drilling bbc heads for extra bolts

[scottmc1813]

anyone know what angle to drill bbc heads for extra bolts in valley

[redvictor]

I'm pretty sure it's 6 degrees.

[bill jones]

-here's a note from when I had to drill a pair of holes up into the underside of a pair of aluminum heads to fit a GM BBC Bowtie block.
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"drill & tap 2 headbolt holes into underside of each of the Dart heads.
---These bolts are special length @1.800” long and use two of the 10° angle bosses (per head) that are cast into the valley of the block.
---Holes are drilled up from underneath into the intake ports .600”deep but they do NOT go clear thru into the floor of the intake port".
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-for what it's worth I also did a 600ci SVO BBF block with a set of SVO prostock style heads--and they were also on a 10º angle.

[MadBill]

----Holes are drilled up from underneath into the intake ports .600”deep but they do NOT go clear thru into the floor of the intake port"....

FWIW, the holes in my 1970-vintage ZL1 aluminum heads do go right through into the ports. The ends of the bolts wind up a little recessed from the port surface, but per an older S/T thread re SB2.2 heads, the pockets do not appear to affect flow.

[bill jones]

-for sure I wouldn't attempt to drill & tap the bolt holes into the heads just using a head gasket for a center point for the drill.
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-if there is any issues at all with milling of the head or decking the block or having to move the heads and/or head gaskets forward or rear on the block to get them sitting over the bores just perfect---all that needs to be done first.

-an example is: on one set of heads I drilled and tapped I had to build special offset dowels to move the gasket one direction--and to move the head a different direction---to get everything sitting right over the 4.625" bore of the block.

-I made up a precise centering pilot to come up thru the bolt bosses to get a proper mark on the underside of the installed heads.

-those bolts (or studs) are virtually impossible to get a torque wrench onto down there in the valley---so coming up with a suitable fastener choice and then a torquing process was also part of my project.

-there really isn't much room inside the blocks bolt bosses to get very far off on the drilling and alignment of the fasteners.

-you should be able to stick a piece of bar stock into the bolt boss hole and use some sort of an angle finder on the deck to the bar stock angle to find the true angle---which may really be something slightly just to either side of a 10 degree mark.

[SchmidtMotorWorks]

I looked through all my notes and can't find anything right now, I know I have it somewhere. Problem is both the 6 degree and 10 degree numbers also sound familiar. Maybe both angles were used at different times.

Time to measure the block.

[MadBill]

...-those bolts (or studs) are virtually impossible to get a torque wrench onto down there in the valley---so coming up with a suitable fastener choice and then a torquing process was also part of my project....

For the aluminum block, GM calls for only 50-55 ft-lb. torque on the valley bolts*, so perhaps exact torque is less critical than for the rest. *AIR, the heads wont come off with studs in these holes, unless you leave a lot of clearance.