WHAT IS THIS DRAIN PLUG FOR?

Drain plug for the transfer gear section of the gearbox. Look at the figure in the "Lubricants and Liquids" of Section 1 of your OM -- it shows how the transfer gear lubricant area is above the main gearbox reservoir area so needs to be drained separately (even though they share the same lubricant). This is why F specifies to add some of the gear oil into the transfer gear fill plug (it can't be overfilled as any excess will just "waterfall" into the main reservoir area).

 

That is the drain plug for the drop down gear set/case. The drop down gear set takes the engine crankshaft output and turns it 180 degrees to feed into the underlying transmission gear set. There are passageways from the drop down gear case into the transmission sump. There is also a plug on the top of the drop down gear case. That hole (on top) serves as the fill hole for the transmission. There is another plug on the back of the transmission case that serves as the gear oil level check hole. When changing the gear oil, it is drained from both the plug in the bottom of the transmission sump as well as from the drop down gear case plug on the bottom. The transmission is then filled from the top of the drop down gear case until it begins to dribble from the hole in the back of the transmission case, which is then plugged. I even add maybe a cup more oil after that back plug is replaced.

 

thanks to Steve for the following diagram I stole some 18 ! years ago

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It may be leaking because the plug is missing a copper crush washer. As far as what oil to use, I use Valvoline 80W-90 synthetic blend, which seems to work well for me. The topic of what gear oil to use has been discussed on this forum ad nauseam. You can do a forum search of the topic to help you come to a conclusion as to what oil to use. Whatever you decide, I guess the best bit of advice would be to use what seems to work best for you. That being said, modern oil formulations are likely a great deal superior to what was used in these cars when they were new. It's easy to overthink it.

 

Redline MTL

Takes exactly 5 qts.

 

I strongly suggest 2 things.
1. The location used by the vast majority of people servicing these is the hole in the back. Its a matter of convenience if you have the car on a hoist. Its a cleaner, neater, easier, faster way to do it. You can add a small amount at the top plug for the transfer gears. It takes forever and is messy to fill from the top. It was never intended to be done that way.
2. Never intentionally overfill. We discovered decades ago that led to excessive leakage. We assumed it was related to insufficient breathing of the overfilled transaxle. And if filled with rear of car on jack stands and front wheels on the ground it will overfill.

 

Tell him I said hello.

 

I was speaking of filling the transmission. If it is not being drained, yes you can just pull and reseal the plug you asked about and reinstall a like amount of oil to the top of the transfer gear case. When running, the inside of a transmission looks like a blender inside. It will distribute the oil equitably.

 

I assume the layout and filling method is the same for 328 from 1986?

 

Yes, same on all 308/328 and many other F models that have transfer gears between the engine output and gearbox input.

 

I have not looked at the 328 but in mhy Dino I like to fill from top drop gear top plug
This allows access to lubrication hole for top bearing behind top drop gear.
I'm sure this is not necessary but I feel better