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Last Update 07/04/07



Everyone wants more power, right? Right! But, more importantly, whether it’s a stock machine or an all-out racer, all of us want an engine that’s going to last longer. Converting a 400 to a 455 is not a new development, although many myths and doubts have surrounded this modification for years. My success in perfecting this modification has made a lot of Pontiac gearheads very happy.

Once and for all, here are the facts. I have personally recorded eleven years of track results using these cut-down 455 (or 421/428) cranks in 400 blocks, and I have some terrific news: Not a single failure occurred during any of the tests. In fact, the last time I talked to the owner the very first crank we prepared this way is still doing nitrous-fed street duty in southern California. Durable? Indisputable. So why cut down a 455’s 3.25-inch mains to fit a 3-inch main 400 block? All questions will be answered in this article. First a little history.


When Smokey Yunick went to work for Pontiac Motor Division in 1959, he was asked to develope a four-cylinder engine for the upcoming Tempest line. These engines also had to win grueling NASCAR-type races to be deemed worthy of Monday morning sales.

Smokey developed the four-cylinder using a 389 crank with bob weights to make up for the missing piston and rod assemblies, and also used a 389 block cut in half. Once he was satisfied, Smokey sent the finished product up to Detroit from his home base in Florida. The engineers at PMD flogged the engine and sent it back to Smokey to test, with one change.

They found that the 180-degree four-cylinder developed terrible vibrations throughout its operating rpm range. To keep the bearings healthy, one of the factory engineers thought their new 3.250-inch diameter journal 421 crank would help.

So PMD sent a couple of new large-journal, four-inch stroke cranks down south for dyno testing. It wasn’t long before Smokey discovered that the larger journal diameter presented problems of its own. At the sustained high rpms the engines endured under racing conditions, the increased surface area of the larger bearing created more heat and, consequently, failed.

Smokey relayed this important information to corporate, but time was running out and the “factory cats”, as Smokey called them, wouldn’t build the cranks to his specs. The debate was so heated that Smokey threatened to quit.

Smokey had been asked to prepare the four-cylinder engine for the 24 Hours of Daytona. He built the first Pontiac “half a V8” race engine that would be driven by Paul Goldsmith, using 3-inch mains. That first four-cylinder ’61 Tempest, using a Smokey-designed “pillow” block to control the rope drive-shaft’s vibrations, won its class at Daytona in 1961 (Yes, if you were paying attention, that engine was half of a 421!). It also had four-bolt main caps.

Fireball Roberts’ legendary winning streak driving Yunick-prepared Pontiacs speaks for itself. All of Smokey’s 421 engines were fitted with 3-inch main journal diameter crankshafts. And, no, the fact that they were forged cranks did not give them an advantage over cast cranks. I’ll explain why later.

A few other engine builders have discovered this secret over the years and have reaped the rewards of longer bearing life. Knowledge of welding and proper crank grinding is the key to longevity. But before we tell you about that, let’s look at the other advantages so you can determine if this mod is for you.


This conversion eliminates problem blocks. Converting a 400 to a 455 can eliminate the overhearing situation that exists in some 455 blocks with very thin cylinder walls by substituting the usually thicker 400 block. Our shop has seen six 455 blocks in three years that sonic tested so thin that a sleeve was required to go. .030 over. We like to see a minimum of .125-inch wall thickness for high-compression or high-cylinder pressure engines. One early 455 block measured .085 at standard bore. That’s too thin to trust.

Now don’t panic and sell your 455 block to your best friend for 25 clams. This just means that the smart thing to do before rebuilding your 455 is to sonic test the cylinder walls for thickness in four locations radially, 4-inch down from the deck, and then again at 1.5 and 3 inches down. Just remember, there is no way to tell if you have a problem block unless you sonic the walls. There are no special part numbers or particular years that are better than others; you just have to check.

Less friction. As we already mentioned, Smokey Yunick discovered that the bearings in the 3-inch mains survived race conditions, and the 3.250inch diameter did not. How does this affect your engine? As far as longevity is concerned, below 6500 rpm we have not seen any advanced bearing wear problems in large-journal cranks as long as the clearance is correct.

The advantage lies in the fact that the smaller diameter journal has less surface area and thus creates less friction. Over a quarter-mile run, 3-inch mains realize 142 feet of less travel. Multiply that be the five mains in the block, and you’ve just eliminated a lot of friction.

If you are building an engine with the kind of airflow and camming that can make useful torque above 7000 rpm, the 3-inch mains should be considered mandatory.

Lighter mass. Removing material by cutting down the journals obviously reduces the weight of the crank. Although minimal when the removed weight is close to the center of the crank, this reduction in rotating mass coupled with the reduced friction of the smaller bearing allows the engine to rev up quicker. Not just higher in rpm, but it allows it to get to its potential quicker. Quicker is what we’re all trying to achieve. Eh?

We often cut down the rod journals from 2.250 inches to 2.100 on more serious engines (to be used with custom connecting rods). This allows us to further reduce friction, but it is not necessary for the conversion. With the main and rod journals cut down, we realize a 3-lb. weight savings. Cutting only the mains yields approximately a 1.25 lb. reduction.

Better oiling. All large journal Pontiac cranks are cross-drilled from the factory; small-diameter journal cranks are not. For the RPM’s most Pontiacs see, as a bonus a 3-inch main conversion provides better oiling. Does cutting the journal to the smaller diameter affect the oil feed hole’s alignment and ability to feed oil properly? No.

Increased strength. Another common question is, “Doesn’t removing .125-inch of material weaken the crank?” On the contrary, when the process is done correctly, the crank is does not loose any strength. It’s exactly the same as before.

Using a competent crank grinder, the corner fillet, or corner radius as it’s commonly called, can be doubled in size. This spreads the load over a greater area thus making the crank less vulnerable to damage.

This concept is easy to understand when you consider the function of a gusset in a roll bar. It supports the corner where the two points meet. Or you can think in terms of a glass cutter. If a very fine scratch is cut into the glass, the glass will break cleanly along that line. If the line cut is a large radius, the glass will shatter inconsistently. The correct accompanying diagram illustrates this.(Not the diagram the hapless Terry McGean approved for the magazine article in High Performance Pontiac without my knowledge. See his diagram. A machining impossibility.


We mentioned that we often cut the rod journals down also, and the same concept can be applied here as well, to further strengthen the crank and reduce friction.

Restores end play. Crank end play has been a sore spot for Pontiac engine builders for a long time. The problem is, if end play is excessive, there is almost nothing you can do to correct it accurately. You cannot force the fore or aft direction of the thrust bearing with a shim without sacrificing clearance on one side or the other, or possibly creating a bind.

Extreme amounts of wear will occur unless the front and back of the thrust are working properly, keeping the crank from moving too far fore and aft. Because we have to resize the thrust area, we have the opportunity to return it to factory specs. We will detail this procedure in the “How To Do The Job” section of this article.

Despite these advantages, there are still several areas of debate regarding this conversion. Let’s address them before detailing the procedure.


Have you always dreamed of owning a forged crank? Stop dreaming. Cast Pontiac cranks are 15 percent lighter than their forged cousins. The ductile cast-iron material of which modern cranks are made also absorbs a certain amount of vibration and so naturally minimizes harmonics.

Forgings are not structurally as rigid as a cast crank. The torsional property of a forging allows it to bend or flex from end to end more easily than a cast crank. Pontiac discovered the benefits of the ductile cast-iron crank in the early 1960s and immediately discontinued its 421 Super Duty forgings in favor of the cast Arma-Steel units for all 1963 Super Duty cars. The cast crank does not flex and it does not bend. Oh, sure, if you spin a bearing the heat generated can bend the crank, but of course this is abnormal use.

If you have an engine that is prone to detonation, a forged crank is for you. It will bend and flex a little farther before it cracks or breaks. But unless you’re running nitro-methane, or are having major tuning problems, a properly prepared cast crank will suffice for any horsepower level achievable with the Pontiac engine today. If you break a properly prepared cast crank, you have other problems.

In “the day,” Smokey Yunick was king of winning races with Pontiac power, and importantly the tireless innovator Mickey Thompson was the ultimate master of early Pontiac horsepower. Consider the nitro/alky mixes being huffed down the pipes of 32 intake runners for five miles at a time in the Challenger I. All of those magnificent race cars Mickey build used ductile cast-iron cranks.! Remember those great stroker crank kits M/T produced in the 60’s? All of them were ductile cast units. Forgings were never offered.


Another misconception that must be cleared up is what happens concerning the stroke when performing this mod. Cutting down the mains of the 421/428/455 crank has no effect on its original stroke, which remains the same as it was in the crank’s respective original block. When you install a larger stroke crank in a 400 block, you are not building a ‘stroker’ per se. (Although many guys just LOVE saying the word “stroker,) Rather, you are simply building the same engine on 3-inch mains. Let me explain.

A 400 has a standard bore of 4.120 inches. A 455 has a standard bore of 4.150 inches. By using a standard bore 455 piston in a .030-over 400 block, you end up with a standard bore 455 on 3-inch mains.

Similarly, a 421/428/455 share a 4.00 inch stroke. A 428 engine shares the 4.120-inch bore of a 400. You use a standard bore 428 piston when using a 421/428 crank in a 400 block, so you can end up with the proper deck height, just as you do when using the 455 piston on a 455 crank.

I haven’t singled out the 421 engine because most guys use the 1967 or later 14-degree valve angle cylinder head. If you’re going to use a 1966 or earlier set of heads with the 20-degree valve-angle heads, you must have custom pistons built. I can do that for you.


The key to performing this modification successfully is knowing how to weld up the thrust area correctly and then reposition it. Educated welders will tell you that welding on cast material is structurally sound and presents absolutely no problems. You just have to know how much heat and what type of wire us use. Experience on the part of the welder and the machinist is a must.

The thrust on a 326/350/389/400 (3-inch mains) main cap is .060-inch narrower than that of a 421/428/455 (3.25-inch mains). The thrust area of the larger cranks must be built up .030-inch on each side to make up the difference. Doing the procedure this way allows you to use a standard available bearing, Clevite 77 part No. MS-483G standard. Your crank grinder must center the position of the new thrust area accurately within its old confines.

We mentioned the restoration of end play earlier. To achieve “a new” end play, your crank grinder will have to measure the width of the new thrust bearing from the Clevite 77 set (No. MS-483G) and finish-grind the thrust area .004-inch wider than the bearing. This will place end play on the small side of factory spec.

The rear seal area must also be resized. Before offering the service to the public, I measured 15 cranks on the rear seal area and decided to spec this area .001-inch larger than the largest measurement we encountered. None of our cranks has ever leaked in this area when properly installing the rear seal. I would rather lose one horsepower (from the additional drag) than gain a quart of oil on my driveway. This flange diameter can be measured and copied from any 400 crank.

Finally, main journals one, two and three need to be turned down to approximately 3-inches. Approximately, you say? When you blueprint properly for rod and main bearing clearances, final crank pin diameters will vary. Space limitations prevent us from detailing the blueprinting procedure her, just go to the “Rod and Main clearances” topic in the FAQ list.

Block preparation consists of roughly five minutes of grinding in one small spot (see photo). The front counterweight will contact the block on the driver’s side if this area is not clearance. You also may find enough casting flash back by the No. 8 cylinder that will need to be removed.

Once the block is clearanced, take your modified crank and install it with the bearings you’re going to use, then check for interference. Once you’ve verified clearances, and you’ve balanced your assembly, the conversion is complete. You now have big cubes with small journals. Proceed with the rest of your engine buildup and enjoy the benefits. If you need help I will be happy to profile your combination for you.

I sell everything you need to build a modern monster that my so called “competition” is just now attempting to copy.