390 w/Serious Knock Under Load

[Redmanbob]

If your going to use hardened seats whats wrong with those heads ? NAPA, pick apart, you name it..Personally I'd look for a whole engine as donor considering the issues so far.

[LedheadELH]

well hardened seats won't make that much of a differece. it just makes it so you don't have to do a valve job ever few thousand miles because of your seats being beat to death w/ unleaded feul, but if U have a knock and only at load usually that means Main bearings. take the spark plug wire off the suspected cylinders to ground them out, and tell me if the knock gets lounder, quieter, or stays the same. if it stays the same, it's probly the mains.

[dlowenst]

Just to close the loop on this thread:

After removing the heads and doing a thorough examination with measurements, the main problem turned out to be (as predicted by several posters here) a cooked head gasket in between cylinders #3 & 4. The mechanic did find a vacuum leak that he believed was at least part of the cause. In addition, the heads had previously been improperly rebuilt without replacing valve guides.

Both heads have now been rebuilt with new valves, guides, seals, seats and head bolts, and bolt holes fully chased and cleaned. It's now running great, with much more power.

Thanks again for all the guidance.

Dave

[Krewat]

Dave, thanks for posting that follow up!

I'm glad it turned out to be relatively(!) simple.

Good luck with it!