Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

Homelite 20042A, 25cc

It was a problem getting the ring in because the bottom of the bore flares out.  You can't use a ring compressor.  I put the top of the piston in the bore then tried pushing the ring in with the tip of a pick.  In the process the ring broke.   What's the way to do it?

Where should the ring gap go?  Is it centered on the longest section of uninterrupted wall?

The crank weight has 11 degrees stamped on it.  How is the timing set?  I have not taken off the flywheel and assume it only goes in one way but not sure.  There is no key shown on the exploded view. 

If this is a two cycle engine, the piston ring groove will have a tiny dowel in it.  That's where the gap goes.  The rings can only fit one way with the notched ends of the ring fitting the dowel.  If that was not all properly aligned when you tried to put it in, that's why it didn't go. 

Regarding the timing.   Many newer engines don't use keyways anymore.   The piston/crank degrees from TDC has to be set by putting a dial gauge in the spark plug hole to measure degrees or mm. before TDC.   However, if you carefully mark the crank and flywheel by using a thin cutting blade on a dremmel, you can mark the crank and flywheel by cutting a fine line across the two surfaces.  That will ensure you have things lined up when you re-assemble.  Otherwise, it's necessary to use the dial gauge.   Good luck and let us know how it goes.  

   Looked at the piston groove and there IS a dowel there and the reason for the no-go.

   This is my first two stroke.  It would not run with gas in the plug or throat.  It would only fire for one revolution and nothing more.  It had a torn gasket on the crank area cover.

Have you got it running?

Pistons with ports in the skirt usually only go on one way.  Generally, there is an arrow embossed on the piston crown that indicates installation direction relative to the exhaust port.  Arrow points to exhaust side of the cylinder.    Another way to figure out piston installation is understanding that the ports in the piston skirt are there for the intake cycle.  Accordingly, the piston skirt ports should also be toward the carburetor side. 

Check out donyboy73.  He rebuilds a lot of chainsaws and other 2 cycle stuff.

http://www.youtube.com/user/donyboy73

trouts2 wrote:

hey trouts...  if the base gasket was torn that would most likely be the reason it wont start...2 stroke need compression on the bottom end to run properly...and crankcase pressure also runs the fuel pump in the carb.

Hey Trouts,

The rear end of that engine is the biggest problem... (the gasket and plastic cover)...

The screws back out of the plastic cover and the gasket leaks... Be shure to use  "perma-tex" gasket sealer on the cylinder gasket and the plastic cover gasket.

I lalso put the Loc-tite on the plastic cover screws too keep them from backing out...

Good Luck,

Friiy

Borat: Have you got it running?

   Not yet, it needs at least a ring and gaskets

 

>>Generally, there is an arrow embossed on the piston crown

   No arrow or mark on the piston.  Maybe it’s not fussy on these models. ?  Which way does the dowel usually face?

Friiy & niper99,

     The compression on this was 70psi which I would think ok for a start and run.  I don’t think there is any compression release on these.   What’s a ballpark psi for these?

 

   The sump area cover gasket was torn and the cylinder gasket in bad shape.  I’m not sure if those were ok before and ripped during disassembly.

 

   One of the screw bosses fell off when the cylinder was unscrewed.  There are only three screws holding the cylinder down, one broken so only two.  There are threads left on the boss so might be enough for grip.  Maybe the boss area could be clamped and glued in place. 

Broken boss lower left.

 

Broken boss lower left

The crank is on roller bearings which was a surprise.

Compression on these small two cycle engines vary considerably.   I'd shoot for at least 90 psi and would be happier with 100+.

The dowel on the piston must line up with a portion of the cylinder that has continuous metal for the entire stroke.  You don't want the ends of the rings catching the edge of an intake or exhaust  port.   Do you have the piece of the boss that was broken off?   If so, you could try some JB Weld to put it back on,then re-tap the hole.   JB Weld is miraculous stuff.  It might just hold if you don't torque the screw too much.

trouts..

ya 70 psi not a good number....it might run with that compression but i suspect with a great deal of power loss.....usually l would like to see 100 psi as a mininum...but l have a homelite myself that has 75 psi and it runs and starts not to bad so they sometimes will run with low compression but usually are hard to start and have no power..and if it has low compression then theres usually alot of blow by past the rings wihich counter acts the crankcase negative pressures...so on the compression stroke (piston moving up) there a negative pressure in the crankcase thus closing the diaphragm in the carb (running the fuel pump)..and on power stroke (piston moving down) the crankcase has postive pressure pushing the diaphragm open alowing fuel through. that why if u have a blowin gasket or worn crank seals they just dont run right or at all....and most times like friily said bolts come loose and that make them run lean which in turn causes scoring of the piston walls and serve piston ring wear.

as for the broken off ear....l would braze it and build it back up...lm a licsened welder so i have some experience in this area...Search on u tube DURAFIX and watch some of those videos u gonna need some pratice though it looks easy but not so much u need to get the hang of it.....l have also brazed about 5 of tecumseh blocks that have throwen a rod and cracked the block, not 1 comeback and one of those was on my 10 hp tecumseh which is still going strong. all u need is aluiminum rod that melts at about 730 deg and a propane/mapp gas torch oh ya and a stainless steel brush to clean the surface before u weld. 

Borat and niper99,

>>>Do you have the piece of the boss that was broken off?  

 

   I do. there’s only a tiny amount of surface to weld to and very difficult or impossible to get at the back sides for welding.   I might try JB weld.  The surfaces are clean, rough (good surface for gluing) and fits back in it’s spot very well.  The surfaces are small though. 

   Possibly JB welding it in place then using a bolt through the crank piece and bolt with a washer on the broken section head side might work.  The washers could grab the remaining ˝ not broken section on the head side.  Even if the weld broke the washer would hold the broken head section in place and hold down the head to the block section. 

 

   Thanks for the Durafix reference.  I watched many videos on aluminum welding.  I never knew that repairs could be done with a small torch.  Very interesting stuff.  Poking around YouTube I found stitch pins and a few other methods of repairing cracks in cast blocks I never heard of before i.e., locking pins, tapered, screw-in, and stitch.  I'll probalby give the aluminum welding a try on some scraps.