Cap It Off — The Main Things You Need to Know About Main Caps

When people start the process of building an engine, they get enthusiastic about exotic connecting rod materials or piston styles. These are sexy choices to discuss and debate where other things — like main caps — aren’t quite as exitinging of a conversation. The reality is, main caps are immensely important in the ability of an engine to survive high-horsepower and high-rev situations.

When a main cap starts to fail, the crank begins to walk away from the bearings, oil pressure goes away, and things go BOOM! There are aftermarket options to replace stock main caps and processes for correctly installing these improved main caps, so your engine keeps on rotating. A rotating engine is a good thing, where one with the inside parts on the outside is a bad thing. Decent aftermarket main caps can keep bad things from happening.

Milodon, known for its oil pans, also manufactures high-performance aftermarket main caps to strengthen the bottom end of engines. They have upgraded main caps for small-block Chevrolets, big-block Chevrolets, Fords, Chryslers, Pontiacs, Oldsmobiles and Holdens (from Australia).

Billet Four-Bolt Mains

Stock main caps can walk and elongate under heavy loading. That, in turn, allows dimensions to change, which can permit a bearing to spin, causing devastating engine failure. An aftermarket four-bolt main cap can keep this from occurring, both through upgraded material and design.

Ken Sink of Milodon provides some insight into why aftermarket main caps are an advantage when building a high-horsepower engine. “Our main caps are made with forged material which provides more cross-sectional strength,” says Sink. “But, the real advantage of our four-bolt main cap is the angled side bolts. These bolts pull from a stronger portion of the block, and a bolt that is angled will require a massive amount of additional load to flex or stretch, let alone break. These Milodon four-bolt main caps are really strong.”

Sink further expanded on the material Milodon uses to build main caps. “We use a ductile material that will expand at the same rate as the engine block as things heat up. Milodon oil pans do a great job of holding in oil, but they don’t do very good at holding connecting rods inside the engine.”

Milodon’s aftermarket four-bolt main caps with angled (splayed) side bolts provide the extra strength needed for a big-horsepower build. They work with standard main bolts (left) or upgraded main studs (right) for even more power-holding capability.

The reason a four-bolt main cap is essential on some blocks is because in the 1970s, manufacturers were so concerned with weight and fuel efficiency that they lightened and windowed particular portions of engine blocks. Those changes made them weaker in those areas.

Unfortunately, those lightened areas in the block happen to be right where some four-bolt mains with non-angled bolts would be bolted-in. Adding a four-bolt main caps with angled bolts to these blocks can help make the entire block stronger.

Milodon’s four-bolt main caps can be installed into two-bolt-main engine blocks with simple machining. The side bolts can be installed by anyone with a hand drill and a tap. To install the Milodon “three cap kit” into a two-bolt main block, you simply use the included drill bushing to drill and tap the angled side bolt holes.

Sink suggests simply using tape on the drill bit to ensure the correct drilling depth. After drilling, tap the block, clean the threads, and install the main caps. According to Sink, Milodon recommends an align-bore once the aftermarket main caps are installed.

Completing an accurate align-bore requires the correct tooling and should be handed off to a reputable machine shop to get the job done correctly.

Post-Installation Machining – More Than Meets the Eye

With Milodon calling for a complete align-bore after installing a set of aftermarket main caps, we decided to speak with engine builder Rich Olivier of TEM Performance Machine Shop to discuss what his shop does when installing a set of aftermarket main caps.

“Our shop supports using good aftermarket main caps for high-horsepower builds,” says Olivier. “There are significant advantages in main-housing stability, clamp force, block strength, heat transfer, main-housing concentricity, crank stability, reduction in harmonics, increase in oil pressure, and cylinder concentricity. All of these things equate to horsepower.”

According to Olivier, there is a lot of machining that goes into setting up aftermarket main caps, but the first step is selecting a quality cap like Milodon. The material and design are critical when asking a main cap to live under extreme performance. As a machinist and engine builder, Olivier says a significant factor in cap selection is repeatability.

When inspecting a cap, whether it’s three, five, or seven caps, all the caps must be the same. It makes for a long day when your engine builder opens the box of seven caps for a 1,000-horsepower engine and finds seven different sizes and shapes of main caps.

Not all engines are created equally. These blocks utilize a total of seven main caps that all need to be machined perfectly and align bored together.

Before installing a set of main caps, TEM Performance Machine Shop starts with carefully measuring the block. They want to know whether there is any core shift that will need to be corrected in the machining process. All procedures are calculated off the crankshaft centerline.

This is why quality planning and work on the main bore is critical. Prior to drilling the block for the additional splayed holes, the machinist plots out the crankshaft centerline and squares the block in the mill. The additional fasteners are just one part of aftermarket caps’ increased stability; the other is how the cap registers in the block.

There is a recess in the main saddles which positions the cap left to right. TEM machines that cap register so the cap fits tight into the block. With the cap tight in the register, then they are ready to mill, drill, and tap for the studs. Olivier always recommends main studs. This advice was echoed by Sink at Milodon, who sells main cap stud kits to complement its caps. The clamping force between a stud and a bolt is significantly different, and studs won’t twist within the block.

Like with cylinder heads, studs — instead of bolts — are recommended for mounting main caps on high-performance engines.

Alignment is Key

Once all of the caps are secured and torqued on the engine block, then they are ready to “align bore.” The definition of bore is “to make a hole in something, especially with a revolving tool.” Although this is a standard procedure in the industry, there are some often overlooked faults in this process.

Boring is relatively accurate, but to maintain accuracy and concentricity, the operator has to keep the boring bar or tool bit under a constant load. Not only is the tool cutting 180 degrees through the main cap (which can be cast iron or billet), but it is also traveling the other 180 degrees through a cast-iron or aluminum engine block. This interrupted cut not only causes tool deflection, but the deflection causes the tool to cut differently upon material change.

Another overlooked detail is, by keeping the tooling loaded in the bore there is material being removed from the “upper” block portion of the main bore as well as the main cap. This ultimately brings the crankshaft closer to the camshaft. Decreasing the camshaft to crankshaft centers can disrupt things.

The preference at TEM Performance Machine Shop is to calculate the material to be removed from the aftermarket main cap. Then, machine each cap individually (off the block). This is a much longer process, but they can achieve their desired main-housing diameter within a few ten-thousandths of an inch. Then, they align hone out to the final size. With the honing procedure, they can control within millionths of an inch if needed — all without altering the cam to crank centerlines.

For proper crankshaft alignment, and to allow each bearing to carry an even share of the weight, it is important that all of the main caps are machined accurately and the tolerances are correct.

Material Matters

Milodon’s Sink prefers using cast-iron main caps for the similar thermal expansion properties as the engine block. Olivier at TEM Performance prefers using a billet-steel main cap. Regardless of the type of material used, both agree that aftermarket main caps are a must for a performance engine.

With a lot of aftermarket support for main caps for older engines, We asked both Sink and Olivier if today’s modern engines could benefit from an aftermarket main cap. “Absolutely. We, at Milodon, just haven’t manufactured them yet,” says Sink. Olivier agreed that new engines could use improvements in main caps.

“Coyote, LS, and Hemi Gen-III engines most definitely need aftermarket pieces,” Olivier says. “Fortunately, the newer engines are utilizing much better designs in the cap orientation and fasteners. Unfortunately, the material in caps and procedures in manufacturing leaves a lot of room for improvement.”

While they might seem like relatively simple pieces of steel, aftermarket main caps are actually a complicated component. Besides having to be immensely strong, a main cap has to be precisely sized in several different dimensions to do its job effectively.

The bottom line is this: the bottom end of an engine needs to be strong for a high-performance powerplant. A good set of aftermarket main caps can add this strength and reliability to the block. With big power-adders and 1,000-plus-horsepower engines becoming more and more of the norm, utilizing well-manufactured, correctly machined, and properly installed main caps can ensure your inside parts stay on the inside under any operating conditions.

Article Sources

About the author

Rob Krider

Rob Krider will race absolutely anything. He is a multi-national champion racing driver and is also the author of the novel, Cadet Blues.
Read My Articles

Hardcore Chevys in your Inbox.

Build your own custom newsletter with the content you love from Chevy Hardcore, directly to your inbox, absolutely FREE!

Free WordPress Themes

We will safeguard your e-mail and only send content you request.

Chevy Hardcore - The #1 Performance Bowtie Online Magazine

We'll send you the most interesting Chevy Hardcore articles, news, car features, and videos every week.

Chevy Hardcore - The #1 Performance Bowtie Online Magazine


We will safeguard your e-mail and only send content you request.

Chevy Hardcore - The #1 Performance Bowtie Online Magazine

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

Hot Rods & Muscle Cars

Corvette Enthusiasts

Street Rods

Chevy Hardcore - The #1 Performance Bowtie Online Magazine

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

  • Hot Rods & Muscle Cars
  • Corvette Enthusiasts
  • Street Rods

Chevy Hardcore - The #1 Performance Bowtie Online Magazine

Thank you for your subscription.

Thank you for your subscription.

Chevy Hardcore - The #1 Performance Bowtie Online Magazine

Thank you for your subscription.

Thank you for your subscription.