Converting from 7/16" head bolts to 1/2"

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Written by AMC258

Please note this is not something everyone can do or has the equipment to do it with, but a machine shop can.

This modification is done to use a 1980 - 1975? heads on a 1981 - 1987 258 Block as well as something to mount the stamped metal valve cover to.

Before reading, note that this is not by any means the only way to tackle this job, just the best.

Step 1: Disassemble the engine. Clean and stone all surfaces that represent the lowest point of the block, and remove any loose dirt from the block.

Step 2:

Mount the block on your turret mill. In my best Stacey David voice: "What? You don't have a turret mill? Whell, you'll just have to go get yourself one!"

Step 3:

Put some clamps in place. Yes, do use them. The block is twisted because it's in a relaxed state. You can twist it back to square either by bolting the head back on or by clamping it down. This is where I put mine (on cylindery #1 and the rear of the block):

Step 4:

Ever notice those machined pads on the top of the driver's side of the block? Don't ever wreck them, they are your friend. Clean and stone them, but do NOT take any material off. The whole purpose of them is to align the block for machining.

Step 5:

Make sure you are going to be drilling square to the deck. This is easiest to check around a cylinder.


Step 6:

Redrill the bolt holes with a 27/64" drill. 500RPM and 3.5IPM works good. Note that on most Series I Bridgeport turret mills, you will have to do this in two setups as there is not enough X travel to do it in one. Fortunately, it's a turret mill, so all you have to do is reposition the turret.

Step 7: Tap the holes with a 1/2"-13 tap.

Done. Note the 1/2" SCHS.