Compression tester for model i.c. engines

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Brian Rupnow

Design Engineer
Project of the Month Winner
May 23, 2008
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Barrie, Ontario, Canada
I'm considering buying a compression tester to use with my model engines, 3/4" or 1" bore. I've been told not to buy a normal automotive compression tester, that it won't work on small engines like I make. Why??? If I have an end that matches my sparkplug threads or even a rubber cone shape tip that can be held tightly against the sparkplug hole in the cylinder head I can't see why it wouldn't work. I'm curious.---Brian
You need a tester or make one that has the check valve right at the tip. The reason being tat with a small engine if you have the check valve upstream you're adding more volume to the already small combustion chamber. Theoretically if your cylinder could pump 85 lbs of compression it will be somewhat lower filling all the volume of the hose on the compression tester. I built a small adapter years ago but found it was more trouble than it was worth. The best way to check a cylinder is with a leak down test but that's really impractical for a small engine. I just do a vacuum test on each port. Usually it's the valves that leak and not the rings.
I've just ordered a compression tester from Amazon that includes a 10 mm adapter which fits my sparkplug threads. It is advertised as being good for automotive, motorcycles and chainsaws. I'll let you know how it works out. Sometimes there is a big difference between what is advertised about a part and what the part actually will do. I thought that all compression testers had the schraeder valve right at the tip that is near the sparkplug.
If the tester doesn't have a shrader valve right at the tip, some don't, then you would have to make your own adapter for a compression gauge shrader valve. There's a difference between tire and compression gauge valves
Thanks Peter. That looks like a worthwhile tool.
Brian, a very important topic !!!

here's what works for me

1) rings - these will work as soon as they're lapped such that there's no visible light between the ring and a test cylinder or a ring gauge, use strong light and a magnifying glass or loupe (in reality neither Trimble "hot formed" rings, nor alternative "cold formed" rings, will pass this test without lapping, "just do it" !).

2) valves - there's no way to visually check valves, and after experimenting with lots of ways to measure pressure I gave up, instead I use a DIY machinists stethoscope !!!, put the end of the stethoscope into an exhaust port and listen while you turn the engine over, if you hear hissing or gurgling before the valve opens it isn't sealing and needs some lapping, ditto with intake ports.

the problem with trying to measure pressure, and/or blow-down time, is it doesn't identify the source of the problem, while a stethoscope does. I used the above method to prepare my V-12 with 4-valves-per-cylinder (thats 48 valves total !!!, also 24 compression rings!) and it started the first time I tried and has been running fine ever since.


I have been thinking about this for a while, I had a look around my parts, had a couple of shrader valves(short), a couple of tyre valve inserts, ( I use them for charging air systems in my models) and a gauge. Yes I know that it is 200psi but that is just what I had in the parts bin.
Machined the valve bodies down, machined the body center, drilled for the valves, tapped for the gauge. machined an adaptor, 1/4"x32 for the glow plug hole. I did not have a 5/16"x32 die, so a brass valve cap soft soldered to the adaptor. Assembled the parts and I have a tester. I will now look for a new gauge.


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Looks good Ghosty. I'm a strong believer in "Do it yourself" stuff, but I've got a bit much on my plate right now. The compression tester I have ordered only cost $28.00 Canadian, including shipping, and it includes a 10mm thread adapter to match my sparkplug threads.
Wet finger over the plug hole seems to work in many instances.
My first was a spark plug outer body, ceramic broken out, brazed to a piece I of pipe t connect with a pipe connector to an oil,pressure gauge off the car. I fitted a bicycle valve in the line, so kicking-over my motorcycle it quickly pumped to a pressure - which in bar-gauge -was the actual compression achieved... Pressure reviewed bu poking at the relief pin on the valve. Then I made an adapter - From another spark plug body - so I could use a pen-type tyre pressure gauge... using an old valve stem from a tyre brazed into the plug body.
One day I actually bought a compression tester... and have used the good gauge to calibrate other less accurate gauges... for tyre pressure!

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