I recently installed a rebuilt M20 and it shifts fine but first gear has a chattering noise starting around 1,500 rpm, second thru fourth sound fine. I was told this was common on rebuilt trans and the noise would be less after break in. Transmission is full of 70-90 gear oil. I would appreciate you guys opinions.

 

First gear should only make a dull whine. There are couple issues that could be your problem. One is the first gear sleeve may be spinning on the main shaft, 2nd: M-20s have a plate that is riveted to the cluster gear, and that plate may be loose. Lastly, it could be the clutch disc.

 

Sometimes the reverse gear idler will contact the first gear and cause a chatter. The remedy is, when rebuilding the trans, grind about a 45 deg angle on the front of the teeth on the idler. Doesn't take much, just enough to keep it from rubbing the first gear teeth. Since yours is already rebuilt and assembled, I wouldn't worry about it. It will not hurt anything and will go away after the teeth polish themselves.