Smoggyboy

I recently lost a 60 size Zero to servo failure. I recovered all but the airframe but have a problem with my Saito 100. There appears to be no outward signs of damage but I have lost 90% of the compression.
A few facts to help.

The engine turns over by hand freely without any adverse friction or sound other than the valves opening and closing.
I have checked and reset the tappets.
The compression improves when I block the crank case breather.
It appears that there is excessive blow-by.

The question is, is it possible that the Piston ring has been damaged in the crash?
If I open the engine what am I looking for regarding other possibilities?
Should I try to start it?
Would the introduction of normal fuel flow through the engine improve the compression to a point where it would run as normal?

Thanks for the help
Dave

Spetz

Maybe your vavle/s have the wrong timing (somehow the cam skipped?)
or they are bent and don't seal properly
Is your piston moving up and down freely without any grinding or friction?

I would open it to check it out. If it's a ring problem it should be clearly visible
At the same time make sure nothing is bent and that the cam timing, valves, valve springs, pushrods etc are all in good condition

XJet

If blocking the breather increases compression then that means the piston is sealing in the bore properly (you're feeling crankcase compression).

After a hard knock, it's not uncommon for the Saito pushrods to pop out of the little dimple in the rocker-arm.

Check to see that each pushrod is correctly seated into the end of the rocker.

The other thing (of course) is to make sure you've got the throttle barrel *open* when you turn the engine over. If the throttle is closed there will not be any compression because no air can get past the carby and into the cylinder to be compressed.

Smoggyboy

I checked the valves first and found on gap x2 large and one push rod had jumped out of its cup seat. Othe rthan that all loked very good at the top end.

The engine turns by hand very smooth with about 10% of the original compression at thr top of the stroke.

One thing is that I do get Very high crank case pressure through the compression stroke.

It seems that the crank case pressurises (the vent blows air) on the compression stroke. Almost is if the piston ring has gone and I get too much blow-by.

I think I will have peak inside. Not sure what to look for apart from a missing or damaged piston ring.

Will the excessive crank case pressure cause any other problems?

Is it worth trying to start it?

Thanks again.

Spetz

I don't think it would start as the way the glow plug works is that when the upward stroke compresses the air it heats up and therefore there is a spark
No compression = no spark pretty much

Also taking into account that it's a 4 stroke engine if there is a valve/pushrod/cam problem there is a chance the valve will hit the piston head

If it was mine I wouldn't attempt starting it until I have made sure the valvetrain is spot on

T.L.A.R. eng

Has the engine ingested some sort of dirt, grass, foreign matter holding a valve open?

Removing the glow plug and using compressed air into the glow plug hole with each valve open can clear out minor debris.

Just a thought as this has happened to me before.

gkamysz

My Feedback: (19)

Even if all of the air in the cylinder made it's way into the crankcase on the compression stroke none would be forced out the breather vent. The piston is displacing equal amounts of air above and below it. Don't be distracted by crankcase pressures. As long as the breather is clear it will run correctly.

Reseating the pushrod didn't restore compression? I vote for dirt also.

daveopam

My Feedback: (9)

Is it possible one of the push rods is hanging in the tube? You said there was no outward sign of damage but could one of the tubes be bent a little?

David

w8ye

My Feedback: (16)

Put a little castor oil in the glow plug hole and let the oil get around the ring and see if compression improves. The cylinder may be dry.

Bladejunkie

Smoggyboy,I had the very same problem with my Saito 65 it was in a Cub I had that went straight into a dirt field.I took the engine out of the remains of the plane and noticed it turned over quite nicely but had no compression.Once I got it home I took it apart and found dirt around the valve seats and on top of the piston.I couldn't find any other problems so I cleaned it up put it back together.I put it in my test stand and it fired right up.I think all the castor oil saved it the dirt seemed to be glued to the valve seats and piston.I always put three oz. of Sig castor oil in each gallon of fuel.By the way this engine looked very clean on the inside.


Jim

Smoggyboy

Thanks guys,

I have just took a better look inside the exhaust port and I think I can see grains of sand. The exhaust was knocked of the engine during the crash, also I am flying in the Kuwait Desert, so it went down in semi-compacted sand. (which saved the engine and RX gear in the first place)

I agree with the dirt theory.

How do I get into inlet and outlet chambers to clean them out, I really dont want to do a full head stripdown, but if that the only way. Can I take the head off in one piece and flush it through with glow fuel.

Dave

Smoggyboy

Well,
I openned her up, took the head off.
Two things wrong, Lots of trash in there, and jamed open exhaust valve. Both quite easy to sort out in the end. I had made a mess of setting the tappets due to ajusting them with debris holding the exhaust valve open. She is back together now and feeling spot on, good compression and SMOOTH.

Thanks to all that pitched in with advice, hope I get a chance to return the favour.

Dave Hart