K&B crankcase not square

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Kez

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Feb 27, 2003
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I decided to run a new K&B 21 outboard that I have in my collection for a couple of decades. The big end of the conrod showed some abnormal wear on both faces. I eventually traced the issue to the crankcase not square. Using my caliper, I measured the thickness between the front housing and the PTO. It was off by 0.007". I am not a machinist and I do not have the tool to check if the cylinder bore is square with the crank shaft. Judging by the machine marks, the cylinder bore was done on a CNC lathe. But the front and PTO faces were done on a regular lathe. SO it may be a setup issue. Is the case salvageable?

Thanks, k
 

Kez

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Feb 27, 2003
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I can probably find another one on Ebay but there is no guarantee that the case is square. Although 0.007 across the seating surface of the front housing seems small, it is having an effect. The engine does not run as strong as my other .21
 

Terry Keeley

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Can't quite figure out what you're saying here, got a pic?

"I measured the thickness between the front housing and the PTO. It was off by 0.007"
 

steve wood

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Dec 6, 2004
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the rod and piston assembly will align itself with the crank in the horizonital direction. if the power takeoff end of the housing is milled parallel with the crank end it should work ok,"IF",the crank end is parallel with the liner bore. you will have to add a shim or gasket to the power takeoff end, to make up for any metal removed, or the power takeoff will bind on the crank,if there isn't enough clearance
 

Mike Hughes

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Feb 23, 2003
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I just measure all 3 cases that I have on hand. They all seem to be off to some degree. You might be able to flat sand them get them the same. I dont think that is the problem thou. You would need to check that the face of the crank house is true to the bore of the sleeve. For it to cause any wobble of crank pin. There is 2-3 thou clearance on the pin as well. What is your wear issue.

I also ran 2 of these cases for years on my outboard hydro before they crack. Never had issues with them then.
Mike
 

Kez

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Steve, the PTO end of the crankcase is not parallel to the front housing. It is off by 0.007". However, I suspect it is not the only issue. With the sleeve out of the case, I turned the crank and checked the rod clearance between the PTO and the crank. There appeared to be enough clearance. I suspect the front housing is not true to the sleeve bore. But I am not a machinist and I do not have the tools to check the alignment

Mike, I can certainly sand the case on a granite block. However, I do not know where the 0.007" difference came from. So I do not know whether to sand the PTO side or the front housing side. After only a few runs, the rod has polished the PTO. The side of the big end to the crank has a polished crescent shaped mark. Judging by the polished mark, the front housing is tilted up. With every revolution of the crank, the rod bind a little at TDC leaving the polished mark.

This isn't the only engine with this wear issue. I had another one with similar issue several years ago. Replacing the case solve the problem. Not every OB is the same. I have an exceptional 12mm crank that runs stronger than the newer 13mm crank. I think alignments like this and p/s fit all make a difference
 

Terry Keeley

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Find someone or send it to someone with a lathe, they can make a mandrel and true up both faces 90* to the inside bore.

It could be either or both faces that are out.

I think I would turn a piece of stock to fit, heat up the case to slide it on then take a cut to true it up. Rinse and repeat.
 

Brad Christy

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Jun 24, 2002
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Steve, the PTO end of the crankcase is not parallel to the front housing. It is off by 0.007". However, I suspect it is not the only issue. With the sleeve out of the case, I turned the crank and checked the rod clearance between the PTO and the crank. There appeared to be enough clearance. I suspect the front housing is not true to the sleeve bore. But I am not a machinist and I do not have the tools to check the alignment

Mike, I can certainly sand the case on a granite block. However, I do not know where the 0.007" difference came from. So I do not know whether to sand the PTO side or the front housing side. After only a few runs, the rod has polished the PTO. The side of the big end to the crank has a polished crescent shaped mark. Judging by the polished mark, the front housing is tilted up. With every revolution of the crank, the rod bind a little at TDC leaving the polished mark.

This isn't the only engine with this wear issue. I had another one with similar issue several years ago. Replacing the case solve the problem. Not every OB is the same. I have an exceptional 12mm crank that runs stronger than the newer 13mm crank. I think alignments like this and p/s fit all make a difference
Kez,

This is probably better approached as a mill job. You need to figure out which end is out of square with the crank bore of the case, if not both, and square it/them up. Terry's approach will likely work, in the absence of a good expanding mandrel, but mount the case on the mandrel by the cylinder bore, not the crank bore. Once the case is secured on the mandrel, with the mandrel mounted in an indexer or in a collet block, indicate the mounting lugs square on the mill, then index the crank bore vertical. You can then check the two faces perpendicular to the crank bore for square. Once you've determined which one is out, if not both, a light skim cut on it/them should cure the problem.

The most important conditions here are the perpendicularity of the cylinder bore to the crank bore and the perpendicularity of the two opposing faces to the crank bore. Checking parallel between these two faces accomplishes literally nothing, as they can be parallel to each other, but out of square with the crank bore, and you'll find the problems you have now. Obviously parallel is better than not, but perpendicularity comes first.

Thanks. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine and Precision LLC
 

Ron Olson

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Dec 26, 2001
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This isn't a new issue as a lot of this happened when MECOA took over. A guy I bought my Aero-Tech 7.5/11 ran into the same problems with the 7.5's. He contacted them but denied any issues.
 

Kez

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Feb 27, 2003
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Thanks all for your reply. This is a job beyond my machining skills can handle. It may be easier to buy a case. However, there is no guarantee that the new case won't have the same issue. Judging by the machine marks, i guess bored the sleeve first on a CNC. The remaining machine work was done on a standard mill. If it was a set up issue, the whole production run would have been affected.

I agreed that the crank bore being square to the cylinder bore is most critical. I have no way of checking it. However the unusual wear pattern on the conrod suggested that they are not perpendicular. If I have to remove a lot of material to square them up, then the other issue is that the front housing will be loose in the case.

Regarding the front housing and PTO mounting surfaces being parallel, I agreed also that it is not a major issue for the outboard. But my past experience is that it can be an issue on an inboard with disc intake. I have a HP .40 Gold Cup in my collection. The same issue caused the drive pin that engages the disc to wear quickly. I attributed the excessive wear to the fact that every revolution the drive pin moves in and out of the disc if they two surfaces is not parallel.

I never thought MECOA would replace the case or any defective parts.

Thanks, k
 

Brad Christy

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Jun 24, 2002
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Kez,

I'm not intimately familiar with K&B engines at all. But I do have a pretty good grasp on machining processes. If I had to guess, I'd say they machined the mounting lugs first, as a primary datum feature, then used those features for all other operations (that's how just about everything is done), and I'd have to assume everything was done on a CNC mill (machining center).

This really isn't a big deal to do this machining, but if you don't have the machinery or fixturing hardware, you're kinda stuck. Are there no machinists in your club? Nobody with a home "shop"?

The big issue is to determine perpendicularity of the cylinder bore and the crank bore. If that is bad, you're not gonna fix it by squaring up the opposing faces of the crank bore.

Good luck. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine and Precision LLC
 

Kez

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Feb 27, 2003
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Hi Brad,

I am not a machinist and my comments about the manufacturing processes K&B used for the case are probably incorrect. Come to think of it, it would make more sense to mill the mounting lugs first as the reference point. However, I can tell the sleeve bore was done on a CNC while the remaining machine work was done on a regular tool. The CNC leaves a much finer, almost polished-like finish.

I will bring the case to our machinist at work and see if he can correct the issue. My guess is that the crank bore and the sleeve bore are off by quite a bit.

Thanks, k
 

Brad Christy

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Jun 24, 2002
Messages
1,118
Kez,

Quick question....

Are the two opposing faces of the crank bore parallel when measured top and bottom? This is the direction that can't be corrected, and an out-of-parallel condition here would be a strong indicator of that.

Thanks. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine and Precision LLC
 

Kez

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Joined
Feb 27, 2003
Messages
1,067
Brad,

The opposing faces of the crank bore measured top and bottom is off by 0.003". This is small and I would let it go. I do not know if the crank bore and the front housing faces are done in the same setup. If so, that would indicate the sleeve and the crank bore are not perpendicular. A machinist at work will check it for me.

Thanks,
k
 

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