The F3 is stripped now and I'm in parts buying mode. While it was a driver though it had a strange clutch problem. It started in neutral fine and idled sweet (although smoky). Step on the clutch though and the engine dropped probably 100 revs and loaded up and a strange knocking came from the bellhousing. I pulled the box and everything looked fine, throw out bearing turned no worries, plate looked good as did pressure plate. In fact it all looks fairly new. Truck was a good driver before being stuck in a shed 25 odd years ago. Any suggestions?

 

Check the height of the fingers on the pressure plate. If they are different height it can cause the throw out bearing to rock as the high point rotates.

 

Is this still a flathead? I'd suspect the rear main bearing thrust surface is shot.

 

Yes, it is still the original flathead and 4 speed box. I did adjust the fingers but it made no difference. Didn't think of the thrust bearing, the engine is being rebuilt anyway due to low compression.