Re-drilling 72 degree angle sbc heads.

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Re-drilling 72 degree angle sbc heads.

Post by rfoll »

I have modified intakes to fit the late 80's heads, and although it worked, I don't like the idea of carving up a nice manifold. I have heard of plugging and re-drilling the heads and it seems easy enough. My thought currently is to carve up an old manifold to manufacture a drill guide. I have a set of transfer punches to get me started in the correct center, and my drill press has a compound angle table to get me to the proper angle. Any thoughts on how to do this, or even what not to do?. Thanks, Rick.
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Re: Re-drilling 72 degree angle sbc heads.

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rfoll wrote: Sun Oct 13, 2019 12:26 pm I have modified intakes to fit the late 80's heads, and although it worked, I don't like the idea of carving up a nice manifold. I have heard of plugging and re-drilling the heads and it seems easy enough. My thought currently is to carve up an old manifold to manufacture a drill guide. I have a set of transfer punches to get me started in the correct center, and my drill press has a compound angle table to get me to the proper angle. Any thoughts on how to do this, or even what not to do?. Thanks, Rick.
I did that back in the early 90's when we got those engines in the yard every week. the carb'd 350's were still really popular but the issue was the only intake came from a monte carlo and at the time was $480 from GM for the rochester 4bbl carb applications for 1-2 years only during the pre TB injection being introduced.

I made a drill jig and bushing from an early head. Transfer punching using a cut up intake would work accurately enough.
Use a intake gasket and drill it to bolt through 3 holes that way you can just flip it to do the other side of the head.

Take a piece of 4130 or 4340 bar stock and drill it 5/16 which is tap size for 3/8 NC and make it about 3" long.
Now take a piece of 1/4" X2" cold rolled flat bar and drill the holes from the 2nd bolt holes from each end of the head.
Thene drill a single hole for the bolt on 1 side or the other beside the exhaust cross over port.
Bolt the plate to a dummy head then bolt the bar stock through the head using a long 5/16" nut and bolts then weld the
bar in place.

Now plug the holes in the new head and bolt the jig on and drill 1 hole flip and repeat till they are done.
We had 100's of engines to do over that 15 year time to sell them as a new factory engine with under 2000k on them otherwise they would sit on the shelf for years till the later engines started to wear out.
I even made a aluminum chip shield/tray deal that fit to the engines to seal off the inside so it could be done without removing the heads.

Other then that just get the right intake that has the angle bushings for the later heads.
Goodson and silver seal sells a drill jig to drill vortec heads to use early intakes as well.
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Re: Re-drilling 72 degree angle sbc heads.

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What were you using to plug the 73 degree holes?
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Re: Re-drilling 72 degree angle sbc heads.

Post by Schurkey »

Curious: Why re-drill those heads? Are any of them worth the effort? The TBI heads with that bolt angle are pathetic.
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Re: Re-drilling 72 degree angle sbc heads.

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Pathetic describes the 76 cc smog heads they would replace. I have found several sets over the years that required nothing more than a quick cleanup. The set I have on the bench currently are 14096217, hardly a pathetic head for a street engine.
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Re: Re-drilling 72 degree angle sbc heads.

Post by Carnut1 »

I believe the 217 is about the same as 083 casting. They cam be made to work with some porting. Very good street performance head.
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Re: Re-drilling 72 degree angle sbc heads.

Post by prairiehotrodder »

re-drilling vortec heads for an earlier intake would probably be a popular move.
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Re: Re-drilling 72 degree angle sbc heads.

Post by novadude »

rfoll wrote: Tue Oct 15, 2019 9:38 am Pathetic describes the 76 cc smog heads they would replace. I have found several sets over the years that required nothing more than a quick cleanup. The set I have on the bench currently are 14096217, hardly a pathetic head for a street engine.
Interesting thing about the '217's.....

I bought a set that was from a ~1996-ish GMPP 300hp 350 crate engine (the predecessor to the 330hp 350 HO with Vortec heads). Because they were from a crate engine and not a production TPI car, they have the old 1955-1986 intake manifold bolt hole arrangement with 90 deg center holes. They are still bolted to a 327 on a cart in my garage.

Doesn't help the OP at all, but since we were on the subject, I wanted to let people know that they DID actually produce these '217's with a 'normal' pattern. :D
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Re: Re-drilling 72 degree angle sbc heads.

Post by midnightbluS10 »

prairiehotrodder wrote: Tue Oct 15, 2019 10:53 am re-drilling vortec heads for an earlier intake would probably be a popular move.
It was popular among the GM turbo 4.3L V6 crowd for quite a while. Since they have a specific intake, they usually redrilled the heads to fit the modified pre-vortec intake. You can see an example of what's done to the intake here

I imagine it's the same thing to fit a pre-vortec V8 intake on vortec heads.

They also have posts detailing the steps involved in drilling and plugging the vortec heads.
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Re: Re-drilling 72 degree angle sbc heads.

Post by Schurkey »

I thought the Vortec heads were so thin on the intake face that drilling for the old 6-bolt pattern didn't leave enough material to have strength for the threads.
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Re: Re-drilling 72 degree angle sbc heads.

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rfoll wrote: Mon Oct 14, 2019 1:54 pm What were you using to plug the 73 degree holes?
I used cast iron plugs I ordered but the real issue is why??
There are sever intakes that are available dirt cheap that will work just fine. I used them for marine engines
for years and they were better then the option of re-drilling for a different intake pattern.

We had 100's of them in our wrecking yard and over the course of a couple years the monte carlo intakes became too
costly to buy to sell an $1800 engine with the re+re time and gaskets etc.
I just came up with a way to do it so we could sell the basicly new engines to guys for projects that were carbureted.
And everyone loved the roller cam engines we could not keep them on the shelf
We had tons of the older Q jet intakes on the shelf but I could drill plug and redrill and tap the holes in an hour and the cost was about $30 for materials. Buying an intake from GM at $480 plus gaskets and shop time just added to the cost.
I have not done this mod for about 15 years now as the after market came up with an intake to save the time of doing it.

Its an easy thing to do after you make a drill jig which is the hard part to get right. it took a few hours to get it right
but I used it on 100's of other engines so that cost was absorbed to the shop. But for a 1 off deal I would just get an intake.

If you don't want it to look stock then just slot the center holes and grind a flat for the bolt to seal on. Been there
done that as well. its not as pretty but its affective.
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Re: Re-drilling 72 degree angle sbc heads.

Post by midnightbluS10 »

Schurkey wrote: Tue Oct 15, 2019 2:27 pm I thought the Vortec heads were so thin on the intake face that drilling for the old 6-bolt pattern didn't leave enough material to have strength for the threads.
It may be for the V8 heads. I have no idea about those. I know I never heard/read any complaints about that specific issue with the V6 heads. I can ask a few of the guys with older swaps to be sure, though it wouldn't make a difference in this application. Being that it's only 4 bolts per side that are all 5/16" and the fact that they're not torqued down very much, it may not need much in the way of threading. That's about the only thing I could come up with.


You can see what's involved in drilling and plugging them at this website
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Re: Re-drilling 72 degree angle sbc heads.

Post by rfoll »

ProPower engines wrote: Tue Oct 15, 2019 3:13 pm
rfoll wrote: Mon Oct 14, 2019 1:54 pm What were you using to plug the 73 degree holes?
I used cast iron plugs I ordered but the real issue is why??
There are sever intakes that are available dirt cheap that will work just fine. I used them for marine engines
for years and they were better then the option of re-drilling for a different intake pattern.

We had 100's of them in our wrecking yard and over the course of a couple years the monte carlo intakes became too
costly to buy to sell an $1800 engine with the re+re time and gaskets etc.
I just came up with a way to do it so we could sell the basicly new engines to guys for projects that were carbureted.
And everyone loved the roller cam engines we could not keep them on the shelf
We had tons of the older Q jet intakes on the shelf but I could drill plug and redrill and tap the holes in an hour and the cost was about $30 for materials. Buying an intake from GM at $480 plus gaskets and shop time just added to the cost.
I have not done this mod for about 15 years now as the after market came up with an intake to save the time of doing it.

Its an easy thing to do after you make a drill jig which is the hard part to get right. it took a few hours to get it right
but I used it on 100's of other engines so that cost was absorbed to the shop. But for a 1 off deal I would just get an intake.

If you don't want it to look stock then just slot the center holes and grind a flat for the bolt to seal on. Been there
done that as well. its not as pretty but its affective.
I have many sets of old style intakes to choose from, some of which were never offer for the 72 degree pattern. When I use Vortec heads, I just use a Vortec intake. In that instance the appropriate intake will almost always work better.
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Re: Re-drilling 72 degree angle sbc heads.

Post by F-BIRD'88 »

prairiehotrodder wrote: Tue Oct 15, 2019 10:53 am re-drilling vortec heads for an earlier intake would probably be a popular move.
Except its a bad idea. The ports don't line up. And the result is lame.
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Re: Re-drilling 72 degree angle sbc heads.

Post by F-BIRD'88 »

Good to see others reconize the perf potential of the 217-083. 350 L-98 cast heads.
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