as yours does. When you connect the fuel line, put your finger over the carb and turn the engine over, can you see fuel (or air bubbles) being pulled along the fuel line? If you then shut the needle fully then repeat with your finger over the carb, does it still pull fuel along the line. If it does, your needle is not shutting off the fuel. As for the design, if I can find the drawings I will run the dimensions through a little prog I have to check timing events, as it may simply be a design fault. Oh, a final thought. Have you tried attempting to run it clockwise as it is a simple mistake to misread the position of the port in the crankshaft!

 

[KenC]

I have just looked at the drawings and I have just a few comments. It has quite difficult transfer ports for a first engine, though I doubt small errors in the build who have much effect. What is quite apparent is that sealing the cylinder liner to the case will not be easy, particularly because of the very small distance between the top of the exhaust hole in the case, and the top of the case. If your cylinder is a drop-in fit in the case it will never seal. The cylinder needs to be a firm "thumb press" fit in the case if it is to have any chance of good crankcase compression; no amount of gaskets anywhere will compensate for a slack liner fit. If you cannot get the engine to pull in fuel with your finger on the carb inlet, a liner/case bad fit is the most likely reason.
I am full of admiration for your tackling this build as a first engine. It is not a design that I would recommend as a first, something like the Boll Aero which is another bar-stock engine would be much more appropriate.
And as a final comment, disregard anyone who advises you to lap a piston to a cylinder by lapping the two together, what ever lapping medium they use. I use solvol autosol as a final polish to both piston and cylinder, but never with the piston in the cylinder. That is simply a recipe for failure; the only thing that should come between a piston and a cylinder is oil. Trust me, I have in excess of 50 engine builds behind me of more than 40 different designs.

 

[c_mario]

Hi Ken,
Thanks for the detailed responses.
Firstly , when I open the needle and place a finger over the carb inlet and turn the prop fuel is definitely pulled up the fuel hose. If I close the needle valve and do the same procedure fuel will not be drawn up the hose. I know this for sure because I also made a second carb and it did not seal at all.
Second, I do think the cylinder to case fit is not good. I have made two cylinders and I still see fuel bubbling up between the bottom of the exhaust port and the cylinder liner.
I have tried sealants but could not seem to stop it even though I felt that the cylinder was a tight push fit into the case.
I did not lap the piston inside the bore, I lapped it with a lap I made.
The piston in it now has a Ring on it, but I did not lap the ring because I don't know how or if I should.
I did try running it clockwise but it made no difference.