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Old 02-24-2005, 09:01 AM
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Post Defective Head/Manifold Studs

This posting covers what to do in the event you discover that the threads in one (or several) head or manifold stud holes are less than perfect, or clearly defective.

Itís obviously very important to thoroughly inspect threads so as to discover problems prior to reassembly. In the case of stud holes in which the threads are observed to be affected by corrosion, damaged by over torquing, or simply exhibit a slightly "sloppy" feel when dry fitted with a new stud, we recommend sealing the bottom threads with JB Weld, and (in the case of head studs) backing off on the torque value a bit.

NOTE: While Universal was still in business, their technical folks told us that they had considerable test data which showed that 30 foot-pounds of torque on head studs would still retain a good safety margin. For this reason, we recommend reducing the torque value on head nuts to between 30 and 32 foot-pounds whenever there is any concern over the condition of the threads in the block. When backing off on the head stud torque value, it's best to use the same torque on all studs.

There are several methods of repairing threads in stud holes which are deemed to be too far gone to risk reassembly, or those that may have failed to hold torque after assembly:

1) During the course of an overhaul (with the engine disassembled), you can re-tap defective stud holes to 1/2" by 13 (coarse threads), install a thread repair bushing. These solid repair bushings have the advantage of returning a block to an "original" look, but they require a bit more work to install, and they can only be installed while the engine is disassembled.

NOTE: See below for detailed instructions to assist in installing these bushings. These instructions will also work for installing repair bushings in defective water jacket side plate holes, although in most cases our stainless water jacket repair kits would be a better choice to repair defective water jacket side plate holes.

2) You can re-tap defective 3/8" threads in the block to 7/16" coarse threads and install a 7/16" repair stud. These repair studs cost a bit more than repair bushings, but their advantage is that they are easier to install and can be installed through the head, even after an engine is fully assembled.

Our 7/16 repair studs are machined to 3/8" fine threads on the upper end of the studs, so the original torque values will still be appropriate. If the 3/8" threads are completely wiped out, you can run a 7/16" tap through the hole without drilling a pilot hole in the block. If the 3/8" threads are still in relatively good condition, it's usually best to drill the hole out to 3/8" before tapping. After tapping the hole in the block, it's best to work a 7/16" drill through the hole in the head before actually installing the 7/16" stud.

3) In case a stud pulls out after the engine is reassembled, a 7/16" repair stud can be installed without removing the head, since a 7/16" tap is only slightly larger than the original holes in the head.

NOTE: Whether a 7/16" repair stud is being installed during a rebuild or afterward (with the head installed), the original holes in the head serve as good pilot holes to keep a 7/16" tap well centered. If the engine is disassembled, the head can be temporarily held in place by installing two or three studs in holes that are not being repaired.

4) Helicoils can be used in place of solid repair bushings; however, it is somewhat more difficult to seal the "coil" design of Helicoils to prevent water from the cooling jacket from working up past the threads of the Helicoil.

INSTRUCTIONS FOR INSTALLING REPAIR BUSHINGS:

1) Drill out the failed hole(s) to 7/16", then tap for 1/2" X 13 (coarse) threads. It's critically important to keep the tap straight as you work. A small carpenter square is helpful. It's usually possible to "adjust" the tap as you go, in the event that you discover that it's leaning a bit one way or the other.

2) Thread the bushing onto the end of a new stud, and dry run it into the hole a few times. You can double-nut the stud to make it easier to turn it in, but be very careful to not get the bushing stuck in the hole at this point. If it gets tight at all, go back in with the 7/16" drill and "worry" the hole a bit larger by wobbling the drill a small amount. Continue to check the bushing for fit, and when you can install it almost all the way in using your fingers, you're ready for the next step.

NOTE: It's good if repair bushings end up allowing a slight bit of movement, so that if the stud ends up being less than perfectly straight, the head or manifold can bring it into alignment (see step 7).

3) Spread JB Weld around the outside of the bushing, and around the inside of the 1/2" threaded hole in the block.

4) Turn the bushing in using a wrench on the double-nuts until only the small round shoulder is exposed outside the hole.

5) The bushings have 4 small slides around the outside which get tapped in between the bushing and the block after the bushing is finally set in place to keep it from turning. Don't worry if one (or even two) of these slides break off. Any two slides should hold the bushing in place while you remove the stud.

6) Grind or file the top of the bushing flush with the surface of the block. If you can get your hands on a small handheld grinder, it makes this part of the job much easier.

7) After grinding the top of each repair bushing level with the surface of the block, and before the JB Weld has had a chance to set up, it's best to install four or five studs (including those in repair bushings), so that you can dry fit the head or manifold over the studs. It's good if repair bushings allow a small amount of movement, so that if the stud is less than perfectly straight, the head or manifold can bring it into alignment.

8) Hand-tighten nuts on the studs used to dry fit the head until the JB Weld has had a chance to fully cure (usually overnight).

Best regards,

Don Moyer
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