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Replace this piston?

Grumpy Bear

Grumpy Bear

Regular Contributor
Hello all. I'm rebuilding the powerhead of a 1974 Merc 500 50hp. One of the pistons I know I need to replace (and not just the rings). Another, however, had good compression but when I looked at it, it was pretty scuffed up (it was next to the other piston that was pretty much fubar). I've attached a pic. Can I get away with just replacing the rings on this thing?

Thanks!
piston.jpg
 
Doubt it will run properly if you follow that method.-----What does the bore look like and what measurement do you get on the bore ?
 
racerone said:
Doubt it will run properly if you follow that method.-----What does the bore look like and what measurement do you get on the bore ?
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Bore looks very clean -- no noticeable scratches at all. I haven't measured the bore, however. I do plan on honing the cylinder some to rid it of any glaze, so I'm pretty sure new rings are in order (which I would replace anyway).
 
Sand her clean with 220 sandpaper; clean out the ring grooves with an old ring (broken in half) and put her back together.

Jeff

PS: Now if it looked like this!
 

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fastjeff said:
Sand her clean with 220 sandpaper; clean out the ring grooves with an old ring (broken in half) and put her back together.

Jeff

PS: Now if it looked like this!
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Haha! Yeah, it's in a little better shape than that one! Thanks everyone for your responses, really appreciate it.
 
2 choices- replace it now, or replace in a short while. Despite what Jeff has said, this piston is finished. Cleaning the ring grooves with an old ring will only remove the debris in the groove, but won't repair the damage to the lands. This will, sooner probably than later, lead to carbon build-up, lower compression, power loss, and eventually severe damage to the bore. I presume you only want to rebuild this engine once?
 
bilge rat jim said:
2 choices- replace it now, or replace in a short while. Despite what Jeff has said, this piston is finished. Cleaning the ring grooves with an old ring will only remove the debris in the groove, but won't repair the damage to the lands. This will, sooner probably than later, lead to carbon build-up, lower compression, power loss, and eventually severe damage to the bore. I presume you only want to rebuild this engine once?
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I don't know anyone who wants to rebuilt an engine twice, so that's pretty much an assumption. That said, like most people, I'm trying to rebuild without replacing everything. Only that which is necessary and that which should be replaced just-for-good-measure. There are lots of "backyard DIY" factors here. Bore size of the cylinder? Don't know. Will new rings work? Don't know. Will I use this boat enough that I will be breaking it down to zero again? Probably not.
 
Don't sand the piston as you remove material from it and make it oval instead of round. The piston is tapered and smaller at top and larger at bottom. My reply from other site....(1) You aint going to clean that piston up (2) The cylinder walls will have some grooves and transfer in it (3) You wont clean it(cylinder) up with a spring or ball hone you will need a ridged hone. I have 2 STD Vertex pistons that still have some of the break-in coating I let you have cheap and I know their good as they have been spun up over 8000rpms
 

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For crying out loud, haven't you people ever actually worked on engines!

Sand the burnt area shiny, clean out the groves, hone the block, re-ring it and put it back together. Done ALL the time.

Jeff
 
I would not even think about using a piston like that. Also a motor of that age it would be very likely the cylinder is out of round and or tapered along with the others. Get it checked by someone that has the tools to do so before you invest your money
 
New motors are not put together with parts like that !!------------But many shops / people work to different standards when it comes to motors and repairs.-------Yes with a piston like that it will run.
 
Manure. Clean 'er up and go boating. Done it a zillion times--and with pistons a lot worse than that.
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You would love to go thru my aluminum pile as your standards are a whole lot lower than mine when I repair a customers motor.I repair em as if they where my own.....
 
Some shops know that as long as a motor starts and runs on a hose it is easy to sell it.-----Most folks can not " hear " the quality in a rebuild.
 
A piston/rings is like $70 for that engine. Even a set of rings is gonna run you $20.

So basically you save 50 bucks.
 
So basically you save 50 bucks.
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Its not about $$$....any time you "sand" something you remove material and change its dimension and engines have min and max specs on cylinders and pistons.
 
Thanks, everyone. I found a piston fairly cheap, so I just bought it. It's not brand new, but looks like it is. Also replacing the rings on all the pistons.
 
fastjeff said:
For crying out loud, haven't you people ever actually worked on engines!

Sand the burnt area shiny, clean out the groves, hone the block, re-ring it and put it back together. Done ALL the time.

Jeff
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I am with Jeff. After all how much can a person afford to invest in a 40 yr old motor that can be replaced for under $500. This motor may run for years, or not.

The perfectionists seem to be saying, "Scrap anything over 25 yrs old." There is a perfect world, but some of us are stuck in the real world. We make do with what we have, or not.
JE
 
Really? I'll bet that motor will run fine fir YEARS with that same piston just cleaned up and reused.

Wanna take the bet?

Jeff
 
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