[Empire087]

So I put new pistons and rings in my sled last year after I bought it because the previous owner blew it up. So I got a good deal on it. I've noticed low compression in both sides at about 105 psi cold. It runs great and has tons of power i'm just really anal about my sleds running at 100%. So where would I lose compression on an engine and what can I do to check them?

Thanks,
Dakota

 

[birtchy13]

I believe the test for checking the rings is to put a capful of oil in your cylinder, pull a couple of times and then re-check the compression. if the compression goes up then your rings are leaking.

Also check with another compression tester as some will read low or high.

 

[sledheaddad]

Compression can be lost through the o-rings, which would be obvious, or from loose tolerences. If the O-rings are bad, then the pressure will blow out the coolant over flow. Tolerence wise, if the piston is shorter than a stock one, or of the diameter of the piston is slightly off from the bore of the cylinder, this can cause lower compression. Bottom line tho, if it runs good and has plenty of power...then leave it be. The more compression the more power, but if its not broke, dont fix it.

 

[Mhawk]

Besides 105 cold isn't too low. Check it warm.

 

[THE_Norwegian]

if both sides have 105 it isent that bad. one way of checking if rings are leaking is to remove carbs/or exhaust y pipe and look if there's much brown stuff below the piston rings.

 

[Empire087]

Well I looked and there's zero brown or black on the pistons. However all of the coating that were on them are gone. There are also a couple of little lines running down the sides.

 

[Empire087]

Any suggestions of what might be robbing me of compression? The pistons have no blow-by and are smooth with no gouges. I'm not afraid to wrench on my sled. I've also tried the ring leak test and it changed about 2 psi. Not much, so that isn't a significant difference. I have 1 of the old pistons and it is exactly the same as the one that went in besides the Teflon coating.

 

[1FastSPX97]

maybe your base gaskets are leaking a little, or your crank seals are on their way out. sled would run good in both scenarios, but you would have a better reading on one cylinder and not the other. food for thought. if it were me, i would do new rings and gaskets for piece of mind.

 

[Luke]

Have you tried a different compression tester? Yours could be off, giving you a bad reading? Ive seen testors read 20 psi different on the same engine cause one was a POS. Did you hold wide open throttle when checking comp and pull till neeedle stops going up, usually 4-5 pulls?

 

[WMarshy]

Putting oil directly in the cylinger will almost ALWAYS increase the compression. No matter how good your rings are sealing, you will always have some blow-by/leak-by and loss of compression. Adding oil will help seal up the inprefections in the cylinder wall and piston increasing your compression.

Like others have said, possible leaks could be the o-rings in the head or piston rings. If you have good power then I wouldnt really worry about it if your not confident in wrenching on your sled. What you could do is a leak-down test, but that will only tell you how long your cylinder hold compression not necessarly where it is leaking from/to.

 

[THE_Norwegian]

Well I looked and there's zero brown or black on the pistons. However all of the coating that were on them are gone. There are also a couple of little lines running down the sides.

do you use spi pistons ? the black stuff wore off after a while in mine too...

y

 

[Empire087]

They are spi pistons, not sure why it came off either. I'm gonna try and find a different compression tester. My sled doesn't leak coolant or burn any. I'll check the over flow to see if any air is coming out the over flow hose. I did hold it wot I only get like a 5-7 psi increase doing that though.

 

[OldCatRider]

Both plugs out too?

 

[picman]

They are spi pistons, not sure why it came off either. I'm gonna try and find a different compression tester. My sled doesn't leak coolant or burn any. I'll check the over flow to see if any air is coming out the over flow hose. I did hold it wot I only get like a 5-7 psi increase doing that though.

The piston fit to the bore size sounds a little tight. The moly coating on pistons is only about 25 microns give or take depending on manufacturer.

I know that does not answer the compression issue.

 

[Empire087]

I was pulling 105 wot with both plugs out. My jugs say 496, but the specs say 497? Could that have anything to do with it? Could having 1 cc to big Of a piston cause lower comp if the rings can't expand properly?

 

[picman]

I would go with Luke's suggestion to try another tester before anything else. That might ease your mind. Sometimes being "perfect" involves a lot of needless tail chasing.

 

[Empire087]

Ill see if my doo dealer will let my borrow their tester tommorrow. Im sure they wouldnt mind holding onto my 300$ ipod to let me borrow the tester. Ill even rent it if i have to.

 

[teamarctic14]

i heard once you rebuild a motor you have to give a little break in time and the compression will go up and you get a better reading cold