MS390 / 039 pop up piston

[Fruecrue]

Received piston from @Gentleman.
Thank you Bruce!
My 039 will either be a pioneer or a guinea pig.
Will be doing video before and after.
Right now is the time for weights and measurements, if you want to know, ask.

 

[Fruecrue]

 

[Fruecrue]

 

[Fruecrue]

 

[Fruecrue]

Correct me if my terminology is wrong.

Deck height (top of piston pin to top of piston at outer edge) .608”

Compression height (center of pin which measures .3925” to top of piston at outer edge)
.80425”

Pop up height .042”

 

[71dart]

I ordered one too. Just to play with and see if there's any noticeable difference.

 

[Fruecrue]

Diameter at crown 1.918” measures the same front to back as side to side.

Diameter at base 1.928”

Skirt width exhaust side measured at pin height
1.465”

Skirt width intake side measured at pin height
1.457”

 

[Fruecrue]

Skirt widens below the full circle portion.
Measured as low as I could.

Exhaust side 1.630”

Intake side 1.510”

Don’t know if these matter much but if I’m widening ports as close to free porting as I dare, it’s nice to have extra piston support down low.

 

[Fruecrue]

So far I’m impressed with quality. Casting flash is minimal.

 

[Fruecrue]

Adding pic with ring locating pins.

 

[Wood Doctor]

Dang, that looks nice. I may have to give my Stihl 390 one of those, just to see if it works. I was considering putting my 390 under a good friend's Christmas tree, but curiosity has me by the throat.

 

[Fruecrue]

Distance from base of pop up to edge of piston is .435”

 

[Fruecrue]

Smallest diameter at top of pop up is .965”

 

[drf256]

Great thread Dave. Thank you.

When will you be checking squish?

 

[Fruecrue]

Great thread Dave. Thank you.

When will you be checking squish?

039 is a fresh rebuild with a non pop up cross piston. I will check squish and compression in it, get some cut videos, then swap pistons.
At that point I can check squish.

I recall the cross piston being very close to OEM in any measurement I took.

I hope to accomplish this all this coming weekend if work doesn’t interfere.

 

[Fruecrue]

Let’s meet the patient.

 

[Fruecrue]

Been running rich since rebuild.

 

[Fruecrue]

Squish with as is- .028”
I’ve seen these vary a good bit, this is a good one.

 

[Fruecrue]

Compression is an impressive 168 psi, please keep in mind what that plug looked like and how much oil is in he jug.

 

[Fruecrue]

Saw has a muffler mod, tank has been repaired with PVC cleaner and glue, and cylinder does have light scoring from a previous life.
All transfer was cleaned.
This saw was built for me to use as a beater out of several carcasses and wrecks.


This ain’t the girl you take out to show off, but she sure does make you smile when nobody is looking.