Piston and cylinder problem

ramboot

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I just took apart my merc inline six head, removed pistons, one (#5) was bad and one (#3)was scored on side, all had 120 compression except #5 which had no compression before tear down. I plan on replacing all pistons and rings but do I have to rebore all cylinders? The cylinder wall on 5 and 3 (number three has next to zero mostly on the piston) have scratch marks that aren’t badly gouged but you can feel them running your fingers over them. How bad do the cylinder walls have to be before reboring?
 

GA_Boater

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Your fingers aren't very good micrometers. Take the block to a machine shop. They can see it, feel it, measure it and tell what's needed. Hard for us to do that over the Internet.
 

ramboot

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Just noticed this, a crack in the exhaust port?
 

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harringtondav

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You're into the job this deep, so go ahead and finish it properly. That motor will last a long time with a good rebuild. Check the price for a new O/B at your power. The machine shop charge will look like peanuts.
 

ramboot

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I can finish up but what about the crack on the inside of the exhaust port?
 

ramboot

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I have a picture on previous post and if you expand on pic then you can see the crack.
 

harringtondav

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I'd still take your block to a machine shop for evaluation. They may be able to get in there and weld it.
Crack.JPG
 

racerone

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Is that an exhaust port or intake " boost port " or what.-----And what is on the other side of that crack.-----Have you taken a look ?
 

ramboot

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It’s on the exhaust port and cracked where the support? Finger comes out from port, cannot see any cracking behind it but will take off exhaust cover to check
 

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harringtondav

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I do ot believe that is the exhaust side of the block !

I tend to agree with you seeing the new pic. The exhaust port looks like its at 6 o'clock in the pic. Its opening is closer to TDC than the four scavenge ports, where it's supposed to be. I don't know what that cracked notch is. I looks like an enclosed little cave cut in with an end mill at an angle. Maybe some kind of swirl feature to assist exhaust and scavenge charge. I'd still see if the crack can be welded and re-contoured. It looks structural, and important.
 

racerone

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The intake is on the other side and I believe here is nothing to worry about.----I suggest the OP to look at the other side by taking the INTAKE cover off.-----Then post more pictures !
 

Faztbullet

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That is the boost port on intake side and its ok. Need to run a rigid hone thru the cylinders to see if it leaves "shadows" which are untouched areas around ports(90% do). Do not use a spring or ball hone to check this. If it leaves shadow cylinder needs bored.
 

jimmbo

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I'd still take your block to a machine shop for evaluation. They may be able to get in there and weld it.

That is the passage used in conjunction with Mercury Power Ported Piston, an addition to the transfer ports from the crankcase
 

racerone

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There are a number of folks who say that is the iintake side.------The original poster should take the time to learn how that motor operates / works.-----Makes repairs of the motor so much easier !
 

ramboot

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Ok now I’m really confused, do you guys think that little crack is nothing to worry about because of where it is? All I have to do is check cylinders if they have to be rebored because of s deep scratches? I plan on replacing all rings and pistons anyway.
 
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