Johnson head reboaring

[commissioner]

I have a 1985 175 Johnson outboard in need of a head rebuilding<br /><br />The piston bore is 3.500 which is standard size<br /><br />wear is about 2/000 in a few spotts with some scuffing.<br /><br />The welch plug shows an "OS" after the model number which my aftermarket shop manual say it means its oversized and can't be rebored<br /><br />I went to my local dealer service shop and they didn't know anything about the OS after the model number.<br /><br />Does anybody know if this is correct or know what the minimum thickness of the actual cylinder wall needs to before the cylinder is junk?

 

[Joe Reeves]

Re: Johnson head reboaring

If any of the present pistons are oversized, the bore will be imprinted into the top of the piston(s).

 

[R.Johnson]

Re: Johnson head reboaring

Depending on what size it is now, you can go .030 over with OMC pistons. If you use Wiseco pistons .040 over. If you get this engine rebored, a couple of things to watch. The aluminum bore at the bottom of the sleeve should be under-cut, the ports have to be chamfered. On OMC engines, there is no supporting boss cast around the cylinders so they kind of hang out in the wind. Have them check the deck hieght that the cast iron sleeve is not below the surface. Also these sleeve's are not always square with the crank cener line. Not all auto rebuilders are aware of this.

 

[walleyehed]

Re: Johnson head reboaring

Please post the results of the piston tops as Joe stated.......that'll tell where we're at here.<br />You may have to carefully brush the carbon from the piston top as to not remove too much material, that the size cannot be read...<br />Let us know!!.......

 

[Solittle]

Re: Johnson head reboaring

You should be able to get it resleeved if necessary.

 

[BRIAN03]

Re: Johnson head reboaring

You need to find someone who bores and hones outboard blocks for a living. The manchine shop should have more knowledge than you. He should be telling you what bore the block will take and what pistons are avalible. You dont need someone learning on your block. I have never seen an automotive style machine shop bore a outboard block well. There is alot of specialize equipment for boring outboard blocks. Find someone who does it every day. They should inspect and measure the bores and tell you what can be done.

 

[rodbolt]

Re: Johnson head reboaring

hello<br /> the 3.5 inch motor was a decent one to rebuild. we had some corroison issues with the big bore version of later years. I would follow the previous posts if I were you and find a reputable shop. There is one in portsmouth Virginia that will assist do it yourselfers. he is an authorized OMC resleeve and reboring shop and does all my head/block work. you will also need to replace the bottom crank bearing. you can UPS the block crank and rods to them and they can turn it around, the name of the place is Ron's marine on rodman Ave in portsmouth VA.<br /> good luck and keep posting. also look very very carefully at the webs in the exhaust ports for cracks. any cracked sleeve will have to be replaced.

 

[Dhadley]

Re: Johnson head reboaring

The OS on the block means it was replaced from the factory as a "policy adjustment" powerhead bored .020 OS (oversize). The block can still go to .030 or more. <br /><br />The Wiseco pistons offered for that motor are Std., .020, .030, .044 and .064 over. The latest series number is 3173. <br /><br />A light deck cut may not be a bad idea. If you have to go over .010" use the "thick" head gaskets. <br /><br />Make sure the heads are a matching set. The port number (cast in the head) will be one digit lower that the matching starboard head. ie -- *****5 and *****6.<br /><br />Good luck!

 

[Mark42]

Re: Johnson head reboaring

You may be interested in what this professional rebuilder has to say on his website about block boaring and how to do it.<br /><br /> http://1outboard.com/Powerheads.htm