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Discussion Starter #1
My 65 389 engine had been rebuilt by the previous owner and since I've had it, there was always a rough idle issue. After checking the compression, valve movement and making sure the plug and plug wires were good and firing as well as making sure all the intake and carburetor bolts were tight as well as carb. and timing adjustments, I was a bit stumped as to what was causing the problem. Fortunately I was able to find a great Pontiac specialist locally and he recommended I remove the intake and check the gaskets. What I found was rather surprising and thought I would share this with the group. The mechanic that built the engine apparently replaced the valley pan, or as it's technically called, "Push-rod and cam cover". This reproduction cover, according to the Pontiac specialist is not only thicker metal than the original, but the shape of it is wider than the original. This caused the intake to partially sit on the valley cover, causing a vacuum leak (see photo with witness marks) instead of sealing flat to the head. The specialist recommended that I trim the valley plate in the areas the intake had interference (see after photos). Now reassembled, no more rough idle! So if you have a recently rebuilt engine or contemplating rebuilding yours and need to replace the valley pan, just be sure you have, or can create the clearance you need.
 

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I think information like this is very important to share. In todays day and age of the interweb, you never know when a bit of information, no matter how small, will help someone. Thanks for posting.
 

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My 65 389 engine had been rebuilt by the previous owner and since I've had it, there was always a rough idle issue. After checking the compression, valve movement and making sure the plug and plug wires were good and firing as well as making sure all the intake and carburetor bolts were tight as well as carb. and timing adjustments, I was a bit stumped as to what was causing the problem. Fortunately I was able to find a great Pontiac specialist locally and he recommended I remove the intake and check the gaskets. What I found was rather surprising and thought I would share this with the group. The mechanic that built the engine apparently replaced the valley pan, or as it's technically called, "Push-rod and cam cover". This reproduction cover, according to the Pontiac specialist is not only thicker metal than the original, but the shape of it is wider than the original. This caused the intake to partially sit on the valley cover, causing a vacuum leak (see photo with witness marks) instead of sealing flat to the head. The specialist recommended that I trim the valley plate in the areas the intake had interference (see after photos). Now reassembled, no more rough idle! So if you have a recently rebuilt engine or contemplating rebuilding yours and need to replace the valley pan, just be sure you have, or can create the clearance you need.
Mine was somewhat similar,but it was the gasket. (Wrong one in the gasket kit.)
 

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Hmmm. You checked most all things except putting a vacuum gauge on the engine. Vacuum gauges can tell you a lot about your engine. :yesnod:
 

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Lucky I could hear my leak. I thought it was my exhaust manifold gasket.

Another example of reproduction parts that are almost like original, but just not quite. Must have been made in China or.........

The other possibility is that the valley pan is correct, but the block was "zero decked" which lowers the deck height and drops the heads lower and changes the fitment of the valley pan.
 

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"...The other possibility is that the valley pan is correct, but the block was "zero decked" which lowers the deck height and drops the heads lower and changes the fitment of the valley pan."


IMO, this is a real good point to consider, when planning an engine build. If it's a super low budget build, then I suppose this is not a possibility. But, for those who have a little more to spend, I'd consider buying Auto Tec forged pistons. They'll probably cost around $550 shipped. But, from that price, you can subtract the price of extra machining on the block, to achieve zero deck height. Won't need any machining except the normal amount required to square up the deck with the crank centerline, & give the surface the correct finish for the type of head gaskets to be used. .

Auto Tec will locate the piston.pin hole exactly where you need it. Last I heard, this pin location change was no extra charge. Obviously, your machine shop, or somebody, will have to measure to see what pint height you need, before you order the pistons. For 400 Pontiac engines, I know that some forged pistons come with a 1.714 pin height, & some come with a 1.720 pin height.

So, I'm guessing that somewhere in the 1.725-1.750 range should get real close to zero deck height, on most engines. Obviously, any particular block might be different, because of previous machine work done, both at the factory & later. Some even like to go .005 above the deck, to achieve a bit more compression. It's quite common to go above the deck. Don't really know how much is too much. You wanna leave sufficient quench distance, between the head & the piston top. And, obviously, you don't wanna get the top piston ring too close to the deck, especially if you have cyl chamfers. Racers have used this above the deck trick for years.

https://www.google.com/search?q=ok+to+run+the+piston+tops+.005+above+the+deck&rlz=1CAHKDC_enUS777US777&oq=ok+to+run+the+piston+tops+.005+above+the+deck&aqs=chrome..69i57.26507j0j7&sourceid=chrome&ie=UTF-8
 

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Discussion Starter #9
It was pulling around 14 PSI of vacuum. The biggest clue was spraying starting fluid around the intake and the engine revved up when sprayed around the right rear bank. Yes I considered the decking issue. One can assume that a 54 year old engine has been decked, which increases the width between the head mounting surfaces. I also opened the mounting holes in my intake, as there were witness marks where the bolt treads had cut in to the inside of the hole which is another indication of of the decking issue. Additionally, I took the sharp edges off the bottom of the intake at the intake ports. It's all good now! Thanks for all the input.
Ron
 

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It was pulling around 14 PSI of vacuum. The biggest clue was spraying starting fluid around the intake and the engine revved up when sprayed around the right rear bank. Yes I considered the decking issue. One can assume that a 54 year old engine has been decked, which increases the width between the head mounting surfaces. I also opened the mounting holes in my intake, as there were witness marks where the bolt treads had cut in to the inside of the hole which is another indication of of the decking issue. Additionally, I took the sharp edges off the bottom of the intake at the intake ports. It's all good now! Thanks for all the input.
Ron
Excellent, success. 14 PSI of vacuum can be a little on the low side, but with a bigger aftermarket cam, it could be normal. The factory cams pulled about 17-22 PSI of vacuum.

The heads when milled will also create a slight misalignment of the intake-to-heads where the bolts are up against the top of the holes they go through. The solution is to have the intake milled a slight amount, but for the average engine, a little misalignment won't really be a problem.

Did you tighten the long water pump bolt to the front of the intake to draw it tight against the rubber O-ring seal before tightening the intake bolts? This ensures a good seal. If not, you might experience a water leak that you can't quite find - it'll be from that O-ring seal if it does happen. I never had an issue, but it can happen. :thumbsup:
 
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