Possible wiped cam lobe, two different cylinder heads.

[rprentiss288]

Just a little bit of backstory,
I picked up a 1978 GMC K15 about a month and a half ago. 350/TH350 Went through the basics, new plugs, wires, oil change, coolant, rebuilt the Edelbrock 1405 and put a new carb spacer on it. (The spacer that the previous owner had on it didn't clear the secondaries and had been driving it around like that.) Rebuilt drive shafts, rear end, had the Mile marker Part time 4wd kit put in the NP203 transfer case. Drove it for a while and noticed a slight tick, pulled valve covers to find the a couple of the rocker arms were loose. Cold lashed the valves to tighten the loose rockers. (Find base circle on the cam for each cylinder, tighten to slight resistance when spinning push rod then gave it another 1/2 turn). With valve covers off after the cold lash I could see Cyl 4 Exhaust wasn't moving full travel once the engine was warm,I loosened that rocker a 1/4 turn assuming I just over tightened it and it moved less, let it run for a short period of time to try and refill the lifter to no change. Shut it down, let it cool and it actually backfired through carb when I revved it and started it again cold. Let it cool again, pulled the push rod and checked it for a bend or damage on the ends and it's fine, cold lashed CYL4 exhaust again and it runs without backfiring but sounds like it's missing. Compression test checked out fine.

Now, I'm assuming the cam has worn a lobe. I planned on swapping it myself with the engine still in the truck. Before I called Lunati or Comp tech support to help me pick the right cam I checked the head casting #'s to know what I was working with.. This is where it gets interesting. Driver side cyl head is a 3973487x and Passenger side cyl head is a 333882. Both are 76cc heads, both are 1.94/1.50 valves. I was trying to decide if I should leave most of the motor as is and just swap cam and lifters or if I should just totally rebuild the top-end.

Do I need new heads?
Should I tear the whole thing down and start fresh?
Can I just swap cam and lifters or do I need Pushrods/Rocker Arms/Valve Springs etc?
On a scale of 1-10 how dumb is it that there's two different cyl heads?



Thanks

 

[yevgenievich]

Equivalent castings with different numbers are common to be used on rebuilds. If you are happy with performance level and everything but the cam is good, pit a new cam/lifter set and keep going. If push rods look good, can reuse. Same for rockers. it would guess it has flat top pistons with four valve relieves to get decent compression.

 

[Daveo91Burb]

Just a point of interest for ya on the 882 head - those were the stock heads on my '76 corvette. I still have them but switched to vortecs when I rebuilt the motor 5 or so years ago. Not the worst head GM ever made but not great either. I think other cars had em besides Vettes.


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[MikeB]

If I was going to the trouble of replacing the cam and lifters, I'd at least want to pull the heads and check their condition. I mean, removing the intake manifold, distributor, and all the associated lines, brackets, and hoses is 75% of the battle to remove the heads.

One another note, I'd drain the oil and then dump lots of solvent in the lifter valley and let it drain out of the pan. Worst case, use a few cans of brake cleaner. Not much you can do if metal particles made their way into the oil galleries and bearings. But there's a good chance that didn't happen, assuming you didn't see a reduction in oil pressure. The particles are going to wash down to the crank, but that doesn't mean they got between bearings and journals. And the only way they'd get into the galleries is if the made it through the pump screen and filter bypass.

When you replace the cam, be sure to coat it and the lifter faces with moly lube supplied with the cam. I'd then use a break-in oil like Driven BR30, but worst case a good 10w30 oil and a break-in additive like Comp Cams P/N 159.

Don't go crazy on the cam. If your engine is basically stock, a stock replacement cam will be fine. If you decide on something else, just keep the duration @ .050" tappet lift to something under 204 degrees. 194/204 generally works well in stock engines. Rich Weyand on this forum has some other more specific suggestions. Might want to do a search of posts by him.

Also, replace the timing set and consider replacing the valve springs and valve stem seals. And use good gaskets such as Fel Pro.