Yamaha 90 cylinder bore question

Marlinman10 · Nov 6, 2008

I'm looking to purchase a Yamaha 90. This guy works out of his Garage and seems to know what he's talking about. He replaced the bottom piston with new and the other pistons were fine so he left them alone. The top cylinder was a stock bore with the middle cly being bored 020' over and the bottom cylinder being 030' over. Is that a problem? By the way it's a 89 model with controls and he wants 1750 for it. What do ya think?

yamamarinetech40 · Nov 7, 2008

Re: Yamaha 90 piston question?

Re: Yamaha 90 piston question?

Stay away, far far away....if you want to give your money to him, then do it, but it's a fool's bet.

Marlinman10 · Nov 8, 2008

Re: Yamaha 90 piston question?

Re: Yamaha 90 piston question?

Can you tell me about the different size bores? Is that a problem in it self?
Thank you

JustJason · Nov 8, 2008

Re: Yamaha 90 piston question?

Re: Yamaha 90 piston question?

bore size isn't a problem in itself as long asl all of the cylinders maintain acceptable compression.
However if its a home job. and the guy didn't use a dial bore gauge, he may have put a nasty taper on the cylinder, and that will can cause a variety of problems.

JUSTINTIME · Nov 9, 2008

Re: Yamaha 90 piston question?

Re: Yamaha 90 piston question?

you want all the same bore size

Marlinman10 · Nov 10, 2008

Re: Yamaha 90 piston question?

Re: Yamaha 90 piston question?

Why? I wish someone could tell me the right answer. Some say it's ok and some say it's not. I'm looking for a expert mechanic to help. Thank You

JUSTINTIME · Nov 10, 2008

Re: Yamaha 90 piston question?

Re: Yamaha 90 piston question?

buy it and find out
as ray, he builds race yami's

Marlinman10 · Nov 10, 2008

Re: Yamaha 90 cylinder bore question

Your a big help. This guy is a Yamaha Certified Mechanic and said it's not a problem. You say it is but can't tell me why.

JUSTINTIME · Nov 10, 2008

Re: Yamaha 90 cylinder bore question

well i replace blown motors not rebuild them
that is done buy the guys that do it day in and day out

post over on screamandfly
they will tell you

i would run from it

sdsaw · Nov 10, 2008

Re: Yamaha 90 cylinder bore question

JMO...I would think that there would be a balance problem with three different sized pistons.

Sounds like a patch job with a limited lifespan.

Marlinman10 · Nov 16, 2008

Re: Yamaha 90 cylinder bore question

I posted it over at screamandfly and a couple of guys said it's not a problem at all. It's about 50/50.

JUSTINTIME · Nov 16, 2008

Re: Yamaha 90 cylinder bore question

what did ray say?
ask hydrotec

rodbolt · Nov 16, 2008

Re: Yamaha 90 cylinder bore question

bore size between cylinders on that motor is irrelavant.
I have seen a few from the factoery with oversize pistons.
my issue is with the first failure,usually due to overheat or lack of oil.
its very very rare to see a used bore that will clean up and still be round but it does happen.
the other issue is what bearings got replaced and who did the crank as there are very very few places that can dissasemble and reassemble a built up crankshaft assy.
were the exhaust covers and water jackets removed and carefully cleaned/inspected?
gearcase seals ok? new water pump,t-stat and pressure valve grommet?
there is a lot more to reconditioning an outboard than simply tossing in a few pistons and some gaskets.
we dont rebuild a lot of them here due to saltwater but I averaged about 30 a year for many years and all were sold with a 1 year warrenty.
I scrapped a lot more than I built due to cooling system corrosion issues.

Silvertip · Nov 16, 2008

Re: Yamaha 90 cylinder bore question

It is perfectly acceptable to have one cylinder on a multiple cylinder engine as much as .030 oversize. Any combination of oversize is equally acceptable. Increasing the cylinder bore increases the volume of that cylinder but the quench area (combustion chamber volume also increases slightly so the net result is compression remains nearly the same and you end up with an engine that has marginally larger displacement. The concern here is how the seller bored the engine. Unless he owns a boring bar which is something you don't generally find in a shade tree mechanics arsenal of tools, it would have taken him a long time to "hog" out a cylinder to accept an oversize piston. Using a cylinder hone is not the way to do that and if that's what he did -- you are well advised to run the other way. For optimum performance and longevity a cylinder must be perfect from top to bottom. It cannot have any taper nor be out of round at any point. You can't do that with a cylinder hone.

twostroke87 · Nov 16, 2008

Re: Yamaha 90 cylinder bore question

Having one or more piston bores be different sizes in a two stroke engine isn't a big deal because the firing order for two strokes is 1-2, 1-2-3, 1-2-3-4, or 1-2-3-4-5-6 depending on the engine size. So the issue of balance (like on boring inboards) wouldn't be a problem, I/O's and Inboards need to have all of the bores the same size. THe real question is did this guy rebuild this thing in his garage properly and what made it fail the first time. Thats the real gamble.

Yamaha 90 cylinder bore question

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