Yamaha C90 Outboard Lower Unit Drain Screw Leak

[cpostis]

There is also the possibility the thread has stripped or corrosion of the aluminium side of the thread (stainless steel/ aluminium electrolysis). The solution to that could be you might Helicoil a new (and better sealing and permanent stronger) thread.

Good point Zenoahphobic!
With that being said, I wonder if the screw and/or the threads in the l/u has been cross-threaded and now the machined surface of the l/u and the sealing surface of the screw are no longer perfectly parallel?

The sealing surfaces are the lower unit machined area around the drain plug and the underside of the drain plug itself (the flange with the fiber gasket being the "seal")., not the threads.

If the plug is no longer parallel with the sealing surface on the l/u, the gasket will compress on one side and not the other, at least not equally. This could cause a leak even when torqued properly, or overtightened (imo).

Whaler82, can you measure the "new used gasket" thickness at several points and determine if it has been (or not been) compressed equally? Maybe you can visually see this? I don't know if the gasket will remain at the thickness while being tight once pressure is released by removing the drain plug though? Hopefully you have a caliper (or other means) to do so.

Maybe there is something in the threaded drain hole that is not allowing the plug to bottom out and seat completely . You could measure the gasket the thickness compared to a new gasket and see if it is being compressed. How about running a thread chaser or tap through the threads to be sure? I would remove the drain screw, run the tap, pull the fill screw to allow any particles to drain with the gear oil.

Sounds like a lot, but you are having a hard time to figure it out. This couldn't hurt!

The new gaskets I have are 1.6mm thick.

Here are some pics:

[whaler82]

Good point Zenoahphobic!
With that being said, I wonder if the screw and/or the threads in the l/u has been cross-threaded and now the machined surface of the l/u and the sealing surface of the screw are no longer perfectly parallel?

The sealing surfaces are the lower unit machined area around the drain plug and the underside of the drain plug itself (the flange with the fiber gasket being the "seal")., not the threads.

If the plug is no longer parallel with the sealing surface on the l/u, the gasket will compress on one side and not the other, at least not equally. This could cause a leak even when torqued properly, or overtightened (imo).

Whaler82, can you measure the "new used gasket" thickness at several points and determine if it has been (or not been) compressed equally? Maybe you can visually see this? I don't know if the gasket will remain at the thickness while being tight once pressure is released by removing the drain plug though? Hopefully you have a caliper (or other means) to do so.

Maybe there is something in the threaded drain hole that is not allowing the plug to bottom out and seat completely . You could measure the gasket the thickness compared to a new gasket and see if it is being compressed. How about running a thread chaser or tap through the threads to be sure? I would remove the drain screw, run the tap, pull the fill screw to allow any particles to drain with the gear oil.

Sounds like a lot, but you are having a hard time to figure it out. This couldn't hurt!

The new gaskets I have are 1.6mm thick.

Here are some pics:

Great post. I do have the same caliper tool as yours. (All P/N's match those in your pics). I can do test on the O ring gasket. I cant find the new/old gasket. A have brand new one I just checked it is 1.56mm.

[pstephens46]

It is a weird gasket to me considering it needs to be watertight. Consistency of wood. Fairly thick also. Drain bolt on my Nissan oil pan is a thin copper washer more or less. Makes you wonder how much pressure might develop in lower unit while running.

[cpostis]

It is a weird gasket to me considering it needs to be watertight. Consistency of wood. Fairly thick also. Drain bolt on my Nissan oil pan is a thin copper washer more or less. Makes you wonder how much pressure might develop in lower unit while running.

I believe the material used is as follows: Vegetable Fiber Gaskets (Plant Fiber Gaskets)

Custom Gasket Mfg. fabricates a variety of die cut parts and gaskets from vegetable (plant) fiber materials. Vegetable Fiber is a treated cellulose fiber material impregnated with protein glue and a glycerin binder. It is commonly referred to as Detroiter. This material works well for a wide range of sealing applications where temperatures can reach 250˚F. It is strong and flexible, resists heavy pressure and is compressible. It is used most often in industrial, chemical, petroleum, petrochemical and automotive industries. Vegetable fiber is chemically treated to be resistant to oils, water, alcohol, grease, air, gas, gasoline and other solvents. It is available in a variety of thicknesses ranging from 0.006” to 0.187” thick. From engineering to design and tooling, to die cutting and fabrication, Custom Gasket Mfg. has the capabilities to meet the tightest tolerances and the highest quality standards.


I believe the problem is not the gasket. More than likely the sealing surfaces and their proximity to each other.

[zenoahphobic]

I believe the material used is as follows: Vegetable Fiber Gaskets (Plant Fiber Gaskets)

Custom Gasket Mfg. fabricates a variety of die cut parts and gaskets from vegetable (plant) fiber materials. Vegetable Fiber is a treated cellulose fiber material impregnated with protein glue and a glycerin binder. It is commonly referred to as Detroiter. This material works well for a wide range of sealing applications where temperatures can reach 250˚F. It is strong and flexible, resists heavy pressure and is compressible. It is used most often in industrial, chemical, petroleum, petrochemical and automotive industries. Vegetable fiber is chemically treated to be resistant to oils, water, alcohol, grease, air, gas, gasoline and other solvents. It is available in a variety of thicknesses ranging from 0.006” to 0.187” thick. From engineering to design and tooling, to die cutting and fabrication, Custom Gasket Mfg. has the capabilities to meet the tightest tolerances and the highest quality standards.


I believe the problem is not the gasket. More than likely the sealing surfaces and their proximity to each other.

Are the fibre orange/red tap sealing washers on water taps this material? If so, the sealing is enhanced by water absorption expanding the material a little. And maybe a new washer is called for as it would be oil covered when removing?

[pstephens46]

I believe the material used is as follows: Vegetable Fiber Gaskets (Plant Fiber Gaskets)

Custom Gasket Mfg. fabricates a variety of die cut parts and gaskets from vegetable (plant) fiber materials. Vegetable Fiber is a treated cellulose fiber material impregnated with protein glue and a glycerin binder. It is commonly referred to as Detroiter. This material works well for a wide range of sealing applications where temperatures can reach 250˚F. It is strong and flexible, resists heavy pressure and is compressible. It is used most often in industrial, chemical, petroleum, petrochemical and automotive industries. Vegetable fiber is chemically treated to be resistant to oils, water, alcohol, grease, air, gas, gasoline and other solvents. It is available in a variety of thicknesses ranging from 0.006” to 0.187” thick. From engineering to design and tooling, to die cutting and fabrication, Custom Gasket Mfg. has the capabilities to meet the tightest tolerances and the highest quality standards.

I believe the problem is not the gasket. More than likely the sealing surfaces and their proximity to each other.

I agree. They work well for me. Interesting info. I am not challenging the selection by Yam engineers. Made of wood no doubt. If we could only zoom in on the opening on OP's lower unit....

[pstephens46]

Are the fibre orange/red tap sealing washers on water taps this material? If so, the sealing is enhanced by water absorption expanding the material a little. And maybe a new washer is called for as it would be oil covered when removing?

No offense, but what are you saying here?

[cpostis]

Are the fibre orange/red tap sealing washers on water taps this material? If so, the sealing is enhanced by water absorption expanding the material a little. And maybe a new washer is called for as it would be oil covered when removing?

I don't think it's the same material if it "absorbs water". This material "Vegetable fiber is chemically treated to be resistant to oils, water, alcohol, grease, air, gas, gasoline and other solvents".

I never re-use fiber gaskets. These are a whopping 92 cents a piece at boats.net.

[zenoahphobic]

No offense, but what are you saying here?

Maybe when the oil was changed last time the washer wasn't. Being oil laden water wasn't able to expand it for proper sealing (low torque of screw was specified, the water coming from the outside cavity to do this).

[zenoahphobic]

I don't think it's the same material if it "absorbs water". This material "Vegetable fiber is chemically treated to be resistant to oils, water, alcohol, grease, air, gas, gasoline and other solvents".

I never re-use fiber gaskets. These are a whopping 92 cents a piece at boats.net.

Cpostis, have you ever changed these washers on a tap? Often when water pressure is turned back on, they darken and may even show some small leak but after sometime the washer dries.
The affect is better(quicker) seen with a hot water line; becomes damp when assembled and the water comes through cold, and then dries up permanently when the hot comes through.
An old plumber can tell you sealing pipe joints with siezle/hemp, a natural fibre, uses the same principle to seal.

[pstephens46]

I think we are over thinking this issue. I have re-used these three times with no issues. The seat or screw, as mentioned before, is the issue.

[whaler82]

I did find the previous gasket and checked caliper. It was even and consistent.

[whaler82]

A little update. On a whim yesterday I stopped in to West Marine to see if they had any drain plug gasket washers of any kind. They had a off brand for all Yamahas. I bought a box of 6. Now the OEM Yamaha - proper part # washers were not working. In fact the ID seemed too big. Normally I have to thread the gasket washer onto the plug. Those just slipped on with ease - even a little sloppy. The generic off brand I bought at West Marine had to be threaded onto to drain screw. I teflon taped the screw. I check it this AM. No leak.... Now I notice the drain screw's first 1 1/2 threads are buggered up. Threads closest to the magnetic end. Screw goes in fine and snugged up tight and even. I wonder if that was the problem to begin with and or combo of defective yamaha gasket washers. Checked again at 7PM still no leak.

[pstephens46]

If you continue to have issues, buy a new screw. Who knows?

[TownsendsFJR1300]

A partially stripped, threaded hole is a disaster waiting to happen.

Your teflon tape sealed it up for now.

Should that drain bolt eventually fall out while underway, big $'s to re-build that LU.

If your not comfy doing a heli-coil, pull the LU and have a machine shop do it.
Might be $35 and the threads will be SS, not aluminum..WAY STRONGER