74 merc 500 tear down

[ebrown999]

I finally got time to complete the tear down. Wow the alternator(?) pully sucked to get off-ended up breaking it before it would come off but clear none the less. That distributor assembly out of the way I removed the crankcase cover and pulled the bad piston. EWWWWW. A clear groove worn in the piston and all the rings were sucked tight against the piston head. kinda weird. Unfortunately, two small grooves are appearant in the #3 cylinder wall. One groove is not deep enough to catch a fingernail on, the second barely catches a nail. My question is: what is the best way to proceed??? I will put in all new needle bearings and all new rings on each piston. but the bad cylinder will need a new piston as well as rings. how do I know when to go to oversized piston and rings? Also, what kind of hone should I use, ball or standard cylinder (the long skinny kind).

 

[ebrown999]

Re: 74 merc 500 tear down

hmph. Two weeks and not one reply. I guess this should be renamed the johnson/evinrude only iboats forum.

 

[This_lil_fishy]

Re: 74 merc 500 tear down

It's pretty much an any engine question. Is your motor sleaved? If so take it in to a shop to get the sleaves replaced. If not have all of the cylinders bored and honed. Even though it feels shallow to a finger nail, the groove could be quite deep. A machine shop will be able to get the cut right to remove the groove. Also an older engine typically has quite a lip at the top of the cylinder wall right about where the rings stop on the compression stroke. It costs about $45 per cylinder to bore and hone around here. Well worth the investment. Get a list of cylinder and ring sizes that are available for your engine, as the machinist will need to know what sizes he can cut to for a finished product. This is all based on my knowledge of 4 stroke engines. I am not certain how water jackets and all that come into play on the 2smokes. A good engine shop will know and help you out.

Good luck, and have fun. I love tearing down old engines...

Ian

 

[emckelvy]

Re: 74 merc 500 tear down

Best way to evaluate cylinder condition is after 'busting the glaze' so you've got fresh metal to look at.

Easiest to use and least likely to cause any problems, is the "dingleball" type of glaze-buster with globs of abrasive in a toilet-brush configuration.

If, after you de-glaze the cylinders, you can catch your fingernail in a score mark, it's too deep and requires bored. Likely you'll end up having that cylinder bored to .015" oversize unless the scratch is very deep.

Note this block can be re-sleeved but that's quite expensive and unnecessary if the problem can be corrected via an overbore.

Also note that if the other cyl's look good there's no need to bore them out to match the one O.S. cylinder. Deglazing is a perfectly adequate method of preparing them for new rings.

Unless you have access to a bore gage or micrometer/ball gages, you can get a good idea of wear/taper on the other cylinders by comparing the gaps of a new piston ring, placed at top, middle, and bottom of each cylinder.

HTH..........ed

 

[ebrown999]

Re: 74 merc 500 tear down

Thank you for the replys. Waiting two weeks for a reply got frustrating. I've been talking to a guy that rebuilds merc outboards. Based on what he is telling me, I think I'll spend the extra cash and have each cylinder machined. I was examineing the pistons and they all have some wear and the rings are carboned up on all of em. I marked each piston making sure to put the same not/screw and the rod cap back in the exact same position. I removed the 25 needle bearings from each one and placed them separately in a plastic baggie with the piston so I don't get anything cross contaminated or out of sequence.

I can post some pictures if it would help your visual.