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Should I bore only one cylinder?

sandman I have a two cylinder with one burnt piston and cylinder wall. I am being told to replace one piston with an oversize after Boring the one cylinder wall to .020 over (Hopefully that is all that will be required). I am being told since the other cylinder wall and piston look fine to leave it alone and only replace the rings and maybe hone lightly the other cylinder wall. Does this sound right? Certainly not what I am accutomed to with cars motors. I still do not know why this one burnt. Carbon on piston indicating good fuel and oil. Burnt on exhaust side the top and side of piston. This was the lower piston on my two cylinder. 2002-03-17
footballplaya get a second opinion on it, whoever is telling you not to bore a second one may be right, but from what i'vebeen taught to do to "small engines"... i have been taught to bore all the cylinders for simply the life of them.. 2002-03-17
Dhadley OK. The cylinder LOOKS fine but how does it measure? Yes, it's OK to bore one but I would bet that for the minimal cost of boring and one piston you want to go that way. You need new rings for the other cylinder after you hone it anyway.

Have you figured out what caused the failure?

Good luck!

2002-03-17
sandman We can not find any obvious cause for the failure. I was not on the boat when it occurred. It was "loaned" out. I don't know if it was overreved, it does not appear to have been lean, any thoughts? 2002-03-17
Joe Reeves Sandman... You don't say what engine you have. If your engine has more than one carburetor, more than likely your engine failure was due to the lower carburetor becoming fouled, resulting in an improper mixture (the carbon build up on the piston does not 2002-03-17
12Footer Yes, because you are 90% there. I would do them both. It will also increase HP (more bore/stroke=more HP),even if only a small amount.And you will keep the original ballance of the crankshaft intact. 2002-03-17
Dhadley Find the cause. The damaged piston is a result -- not the cause. If you don't you are setting yourself up for a repeat failure.

Old fuel, wrong plugs, dirty carb, double firing pack -- something caused the piston to "burn".

Good luck!

2002-03-17
sandman I agree, something caused this but we have been unable to determine why. Are there suggestions or steps to follow that may help us to rule out things as we go along? The manual helps us in repairs but I do not see much help in trouble shooting. Can you suggest steps in determining what went wrong? 2002-03-17
J.B. Cornwell Howdy, Sandman.

I agree with those above. Whatever caused your problem is probably still there. Your carburetion system is a strong suspect. Be sure to fix the cause before running the engine again.

Overboring one jug is a sure way to unbalance your engine. BAD news. Don't even consider it.

Red sky at night. . .
JB

2002-03-17
almost retired again Hey Sandman here comes my two cents worth. I agree with Joe Reeves, I've overhauled a lot of outboards; it don't make any differance. As far as hopping it up, I don't think that you will notice any differance. One of the reasons that you can get away with boreing just 1,2,or ever how many is that each cyl. is a little engine in itself, and fires each time that the piston approches top dead center. A 4 stroke fires ever other time the piston aspproches TDC and all cyl.s interact with each other through a common crankcase. I
have just completed a 90 HP that 2 cyl.s have been bored to .064" one at .020" and one at .044". I don't think that I would do my own quite so radical, but this is a rental engine and the owner just wants to get it back in service.
I know that some people might dissagree, but that's what I've always been taught, I have never had any trouble with it.
2002-03-17
Pursuit2150 Only 2 cyl. 2002-03-17
sandman Yes, it is a Johnson 1990 40HP 2 cylinder. 2002-03-17
J.B. Cornwell Hi, Joe and ARA.

I don't know where you guys learned your physics.

A two, three, or any number of cylinder engine is ONE engine, not a bunch of little independent engines that don't affect one another or contribute complimentary and opposing forces to the whole. The same is true of the reciprocating masses in that engine. A +.030 piston weighs more than a stock piston. If you have one of each you have imbalance.

Your crank, seals, etc. may tolerate an imbalance. . . for a while. That doesn't make it a correct thing to do, but if it's your engine, take whatever chances you wish.

If it's someone else's engine, that is a different matter.

Red sky at night. . .
JB

2002-03-17
Dhadley Just a thought -- Wiseco has made sure their oversize piston assemblys are within weight tolerence of standard pistons. That's why you can run different oversize pistons in a two cycle outboard motor. Go by cylinder measurements rather than weight. I still think you will end up boring both anyway.

By the way -- did we figure out what caused the failure?

2002-03-17
jwhite I completely agree with joe and almost retired, it won't hurt anything to just bore one cylinder, I have done it hundreds of times and never had a problem. Balance in a 2 stroke is not nearly as important as it is in a 4 stroke, unless your racing or turning the engine 9000 RPMs. In fact when Johnson built the first 3 cylinder everybody told them it would never last because it wasn't balanced but look at them now, they are a very popular engine for reliablilty. 2002-03-17
J.B. Cornwell Well, now. If your +.030 piston is within weight tolerance for stock pistons I take it back.

I guess there is still too much racer building 13,000 to 15,000rpm engines in me.

Thanks for setting me straight, DHadley.

That said, I would still bore all cylinders if I bored 1. More bore = more cc = more torque = more faster go de boat.

Red sky at night. . .
JB

2002-03-17
sandman measured the inside of the good cylinder wall and it is within .001 of being completely round. I will ask the machine shop what they think. No, I still have not determined what caused the failure. All suggestions for steps to take to determine failure would be greatly appreciated. 2002-03-17
12Footer Yes, if you can get by with honing and original piston with oversized rings. But no if you need to just BORE one cylinder. By boring, you must go with a larger,heavier piston (or lighter,depending on original and replacement piston material). This would create the imballance situation that J.B. is refring to. So you would have to do both if you must bore for oversized piston.
Also, as J.B. stated, look for a lean carb as the cause. or maybee a batch of bad mix. If oil injection equipped, the pump and it's spaghetti are suspect too.
Check plugs for proper heat range.
2002-03-17
alcan Hello Sandman
As you can see some times opinions vary. I recently aquired a 89 evinrude 40hp wjet pump. It also had no compression on the bottom hole. I was some what lucky. I was able to get acceptable compression (not perfect) by the decarb method. So there is some damage to that cylender. I never did actualy find the problem that caused it. I bought the unit as junk. The VRO system was a mess. Oil bag full of water, slime, etc. I disconected it, in favor of mixing my own fuel. I have had no further problems. Motor runs good.I agree with Joe and almost retired. I have done the one hole thing numerous times with no ill affects.
2002-03-17
ODDD1 Sandman, the only reason to bore a cyl oversize is because the cylinder is 'out of specification'....meaning scored,worn beyond acceptable clearances in taper from top to bottom and out of round....the manufacturer will publish a standard bore size and a wear limit....if that cyl is under wear limit, leave it alone [aside from a light hone and new rings]
power increase on that cyl will be unnoticable....proper setup/tune will give you a larger power increase than a measly 030 over....
2002-03-17
Dhadley Very true ODDD1. I once had a guy send me a perfectly good standard V6 crossflow block he wanted bored to .064 over. He thought it was going to be some kind of hot rod. I told him he was going to be very disapointed, which he was. He gained nothing. In the end we got the boat faster but did it with set up and a good prop.

Oh well!

2002-03-18
sandman Thanks all. I will visit my machine shop today. I will look at my carberators as my #1 suspect. Any suggestions on how to go about eliminating the carberators as the prime suspect? I disconnected my VRO last year and mixed my own afterward. I guess I could have gotten a bad lot of gas at my local marina. If you could give me some steps to follow on the carbs, I would appreciate it. 2002-03-18
clanton The weight of the OEM .030 piston is same as a standard piston, and the replacement/ boreing of one cylinder was approved by the old OMC for warranty repair. If you use a weisco piston it needs increased clearance or the piston will scuff, most of the machine shops here and Tampa add about .002 to a 150 piston. The men at WWW.screamandfly.com use a lot of weiscos and tell you the good and bad, and I think they have a new piston with a change in the ring locateing pin. If it was me I would bore both holes. 2002-03-18

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