gmc 305e v6 head gasket replacement

[bigrigpardo]

ok I figured the 60-66 guys have a bit more knowledge on this. I have a 68 gmc with a 305e in it. cylinders 1 and 3 are weak. I have checked all the usual stuff I can think of and am down to either internal damage or a blown head gasket between the 2 cylinders. it runs better when its hot so that leads me to further the bad gasket and its sort of sealing when the heads expand. I will be pulling this later in the week but have a few questions.

1 is there any special method to installing the head gaskets that I may not know of

2 what are the torque specs and sequence for the heads and intake as I cant seem to find them with all of my googleing

3 is there any common issues with the v6 that would cause 2 adjacent cylinders to be week? bad valves? the truck has 76k original miles and isn't all sludged up inside the valve covers.

[TJ's Chevy]

Originally Posted by bigrigpardo

ok I figured the 60-66 guys have a bit more knowledge on this. I have a 68 gmc with a 305e in it. cylinders 1 and 3 are weak. I have checked all the usual stuff I can think of and am down to either internal damage or a blown head gasket between the 2 cylinders. it runs better when its hot so that leads me to further the bad gasket and its sort of sealing when the heads expand. I will be pulling this later in the week but have a few questions.

1 is there any special method to installing the head gaskets that I may not know of

2 what are the torque specs and sequence for the heads and intake as I cant seem to find them with all of my googleing

3 is there any common issues with the v6 that would cause 2 adjacent cylinders to be week? bad valves? the truck has 76k original miles and isn't all sludged up inside the valve covers.

You check the valve clearance on the 1 and 3 cylinder? Run the engine until it's hot and check for a .012 intake and .018 exhaust clearance? Tight valves are common on these as your shade tree mechanics think tighter the better. lol Ok....Most do.....I'm the exception.

No special method, take a square edge and check for head and block warping, that is very rare....but it Can happen. Buff the surfaces with care and install a new gasket. Perrty simple. Picture below on a head torque sequence.

Bad valves yes, these engines were designed to run off of leaded fuel, the intake valves have a cast iron seat...in other words...no seat at all...just a round seat ground into the head. The exhaust valves have an installed seat but it is not hardened. Though folks have ran these without leaded fuel for years with no problems...always another possibility.

With only 76,000 I highly doubt the rings are Weak as they are industrial and would only prematurely wear if oil was never changed and/or the engine inhaled a ton of dirt or was over revved all of it's life....as many like to do an then complain and call the engine junk.

EDIT: Forgot to mention, head torque spec is 65-72 foot pounds.

Intake is 20-25 foot pounds. No sequence, but I do a criss cross pattern.

[bigrigpardo]

Yup did the valves to spec and then again even looser til they clacked justincase they had carbon that needed to be beat off. But yea thanks for the specs. It would be a wierd situation to have bad valves in adjacent cylinders but who the hell knows I will yank the head and go from there.

[TJ's Chevy]

Originally Posted by bigrigpardo

Yup did the valves to spec and then again even looser til they clacked justincase they had carbon that needed to be beat off. But yea thanks for the specs. It would be a wierd situation to have bad valves in adjacent cylinders but who the hell knows I will yank the head and go from there.

Another common thing I forgot to mention was due to the location of the spark plugs....all the crap you can think of builds up around the plug....even with air sometimes some of the crap stays there....pull a plug and it falls into the cylinder. I had that happen once...luckily the engine was fine.
Good luck! I've had heads off these...you may want to get an engine hoist just for the head. haha They are heavy....Took all I had to lift it off and out with just my arms. One quick thing you might do is put some oil in the cylinder...if the compression increases...then you'll know for sure it's rings...if it doesn't....head gasket or valves.