Stihl 084 av low compression

[andyli]

Hi guys.
I checked my 084av compression recently and found it to be only 105 psi. From what I have learned 140 psi + should be the mark.
The saw runs like a dream but has it's arse kicked easily by my MS 660 when it comes to cutting posts and planks.
I see after market piston and rings kits are easily available on eBay but cannot find a new cylinder.
1. Are after market cylinders available and if so can someone suggest a supplier?

On the other hand ...Is it too difficult and expensive to have the original cylinder honed and match a new piston and ring set to suit?
2. If someone here has done this already is it feasible and could you give an approximate costs for honing and the oversize piston/rings?
Advise would be appreciated.
Thanks
Andy

 

[tallfarmboy]

Probably not necessary for a new cylinder. You need to pull the cylinder and inspect the piston and rings to see if they need to be replaced. There are several aftermarket companies that make piston kits, and your cylinder will probably clean up with some muriatic acid if there was some aluminum transfer onto it.

This site is a great resource... keep asking questions and experts will chime in to help you at just about any hour of the day.
TFB

 

[roostersgt]

Unless the cylinder lightly scored, why not simply lightly sand it with fine sandpaper and re-ring the piston? That should bring up the compression and freshen up the saw based upon what you've posted. I'd save my "complete" rebuild money for when it doesn't start, or run.

 

[splitpost]

Firstly ,is your comp tester accurate and did you give it a good couple of pulls on the starter,4 or 5 usually is enough to get a reading?

 

[andyli]

Probably not necessary for a new cylinder. You need to pull the cylinder and inspect the piston and rings to see if they need to be replaced. There are several aftermarket companies that make piston kits, and your cylinder will probably clean up with some muriatic acid if there was some aluminum transfer onto it.

This site is a great resource... keep asking questions and experts will chime in to help you at just about any hour of the day.
TFB

Thanks for tip on "muriatic acid". Sounds like one way or another the bore needs a close inspection.

 

[andyli]

Firstly ,is your comp tester accurate and did you give it a good couple of pulls on the starter,4 or 5 usually is enough to get a reading?

Yeh good question and thanks for taking an interest
1. I drained the fuel, removed plug and pulled over several times and air blasted through plug hole to remove all possible fuel/oil mix. Local Stihl fella said a dry bore is the only way to get a true compression reading.
2. I bought a new compression tester and made sure it was nipped up tight.
3. First pull read 35 psi and then after 6 or 7 pulls reading remained constant on 105 psi. I am assuming that a new tester would be accurate. I'll hook it up to the MS 660 and see if it's up around 150 psi. If it is then I'll know it's accurate enough.

 

[cbfarmall]

I've heard excellent Stihl techs express serious reservations (in the past) about aftermarket pistons for this saw in particular. Something about cooling and modifications (interpret as 'cost-savings') to the original design. It's possible the current aftermarket stuff is close to OEM but this is an expensive saw to experiment with.

Do a quick search of AS and you'll see some of the discussion.

Chris B.

 

[splitpost]

check youre decomp valve,make sure its not leaking,if everything looks fine next I would measure piston to bore clearance,if ok just re-ring the old girl

 

[Frank Boyer]

As previously posted double check your de comp valve. Take it out and clean it, and test the compression with a plug in the hole.

 

[scallywag]

what should the piston to bore clearance measure and are different size Stihl saw's around the same ?

 

[andyli]

check youre decomp valve,make sure its not leaking,if everything looks fine next I would measure piston to bore clearance,if ok just re-ring the old girl

That makes good sense too! Will do. Thanks

 

[splitpost]

what should the piston to bore clearance measure and are different size Stihl saw's around the same ?

I work with .001" to .0015"per inch of bore size

That makes good sense too! Will do. Thanks

check the easy stuff first,could be a simple fix

 

[andyli]

I've heard excellent Stihl techs express serious reservations (in the past) about aftermarket pistons for this saw in particular. Something about cooling and modifications (interpret as 'cost-savings') to the original design. It's possible the current aftermarket stuff is close to OEM but this is an expensive saw to experiment with.

Do a quick search of AS and you'll see some of the discussion.

Chris B.

Now you make me think! When something is moving at 10,000 rpm (stopping and starting 20,000 times) I would want to have everything absolutely perfect. I guess it wouldn't take much for a very weeny little difference in tolerance or composition somewhere to effect enough to cause an imbalance. I have checked the decomp valve and replaced with bolt to end up with same 105 psi. After reading your post yesterday I ordered from the Stihl man a new set of rings to replace. That seems to be the logical first move if everything else is constant once I lift off the cylinder. Thanks for that thought.