Advice on Compression Gauge

yorab

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Jul 6, 2002
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I want to run a compression test on my motor with the gauge that I have. It is a K-D Tools #2428. It's a basic gauge that I've used before on cars and on my boat motor. However, I realized that I can't tighten it properly because of the depth of the recessed spark plug hole. I can't get a wrench down in there to tighten it enough to prevent air leakage at the spark plug hole threads. I tried to simply turn the hose, but there is a free-spinning connection between the hose and the end that screws into the plug hole. Therefore, turning the hose does not tighten the end.

I checked the compression on this motor about 7 or 8 years ago and I believe that I used the same gauge. I don't remember ever buying a new compression gauge between then and now. I can't remember how I tightened it then. I'm wondering if the connection should not be free-spinning and perhaps the tool has broken. I should also note that the hose connection closest to the gauge also spins. Could it be that both hose connections have broken?

The tool came with two adapters that are metric. Obviously these won't work for me, but I thought that if I could find an extension that is similar to the adapters, I would be in business since they extend the end enough that I could get a wrench onto them. So I went to the local fastener shop and picked up two fittings that essentially act as an extension. However, I can see one potential problem; the threaded end of the homemade extension is longer than the original threaded end of the gauge by a few millimeters. I'm afraid that I'll screw the extension into the hole too far and bang a piston against it.

Any thoughts? I can certainly post pics if any of this is unclear. Just let me know.
 

boobie

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Re: Advice on Compression Gauge

I've got 3 or 4 compression gauges and all the hoses are attached solidly to the end you screw into the plug hole. Must be something wrong with yours. The end of the hose nearest the gauge on mine will spin because of a quick disconnect.
 

wilde1j

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Re: Advice on Compression Gauge

Cut the adapter to the correct length, and clean up the cut on a grinder. Easy. But if the hose and fitting aren't supposed to turn, the gauge may not be very accurate. I've got an old gauge set that is not screw in and it's never failed me. But you do have to hold it in place while cranking.
 

rickryder

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Jun 24, 2010
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Re: Advice on Compression Gauge

Put an o-ring on the adaptor piece so it will be shorter. Plus the o-ring will help seal it and you don't have to wrench it down.
 

levi_tsk

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Dec 26, 2010
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Re: Advice on Compression Gauge

Put an o-ring on the adaptor piece so it will be shorter. Plus the o-ring will help seal it and you don't have to wrench it down.

times 2 mines got orings and it doesnt need to be very tight to make a good seal
 

Joe Reeves

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Feb 24, 2002
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13,262
Re: Advice on Compression Gauge

Is this being used on the 1973 75hp mentioned in your post? If so, why not simply purchase a cheap hand held gauge that uses a rubber tip. Most of us do, quick and easy.
 

yorab

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Re: Advice on Compression Gauge

1963 75hp. Will the hand held gauge work for that?
 

boobie

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Re: Advice on Compression Gauge

It will work just fine. Used to use them before I got lazy. LOL
 

wilde1j

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Re: Advice on Compression Gauge

It'll work on any motor. Use a remote starter switch to crank.
 

boobie

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Re: Advice on Compression Gauge

It won't work on a Yamaha 4 stroke unless it's kinda long.
 

wilde1j

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Re: Advice on Compression Gauge

My oldy has a collection of straight and offset adapters of several lengths. I've yet to find a motor it won't accomodate, but I've seldom used it in the past 25 years.
 

boobie

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Re: Advice on Compression Gauge

I don't even know where my oldy is anymore. Glad this subject was brought up 'cause now I can start looking for it. LOL.
 

yorab

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Jul 6, 2002
Messages
960
Re: Advice on Compression Gauge

The extension that I found was NPT. Even though it mated nicely with the compression gauge, I learned that the plug threads are not NPT. Good thing I didn't screw the extension into the head. I searched the web to find out what threads are on a spark plug before I bought the extension but I had no luck. I had to call a local shop. Anyway, I've attached a pic of my old gauge end. You can see the threaded end that rotates relative to the hose. You can also see remnants of epoxy that I put on there to try to keep the end from spinning. It didn't hold. I also put an oring on the end but it still didn't seal up enough because I couldn't tighten more that finger tight or else the hose would spin but the end would not screw into the plug hole any further.

I went cheap and got a loaner from Pepboys Auto. It's in the second pic. The hose and threaded end are one so I could tighten it down well. However, I think that it was all for nothing though because I am bench testing my powerhead and it's pretty difficult to spin the motor quickly enough and with consistency from cylinder to cylinder. On the first cylinder, I got 100 psi and the second I could only get 80 psi. It was a total PITA. I gave up after the second cylinder. I think that I'll wait until I get the powerhead mounted and try again with the starter motor.
 

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wilde1j

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Re: Advice on Compression Gauge

Get a gauge like this ... it's just like my oldy ... KD still makes this:


KD Tools 3305 compression tester.jpg
 
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