Re: 305C-305E Difference?
 

Engine number stamped on the block, check this page of My site for more info; Engine Data Page.

Re: 305C-305E Difference?
 

The 305C engine was the heavy duty version of the 305 line. It was mostly found in the 3000-4000 series 60-66, then in 4500-6500series through '74. The main differences with the E series engines is all sodium filled valves on the Exhaust (called stellite), rotators under the valve springs, and 4-ring pistions. The E series was the light duty or Economy (pick up) version and lacked the things that the C engine had and also was equipped with 3-ring pistons (the only engine in the V6 line to come that way factory).

Re: 305C-305E Difference?
 

My understanding is all GMC V6 engines 351 through 478 came equipped with Sodium cooled valves, all 305 engines did not and my factory GMC manual confirms this, stellite is a hard alloy used on the valve faces that come in contact with the valve seat. There was an HD305E V6 that was available in pickups, my 68 was equipped as such and this engine used valve rotators.

Re: 305C-305E Difference?
 

Thanks Pete, for reminding me about the valves, in that regaurd you are right. However only the "E" series ever came with 3-ring pistons, which as you have said before have been replaced with the 4-ring pistons of the A-B-C-D engines. I hope you have had a blessed day today. BTW, how is that 478LD of your running??

Re: 305C-305E Difference?
 

Lewis, the 478M finally got fired up on this last President's day, after getting things dialed in like the valve lash and timing the engine sounds great, it's good to hear it running after some 16 years and the 478 does sound somewhat different from the 305. I have yet to finish the power steering conversion before I get the truck on the road, working 6 days a week is making things difficult at this time but hopefully next month will be better. As far as the 4 ring piston issue, what I was talking about was in the early seventies GMC issued a service bulletin that stated 3 ring 305 pistons were discontinued as a replacement part. From that point the 4 ring 305C pistons were to be used in prior 305A,D and E engines for replacement and the purpose was for standardization. This all made sense at the time since only 305C engines were in production. Of course aftermarket suppliers more than likely offered the 3 ring piston.