reboring

[mike pulcella]

1991 3 cylinder mariner 75. bottom cylinder badly scored. is it necessary to rebore all 3 cylinders.

 

[orca]

Re: reboring

G,Day, the answer is no, the oversize pistons are balanced to the same weight as the original and you will have no problems

 

[mike pulcella]

Re: reboring

Thanks Captain whaley and good luck in the world cup

 

[Forktail]

Re: reboring

"oversize pistons are balanced to the same weight as the original"<br /><br />Not really. You'd have to remove the other pistons and weigh them to be sure. It's highly unlikely they will weigh the same, especially considering any carbon build up.<br /><br />When OEM pistons are manufactured they end up with various weights. Partly due to the fact that they are cast. Prior to assembly the factory matches the pistons in sets. This ensures smooth and long lasting operation. An inbalance of piston weight can cause undue stress on cranks, con rods, bearings, etc. Besides, there will be a slight compression change which will affect fuel timing for that cylinder. And the volume of air/fuel mixture will increase for that cylinder. Thus without jetting compensation that cylinder will run leaner. <br /> <br />It is true that most replacement pistons can be bought one by one and are claimed to be the same weight. This IMO, is a compromise and shortcut. Going through the trouble of disassembling an outboard's engine and boring only one cylinder makes little sense if quality work is a concern.<br /><br />Aftermarket piston sets are balanced and matched as exact sets. Boring 3 cylindes rather than 1 isn't that much more work. Sure, doing a one-cylinder job will work, and it may be the way to go on an older "junker". But if you want a quality job with long-lasting results, do them all. JMO

 

[dmessy]

Re: reboring

Have to disagree with Forktail, just bore the effected cylinder, replace with oversized piston(Wisco is hard to beat). If other cylinders are within limits,clean them up(decarb.)and leave them be. <br />Two old sayings that have a ring of truth;<br />If it isn't broke , don't fix it and, better is the enemy of good. What this means to me is, why waste money on replacing in -spec. pistons/rings just because you've diassembled the motor. Two extra bore jobs and piston/rings/wristpins/bearings will increase your rebuild cost around $250, thats a pretty big chunk of what your 12 year old motor is worth. The new pistons are just as likely to break as the originals. I've done the above on several motors I've owned and theyare still running just fine several years down the road. It's your motor, I'm sure this forum will steer you in the right direction.

 

[Forktail]

Re: reboring

Certainly do what you want. <br /><br />"...replace with oversized piston(Wisco is hard to beat)..."<br /><br />Now you have 2 cast pistons (OEM) and one forged piston (Wiseco). They will not be matched. As cylinder temperatures increase, forged pistons expand further and at a higher rate than cast pistons. Therefore they generally require slightly more bore tolerance. Forged pistons require better warm-ups to avoid "cold seizure".<br /><br />"If it isn't broke , don't fix it..."<br /><br />It is broke. That's why he's rebuilding. How about "fix it right the first time". <br /><br />"Two extra bore jobs and piston/rings/wristpins/bearings will increase your rebuild cost around $250"<br /><br />There is a cheap way and a right way to rebuild an engine. $250 is chump change compared to another rebuild and peace-of-mind.<br /><br />"The new pistons are just as likely to break as the originals."<br /><br />How do 'ya figure that? The originals will obviously have some wear and stress. Probably a little scuffing. After 12 years the cylinders are probably out of round, the cylinder top will most likely have a ridge, and most of the honing will be gone. Maybe even a little damage from the cause of the rebuild itself.<br /><br />Again, certainly do what you want.

 

[dmessy]

Re: reboring

Hey Gotcha<br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=008120. <br />This recent topic has the advice you need from some very good wrenches on this board. You know my and Forktails opinions so i guess you should do what's right for you. Hope you get her up and running soon as the season draws near(or in my case, never stopped!) <br />PS-Ooopps, tried the link but no go, just do a search using "piston replacement". All you'd ever want to know.

 

[Hooty]

Re: reboring

Could there be two right answers here? Probably. I've done it both ways and I would say it just depends on what the other two cylinders look like.If, like Forktail sez, the other two are out of round, have a ridge at the top OR bottom, piston skirts showing signs of hi-milage, go through the whole engine. Re-bore, new pistons, rings, etc. But if it's a 12y/o engine with a hundred or so hours, bore the one hole and replace one piston, hone the other two cylinders and re-ring. If you do decide to replace just one piston, I would use a cast one. In fact, regardless how many pistons you replace, I would replace with cast pistons<br /><br />c/6<br /><br />Hooty

 

[ODDD1]

Re: reboring

gotch, you can do just the damaged one...mercs pistons are said to weigh out the same, and any difference would be a minor thing anyway....if you dont beleive me, get a mercury service manual for your engine and read it.

 

[dpminc]

Re: reboring

The above post is correct. The book does say replacing one piston is ok,HOWEVER, the factory manual is written with warranty service in mind (read: save us money on claims) Some factory manuals also say to reuse seals and o-rings if they look OK. This does not mean this is the best course of action on an older outboard. If you are on a tight budget, and doing the work your self, just bore the one hole, and re-ring the other two. Have the two good holes honed when you have the bad one bored. If you are paying labor on top of parts, it would be false economy not to have a complete job done. A few tips if doing this your self: Check the bore size after boring! Clean the bores with HOT SOAPY WATER, not carb cleaner,wd-40,solvent etc these will not remove the abrasive left from the machining process. Check the ring gap(most important). On that motor of yours, check that the sleeve has not moved downward. this is common on these motors, and a couple of well placed beads with a tig welder will prevent these from moving. Discuss this with your rebuilder. Good Luck.

 

[Hooty]

Re: reboring

Another thing. When honing a two stroke cylinder, don't use a ball hone. It seems those little balls whipping around in there will ding the edges of the ports.<br /><br />c/6<br /><br />Hooty

 

[Dhadley]

Re: reboring

A couple of things here just to clarify - Wiseco's are matched to the factory OE specs (weight tolerence of + or -) for each and every model. <br /><br />Using a ball hone is OK. It actually deburrs the sharp port edges after boring. <br /><br />I still think that on a 12 year old motor you will find the wear tolerence at or past the max on the other two. Measuring will determine which ones to bore. Its hard to say by looking if the cylinder is simply worn and not scored.<br /><br />Good luck!

 

[mike pulcella]

Re: reboring

To all the gentlemen who responded to my query, I thank you all for your valuable input. I have decided to rebore all 3 cylinders. It may cost more, (especially in SA Rands) but it will be cheaper in the long run.