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Old 11-08-2009, 03:27 AM
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porting head suggest

Hello guys!

I must remove heads from my '81 k5 Blazer 305 engine, 'cause i must replace old lifters with new ones OE.

I'm thinking that it could be a good thing check-out heads for crack or damage and doing "porting heads".

With "porting heads" process, which other process it's important?

if i improve air flowing, intake manifold must be replaced with better new?

or for example with new exhaust manifold?

If air runs better but it's blocked by tight exhaust manifold, i think could be just an hald good job.



Thanks guys for attention.

Fabio

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Old 11-08-2009, 09:17 AM
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It is not necessary to remove the heads to remove/replace the lifters on your engine. The intake manifold has to come off and the valvetrain disassembled.

There are other 305 heads that would offer more potential compared to your truck heads but they may not be readily available locally (416 & 601 castings).

I would suggest you stay with the factory intake manifold.

305 engines of your vintage were notorious for 'losing a lobe' or two on the camshaft also - they were poor quality cores.

As for porting the heads, look for some books such as those written by David Vizard regarding porting Chevy heads. There is a lot written about the small-block chevy and it would be worth researching.

The exhaust manifolds can also be ported to some extent or replaced with tubular headers.
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Old 11-11-2009, 12:02 PM
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thanks for answer!
i'd like to ..."refresh" this engine.
it's important not changing many parts. i don't want a performing engine, i think in this case 305 engine it's not so good to tune-up.

as you told me, if i can place same intake manifld and heads it would be a good thing.

i replace alternator, rochester carb, air filter, distributor, wires system, 'casue they are so old and weak.

now it runs better
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Old 11-11-2009, 12:07 PM
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Since the engine is stock I suggest you don`t port the heads at all. If you had experience in porting them first hand and know where to port them that`s different. When porting a head it`s easier to screw it up than it is to get it correct when you don`t know where to port or how to shape them or how much metal to remove. The simplest way to give any 305 more power is swap in a 350.
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Old 11-11-2009, 12:23 PM
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but is possible to put 350 heads on 305 engine? or combustion chambers are different about diamater?
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Old 11-11-2009, 12:49 PM
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350 heads on a 305 would cut the compression ratio down to about the 6:1 range and the engine wouldn`t have enough power to spin on mud.
The biggest issue with a 305 is it has a very small bore size and a long stroke. The 305 and 350 share the same stroke depth of 3.480. However, a 305 has a bore size of 3.736 and a 350 has a bore of 4.000 inches and the larger bore is what gives the 350 it`s extra 45 cubic inches. You can do things to a 305 to give it more power but unless your in a racing class restricted by cubic inches or your shooting for good street fuel economy then there`s no reason not to use a 350. I never understood why GM ever put 305`s in 4x4`s to start with.
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Old 11-11-2009, 01:04 PM
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Another thing to point out is that you can't put new lifters on an old cam. You'll wipe the cam in about 3 minutes. New cam is a must if you're doing new lifters.
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Old 11-11-2009, 03:03 PM
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so guys,,, i think that best thing are:

- open intake manifold ok?
- change lifters and cam, but if replace them i must replace pushing rod, roller arms and everything other?
- if i change cam maybe it's a good thing to change timing chain!?
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Old 11-11-2009, 05:07 PM
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Quote:
Originally Posted by Cerry
so guys,,, i think that best thing are:

- open intake manifold ok?
- change lifters and cam, but if replace them i must replace pushing rod, roller arms and everything other?
- if i change cam maybe it's a good thing to change timing chain!?
The reason for replacing lifters should drive whether or not the cam needs to be replaced. If the lifters are internally worn out to where they no longer maintain zero lash and therefore click; then they can be replaced on the old cam. If the cam has lost lobes in part or completely, the cam and lifters should be replaced as a set. The problem you could have is not knowing what condition the cam is in. Obviously, really worn lobe and lifter combinations will show as lost lift at the valve, but cams that are pitted and lifter's that are slightly worn may not yet show a measurable loss of lift at the valve, but while everything is apart it would be cost and time foolish to reuse such parts as they would have to be addressed again somewhere in the near future.

The push rods and rocker arms don't have to be replaced if they are in good condition. For a high mileage street engine the major wear on the rocker will be on the end rubbing the valve stem. These ends are case hardened, so while it's possible to dress the end to a nice finish, once the hard case surface has been worn thru, the wear continues at a high rate afterward which reduces the time these parts will continue to function.

When disassembling the heads, it is likely the valve stem tips have been burred by the wearing process and can't be driven thru their guides without damaging the guides. So before taking a hammer to the valve stems, take the time to dress the edges of the tip with a file or die grinder with a fine stone till the stem will freely drop thru the guide. The guides are probably worn, this needs to be checked by a competent machinist and the proper clearances restored. Typically this is done with an insert of hard brass alloy, boring to an oversize and replacing all the valves with those having oversized stems, or by knurling the guide and reaming them to a proper bore size. The latter process is the least expensive but also the shortest lived. The valve springs need to be checked to see if they are up to spec on lengths and pressures. For a street rebuild, any that are a little weak can be shimmed. A Chevy this old will have rubber umbrella seals or steel shields and an O ring to direct oil away from the top of the guide. These rubber parts need to be renewed and, if equipped, the steel shields should remain. Removing these parts will greatly increase oil consumption down the guides unless the guides are modified for hard plastic seals.

You asked about porting your 305 heads. Yes this will pick up some power and a mild port job will make the engine react as if it has more cam without the negatives of a faster idle, less manifold vacuum and stuff like that. The 305 heads respond nicely to a mild pocket port, but there isn't a lot of material in the walls so you want to be conservative. For the intake mating surface to the throat of the turn, I'd recommend just dressing off the cast surface and truing up the dimensions of any casting globs that change the intended shape/dimension of the port. The goal is to improve swirl and overall flow, most of this comes in the pocket. To start with on the intake side you want most of the flow to be on the port wall adjacent to the cylinder wall. The major flow then scoots across the wall behind the guide toward the valve side that opens toward the spark-plug (center of the cylinder). But there's a big chunk of valve guide in the way of putting the major air flow on the cylinder wall adjacent port wall which we'll call the outside wall. So your porting efforts need to concentrate on streamlining the guide, reducing it's overall size, and increasing the width between the guide and the outside wall which will come mostly at the expense of the guide's bulk as the wall isn't too thick and shouldn't be cut deeper than the paste line that shows the boundry joint between the upper and lower casting boxes (cope and drag). The exhaust port is cut in the same way but with anticipation that the flow will enter from the center of the cylinder and proceed toward the outside and top walls. The guide should be streamlined as much as possible, the space between the guide and the outside wall widened as much as as safe. The pocket side of both the intake and exhaust seat should exhibit a small venturi shape opening the seat smoothly to a larger diameter of the port walls. This is about as crazy as you should get without a flow bench, these contours deliver a pretty good flow increase, beyond this one needs to work very carefully for the returns and it is easy to make a port having a lot less flow than the original as cast port. Once this is done you can consider bigger valves, the 305 will accept 1.9 and 1.5 350 valves.

I would not recommend going away from the 180 degree two plane intake. This enhances torque and that is what the 305 is all about.

Yes while you're there most definitely replace the timing chain and gears. Keep in mind that the SBC when setting up the timing gears with both gear dots aimed at each other, the engine is ready to fire number 6. You can leave it like that as long as you set the distributor to fire #6 or before you close the timing case, rotate the engine till each gear's timing dot is in the straight up position, then it's set to fire number 1.

Finish this off with a set of long tube headers and duals.

Bogie
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Old 11-11-2009, 05:11 PM
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I see now your in North Italy, so this may make getting ahold of other small block pieces somewhat harder, I`m not sure. However, if you can get pieces for it, there are some things you can do to help the 305 greatly, some of the things that Bogie suggested on the previous post as well as other things such as how to tune it up properly and what size cam to use that will result in the greatest benefit. But first I ask, how many miles are on your engine? This will give us a starting point on where to start.
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Old 11-14-2009, 08:39 AM
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thanks for suggests guys!
it's hard to know how many km the engine has.
but i think not more of 200.000 km.

my mechanic checked out the engine 1 years ago and he said me that engine runs good, just the problem about lifter.

the initial problem concerned the exhaust gas recirculation, engine seemed to go with half the cylinders.
we removed all parts excluding gas recirculation and engine was fine.

actualy engine runs good but i know surely it could be better!
i replaced old distributor with new one and i think that engine has gained 20 hp than before, alternator, plugs and wire plugs, filter, new Q carb with new rochester carb...everything as it was originally.

i don't want performance parts, I wish my truck was fine as 30 years ago.

Fabio
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