Dimpled piston on Stihl 084

[SGaron]

Hey everyone, my name's Shane and this is one of my first post here on AS. I got the chainsaw fever about a year or so ago and this site has helped me alot with info on all my old saws. Other than this site, I usually text Kensie1988 all my questions but today I came across an unusual thing on an 084 I just bought. Long story short, the 084 I bought was from the Stihl dealership locally, I've become good friends with the owner. It was a 2nd owner saw and years ago they sent the piston and cylinder off to a machining company in california where they ported it and worked on the piston. The muffler was a tuned piped version but someone stole it so they put a regular front plate on it and drilled a bunch of holes in it to get it to run right again. It's been sitting for years now, but still cranked on the third pull today. My question is, the piston has dimples drilled in them and I can't find much info about this online. Any information is appreciated. Thanks again everyone.

 

[Kensie1988]

Hey everyone, my name's Shane and this is one of my first post here on AS. I got the chainsaw fever about a year or so ago and this site has helped me alot with info on all my old saws. Other than this site, I usually text Kensie1988 all my questions but today I came across an unusual thing on an 084 I just bought. Long story short, the 084 I bought was from the Stihl dealership locally, I've become good friends with the owner. It was a 2nd owner saw and years ago they sent the piston and cylinder off to a machining company in california where they ported it and worked on the piston. The muffler was a tuned piped version but someone stole it so they put a regular front plate on it and drilled a bunch of holes in it to get it to run right again. It's been sitting for years now, but still cranked on the third pull today. My question is, the piston has dimples drilled in them and I can't find much info about this online. Any information is appreciated. Thanks again everyone. View attachment 658485 View attachment 658486 View attachment 658487 View attachment 658488 View attachment 658489 View attachment 658490 View attachment 658491

Thanks for the shout yes that is quite an interesting modification and I'm looking forward to what answers you get!

 

[cuinrearview]

I have three guesses: Lightening of reciprocating mass, decreased friction, increased ability to carry oil.

 

[ericm979]

That sort of thing is sometimes done for oil retention. For example Stihl puts dimples on the drivers on their Oilomatic chains. This is the first time I have seen it on a piston.

 

[SGaron]

Wow, thanks for the quick response guys.
In terms of power output, would this translate to something along the lines of being able to tune it to higher RPMs with less chance of burning the P&C up? He did say the guy who had it had burned up a few motors and wanted this one to be built, well, not to do that.

 

[Kensie1988]

I have three guesses: Lightening of reciprocating mass, decreased friction, increased ability to carry oil.

This was my thought as well, but it's nice to see someone come to the same conclusion as I did.

 

[cuinrearview]

I bet @Big_Wood could shed some light on this...

 

[ericm979]

Wow, thanks for the quick response guys.
In terms of power output, would this translate to something along the lines of being able to tune it to higher RPMs with less chance of burning the P&C up? .

I would not depend on that. I think it'll make only a small difference in the amount of oil on that piston skirt. It will not address any of the other things that can cause seizure. If it was a common inexpensive part I'd recommend getting a stock one. As it is there's a somewhat higher chance of the piston skirt breaking but I think that's still low. The force on the power stroke is on the intake side due to the rod angle.

 

[ericm979]

There's some mention of dimpling to reduce friction in this paper: http://eprints.whiterose.ac.uk/101420/3/Reducing in-cylinder parasitic losses.pdf
I did not read the whole thing. They cite a 2-4% reduction in piston to cylinder friction from dimpling. Dunno how many or large the dimples were but it might be in porportion to the reduction in contact area. Seems rather a waste of time.

I wonder what other ad-hoc garage tuning's been done inside the motor?

 

[blsnelling]

I doubt that will have much affect if any on how it runs.

 

[ironman_gq]

Somebody probably figured it would carry more oil between the piston and cylinder, you can see that the dimples are carrying oil. How much difference it makes would be debatable.

 

[cus_deluxe]

That looks like a friggin hackjob on that exhaust port...

 

[Kensie1988]

That looks like a friggin hackjob on that exhaust port...

I was actually looking at how un even the exhaust port looked

 

[cus_deluxe]

And why in the heck would you grind the piss out of the exhaust floor at all. Im not a pro by any means, but i know enough to not do that...

 

[cuinrearview]

That looks like a friggin hackjob on that exhaust port...

I was admiring that, as well as how uneven the dimples are cut.

 

[blsnelling]

That's one of the worst I've ever seen!

 

[pioneerguy600]

Hack Job!!

 

[junglejohnny28]

I think guy who did it thought the piston was worn snd all texture was out of piston and thought it would bring more lubrication up . Looks rough but might work well depending on port and timing numbers . I would not do,but interesting to see it done and run . I can tell u on stihl 462 and 661c the rubber intake boot has a dimpled area inside to create more turbulence in the fuel mixture , but never have seen that before ,