new piston enough?

[hisey1]

my '79 evinrude 85 horse has bad compression on one cylinder. i have begun taking the motor apart and the upper ring on the #2 is toast. the thing is that the wall looks and feels like new---well, looks and feels like the others do. i don't see any damage on it. am i safe to assume that all i need to do is replace the piston and rings and reassemble for a good motor? also the carbs look like they are new too. there is a lot of black junk in the exhaust manifold---whats the best means for getting that stuff off there?

 

[WillyBWright]

Re: new piston enough?

The cylinder isn't round anymore. The new rings may never seat to he walls. At the very least you need to hone the cylinder. But a motor of that age ... the cylinder ain't round anymore.<br /><br />Bust off the chunks of carbon as best you can, but don't sweat the thin stuff. Carbon is going to form again anyhow.

 

[hisey1]

Re: new piston enough?

so let me ask this. if i replace the damaged parts (piston and rings and hone the cylinder.) do you think this motor will last through a full summer and fall season? the boat was cheap and my initial thought was to run the boat as long as possible then sell it.

 

[OBJ]

Re: new piston enough?

If done correctly, it should last a lot longer than that. Do pay attention to WillyB's advice. You will want to have the cylinders micrometered to see if they are still in spec for diameter and roundness. Boring may be required. <br /><br />If you plan to do the job, an OEM service manual is a must to have.

 

[Silvertip]

Re: new piston enough?

The more important question is why did the damage occur in the first place. Something caused the ring damage. Incorrect timing and/or lean fuel mixtures are the two usual causes. Because the carbs look clean doesn't mean they are. They need to be totally disassembled, every passage cleaned thoroughly and blown out with air. Then new kits installed. Upon reassembly, ensure idle and WOT timing is set to spec. Otherwise you may be doing this job again in a very short time.

 

[gatorred]

Re: new piston enough?

you do one piston make sure the other three are same weight, it a blance factor,do what upinsmoke is right clean is everthing do a ring job on all four piston's to hone all four cylinder you want the motor to run even and run for long time to then run tank of 1 quart oil to six gallon's first run tcw 3 type out then 1pint to six gallon's of gas the first tank is brake in tank it like a new motor after rebuilt it.<br /><br />hope this helped

 

[hisey1]

Re: new piston enough?

thanks guys. i have a manual and have been tearing down the motor. i have the heads off and the carbs and intake manifold and exhaust covers off. also starter and solenoid. i'm to the point of taking off the flywheel but i have no "appropriate flywheel holding fixture" as mentioned in my manual. is there any other acceptable way of holding it while i try to break the nut free? i have soaked it in penetrating oil to help free it some and tried tapping the wrench with a hammer to get it broke, but the flywheel gives just enough that i dont think i can break it without holding it someway.

 

[Paul Moir]

Re: new piston enough?

Just use an inexpensive automotive flywheel holder/turning tool. You're going to need it to torque the nut later anyway.

 

[hisey1]

Re: new piston enough?

thanks paul, i figured i was going to have to buy some expensive special outboard tool. i'll be on my way to the auto parts store.

 

[gatorred]

Re: new piston enough?

how i do it hold the flywheel with vice grip or big screw drver then force the wrench or tapping the wrench well holding the flywheel after it frees up let nut on thread's even with top of nut take screw driver pry the flywheel up even on two side's with screw driver's or prybar's two people job then tapping the nut maybe a hard rap with hammer then split case in too hope you split from case from lower unit it real job to do.

 

[hisey1]

Re: new piston enough?

Ok- upon further disassembly, i found a nice, deep gouge in the wall of that #2 cylinder. now, being that i haven't the means for boring the block or the budget to afford a rebuild right now, would i be a fool to just clean everything up as best i can and put it back together? as mentioned above, the boat was cheap and i'm thinking about running it as long as possible then selling it. advice? feedback? anyones' thoughts on that idea? would i be wasting money if i went ahead and did a ring job?

 

[Paul Moir]

Re: new piston enough?

I'm a big cheapskate, but I just don't get it. You're talking about installing a new piston and ring (~$100 or so, plus gasket set, plus sealers, plus tools plus your time). The price is exactly the same for an oversized piston and ring set. Now you're asking if you can slap a $50 gasket set and a $20 set of rings on it and run it as is, all for the sake of saving $80 or whatever it'll cost to have that one cylinder bored out and honed?<br /><br />It really seems like good money after bad. I don't think you'll find anyone here that will recommend running an outboard with a half-blown piston. It's just going to get worse until it fails catastrophically. Which always happens furthest from shore in the worst weather. <br /><br />I understand it was a cheap boat. Of course it's a cheap boat: it's motor needs work. You sound like a smart guy - why not just fix it properly yourself and when you do sell it you can sell it with a perfectly good engine? If you want to get a feel for what's needed, have a look here:<br /> http://forums.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=28;t=024177 <br />Now that's a V-4 and he did all four holes (plus a bunch of extra stuff like bearings), but it gives you the idea.

 

[hisey1]

Re: new piston enough?

WOW!! i just read that entire thread. what a story!---and inspiring too. i have never tried to tackle anything like this and i guess i'm just feeling a little overwhelmed. i guess you're right about just doing what needs to be done and that way i know everything is in working order. i will probably press on tomorrow night after work. thanks for the advice.

 

[emdsapmgr]

Re: new piston enough?

Check the head for damage. Typically, when a ring lets go, it pounds itself into the head and the top of the piston. If the head is pitted, it needs to be replaced. Each pit can cause a hot spot, and preignition. I would not suggest running an overhauled engine on a bad head. Let us know what you find.

 

[hisey1]

Re: new piston enough?

the head has just a few--2, maybe 3--small nicks in it in one place. does it need replaced or can i just grind them smoothe with a dremel or something like that? no very deep either. i can take a picture and post and maybe you guys can tell me what you think??

 

[OBJ]

Re: new piston enough?

2 or 3 small nicks won't hurt. Like you said, take a dremmel and smooth them over a little. What you don't want is a bunch of nicks that can get carbon built up in them. The carbon deposits can get hot enough to glow and cause pre-ignition.

 

[hisey1]

Re: new piston enough?

if i have that one cylinder bored out and honed and install an oversized piston, is that going to knock the balance of the motor off? if the others look good, do i need to have them done too just to keep everything balanced or is it aceptable to just do the one?

 

[Goldfish]

Re: new piston enough?

From other threads i have read, no, omc or the manufacture of the piston makes them all weigh the same. You should be good to go

 

[hisey1]

Re: new piston enough?

thanks again---gonna try tearing it down some more this week and call some machine shops in the next couple days. i am sure i'll be back to ask for more advice soon.

 

[hisey1]

Re: new piston enough?

ok----after much work and fumbling through, i have my motor tore down all the way. i have taken the block to a machine shop so he could look at it and he said i'd be good buying a .020 over piston and he'd bore the hole for me. i also found that the rod bearing on that #2 is shot. i'm trying to get parts on order now and would like someone else to take a look at what i've picked out so i don't buy the wrong things. i have decided to buy from boatpartstore.com. the piston item number is 85-3173p2. bearing is 85-18-1361. and overhaul gasket set is 85-18-4303-1. it would be highly appreciated if someone could assure me i have got the right parts here. once again----thanks a million guys!