Cylinder Worn Out ??
10-26-2005, 10:45 PM
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Cylinder Worn Out ??
I have a Saito 56 with ALOT of run time on it. Lately I've had trouble getting it started and it seems to be down on power and surges at full throttle. I took the engine apart and can't really find anything mechanically wrong with it. Tried a different carb but that didn't help. The engine seems to have good compression and when I hold my finger over the glow plug hole and slowly turn the engine to compression, it holds compression for a few seconds before going down. The only thing that doesn't quite look right is the cylinder bore. It has some pencil thin lines (scratches) running from the top to the bottom that look they are dark colored instead of shinney. The bottom skirt of the piston also has some wear lines going from top to bottom. The cylinder also has some spots that are shinnier than other areas on the cylinder walls. The cylinder on this engine is just chrome plated onto the aluminum and doesn't have a sleeve. How can you tell if the cylinder is worn out??? Is it normal to have these "scratch" lines on an old engine cylinder and when would this be a problem????????
10-27-2005, 05:02 AM
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RE: Cylinder Worn Out ??
Stevens:
You have probably been running in dirty air, and have sucked some trash in the engine.
Run your finger around inside the cylinder. If the scratches aren't deep enough to feel they wont hurt you. If you can just barely feel them it's worth trying to reuse it.
If the scratches in the piston are deep enough to catch a fingernail in, replace it. If not, check the skirt clearance in the cylinder. On such a small engine the easy way is measuring the bore with a "Snap" gauge, and the piston with a micrometer. If you have the tooling you can put the piston in the bore and use a dial indicator to measure the side movement. DO NOT use a feeler strip to check it. While the chrome in the bore is hard, if you get a little careless with a steel thickness gauge you can literally dent the base metal of the bore, and crack the plating. Then you will have to replace the cylinder.Up to 0.002" is OK, at 0.003" replace the piston.
Check the valves carefully, the seating face should not be convex in the least. If you have any doubts about the valves, replace them. DO NOT UNDER ANY CIRCUMSTANCES LAP THE VALVES TO SEAT. The AAC cylinders have a thin chrome plating over the base aluminum as valve seats, any damage (for example, cutting them while lapping valves) will thin the chrome and cause you to buy a new cylinder.
Eyeball the cam lobes. A slight concavity worn into the peaks of the lobes is OK, any more than that, or any pitting in the rubbing faces means a new cam. Always replace the tappets when you replace the cam.
The two things (three) that you definitely will want to replace are the piston ring and the (two) valve springs.
When you order your parts add an intake stack and a fine mesh Bru-Line air filter. The stack will trap almost all of the fuel that's spit back out of the intake, and the air filter will trap the rest as well as keeping the trash out. Engine life will go up and fuel economy will at the same time.
HTH.
Bill.
You have probably been running in dirty air, and have sucked some trash in the engine.
Run your finger around inside the cylinder. If the scratches aren't deep enough to feel they wont hurt you. If you can just barely feel them it's worth trying to reuse it.
If the scratches in the piston are deep enough to catch a fingernail in, replace it. If not, check the skirt clearance in the cylinder. On such a small engine the easy way is measuring the bore with a "Snap" gauge, and the piston with a micrometer. If you have the tooling you can put the piston in the bore and use a dial indicator to measure the side movement. DO NOT use a feeler strip to check it. While the chrome in the bore is hard, if you get a little careless with a steel thickness gauge you can literally dent the base metal of the bore, and crack the plating. Then you will have to replace the cylinder.Up to 0.002" is OK, at 0.003" replace the piston.
Check the valves carefully, the seating face should not be convex in the least. If you have any doubts about the valves, replace them. DO NOT UNDER ANY CIRCUMSTANCES LAP THE VALVES TO SEAT. The AAC cylinders have a thin chrome plating over the base aluminum as valve seats, any damage (for example, cutting them while lapping valves) will thin the chrome and cause you to buy a new cylinder.
Eyeball the cam lobes. A slight concavity worn into the peaks of the lobes is OK, any more than that, or any pitting in the rubbing faces means a new cam. Always replace the tappets when you replace the cam.
The two things (three) that you definitely will want to replace are the piston ring and the (two) valve springs.
When you order your parts add an intake stack and a fine mesh Bru-Line air filter. The stack will trap almost all of the fuel that's spit back out of the intake, and the air filter will trap the rest as well as keeping the trash out. Engine life will go up and fuel economy will at the same time.
HTH.
Bill.
10-27-2005, 10:26 AM
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RE: Cylinder Worn Out ??
Bill
Thanks for your expertise. The scratches appear to be somewhat deep since they look black. I presume that they have gone thru the chrome and what I'm seeing is plain aluminum that has turned dark. The back of the exhaust valve looks good, a shinny round ring can be seen and the valve seat in the head looks O.K.. The intake valve doesn't have the shinny ring but a flat dull appearance and the valve seat also looks a little strange. I never knew that you shouldn't lap the valves on this engine. Is this because the head/cylinder is one piece and everything inside is chromed ??? Is that why it is O.K. to lap valve head seats in engines with a removable head ??? Cam also looks funny. You can see a slight grove where the cam follower has worn it on the upside of the lobes but you can also see a creater on the back (downhill) side where it looks like the follower has bounced back onto the cam after it "jumped" over the cam lobe (maybe weak valve springs here??). Anyway repair parts would probably cost more than half of the cost of a new engine (new about $170) so I'm better off buying a new one. This has been a great running motor and has provided me with many many hours of fun. I've more than got my monies worth out of it. I was mainly courious to find out what was causing my running problems since I really couldn't see anything mechanically wrong. Maybe my simple compression test didn't tell the whole story. Maybe it should have held compression for more than a few seconds or maybe when the engine got hot the worn thru spots on the cylinder walls caused it to heat up and surge?? Still not sure why it would be so hard to start?? Thanks again for you help and allowing me to benefit from your experience. That's what makes this hobby great, the people!!
Thanks for your expertise. The scratches appear to be somewhat deep since they look black. I presume that they have gone thru the chrome and what I'm seeing is plain aluminum that has turned dark. The back of the exhaust valve looks good, a shinny round ring can be seen and the valve seat in the head looks O.K.. The intake valve doesn't have the shinny ring but a flat dull appearance and the valve seat also looks a little strange. I never knew that you shouldn't lap the valves on this engine. Is this because the head/cylinder is one piece and everything inside is chromed ??? Is that why it is O.K. to lap valve head seats in engines with a removable head ??? Cam also looks funny. You can see a slight grove where the cam follower has worn it on the upside of the lobes but you can also see a creater on the back (downhill) side where it looks like the follower has bounced back onto the cam after it "jumped" over the cam lobe (maybe weak valve springs here??). Anyway repair parts would probably cost more than half of the cost of a new engine (new about $170) so I'm better off buying a new one. This has been a great running motor and has provided me with many many hours of fun. I've more than got my monies worth out of it. I was mainly courious to find out what was causing my running problems since I really couldn't see anything mechanically wrong. Maybe my simple compression test didn't tell the whole story. Maybe it should have held compression for more than a few seconds or maybe when the engine got hot the worn thru spots on the cylinder walls caused it to heat up and surge?? Still not sure why it would be so hard to start?? Thanks again for you help and allowing me to benefit from your experience. That's what makes this hobby great, the people!!
10-27-2005, 10:14 PM
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RE: Cylinder Worn Out ??
If your engine is as old as you say it might pay to check the O-ring(s) in the intake system. If the O-ring(s) has/have hardened up and isn't/aren't sealing then you'll get hard starting and inconsistent running just like you describe.
For the sake of a couple of bucks it would be well worth replacing them and seeing if it makes any difference.
For the sake of a couple of bucks it would be well worth replacing them and seeing if it makes any difference.
