What are tolerances when reboring Johnson Outboard?

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Hi, this forum has been very helpful. I just had another question I need help with. I have an '86 Johnson 40 horse outboard that I am rebuilding. I took the crankcase to a professional to get bored .030 oversize. The pro wants to know what is the tolerance when he bores the cylinders? What do I tell him?

 

[R.Johnson]

Re: What are tolerances when reboring Johnson Outboard?

The standard bore on your engine is 3.1875, so .030 over would be 3.2175. The factory recommends you hold this tolerance. If the shop you are going to has not done 2 cycle blocks. Be aware that a 2 cycle has the open ports. To a machinist, this means an interupted cut, which can mean chatter, or a rough cut. Ask if they have done this before. Also the bore has to have a relief cut at the very bottom. When finally rebored, the port's need a very slight chamfer. I know this is going on quite long, but you need a knowlegeable machinist for this type of work.

 

[tink]

Re: What are tolerances when reboring Johnson Outboard?

Ok, I'll be sure to show the pro your post. But is there no tolerance then? Or is the tolerance .001 or .005 or more or less? I told the pro that it needed to be 3.217, and he wanted to know what the tolerance was.

 

[Walker]

Re: What are tolerances when reboring Johnson Outboard?

What pistons are you going with. Its been a while since I did one but if I remember right Wiseco calls for slightly larger tolerance than OMC

 

[CorB]

Re: What are tolerances when reboring Johnson Outboard?

To R. Johnson:<br /><br />Could you explain what this relief cut means, and what the purpose is? And how should the ports be chamfered, is there a special tool? I just had my '78 55 HP rebored and found your suggestions very interesting, so I want to check my rebored powerhead.

 

[Chinewalker]

Re: What are tolerances when reboring Johnson Outboard?

Hi CorB,<br /> When I redid my 75hp Johnson some years back, I used what's called a "dingleberry hone" to remove the file couple thousandths of an inch from the bores and also put a slight radius on the ports. You need that radius so that you don't catch a ring on them. I'm not sure what the book says, but it does list a ring end gap tolerance, so you'll want the final bore to allow the ring gap to fall within those tolerances (usually somewhere between .005 and .015 on most engines - check your manual). Ideally, you'd like the guy doing the boring to come in a couple thousandths under and take the rest with the dingleberry hone. The dingleberry hone can remove a lot of material in a hurry so work slowly, measuring freqently. <br />- Scott

 

[Droll]

Re: What are tolerances when reboring Johnson Outboard?

Do not know your outboard , but I got my OLD V4 bored this spring .<br /><br />STD bore 3,375"<br />Oversize 3,395" +/-0,0005" ( 020 over )<br />I would think your tolerance is around the same .

 

[R.Johnson]

Re: What are tolerances when reboring Johnson Outboard?

The relief cut is made at the very bottom of the bore where the cylinder liner ends in the aluminum casting. When the cylinder is bored out, the liner, and the aluminum wall at the bottom end up the same size. The aluminum is slightly under-cut to give clearance. The port's a lightly chamfered with a small ball shaped grinder, held in a die grinder. Most any shop that does reboring, also will do the finish honing with a Sunnen hone. This hone will leave a much better finish as to size, and straightness than a spring loaded loaded, or ball hone held in an electric drill. The figure 3.2175 is what the bore should finish at when using an OMC piston. If you go with the Wiseco piston, make the bore .001 bigger. The method used to be, and probably still used by some shop's was to bore to each individual piston. Piston's these days' are cam ground, and not perfectly round, this can make measuring difficult. This is probably why OMC suggests going exactly .020, .030, or.040 over, what ever the case might be.

 

[Hooty]

Re: What are tolerances when reboring Johnson Outboard?

Use the boring specs that come with the new pistons. If at all possible, use a machine shop that has experience boring o/b, 2 cyl. engines. They ain't automobile engines. <br /><br />c/6<br />Hooty

 

[CorB]

Re: What are tolerances when reboring Johnson Outboard?

Ok, sorry for my disturbance of someone else's thread, but thanks a lot guys for your responses.<br /> <br />R. Johnson: I only have one thing that I don't completely understand. You say that the aluminium is slightly under-cut to give clearance, but could it be you mean that the aluminium is slightly over-cut? I can't imagine to under-cut a piece what was previously bored to a larger diameter. Hope you understand what I mean?

 

[R.Johnson]

Re: What are tolerances when reboring Johnson Outboard?

CorB: I'll try to explain this! In machinist terms, when boring a hole, as an example the cylinder under discussion at 3.2175, at the bottom of the bore where the cylinder liner meets the aluminum casting, The casting would be cut an additional, let's say .005 to .010,this would be called an under-cut. If it would help to make it plainer, you could call it an over-bore. You are simply making the hole bigger at this point. The idea is to make up the difference in exspansion rates between cast iron, and aluminum. To go a little further with boring OMC block's, it's a good idea to check the deck height of the cylinder liner, and the aluminum casting, this would be where the cylinder head bolt's on. The liner can sometimes be slightly lower, if so, they should be brought into agreement with just a skim cut in a milling machine. The reason being, If you look at the inner cylinder of an OMC engine, they kind off hang out in mid-air with no support except at the bottom of the bore. Bringing the liner, and block into agreement, and proper torqueing of the head bolts help's this condition. I will probably have some disagreement on some of these point's, but it seem's to work for me. I hope to here of other method's.<br /> ,

 

[tink]

Re: What are tolerances when reboring Johnson Outboard?

R. Johnson and everyone else, thanks for your excellent advice. I called one of the better marinas in Des Moines and they told me who they use to bore out all of the engines they rebuild. I am going to use the same company. I called and talked to them today. They've bored many 2 cylinder outboards and knew the tolerance and everything else. They also happen to be cheaper than the first "pro" I was going to use. Thanks again for your all your great advice!

 

[CorB]

Re: What are tolerances when reboring Johnson Outboard?

Yeah, I also want to say thanks again. I understand the under-cut part and the rest of the points of concern completely now, great explanation!