MG Car Repair: MGB transmission, engine temp, fresh oil

Question
Hi Howard,

the car is a 1968 MGB.  I am unable to speed shift into fourth gear.  When you try it grinds, and generally will not go in.  If I shift deliberately (slowly) it will go into fourth, but sometimes still with a clunk.  Fresh oil, level is good.  Any hints?

Thank you in advance for any and all information! :-)
Dan

Answer
Hi Dan,

Most likely it is a worn syncro ring on 4th gear. The syncro rings are small cone shaped clutches that get the gear and a sliding hub spinning at the same speed before engaging. When a syncro ring gets warn the two splines are not spinning at the same speed and either grind or just not engage.

There are other things that can cause it but that is the most likely and most common. If you use the factory recommended gear oil in the gearbox, there is a simple test you can do to see if it is a warn syncro ring. Drain that oil out and put engine oil in the gearbox and drive it for a while to get the engine oil all through the gears and get the oil warm. (takes longer then engine temp to come up in temp)

If it improves the shifting then that is what is wrong and you can just leave the engine oil in and use it that way until you need to go into the gearbox. If it does not improve the shifting into 4th, it can be a worn spacer on the main shaft or several other things. All of which are inside and none accessible from the outside.

Howard