Grinding Noise When Shifting into Gear

[Mako19]

Let me start this thread by admitting that I am the opposite of mechanically inclined. I am an electrician not mechanic.


The motor is a 2001 model Yamaha 150 hp.
We have the motor serviced every year by a professional mechanic who changes water pump, lower unit oil, spark plugs, checks compression, and does a pretty thorough tune-up. He always says that the motor is very clean and in great shape.

Have had the boat for 3 years now.
Since owning the boat we have noticed when shifting into gear there is a grinding sound that comes from the motor.

The first time that the mechanic serviced the motor he told us that he found a small amount of metal shavings in the lower unit oil. He recommended that when shifting into gear we should do it very Deliberately to avoid this grinding sound. We have followed this advice for the last three years but this summer I have noticed that the grinding is getting worse.

My questions are:

What is actually grinding in the motor? I assume some type of gears but would like a better understanding of where the metal shavings are coming from.

Is there a way to check these parts to see how badly the grinding has worn or damaged the gears? We take this boat offshore and try to be as proactive as possible with maintenance issues.

What will eventually happen when gears are worn down too much? How will failure take place? Could it cause further damage to motor?

How much should it cost to repair these gears?

What causes the gears to start grinding like this? It has done this since we have had the boat. Is it just age that makes the gears start to wear?

I know this is a long question but when we were 40 miles southeast of grand isle Saturday and I heard the grinding I knew it was time to address issue.

Overall is this a serious issue that we should be concerned with?

Thanks in advance for any advice.