Low Compression With New Piston/Ring

[Oli]

I am trying to rebuild a Husqvarna 42 Special. During tear down, the saw appeared to be very clean suggesting that it was lightly used. It has an open port cylinder with a non-window piston. The piston and cylinder were scored when I received it. I cleaned up the cylinder with muriatic acid and a Scotch brite pad. There were no observable scratches that I could see or feel after I finished, but there was some discoloration where the cylinder was scored. I replaced the piston with a new Meteor piston and ring that was specified for a model 242. The new piston is a straight skirt, non-window piston and appears to be a good match to the old piston The only difference is the old piston had a a depression/cavity around the wrist pin hole and the new one does not. Upon re-assembly, the compression was only 60 psi even with two cycle oil in the cylinder. This was my first attempt at trying to salvage a cylinder. Was this cylinder too far gone to resurrect, or do I need to check other parts on the saw? Would an OEM piston ring make any difference?
Thanks for any suggestions.

 

[greg409]

A member named scandy14 asked this in his thread "husqvarna 42/242 piston question" back in 8/2009

Try searching that

I hate to state the obvious, but you had the throttle wide open and pulled it at least (6)times?

I agree, that's suspiciously low.

Compression release?

luck,greg

 

[Roanoker494]

Did you check the ring end gap before installing the piston? If the ring gap was good then I don't see where a oem piston kit would produce any more compression. How many times did you pull the saw over to get the 60psi reading? As mentioned already it can take 6-8 pulls to get a true reading. Does the compression feel low when pulling it over with a spark plug installed? If your compression tester is lacking the little valve in the hose, or your valve is bad, it will give a false reading.

 

[blsnelling]

Check your squish. With that low of compression, it sounds like the piston is much shorter than the original. Meteor pistons use Caber rings. They are very good.

BTW, it is not necessary to hold the throttle open on these 2-stroke engines.

 

[nmurph]

Check your squish. With that low of compression, it sounds like the piston is much shorter than the original. Meteor pistons use Caber rings. They are very good..

Yeuuup

BTW, it is not necessary to hold the throttle open on these 2-stroke engines.

and double yeuuuup yeuppppppppp!!

 

[Oli]

I checked previous threads ref. model 42 vs. 242 piston interchange and it seemed to work for other people. Multiple pulls with a known good pressure gauge resulted in 60 psi. I could not check ring end gap. The ring end gap is not visible from either the intake or exhaust port with the new Meteor piston, but the ring end gap on the old piston could be observed from the exhaust port. I need to borrow a micrometer before I can advise on the squish. Thanks for the replies

 

[PES+]

How much clearance was there between the piston and the cylinder without the ring?

Are you sure you installed the piston the right way round?

Some meteors I have seen with an arrow pointing to the intake side rather than the exhaust.

I can't remember what saw but it explained it in the little paper in the box

 

[Fifelaker]

End gap is measured with the ring off the piston. Stuff the ring in the cylinder and push it in with the piston.

 

[PES+]

How much clearance was there between the piston and the cylinder without the ring?

Are you sure you installed the piston the right way round?

Some meteors I have seen with an arrow pointing to the intake side rather than the exhaust.

I can't remember what saw but it explained it in the little paper in the box

It wasn't a saw it was a bike piston but the arrow was reversed from normal.

I am getting senile

 

[Denis Gionet]

I am getting senile

That's ok... admitting you have a problem is the first step towards cure ;-)

 

[greg409]

That's ok... admitting you have a problem is the first step towards cure ;-)

Are you just leading me on?

I thought my forgetfulness was only gonna get worse!!

What was the problem???

luck,greg

 

[greg409]

Check your squish. With that low of compression, it sounds like the piston is much shorter than the original. Meteor pistons use Caber rings. They are very good.

BTW, it is not necessary to hold the throttle open on these 2-stroke engines.

Brad- Murph, all my life I've done compression checks @ WOT (including 2-strokes)

I honestly don't know why it's not necessary, Where's the adequate air come from for the compression?

Luv to learn, honestly

luck,greg

 

[nmurph]

Because atmospheric air is pulled into the cylinder when the piston falls. Try it: i.e. it might take 6 pulls to get to 120psi with the throttle open and 7 without, but it will get to the same final reading and a bigger factor in how fast it reaches its peak is how hard you are pulling.

 

[blsnelling]

It'll pull it in the exhaust port because there's no valve to block it off like in a 4-stroke.

 

[Sprintcar]

To the O.P.

Take some pictures of the two pistons side by side.

Measure from the top of the original piston to the center of the wrist pin boss.

Now measure form the top of the replacement piston to the center of the wrist pin boss.

Compare readings.

Report back with findings.

 

[greg409]

It'll pull it in the exhaust port because there's no valve to block it off like in a 4-stroke.

I know now that I thought about it.

Both of you better not tell anyone I asked that question:msp_mad:

Somebody say sumpin' about senile?

Thanx

luck,greg

 

[bplust]

For compression readings, I like using a drill with a 3/8" bit and socket on it and watching the gauge climb until it stops. I find it to be quicker and ensure that I didn't cheap out on the pulls.

 

[wyk]

The last 42 special I saw had a compression valve release on it. Perhaps that could be the problem.

 

[Callum]

sorry to wake up a dead thread, but did you find the solution, I have a 010 with a similar problem