Piston not moving when turn flywheel

[gr8rg8r]

I have a 1998 Evinrude Commercial 100 engine that I inherited from a tenant when he left (long story). He said he never ran it and he was going to check the compression when he noticed, through the spark plug hole, that one piston wasn't moving when he turned over the engine. I guess the problem has to be a broken crankshaft or a broken connecting rod. Anything else possible?<br /><br />I was thinking of pulling the head and attaching a suction to the piston and see if I can pull it out to assess the problem. Or, alternatively (translation: a lot more work), I can pull the front of the engine off, exposing the crankshaft and connecting rods. Does this make sense?<br /><br />Also, what is the difference between a commercial and non-commercial engine? Is a commercial more durable or just have less gizmos so the fishermen and public sector folks will buy them?<br /><br />Any and all advice/suggestions are appreciated.<br /><br />Richard

 

[Hooty]

Re: Piston not moving when turn flywheel

Yep, if that piston ain't movin' it's got itself a problem okay. <br />My guess iz a rod but I've broken crankshafts too. Removing the head would be easiest but the front's gonna have to come off sooner or later and that's where you need to look anyway so that's where I'd start. Any external damage that you can see? Like where a rod came through? <br />Get back and tell us what ya find.<br /><br />c/6<br />Hooty

 

[lark2004]

Re: Piston not moving when turn flywheel

You still nedd to take the head off to remove the piston, but as hooty said, you need to take the front off to be able to reassemble the engine. good luck

 

[rwise]

Re: Piston not moving when turn flywheel

still a full tear down but I have heard of snapping the top off a piston. the top would be at the top of the cylandar.

 

[gr8rg8r]

Re: Piston not moving when turn flywheel

Hooty,<br /><br />No external damage. I am leaning, like you, that it is a broken rod just based on the shear size and thickness of a rod versus a crankshaft.<br /><br />Rwise,<br /><br />Hmmmm, never heard of a piston breaking off. Interesting.<br /><br />Replacing a rod can be accomplished by removing the front guts to expose the crankshaft, unbolting the rod, and pushing it out. Much easier than replacing the crankshaft.<br /><br />I'll keep you apprised of what I learn.<br /><br />Richard

 

[BF]

Re: Piston not moving when turn flywheel

Have you looked for yourself to see if it's REALLY not moving?? I would confirm that before tearing into it. Sometimes folks diagnosin' skills suck to a surprising degree (or they may have been going on somebody else's bad vision).

 

[stairliftoheaven]

Re: Piston not moving when turn flywheel

i agree with BF; you should have some horrible noises with a broke rod when you turn the engine over. not familiar with ur engine: how many cylinders? which one aint movin?

 

[gr8rg8r]

Re: Piston not moving when turn flywheel

BF,<br /><br />Per your Excellent advise, I checked it this evening. The starboard top piston does not move when I turn the crankshaft - the other 3 pistons turn. But, there is no horrible noise. It is almost like there is no connecting rod in there! When I tried to push in on the piston under moderate pressure, it wouldn't move.<br /><br />The bad cylinder is the uppermost position on the crankshaft (closest to the flywheel). This would suggest that the crankshaft did not break because the lower pistons are moving.<br /><br />Also, there is no evidence of overheating.<br /><br />Tomorrow, I'm going to pull that head and see if I can see the problem. I don't have time to fix it now, but the diagnosis is driving me nuts.<br /><br />Richard

 

[Hooty]

Re: Piston not moving when turn flywheel

I broke a V6 crankshaft once, between nr.1 & nr.2 rods and the engine still ran. It did knock a little but it ran.<br />I bet you lost a rod cap and the crankshaft just pushed the rod and piston up out of the way.<br /><br />c/6<br />Hooty

 

[gr8rg8r]

Re: Piston not moving when turn flywheel

Well, curiosity got the better of me and I pulled the head. Aluminum shavings from the top of the piston greeted me! And the cylinder wall is scored.<br /><br />Without tearing it apart, I know it needs a total rebuild with at least one new piston and rings, bore one cylinder and hone the others, and either a connecting rod or a crankshaft (connecting rods are much cheaper than a crankshaft). These are repairs I can, and have done in the past, myself. I started a new thread to ask for opinions on whether I should rebuild it myself and have decided to overhaul myself beginning in a week or two.<br /><br />The more nagging question, is why it happened in the first place.<br /><br />Thanks for input

 

[BillP]

Re: Piston not moving when turn flywheel

gr8rg8r,<br />I don't know specifically about 1998 OMC commercials but my older 65hp commercial was advertised with the following changes compared to a "domestic 70 hp" motor (same head).<br /><br />Low compression head - For running down to 67 octane gas. A reality in foreign countries where most fuel quality is iffy. It also dropped 5hp on mine.<br /><br />More splines on the driveshaft - for constant and fwd/reverse shifting (full throttle to full throttle). <br /><br />Larger fuel pump <br /><br />No vro<br /><br />Chromed water pump housing - longer life for impeller<br /><br />OMC advertised some of their commercials as having different gear ratios. The rep (warranty manager for OMC Commercial Products in Sunrise, FL) I bought this motor from told me it was the same lower unit as a standard motor.