1977 70 johnson is it possiable to just hone one cylinder and put in over size piston

[forty5goingon65]

im new to rebuilding a boat motor but i am going to give it a try since i rebuild my snowmobile engine i had a 70 horse johnson given to me it had a lot of play in the top cylinder i took it apart and the piston was scraping the side of the clyinder its not gouged to bad but i think it should be honed out the head was already apart so i could not check commpresson in the other 2 cylinders but there is no play in them the piston i took out you could spin the bottom ring alway around so i was thinking this was the main problem since the roller bearing seemed good none missing or wore so my question his could i hone the one cylinder and over size just the one piston i dont want to put to much money into such a older outboard when given the motor i was told it ran pretty good until that happened the bottom end was replaced just recently thanks

 

[wilde1j]

Re: 1977 70 johnson is it possiable to just hone one cylinder and put in over size pi

You can't "just hone" it. You need to measure the bore and check for roundness. The bad holes need to be bored, not just honed, and the correct oversize pistons and rings installed. Honing is only useful when there are no scores in the cylinder walls and wear is within service limits.

However, before that, you need to determine why the failure occured, or it surely will happen again. Most often where a single hole is damaged, it's caused by lean running in the hole, which in turn is often attributable to crap in the carb feeding that cylinder. But it can be a host of other thing as well.

The boring should only be done by a shop that does outboard blocks, or you risk a bad (and expensive) job.

It's acceptable practice to do only one cylinder, but for my own motors, I"ve always done all.

 

[forty5goingon65]

Re: 1977 70 johnson is it possiable to just hone one cylinder and put in over size pi

You can't "just hone" it. You need to measure the bore and check for roundness. The bad holes need to be bored, not just honed, and the correct oversize pistons and rings installed. Honing is only useful when there are no scores in the cylinder walls and wear is within service limits.

However, before that, you need to determine why the failure occured, or it surely will happen again. Most often where a single hole is damaged, it's caused by lean running in the hole, which in turn is often attributable to crap in the carb feeding that cylinder. But it can be a host of other thing as well.

The boring should only be done by a shop that does outboard blocks, or you risk a bad (and expensive) job.

It's acceptable practice to do only one cylinder, but for my own motors, I"ve always done all.

okay thanks jim

 

[F_R]

Re: 1977 70 johnson is it possiable to just hone one cylinder and put in over size pi

Boring cylinders that don't need boring wastes precious metal that you may need in the future. You can't put it back. Well, you can by sleeving it, an expensive proposition.

 

[forty5goingon65]

Re: 1977 70 johnson is it possiable to just hone one cylinder and put in over size pi

so are you saying i should just clean up the cylinder and try a new piston and ring my son inlaw is a diesel mechanic and he said he could probably get me the right honing tool. the cylinder is only scratched up a bit no real big gouges on the walls thanks

 

[F_R]

Re: 1977 70 johnson is it possiable to just hone one cylinder and put in over size pi

so are you saying i should just clean up the cylinder and try a new piston and ring my son inlaw is a diesel mechanic and he said he could probably get me the right honing tool. the cylinder is only scratched up a bit no real big gouges on the walls thanks

I am saying that if it needs to be bored, then bore it. But I am also saying that boring cylinders that don't need to be bored is a waste of valuable metal. To answer your first question, yes it is perfectly OK to bore just one of them.

 

[forty5goingon65]

Re: 1977 70 johnson is it possiable to just hone one cylinder and put in over size pi

okay thanks im going to hone it its only scratched i dont want to put alot of money into the motor since it is a 1977 if it gets me through a season i would be happy thanks again