07-07-2011, 10:28 AM #1
What a compression test will tell you and whats REALLY going on is still a mystery.
So, Heres some food for thought..
Everyone always says.."Do a compression test and that will tell you basically how good your engine is"
Thats not true. On my SLX, all 3 cylinders said. 130PSI on each hole before running it... sounds perfect right? WRONG!
I have 3 Pistons ALL with chunks missing on the exhaust port side..2 of them STILL had 130 PSI in them! The third was gone.. How so? The piston chunks that were missing were ABOVE the rings and so it read a 130 PSI sounding like its fine..
The one cylinder that was lost had 0 Psi and had broken both rings and lost compression. Just a friendly reminder to not take the compression test as the be all test for condition of your pistons compression or condition of the piston tops.
The other 2 had the rings intact, hence the perfect compression. although, they had chunks missing on the exhaust side top of the piston..
The center piston had serious shot-peening on top also. but still perfect compression..
Actually, if you had a super accurate gauge, the chunks would of increased the volume in the combustion area and you could pick that up on the gauge if it were sensitive enough. I doubt they make a gauge capable of sensing the volume change. but it would show up.
The only true way of checking your condition of the pistons is to take the head off and actually LOOK at the pistons.. This alone will not tell you the condition of the skirts, piston walls,ring condition,piston to wall clearence on the rings, port condition, out of round, or tapered bore. it will only tell you that your pistons aren't missing chunks.
This will also give you a chance to inspect the domes, otherwise, your just guessing..
A boroscope would have seen the problem but then again, you still cant inspect the domes.
Just wanted to share that with ya.. I was under the assumption that a quick compression test would be all I need to check the condition of the motor.. I know better now..
07-07-2011, 10:36 AM #2
Personally, I'm glad you put this out there. As a newb, and someone who has replaced a short block, wear ring, carb overhauls and other nifty stuff with a ton of help from this forum, I was not aware of this possibility. In fact, when I went to by my most recent project ski, I was thrilled when the cheap harborfreight compression test tool registered 130psi on a junk battery with a ski that had not run in a long time. Is a boroscope an expensive tool?
07-07-2011, 11:12 AM #3
You did a compression test on a recently rebuilt engine, correct ?
If that engine was run for a while-the compression test would have shown a serious problem.
07-07-2011, 11:27 AM #4
Thats why you do a compression test and THEN lake test it. The compression test just prevents you from wasting everyones time and tells you the motor should be good. A test run on an engine that's missing chunks of piston won't last long. Also anyone can fool a cold compression test on a lightly damaged engine by pouring 2-stroke oil in the cylinders. But the test ride will likely reveal the problems
07-07-2011, 12:45 PM #5
Hey Casey.. I was under the impression that this engine had a new crank put in it.. The guy didn't go into great detail with me about it when I bought it. I dont think he knew a whole lot about the ski in general. he just bought it himself from a new york dealer so anything could of happened. Then his house flooded out and he put it up for sale needing the money.
still...500.00 for the ski is still a good deal.
Artic, why would you do a compression test, then ride it? Thats what I did before I ran it. and look at the result. It didnt tell me anything but 130 across the board. I bet when it warmed up a lil, thats when the chip fell off the mag piston and busted busted the rings. it was a 130 across the board before that. the comp test didnt show the pistons being bad from low compression.
I believe the pistons were chipped or cracked there way before I got to it.
I did test it cold but without using any oil while testing. that would give you a false reading.
Gigem....a boroscope is about 100 to 400.00 depending on what your looking for. some have video screens to look at, other have just a eye scope to look into. some have led lights on the tips.. some guys use doctors ear scopes too.
07-07-2011, 01:49 PM #6
- Join Date
- Aug 2006
- Homer Glen Il.
post pics of the pistons they sound like they where installed backwards
07-07-2011, 03:23 PM #7
- Join Date
- Sep 2006
It's easy enough to take a pen light and peak with the plugs out when you're doing a compression test. I always have my little LED light with my compression kit.
07-07-2011, 03:49 PM #8
Absolutely a compression check is not the tell-all end-all, but if you do a compression check and come up with some super low readings, then you know that there is a problem for sure.
I also think Lugs might be on to something. I thought about that yesterday. For all 3 to be F-uped, there should be some common denominator somewhere.... maybe the piston installation.
07-07-2011, 05:29 PM #9
I'll take some pics of the 2 pistons I have here. the third is in the ski still.. the mag... I couldn't get to the circlip with the tank in. But the mag looks like the pto pretty much.
The center one is the shot-peened one.
I'll post a pic in a few mins..
07-07-2011, 05:41 PM #10
The first one is the Pto and the second one shot-peened is the center. the mag looks real close to the pto in the pics.
Theres a number 83.83mm and the other stamped says AUS> with a arrow pointing out.. They had the arrow pointing at the intake side when installed.
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