The 6602 (602 ci) has the most power of the three 39 series trucks, this summer, we are going to pull the heads from a 6602 powered m62 (wrecker), I had another set of heads from our spare 6602 milled with a 3/16 cut, this will increase the compression so using modern gas will decrease the fuel consumption under load, also plan on filling in the carb jets and redrilling, this along with electronic ignition should make a very usable 6602, we are going to install a co2 sensor so things can be dialed in.
Other then the low compression, the EXHAUST guides ARE the main issue in these engines, they are made of bronze, the 6602 makes a lot of carbon (due to the over fueling with the low compression), this with some oil film becomes valve grind compound which eats the bronze valve guides, I had valve guides made from high impact, high strength, ductile iron so no more eaten up exhaust valve guides, the original intake valve guides micked out to almost new specs., the exhaust was another story, they were worn so bad that the stems almost wobbled. There is NO seals for the guides, just guide to stem clearance, so keeping a close stem/guide clearance is IMPORTANT.
The 6602 has one of the nicest carbs I have ever seen, it is TWO complete ONE barrel carbs in one housing, the fuel bowel is suspended from the top which allows air to flow AROUND it, PREVENTING vapor lock. The after market zenith replacement for the original holly is a POS compared to the holly, and not rebuildable.
If you get the truck, change the coil to 12v from the stock 24v, the reason for this is that 24v coils are NOT made anymore, so the replacement 24v coil is nos, it may be NEW in the box but is OLD by date, allowing it to collect moisture over time and old age deterioration = failure, run 12v to a second ignition switch, the points do not care if 12 or 24 is run though them, this mod WILL save you a tow bill when the 24v coil dies.
