1999 Yamaha F100 oil pump stripped splines

[speed_demon]

I bought a used Yamaha 100hp four stroke needing work. The guy said the engine started to vibrate and the alarm went off so he quickly shut the engine down. He suspected oil pressure so he pulled the filter thinking it would take a few minutes for all the oil to drain back down. The filter was empty so he thought oil pump. He pulled the oil pump but because the bearing felt good he thought it was good. Instead of wasting time in the middle of the fishing season he bought a new engine. After getting it home I inspected the pump. At first glance it looks good but if you look closely the splines are almost completely stripped out.

I'm curious if this is common and if it's wear and tear or maybe a blockage in the engine may of caused a spike in oil pressure. Should I be worried about the powerhead running briefly without oil?

[boats.net]

I haven't seen any with stripped gear but if the engine was shut off when buzzer sounded it will be ok, run a compression test if possible, if not then pull cylinder head and inspect pistons and cylinders. If looks good remove oil pan, clean and replace oil strainer to get debris from gear out.

[speed_demon]

Thanks for the reassurance. I'm very happy to have confirmed a bad pump. At first I figured worse case scenario rebuild the engine.... wrong! None of the local machine shops would grind the crank nor does Yamaha sell oversized bearings. The only shop that would bore it wanted $100 per hole. They insisted it was a "blind hole". I explained it was a four stroke and they started asking me about ports in the bores. If they don't know the difference between a four stroke and a two stroke I don't want them working on my stuff. I'll do a compression test and scope the cylinders. If I have time I may also replace the rod bearings.

How much compression should I expect dry?

How many hours do these engines typically last?

Thanks!

[99yam40]

leak down test will tell you more than a compression test

[BQUICK]

I saw one last fall with a stripped oil pump. Owner admitted running full throttle in 40 degree weather after only a couple minutes warmup. New oil pump and removal of as much debris as possible without teardown and he was back on the water. I suspect excessive cold oil pressure was the issue putting too much load on the oil pump?

Another reason I like my old 2 strokes......they seem to be a little more forgiving.

[speed_demon]

I saw one last fall with a stripped oil pump. Owner admitted running full throttle in 40 degree weather after only a couple minutes warmup. New oil pump and removal of as much debris as possible without teardown and he was back on the water. I suspect excessive cold oil pressure was the issue putting too much load on the oil pump?

Another reason I like my old 2 strokes......they seem to be a little more forgiving.

That makes a lot of sense. The only way this could have happened is if the pump froze up or excessive oil pressure and the oil pump spins freely. I was afraid to put it together with a new pump to find out I have an unconfirmed blockage in the powerhead. I'm pretty sure this happened early spring on the Kenai river which is rather cold. Thanks for the info.

Did the other guy damage the shaft splines too? I hope I don't have to replace my shaft.

btw, I love 2-strokes but if you want to dipnet on the Kenai you have to run DFI or 4-stroke. The other 49 weeks out of the year apparently 2-strokes are fine. It's really the fisheries trying to disqualify as many boats as possible during the brief late salmon run. Up here you have to pay to play.

[BQUICK]

No just the oil pump. I'm sure the shaft gear is not perfect but it's still running. Told the guy if he wants to run hard cold go to thinner oil or make sure it's warmed up. People don't realize how long it takes the oil to warm up vs water temp. Oil temp gauge would tell the story......

[speed_demon]

No just the oil pump. I'm sure the shaft gear is not perfect but it's still running. Told the guy if he wants to run hard cold go to thinner oil or make sure it's warmed up. People don't realize how long it takes the oil to warm up vs water temp. Oil temp gauge would tell the story......

Yeah, I think you're right. The oil pump splines aren't torn up, they are completely gone. It wasn't an even fight if you know what I mean. The shaft is probably harder metal. It's a shame. The internals of the pump look great. All it needs is the spline insert but as far as I know you can only buy the whole assembly.

[BQUICK]

Right I've only seen whole pumps. Maybe you could get a good used driven gear...