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Plugged In And Ready

Block Plug Basics

Bob Mehlhoff Mar 1, 2002

Step By Step

This kit for a 350ci small-block contained: eight 1-5/8-inch freeze plugs, three 1/2-inch expansion plugs (oil galley), one 23/32-inch expansion plug (cam plug), and five 1/4-inch NPT pipe plugs (oil galley).

At the front of this small-block are three 1/2-inch pipe plugs. Many engine-builders will tap and install 1/4-inch NPT pipe plugs in these three holes for sealing security. If these holes are tapped, it’s important to check for proper thread engagement.

For proper block-cleaning, every plug should be removed. For stubborn plugs, heat the plug (not the block) with a torch until it’s cherry red. Then with the torch removed, allow the plug to slightly cool down and squirt engine oil around it. The oil will penetrate the threads, allowing the plug to be removed easily. To install the new one, remember to add liquid thread sealant. Teflon tape should not be used because it can sometimes work loose and travel into the engine’s oil passages.

Did you know a 1/2-inch expansion plug is installed under the rear main cap?

Most small-blocks feature four 3/8-inch threaded holes (two per side) at the front of the block. The upper hole adjacent to the fuel pump passes into the crankcase area and if left open will allow oil to leak out from the block. Be certain to plug this hole only with a short 3/8-inch bolt (1/2-inch long). Because this hole aligns perfectly with the fuel pump’s pushrod, a longer bolt will contact and seize the pushrod during engine operation.

Freeze plugs are easily installed with a large hammer and the proper-size socket. In this case a 1-inch socket is used. Most freeze plug kits are available in brass or steel. Brass may cost more, but they don’t rust so they live longer. Remember to add sealant around the contact area. If you’re building a 400ci small-block, remember that many used two more freeze plugs (one extra per side) than other small-blocks.

Most factory plugs used a 1/4-inch square drive plug. Replacement plugs generally are 1/4-inch hex (left).

At the rear of this small-block Chevy, there are two freeze plugs, a camshaft plug, and three oil galley plugs.

For additional timing chain lubrication, some engine-builders drill one oil galley plug (0.030-0.035-inch) at the front of the block. This allows oil to spray onto the timing chain during engine operation.

After frequent rebuilds, the dowel pins may become worn and need to be replaced. On each cylinder bank, two dowel pins are press-fit into the deck surface and are used to locate each head.

At the rear of the block, two guide pins locate the bellhousing into position.

At the beginning of any complete engine buildup, the block needs to be cleaned. Serious engine-builders know that hot tanking, block machining, and a host of other engine-building tasks all require the block to be stripped first. To do that all of the block’s plugs must be removed (even with a new engine) to allow cleaning access to areas that allow oil or coolant to flow. But did you know that the typical Chevrolet V-8 contains not only a set of expansion plugs, but also a variety of other plugs in some hidden areas? Leave one of these small plugs out or install one improperly, and your new engine could fail during the first few minutes of break-in. To make sure that doesn’t happen, we’ve compiled some information showing just what to look for during your next block-rebuilding excursion.

The typical block-plug kit found in most auto parts stores is not complete for a thorough rebuild. It’s usually missing several ½-inch expansion plugs and ¼-inch NPT pipe plugs. Sometimes two kits will be sufficient to replace all the plugs in your block. Remember also that many 400 small-blocks had one additional freeze plug on each side, totaling 10 for the entire engine instead of 8.


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