1967 gmc

[dbrown1992]

So can I block that hose if I run a breather on the valve cover?

[BobBray]

Should be O.K.. The PCV valves are under the valve covers, so you have to allow fresh air into the crankcase. Most V-6's have the can with the small filter in it behind the left head. On California 'closed' systems, the can was replaced with a pipe to the air cleaner. Later on (about '68 and up) the hole where the can or pipe attached was blocked off with a freeze plug and a breather was added to one of the valve covers, usually with a hose to the air cleaner. The filter in the can, pipe to the air cleaner, and breather on the valve cover all do the same thing: they let air into the crankcase.

[GMCNUT]

Originally Posted by BobBray

Should be O.K.. The PCV valves are under the valve covers, so you have to allow fresh air into the crankcase. Most V-6's have the can with the small filter in it behind the left head. On California 'closed' systems, the can was replaced with a pipe to the air cleaner. Later on (about '68 and up) the hole where the can or pipe attached was blocked off with a freeze plug and a breather was added to one of the valve covers, usually with a hose to the air cleaner. The filter in the can, pipe to the air cleaner, and breather on the valve cover all do the same thing: they let air into the crankcase.

Apologies for hijacking the thread but I have to ask Bob a very closely related question right quick while he's on this topic - Bob, since you know a lot about how these engines breathe, I think you can answer this question: the 305A engines have a road draft tube and breather caps in BOTH valve covers - if someone wanted to add the 305E type valve covers to a 305A (Plaid covers for example) then doing this would eliminate one of the two stock breathers because the 305E only used a breather cap on one of the valve covers - not both like the 305A. So my question to you is, would the 305A still "breathe" OK with only one valve cover having a breather cap?

[TJ's GMC]

On mine all I have is a breather on the cover. Works fine.

[BobBray]

Originally Posted by GMCNUT

Apologies for hijacking the thread but I have to ask Bob a very closely related question right quick while he's on this topic - Bob, since you know a lot about how these engines breathe, I think you can answer this question: the 305A engines have a road draft tube and breather caps in BOTH valve covers - if someone wanted to add the 305E type valve covers to a 305A (Plaid covers for example) then doing this would eliminate one of the two stock breathers because the 305E only used a breather cap on one of the valve covers - not both like the 305A. So my question to you is, would the 305A still "breathe" OK with only one valve cover having a breather cap?

Good question! The deal is that the early 305A's with a road draft tube did not have PCV valves. The air came in through the valve cover beathers and was sucked out through the draft tube. Considering that modern engine oil is a lot less likely to turn to sludge than oil was in the old days, you could probably get by with one breather if you change your oil regularly and don't have a lot of blow-by. I would periodically check under the closed valve cover to make sure you don't have a problem developing. Also, put the cover with the breather on the opposite side of the draft tube.

[bigblockv6]

What I would do is update the system to a 62 305D where one PCV valve was used that ran in back from the carburetor to the rear filter and eliminate the road draft tube.

[BobBray]

Originally Posted by bigblockv6

What I would do is update the system to a 62 305D where one PCV valve was used that ran in back from the carburetor to the rear filter and eliminate the road draft tube.

This would be the best solution if you can find the parts. Road draft tubes only draw the fumes out of the crankcase when the truck is moving.

Peter, what year did they start putting the PCV valves in the heads?

[bigblockv6]

I would say it was 1963 with the introduction of the 305E, I know the biggest difference between the 305D vs the 305E was the emissions equipment.

[GMCNUT]

Originally Posted by BobBray

Good question! The deal is that the early 305A's with a road draft tube did not have PCV valves. The air came in through the valve cover beathers and was sucked out through the draft tube. Considering that modern engine oil is a lot less likely to turn to sludge than oil was in the old days, you could probably get by with one breather if you change your oil regularly and don't have a lot of blow-by. I would periodically check under the closed valve cover to make sure you don't have a problem developing. Also, put the cover with the breather on the opposite side of the draft tube.

Thank you Bob - I wasn't sure what the negative affect might be if I did that. I have a nice set of plaids I'd like to stick on the 305A maybe, so might consider doing that after I get the truck painted.

[GMCNUT]

Originally Posted by bigblockv6

What I would do is update the system to a 62 305D where one PCV valve was used that ran in back from the carburetor to the rear filter and eliminate the road draft tube.

Pete, would the same positive improvement be the case if I were to use a 351C? I incidentally have a good one that ran great when pulled, but haven't had a good use for it since. Not sure you could get the GMC dealer to install a 351C as optional equipment in a 1960 1000 series truck or not - came out of a 1962 GMC 1000 series truck, and the dealer did replace the 305D that came in it from the factory with the bigger 351C at the time the truck was being bought, so might have been available in 60 too? Do you know?