MS460 Compression 120 psi

[terrifictimbersllc]

Haven't worked on a cylinder before.  Saw runs fine.  Is it a good time to do something if so what?

[ladylake]

 If it runs good , has decent power leave it alone unill it doesn't.   Steve

[SawTroll]

Try to change the piston rings only, if there are no sign of scoring.

[JohnG28]

If its not broke...

[ladylake]

Don't fix it.      Steve

[chevytaHOE5674]

Try to change the piston rings only, if there are no sign of scoring.

I agree if there's no signs of scoring then it is a perfect candidate for a ring job. If it goes too far then you will be looking at a piston and cylinder replacement.

[terrifictimbersllc]

When does one use a cylinder hone to resurface the cylinder walls?

[ladylake]

 Make sure to use your compression tester on a different saw before ripping it apart, testers can vary quite a bit.    Steve

[terrifictimbersllc]

Make sure to use your compression tester on a different saw before ripping it apart, testers can vary quite a bit.    Steve

I did, thanks, and then checked it by blowing into it with my air compressor line at 2 different settings.  The compression gauge has a valve that traps the highest pressure reading.  It was right on.

[tlandrum]

stock ms460 are around 150-155 your getting a little low. id rering it and drop the cylinder down so that the squish is at about .020 and she'll pep right up. have you opened up the muffler on it. if not try opening it up to about  70% of cylinder opening. jmho

[terrifictimbersllc]

drop the cylinder down so that the squish is at about .020

Thanks.  What does this mean?    Didn't open up the exhaust yet, also what do you mean open up to 70% of cylinder opening? 

[JohnG28]

That refers to the exhaust port on the cylinder...open the exhaust port on the muffler until its size is roughly 70% the size of the exhaust port on the cylinder

[tlandrum]

dropping the cylinder down with a thinner base gasket will decrease the clearence between the piston crown and cylinder top. this will increase compression.you want about 20 thousands between cylinder top and piston crown.

[Al_Smith]

Haven't worked on a cylinder before.  Saw runs fine. 

Key words--saw runs fine ----.Leave it alone then . ;)

[boobap]

it may be running fine, but i doubt it is creating its full power...how many hours would you guess are on it? i would start by pulling the decomp and checking that it is not all carboned which prevents a full seal and would also lower compression...

[terrifictimbersllc]

it may be running fine, but i doubt it is creating its full power...how many hours would you guess are on it? i would start by pulling the decomp and checking that it is not all carboned which prevents a full seal and would also lower compression...

Guessing maybe 100 hours?  5 yrs old with 2 yrs of 3 cords per yr and 3 yrs of portable sawmill business.  Thanks for the decompresser idea would like to go looking for an easy out.

[boobap]

I have the 460 service manual and IPL if you want them...just PM me your email address

[boobap]

with that much use, i would guess it is time for a ring job...if not more. rings are fairly easy to do on those, and even more now that you have the service manual!!  8)

[ladylake]

 A 460 should last 1500 to 2000 hours before it needs rings, If it runs good and has good power leave it alone, maybe check the decompression valve as mentioned above .   Steve

[SawTroll]

A 460 should last 1500 to 2000 hours before it needs rings, If it runs good and has good power leave it alone, maybe check the decompression valve as mentioned above .   Steve

Different saws live different "lives" - so the hours isn't really relevant when there are indications of a problem.... :

[ladylake]

Troll You know as well as me that a lot of 460 saws will last 2000 hours, unless it injested a lot of dirt or something  I'd be looking at a faulty comp test, leaky decompression valve  . Maybe pull the muffler off and take a look, it could half burned up. If not scored put the muffler back on and run it.   Steve

[SawTroll]

Yes, that is basically what I said - each saw has a "life" of its own, depending on several factors - just counting hours  :P is meaningless!   :

[terrifictimbersllc]

 I was in to the Stihl dealer Saturday and the mechanic who has worked on saws a long time said that 120 psi is good.  I asked him what the compression on a new MS460 is and he said 120-130. So there's nothing needed to be done.    Thanks to everyone for ideas.

[ladylake]

 Most new saws run around 140 to 160# cold, when warm maybe 10 to 15# less. Also when testing several hard pulls with the throttle open untill the gauge quits going higher get the right reading. As mentioned above if it runs fine run it, no way it should be wore out at 100 hours unless it's started to melt down. It take about 5 minutes to pull the muffler of and look at the piston and rings.   Steve

[chevytaHOE5674]

I asked him what the compression on a new MS460 is and he said 120-130. So there's nothing needed to be done.    Thanks to everyone for ideas.

He lied to you. I have a brand new 460 in the box and it pulls 155psi, and my 3 other 460's pull between 150 and 160 with hours upon hours of use.

[sharkey]

The other consideration when replacing only the piston rings in a two stroke engine is if the engine is a piston ported design, and this one is, that the piston skirts also wear.  Maybe finish out the season with it as is and then replace the piston and rings, crank seals, fuel lines and filters, kit the carb, etc and set it up fresh for next season.

Baileys should have a Meteor piston kit for the saw and also have the gasket set, seals, filters etc     

[boobap]

Sharkey, so every time you replace rings, you replace the piston? Even after a visual inspection reveals no transfer or any other signs of "OBVIOUS" wear? Just curious...my thinking has always been that a piston should never touch a cylinder wall. That being said, I understand the clearances are tight and when I say "never touch the walls" I mean that oil is between them...so in theory wear should be minimal.  Just curious... 

[Michael]

I bet none of you guys knows what the compression is on your car and yet you drive it every day. Let that compression fall a little on a $400 saw and it's "let's change the rings!" Scheese. And in case anyone doesn't know (and seems like a lot don't) the main thing needed is a good piston. A  badly worn piston won't come up with additional compression with new rings. I know you don't believe it because you are used to four strokes but I have put together 100s of saws. Have you?

[boobap]

you still haven't answered the question...how does a piston wear when everything else is working properly? the rings and cylinder will...come on, help me out! 

[Michael]

you still haven't answered the question...how does a piston wear when everything else is working properly? the rings and cylinder will...come on, help me out! 

Well the piston is the softest thing there. The nikisil is very hard. The rings are iron. When the dirt, worn iron from the rings and contaminates in fuel interact with the piston, running up and down the cylinder at 13,000 rpm, it wears on the piston. Every up/down stroke causes the piston to go from pushing on one side of the cylinder to suddenly and violently rock to the opposite wall (once at the top and once at the bottom). That is 26,000 cycles a minute. It is a wonder that the piston lasts as well as it does!