Chainsaw Repair

Thanks!
Shep

Joe,  whats your background? Machinist,computer programmer, engine builder?? I'm very impressed with all the cool stuff you do on this site, especially the motor tuning models!   Regard, Adam

CBGT is not IME an easy measurement to get accurately. I usually measure the thickness on a new gasket, smear a light coat of grease on it, then install it, and torque the cylinder to spec, wait an hour, then remove the cylinder and measure immediately again. If I let it sit on the bench for more than a few minutes and measure again, it has changed thickness back toward the original measurement.

I am wondering if there is a limit as to how far the gasket will compress over time. OBTW, these are factory or after market standard gaskets, not reinforced PTFE gaskets.

That is why I don't use base gaskets.  Flange sealant adds 0.001".

updated 12/01/14

Figuring out clearances and machining cuts MATH:

The following are for closed end cylinders only.  (Not for separate head/jug combos).

Group:
1.  Use these if measuring initial squish with no base gasket installed.
2.  Use these if measuring initial squish with a base gasket installed.
3.  Use these if no machining at all will be done.


------------------------------------------------------------------------

Group 1 - (if measuring initial squish with no base gasket installed).



TARGET sQUISH = INITIAL SQUISH + SQUISH CUT + C.B.G.T. - BASECUT


cYLINDER BASE CUT = INITIAL SQUISH + SQUISH CUT + C.B.G.T. - TARGET SQUISH


SQUISH CUT = BASE CUT + TARGET SQUISH - INITIAL SQUISH - C.B.G.T.


C.B.G.T. = BASE CUT + TARGET SQUISH - INITIAL SQUISH - SQUISH CUT


WHERE:

C.B.G.T. = COMPRESSED BASE GASKET THICKNESS

C.B.G.T. = 0  IF NO BASE GASKET WILL BE USED
 
SQUISH CUT = 0  IF NO SQUISH BAND CUT WILL BE MACHINED

BASE CUT = 0  IF NO CYLINDER BASE CUT WILL BE MACHINED

INITIAL SQUISH = PHYSICAL SQUISH CLEARANCE MEASUREMENT ON ASSEMBLED MOTOR

-------------------------------------------------------------------

BEFORE MACHINING:  CHECK FOR MAXIMUM ALLOWABLE BASE CUT THAT CAN BE MADE
                   TO PREVENT FREE PORTING CONDITION.


IF MEASURING FROM DECK (SQUISH BAND):

MAXIMUM BASE CUT = (SQUISH CLEARANCE + TOTAL PISTON HEIGHT) - (SQUISH CLEARANCE
                   + LENGTH FROM TDC TO EXHAUST PORT FLOOR)

OR


IF MEASURING FROM TDC:

MAXIMUM BASE CUT = (PISTON PIN HEIGHT + SKIRT LENGTH) - (LENGTH FROM TDC TO
                   EXHAUST PORT FLOOR)


----------------------------------------------

NOTE:  IT MAY BE OF BENEFIT TO SUBTRACT A 0.1 MILLIMETER OR SO FROM THE
       MAXIMUM ALLOWABLE BASE CUT TO BE SAFE, AND ACCOUNT FOR MACHINING
       TOLERANCES.

NOTE:  "MAXIMUM BASE CUT VALUE" (MINUS MACHINING TOLERANCE IF USED), CAN BE
       SUBSTITUTED INTO THE FORMULAS FOUND AT THE TOP OF THE PAGE......
       WHEREVER "BASE CUT" IS USED.

---------------------------------------


----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------

Group 2 - (if measuring initial squish with a base gasket installed).


TARGET SQUISH = INITIAL SQUISH + SQUISH CUT + NEW C.B.G.T. - BASE CUT - OLD C.B.G.T

BASE CUT = INITIAL SQUISH + SQUISH CUT + NEW C.B.G.T. - TARGET SQUISH - OLD C.B.G.T.

SQUISH CUT = BASE CUT - INITIAL SQUISH - NEW C.B.G.T + TARGET SQUISH + OLD C.B.G.T

NEW C.B.G.T = BASE CUT - INITIAL SQUISH + TARGET SQUISH + OLD C.B.G.T - SQUISH CUT


WHERE:

OLD C.B.G.T = EXISTING COMPRESSED BASE GASKET THICKNESS

NEW C.B.G.T = NEW " " " "

---------------------------------------------------------------------------------
---------------------------------------------------------------------------------
---------------------------------------------------------------------------------

Group 3 - (if no machining at all will be done).


TARGET SQUISH = INITIAL SQUISH - OLD C.B.G.T. - NEW C.B.G.T

NEW C.B.G.T = INITIAL SQUISH - OLD C.B.G.T - TARGET SQUISH

INITIAL SQUISH = TARGET SQUISH + OLD C.B.G.T. + NEW C.B.G.T

OLD C.B.G.T = INITIAL SQUISH - TARGET SQUISH - NEW C.B.G.T


AS BEFORE, IF NO BASE GASKET WILL BE USED, NEW C.B.G.T = 0




HOPE IT HELPS

-joe

Cool thanks!!!

this isn't really cool...  next thing up, within a week or so might be cool worthy?