Outdoor Power Equipment and Small Engines - homemade leak down tester

I had come up with the idea of taking an old spark plug and gutting it out so that all that is left is the threaded part with the nut and brazing a tire-like air valve to it. That way I could air up a cylinder with a normal tire chuck and monitor the air pressure with the built in tire guage that's on the air chuck. The air valve I had in mind is some sort of valve found on hydralics and it is threaded to 1/8" pipe thread. I had asked this question on a discussion board about tractors and I got a few replies but what do the small engine repairman think about this? I know that people not only use leak down testers to check for compression leaks but also to hold valves in place when changing seals or springs. About how much air pressure is safe to put into a cylinder? Also, what types of threads are found on spark plugs? From what I catch, it is mostly 14mm and 18mm pipe threads, it this right? What you ya'll think about this set up? I am going to use it on small engines and tractors in the future. It sure beats spending money on another tool when just starting out. Also, how exactly is a leak down test performed? Is a cylinder supposed to maintain a certain amout of air pressure for a certain amount of time? I know a leak down test is commonly used after a compression test. Any comments are more than welcome!!!!!!:D Thanks.

Sounds like a perfectly novel idea to me. The guy i used to work for in the small engine shop had alot of homemade gadgets, and that was one of them. Your best bet is to weld an air compressor fitting onto the end, and get a guage and an air intake of some kind, using a splitter. A tire chuck would work also, just make sure it's sealed. Good luck gutting the plug tho, its not easy, and 14mm is the best for small engines/

sorry, forgot to answer all the rest of your replies. Spark plug thread is metric fine or very fine thread. I think 14mm by 1.25is what they use, but it could be 1.5. To perform a leakdown test, put the engine at EXACTLY TDC on the cylinder you're testing. This will test the valves and rings. you let air into the cylinder, using a compressor, and then stop. The cylinder should retain about 95% of the air, youre gonna have to do the math. For example, if you put in 100psi, it should go to 95 or so. I thing anywhere above 80 isnt too bad. If you put the cylinder at bottom dead center, it works too, and this will find deep bore scratches not evident at the top of the hole. make sure the valves are closed too. Good luck, and make sure your leakdown tester isnt the source of the leak.

ps, as much pressure as the compressor will make is safe to put in the engine, as cranking pressures sometimes exceed 180psi

I agree with all Gill has to say about this. You will probably have a hard time keeping the piston at TDC though. Don't try to hold it by hand. I think this test would be easier to perform at bottom dead center. You will need a gague to measure how much pressure the cylinder is retaining.

Do a search for “leakdown tester”, “making leakdown tester” building leakdown tester”, and “differential compression test”. You can learn more than you may want. It is imperative the crankshaft be securely held in place while performing a leakdown test or you may severely injure yourself. Briggs and Stratton makes a tool for securing the crankshaft. On small engines the tester is generally used to determine cylinder pressure loss. From my experience with a homemade tester you first need only to start with 30PSI. If you have cylinder pressure loss it will show up quickly at this pressure. If needed then increase the pressure.

http://www.motorcycleproject.com/motorcycle/text/leakdown.html

http://130.58.81.206:8080/SOHC4/library/Engine/tech009.html

http://www.lycoming.textron.com/main.jsp?bodyPage=/support/publications/keyReprints/maintenance/compressionCheck.html

http://www.littleflyers.com/engcomp.htm

Wow!!!!!!!! Thanks for the links. Just one last question, what is the easiest way of keeping the crank from turning, other than the tool you mentioned? Thanks again.:D

Mower,

You asked, "Just one last question, what is the easiest way of keeping the crank from turning, other than the tool you mentioned?"

Large pair of vicegrips on the crank. Clamp on an area where if you mar the crank it won't hurt anything.

When you say the cylinder should retain a certain percentage of the air that was put into it, how long should it hold this amount of air? Won't the air pressure continue to leak out constantly? Also, could I check the cylinder and valves with the piston at bottom dead center? Would the valves be completely closed both on small engines and large engines?

You can't do a leak down test at bottom dead center because either the intake or the exhaust valve will be open depending on what cycle you happen to be in. There will always be some air escaping through the rings and maybe some through the valves. The idea is that a percentage of the air pressure will be retained for a specfied amount of time. Aircraft engines have a known specification that must me met. Smaller engines can only be guessed at. If you suspect that either the rings or the valves are bad the leak down tester can be employed to quickly comfirm that fact without tearing down the engine. That may be it's biggest advantage. With the cylinder pressurized a quick listen to the crankcase, intake port, or the exhaust port will quickly reveal where the problems are.

Good point, and you're right. A leakdown test won't work on bottom dead center. It will leak constantly, which is why you need a constant regulated air supply from a tank/compressor to do a leakdown test.

I have a commercial leakdown tester I bought several years ago when I was overhauling the engine in my chevy pickup. Maybe the pictures will show up OK. The left gauge shows regulated air pressure and the right gauge shows the pressure with the leak. The percentage leak is known from the pressure on the right gauge. See the picture of the instruction sheet for those figures.

http://ronj48.home.mchsi.com/leakdown.jpg
http://ronj48.home.mchsi.com/ldinstruct.jpg

I've followed the directions as best I can but I can't seem to get the actual pictures into the post. All I've been able to do is get a link to the pictures. It would be nice to put some thumbnails in the actual post so you could click on them for a full sized version.

Thanks for the pics and the info.:D

Wow mower17! You have a big prodject here. And lots of people have given you great advice. I hope that you don't find out you have a whole bunch of problems with your engine. I hope that everything will be just fine and you will be having a lot of fun doing this operation and learning it at the same time. Just be careful and don't injure yourself!