Transmission Rebuilding sucks by the way.
So you think you might want to rebuild your transmission, you probably are having one of those moments where you are not thinking correctly. I have rebuilt several transmissions in my life as well as built some from scratch for racing. Not a great idea for a novice. If you have no idea how it works you should never try this idea of rebuilding a transmission especially the fiat x1/9 4 speed or 5 speed for that matter.
OK, enough about the warnings, now on to business.These transmissions use very strange materials versus newer transmissions or most other transmissions. It is more like a transmission you might find in a motorcycle. The gears are not very hard as far as gears in a transmission go, and they use a coated spring steel thing to slip in and out of gears to lock them in place. If they are worn you will grind gears. This is very important to note that part. Also When choosing oil it is almost impossible to find oil that will not tear them up when you decide to try to drive it. From all the information I have read on the topic and all the people I talk to and oil manufacturers, you need to find a oil that has low sulphar and phosphates. It should almost be like mineral oil or at least use that as a base for the oil. Also it must also not be a gl4 or gl5 oil, because those will tear it up very fast. It may drive for a little bit but then you will be buying a new transmission and no one wants to do that every few months.
Take special care to inspect all the gears for wear and the shifting forks must also not be worn too much other wise you run the risk of them getting hair line cracks and breaking off.
There is a plate on the side of the transmission that holds some springs and behind those are some ball bearings that they used to mark positions for the shifting rods to move the shifting forks around. DO NOT LOOSE THOSE BEARINGS! They are hard to get out and they love to fall out at the wrong time.
You should also make sure all the bearings in the transmission are in good shape. They play a very important part in keeping the gears running smoothly together.
The most important thing I can say is to pay close attention to the type of oil. I know I said that a minute ago, but OIL TYPE IS IMPORTANT!
I have found a few places that can get the right kind of oil for you fiat x1/9 transmission. One of the best places I found to have it available is Penrite.
They suggest HPR30 20W-60 for both engine and transmission or MC-4ST HD 50 Four Stroke oil for transmission only. Either one of those is fine. I did ask the company for advice and their response is to use the HPR30 because it works for both the engine and the transmission.
Here is a excerpt from Midwest Bayless:
“For transmission fluid, you have 4 choices:
GL-1 (90wt Mineral Oil) – Sold at NAPA in gallons under part number NHF65201
20W50 Motor Oil
You should never use any transmission fluid which is identified as GL-5 grade, with EP modifiers.
This stuff is easy to identify, it’s usually greenish/blueish in color, and smells funny (like trans fluid does).
This stuff is for modern gear boxes, and the EP modifier will soften and strip the gritty coating off of the synchronizer
rings in our transmissions.”
The reason that oil is so important in this section is because between the brass parts and the coating on the spring steel both of them are very easy to get destroyed by incorrect oil types.
If you put a oil into the transmission that is a HP GL-5 it has too high of sulphates and phosphorus and will eat the brass and the coating away quickly and not provide the right weight of oil at the right temperatures. The GL-5 is actually too thick at start up and thins out too fast for the fiat transmissions due to the heat range of GL-5. The fiat transmission needs a oil that stays at a medium weight at start up and can stand up to the heat that comes from a hot fiat transmission. Fiat made a transmission that was sort of too small for the car, this causes it to heat up much hotter than most transmission from then and now, plus there is not transmission cooler.
So overall it is a royal pain to deal with the transmission on this car but if you think you are a pro still after reading this, OK go for it but do not blame me if it does not go well for you. I am not responsible for any damages or anything else by following my guide, so keep that in mind.