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Yamaha F100 slightly bent rod

Old 11-30-2019, 08:15 PM
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Default Yamaha F100 slightly bent rod

Gents,
I'm at my wits end with a 2000 Yamaha F100, long story short, after much investigating Shes had a cracked head due to corrosion, its now at the machine shop getting welded and surface grinded. Looking at the cylinder bores seems the cylinder number 4 which copped the water ingress from the cracked head sustained mild hydrolocking. Its now 5-7mm from top dead centre, so ive deduced the conrods kinda bent.

anyone here ever had a bent rod and the motor kept going? Im reluctant to pull the bottom end apart and replace the conrod for many reasons, time and money etc.

thanking in advance



Old 11-30-2019, 08:38 PM
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Once you’ve bent a rod you have induced a stress riser to the metal. It is just a matter of time before it will fail premature compared to a rod without the stress riser. It is a question of how long that is.

In addition, if you are that far away from TDC when compared to the other holes, you will end up with low compression, incomplete burn, poor harmonics, and low power.

So in summary, if it really is that far off, I would change the rod.

JD
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Old 11-30-2019, 09:18 PM
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If the rod is bent, what condition might the bearings be in? Or the piston? Or the crank shaft?
Old 12-01-2019, 04:04 AM
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5-7mm is massive. You run it, you will soon have a ventilated block.
Old 12-01-2019, 04:05 AM
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5-7mm is massive. You run it, you will soon have a ventilated block. The time and money to replace the rod and check the bearings and crank will be very low compared to a new powerhead.
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Old 12-01-2019, 12:19 PM
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A slightly bent rod is kind of like being slightly pregnant or slightly dead! it's bent, it's replacement time.
Old 12-01-2019, 01:15 PM
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Originally Posted by brendanm View Post
5-7mm is massive. You run it, you will soon have a ventilated block.
😂😂 ventilated bock!! Never heard it put that way!
Old 12-01-2019, 01:16 PM
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Originally Posted by Tossedabout View Post
😂😂 ventilated bock!! Never heard it put that way!
Aka "helicoptered"
Old 12-01-2019, 01:25 PM
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Boat engines are a little different because they live at constant high RPM.


Old 12-01-2019, 01:39 PM
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Old 12-01-2019, 01:58 PM
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If the rod is bent, at bare minimum the loads on the big end bearing are likely to be uneven and cause an early failure. That might be your best case scenario. A rod through the block is just as likely. Really no choice but to fix it now.
Old 12-01-2019, 05:07 PM
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Originally Posted by Elgreco809 View Post
Boat engines are a little different because they live at constant high RPM.

It is basically impossible for the cylinder wall to bit be marked by that, the piston would be cocked sideways constantly = zero piston to bore clearance.
Old 12-01-2019, 05:20 PM
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Once a rod bends its only a matter of time before it breaks and exits the block. Experienced this personally many times. Customer amazed engine runs again after hydrolock driving through deep water...then its towed back in a week with a huge hole and needs a short block. You have it apart now its a no brainer.

edit: to add, cost will obviously be way higher once the block is toast.
Old 12-01-2019, 05:20 PM
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Bent rod?....what could possibly go wrong....run it and run it hard....make sure you video it please...
Old 12-01-2019, 05:52 PM
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I like the analogy of “slightly pregnant” lol. If you already paid to repair the cyl head it’s foolish not to go through the bottom end!
Old 12-01-2019, 05:58 PM
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5-7mm lacking on tdc piston height means the rod is REALLY bent. That is huge. Pull the short block, split the case and fix it right. Run it like that and it will "ventilate" or "window" the block.
Old 12-01-2019, 06:20 PM
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That’s like close to 1/4”, huge!
Old 12-01-2019, 06:41 PM
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Not worth fixing . The block will need to line bored, cylinder sleeved and I bet 100% you need a new crank, all new rod and crank bearings..$$$
Old 12-01-2019, 06:42 PM
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Thanks Gents,
Appreciate the advice. Its a pretty daunting thought and at times overwhelming to think what the task ahead entails (I have two left hands). Pulling this motor apart was always going to be a steep learning experience, I've made it as far as the head off etc. but the bottom block, looks far beyond my capabilities and honestly motivation. The verdict is in - re-assemble and sell as a wrecking motor, thanks everyone!!!



Old 12-01-2019, 08:40 PM
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Originally Posted by nigz View Post
Thanks Gents,
Appreciate the advice. Its a pretty daunting thought and at times overwhelming to think what the task ahead entails (I have two left hands). Pulling this motor apart was always going to be a steep learning experience, I've made it as far as the head off etc. but the bottom block, looks far beyond my capabilities and honestly motivation. The verdict is in - re-assemble and sell as a wrecking motor, thanks everyone!!!
I would sell as is. I'd much rather be able to see for myself that it has a bent rod, and look at the bores etc before it's assembled if I were buying it.

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