New cylinder bad compression

Arborist Forum

Help Support Arborist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Pim

ArboristSite Lurker
Joined
Nov 14, 2017
Messages
24
Reaction score
8
Location
The Netherlands
Hello all!


I'm new to the forum.
I got a Stihl MS181 with seized piston. First tried to revive cylinder but no luck
As these are very cheap saws I decided to buy a cheap Chinese cylinder (including piston etc. ) for $20 on eBay.
After rebuilding it has low compression about 90psi.
I starts, but has very low power, even for a MS181 J
Can I do something to fix? Try to hone it?
Or was it a waste of money?
 
I would say the supplied rings are no good and possibly the piston is too short and the squish is too much.


Sent from my iPhone using Tapatalk
 
the piston is too short and the squish is too much.
Most likely IMO. Ive seen/installed aftermarket kits with squish in excess of .060. Would run decent but with low compression and power. Tho i would first check to make sure stuff isnt scored or anything else obvious.
 
Thanks for the quick response.
I checked the piston through the exhaust, no scoring looks perfect.
I will disassemble the engine tomorrow or day after and report back.
 
Most likely IMO. Ive seen/installed aftermarket kits with squish in excess of .060. Would run decent but with low compression and power. Tho i would first check to make sure stuff isnt scored or anything else obvious.

Stens kit on a ms260 was at 57 on squish and was pretty pathetic in the power dept...pretty crappy rings too.
 
I’ve had an aftermarket 028 super piston in a oem cylinder with .060 squish and 120 psi compression. I took it out and used a Tecomec (made in Italy) aftermarket piston and the compression was 155 psi. I didn’t check the squish with the tecomec piston.


Sent from my iPhone using Tapatalk
 
Cant Hone cylinder they have a coating. Need to pull cylinder and ring off piston push the ring into cylinder and measure your end gap
*
I wouldn't be at all suprised if an MS171 37mm ring set got stuck in an MS181 38mm kit. lol
It's easy to catch on a 2mm jump during an install, but a 1mm might slip by.
 
The squish I measured is 1,4mm (0.055”). Is this too much?
I don’t have tools to measure the gap when the ring is in the cylinder, so made a picture.
The piston ring is 1,5mm x 1,2mm, so the gap is about 0,4mm derived from the picture.
The piston is 37.85mm
Cylinder measures 37.9mm (although difficult to measure)

Knipsel.JPG
 
I would say the squish is too much. Could the cylinder be out of round?


Sent from my iPhone using Tapatalk
 
Seems like that squish is pretty typical of AM top ends. Ring end gap seems pretty close, close prob be a little tighter, but the excessive squish will, i think, hurt performance more. As said above, you may be able to find a different piston that has more compression height.
 
Put that cylinder on your belt sander. Keep the bottom square and bring the squish down to .020. Your saw will run!

Nice idea but it will not work for this type of cylinder. Sanding the bottom will not lower the seating of the bearings so this will have no effect on the squish.
 
I would compare the dimensions of the old piston and the new one and see what you come up with.


Sent from my iPhone using Tapatalk
 

Latest posts

Back
Top