How big is the cylinder to piston clearance?

[Mark42]

I was looking at the cylinders on the '93 40hp (trying to determine cause of cylinder scoring)and noticed that they fit quite "loose" in the cylinder. I can press my finger on top of piston and rock the piston side to side, like there is 0.020 - 0.030" clearance. Seems excessive to me, don't remember any other motor I worked on having that much side play. <br /><br />This block was probably bored oversize as part of the rebuild. I'm wondering if the cause of the cylinder failure is from standard pistons being installed with oversize rings in an oversize bore? Could cause the rings to break and score the cylinder wall? <br /><br />I know this much: <br /><br />* No water in fuel line.<br />* No water in Oil tank.<br />* Oil shows at sight tube under VRO pump.<br />* Motor was pumping water very well from tell-tale and thermostat was just removed to ensure no head blockage.<br />* Temp guage showed 120* steady, dropped after taking out thermostat.<br />* Lower cylinder still reads 140psi compression just like it did last month when everything was running well. Doesn't sound like VRO problem right?<br /><br />I want to know what happened BEFORE I ship this power head back to Blackbird for inspection because they indicated that they would compensate me if the cause was materials or workmanship even though I am out of warrentee due to my claim of low hours on motor. <br /><br />This photo shows the top piston fitting tight on the port side.<br />

<br /><br />This photo shows the top piston with large gap on starboard side.<br />

<br /><br />This is a dead on photo showing that both pistons seem to have the same gap on the starboard side.<br />

<br /><br />What do you guys think?

 

[rodbolt]

Re: How big is the cylinder to piston clearance?

from the looks of the piston and the rust on the cyl walls as well as the looks of #2 piston dome you have a water intrusion problem. also the intake and exhaust ports are nasty looking. like they were not chamfered properly. but the white flecks on the ex port also indicate water.<br /> if it was a VRO issue you will normally see damage on both pistons as well as the crank/rod bearings. usually they turn blue.

 

[Mark42]

Re: How big is the cylinder to piston clearance?

Thanks, Rodbolt.<br /><br />I'll drain the carbs and see what comes out. Saw that rust color and have been looking for water but haven't found it (yet).<br /><br />Maybe the head gasket? What are the telltale signs of the head gasket leaking?<br /><br />I take it the piston fit looks ok to you? I don't have a guage to measure for out-of-round or other micro measurements.

 

[clanton]

Re: How big is the cylinder to piston clearance?

The numbers work out to .004.4 for standard piston and .004.5 for .030. over piston. These specs for OEM piston. I would not have removed the thermostat. The piston clearance is not measure at the top.

 

[R.Johnson]

Re: How big is the cylinder to piston clearance?

That engine look's just like what you did! Removed the thermostat.

 

[R.Johnson]

Re: How big is the cylinder to piston clearance?

That engine look's just like what you did! Removed the thermostat.

 

[G DANE]

Re: How big is the cylinder to piston clearance?

I never had one of these apart, but what is that hole above exhaust ?? Piston and cylinder looks like its steam cleaned or burned lean. All oil is gone.

 

[Mark42]

Re: How big is the cylinder to piston clearance?

G Dane, Yeah, it does look clean. But then again it only has about 50-60 hours on it. I'm pretty sure there must be water in the gas tank. I'll have to siphon it out and see what I can find this weekend.<br /><br />That hole is connected to the exhaust port and I believe it is part of the looper design. <br /><br />Live and learn. Like my neighbor Mr. Richbritches often says... "It's only money right?". Yeah, right.

 

[OBJ]

Re: How big is the cylinder to piston clearance?

It's called "cold seizing". This is when the piston expands faster than the cylinder walls during operation and you get those scuff marks on the sides of the walls. It's caused by the water around the cylinder being to cold and not allowing the cylinder to heat up and expand.

 

[Dhadley]

Re: How big is the cylinder to piston clearance?

That hole is a compression relief hole. Theory has it that it helps the motor to idle better.<br /><br />Sounds good, huh?

 

[Mark42]

Re: How big is the cylinder to piston clearance?

Originally posted by OBJ:<br /> It's called "cold seizing". This is when the piston expands faster than the cylinder walls during operation and you get those scuff marks on the sides of the walls. It's caused by the water around the cylinder being to cold and not allowing the cylinder to heat up and expand.

I would buy that but the motor was already warmed up, in use for close to an hour and started running hot. Like I said, 120* maybe a little more. After removing the thermostat, the temp guage dropped back to normal. My guess is that lack of lube was causing friction heat buildup. Sound good?

 

[willamettejeff]

Re: How big is the cylinder to piston clearance?

Sure would be interested in seeing what the bottom cylinder looks like in comparison. Any chance of getting a picture of it Mark? This has been all just too interesting and I have been following it very closely. BTW Mark, did you have a fuel filter/water separator installed. My outboard mechanic friend advised me to do this when I installed my 18 gallon permanent tank. Don't know if it means anything that your problem seems to have occured shortly after installing your permanent tank.

 

[Mark42]

Re: How big is the cylinder to piston clearance?

Willettejeff,<br /><br />Can do that this weekend. But I can tell you the bottom cylinder wall looks perfect. Just like a good running cylinder should... no scuffing or discoloration. Don't know about the flakes that rodbolt pointed out earlier. Will have to see if they are on the lower cylinder exhaust port too.<br /><br />I will be adding water intrusion/filtering hardware. Just posted about it in outboard discussions.

 

[Dhadley]

Re: How big is the cylinder to piston clearance?

Is that how dry the cylinder was or did someone wipe it down?<br /><br />What plugs were you using?

 

[Mark42]

Re: How big is the cylinder to piston clearance?

Dhadley,<br /><br />Actually, the walls were drier. I shot in a squirt of oil to do the compression test because the plugs came out dry. <br /><br />The plugs are Champion, the normal operating range, not the extended hi speed range for that motor. I don't recall the plug number. It is whats listed in the Johnny/Evinrude manual.

 

[Dhadley]

Re: How big is the cylinder to piston clearance?

They should have more gas & oil. It just looks awful dry. Have you looked at the carbs yet?<br /><br />Do you think this may have happened when it ran hot?

 

[Mark42]

Re: How big is the cylinder to piston clearance?

No, have not looked at carbs yet. Just what you see in the pictures. Will get to it this weekend when I remove it for crating.

 

[G DANE]

Re: How big is the cylinder to piston clearance?

Mark I rember that you mentioned that on initial startup after winhterizing, it iddled rough from start and luckily smothened out after you ran it some. I think the upper carb got somewhat plugged after winther. <br /><br />Dhadley - sounds strange with the hole - but they have to invent something once and in a while, dont they. A little heavier flywheel would help more, I think, but what do I know. Do all loopers have that hole ? <br /><br />The reason I mentioned the missing oil is that cast iron actually sucks up oil if there is oil present. Your scorings look "dry". I dont think you have water in tank, but your exhaust water jacket gasked could be leaking. But is looks like lean burn to mee.

 

[DJ]

Re: How big is the cylinder to piston clearance?

Mark,<br /><br />Water in the fuel tank or oil injection system would effect both cylinders, as mentioned by rodbolt.<br /><br />However, it sure looks to me that water was definetely getting in that cylinder, thus washing everything down, including lubrication. I had excatly the same thing happent to V-4 crossflow. Luckily, I caught it before it scored anything. It was a headgasket.<br /><br />If that engine came from the rebuilder with the head on and the exhaust cover on, I think you have a claim.

 

[Dhadley]

Re: How big is the cylinder to piston clearance?

No, not all loopers have that hole. And youre right about the engineers having to justify their job. They swear it makes the motor idle better but if you sleeve a cylinder that hole gets covered up. Guess what? It idles the same. Now, some guys claim the motor loses hp with those holes. They're absolutely right. On a V6 at 6000 rpm plugging all 6 holes adds 1/2 of 1 hp. Try measuring that.