Main caps locate the crankshaft bearings in the engine block. Their fasteners must create controlled clamp load while preserving the alignment and diameter of the main housing bores.
That is why a stronger fastener cannot simply be installed and forgotten.
Changing from factory main bolts to aftermarket studs can alter how the caps, block threads, and bearing tunnel respond to preload. The correct decision must be made with the engine builder and machine shop before final machining begins.
What Main Fasteners Actually Do
Main fasteners clamp the main caps to the block so the crankshaft bearings remain correctly located under engine load.
The joint must resist:
- Combustion loading
- Crankshaft bending and vibration
- Main-cap movement
- Thermal cycling
- Repeated fatigue loading
More clamp load is not automatically better. Excessive or uneven preload can change the shape of the housing bores, affect bearing crush, and alter oil clearance.
The goal is controlled, repeatable clamp load with correct block, cap, and bearing geometry.
Main Bolts and Main Studs at a Glance
| Factor | What It Means | What to Verify |
|---|---|---|
| Factory main bolts | Developed with the production block, caps, and factory tightening procedure | Whether the bolts are reusable, torque-to-yield, or mandatory replacement items |
| Aftermarket main studs | Thread into the block while separate nuts apply clamp load to the caps | Application approval, final preload procedure, serviceability, and resulting bore geometry |
| Torque value | Tool input used to create fastener preload | Lubricant, thread condition, washer surface, and specified sequence |
| Main-bore geometry | Controls bearing alignment and crankshaft support | Measure with the final caps, fasteners, lubricant, and preload procedure installed |
| Line honing | Corrects main-housing alignment and diameter when measurement shows it is required | The process must use the final fastener setup intended for assembly |
Why Main Studs Can Change Block Geometry
A stud-and-nut system does not load the joint in exactly the same way as a bolt tightened directly into the block.
Differences can include:
- Thread loading in the block
- Under-head versus nut-and-washer friction
- Fastener stretch
- Clamp-load distribution
- Final preload
Higher or more repeatable clamp load may be useful in a high-output engine, but it can also alter main-housing diameter or alignment.
The machine shop should therefore measure the block with the exact:
- Main caps
- Studs or bolts
- Washers and nuts
- Assembly lubricant
- Tightening sequence
- Torque or stretch procedure
Measuring with one fastener arrangement and assembling with another defeats the purpose of precision machining.
Factory Main Bolts Can Still Be the Correct Choice
A stock or moderately modified engine operating within the original design may be best served by the factory clamp strategy.
New OEM bolts may be appropriate when:
- The engine remains near factory output
- The block and caps are in good condition
- The factory procedure is well documented
- The machine shop is retaining factory main-bore geometry
- The engine builder does not require additional clamp-load capacity
Determine whether the original fasteners are torque-to-yield or otherwise classified as single-use.
Reusing a fastener that the manufacturer requires replacing can compromise preload and repeatability.
For EJ20 and EJ25 Subaru builds that will use a bolt-based main-cap strategy, ARP’s replacement block-bolt kit provides an application-specific option. The machine shop still needs to establish its final machining and tightening procedure around that exact hardware.
ARP Subaru WRX EJ20 & EJ25 Block Bolts
When Main Studs May Make Sense
Main studs are commonly considered when the engine builder needs greater or more repeatable main-cap clamp for a defined operating condition.
Potential examples include:
- High-cylinder-pressure builds
- High-output turbocharged or supercharged engines
- Competition engines with frequent disassembly
- Applications where the builder has validated a specific stud-and-machining combination
Studs do not add horsepower by themselves.
Their purpose is to support the main-cap joint under the intended load while maintaining correct bearing geometry.
ARP’s main stud kit for the 2003–2006 Evo is a common starting point for a performance 4G63 bottom end. It should be supplied to the machine shop before the main housing bores are finalized so the block is measured under the intended preload arrangement.
ARP 03-06 Mitsubishi Evo Main Stud Kit
The VR38 uses its own application-specific main stud package, reinforcing why the fastener system must be selected by engine family rather than treated as generic hardware. The builder should confirm the required machining, lubricant, and tightening process for the exact Nissan block.
Choose the Fasteners Before Machine Work
The engine builder and machine shop should choose the main-fastener system before final main-bore machining.
A reliable sequence is:
- Clean and inspect the block.
- Inspect the main caps and registers.
- Select the final bolts or studs.
- Install them using the specified lubricant and procedure.
- Torque the caps in the required sequence.
- Measure every main housing bore.
- Line hone only when measurement shows correction is required.
- Reclean the block and verify final bearing clearances.
Changing fasteners after line honing can alter the geometry that the machining operation just established.
Evo X builders retaining a bolt-based 4B11 arrangement can use ARP’s 4-bolt turbo main-bolt kit as the selected hardware for mockup and final measurement. The kit should be present before the machine shop establishes the housing-bore geometry.
ARP Mitsubishi 2.0L (4B11) 4-Bolt Turbo Main Bolt Kit
For a 4B11T build where the engine builder specifies studs instead, ARP also offers a dedicated main-stud kit. Moving between the bolt and stud options after machining is exactly the kind of late change this guide warns against.
ARP Mitsubishi 2.0L Turbo (4B11T) Main Stud Kit
Important: Do not substitute internet torque values. Fastener material, diameter, thread, lubricant, washer surface, and joint design determine the correct procedure.
Line Honing Is a Measurement-Driven Decision
Installing main studs does not mean every block automatically requires line honing.
It also does not mean line honing can be skipped safely.
The correct answer comes from metrology.
With the final fastener system installed and tightened correctly, the machine shop should inspect:
- Main-bore diameter
- Out-of-round condition
- Bore-to-bore alignment
- Cap register condition
- Bearing housing consistency
If the measurements remain within the required specification, corrective machining may not be necessary. If they do not, the machine shop can establish the correct geometry using the final assembly configuration.
Torque Is Only an Indirect Measure of Preload
A fastener acts like a spring. Tightening stretches it, and that stretch creates the clamp load holding the joint together.
Torque is only the tool input used to produce that stretch.
A substantial portion of applied torque is consumed by friction at:
- The threads
- The bolt head or nut
- The washer surface
That is why lubricant matters.
Using generic oil in place of the specified assembly lubricant can change the resulting preload even when the torque wrench reaches the same number.
Follow only the procedure supplied for the exact fastener and application.
Torque-to-Yield, Torque-Angle, and Stretch Procedures
Different fastener systems use different methods to control preload.
Torque-to-Yield Fasteners
Torque-to-yield bolts enter a controlled plastic range during tightening and are commonly treated as single-use components.
Torque-Angle Procedures
A torque-angle procedure begins with an initial torque and then adds a controlled amount of rotation. This can reduce some of the variation caused by friction.
Direct Stretch Measurement
Some performance fasteners are installed by measuring elongation directly because stretch relates more closely to preload.
Use only the method specified for the exact fastener and joint. Do not convert casually between torque, torque-angle, and stretch procedures.
Install Studs According to the Manufacturer’s Procedure
Main studs are not normally driven hard into blind holes unless the instructions explicitly require a particular installation method.
Verify:
- Block-thread cleanliness
- Stud engagement depth
- Whether the hole is blind or enters a fluid passage
- Sealant requirements
- Washer orientation
- Nut clearance
- Cap and girdle fitment
- Tightening sequence
If a stud bottoms in a blind hole before the cap is clamped correctly, the joint can produce misleading torque and potentially damage the block.
Thread Preparation Matters
Dirty or damaged threads change friction and can prevent complete engagement.
Machine shops clean and inspect block threads using a method appropriate for the application without unnecessarily removing material.
Blind holes must also be free of trapped liquid.
Oil, coolant, cleaning solvent, or assembly fluid trapped beneath a fastener can create hydraulic pressure as the stud or bolt enters the hole.
Check Girdle, Windage-Tray, and Oil-Pan Clearance
Main studs may extend farther above the caps than factory bolts.
That can create interference with:
- Main girdles
- Windage trays
- Oil-pump components
- Oil pickups
- Oil pans
- Internal baffles
Clearance should be confirmed during mockup—not after final cleaning and bearing installation.
Any modification to a tray, girdle, or pan should preserve its intended structure and oil-control function.
Main Studs Do Not Correct a Weak or Damaged Block
Stronger fasteners cannot repair:
- Out-of-round main bores
- Damaged cap registers
- Main-cap fretting
- Crankshaft misalignment
- Poor balance
- Incorrect bearing clearance
- Inadequate block stiffness
Main studs belong inside a measured bottom-end assembly process. They are not a substitute for block inspection, machining, crankshaft preparation, or oil-system planning.
Main-Cap Walk and Fretting
Under high load, movement between the main caps and block can leave fretting marks on the mating surfaces or disturb bearing alignment.
Fasteners are only one part of controlling that movement.
Other factors include:
- Cap material
- Register fit
- Girdle design
- Block stiffness
- Crankshaft balance
- Combustion pressure
- Engine speed
Evidence of fretting should be evaluated by the engine builder rather than covered up by installing stronger fasteners alone.
Verify Bearing Clearance After Final Machining
After the main bores are measured or corrected, the block must be cleaned thoroughly.
The builder should then verify bearing clearances with the final:
- Crankshaft
- Bearings
- Main caps
- Fasteners
- Lubricant procedure
- Torque sequence
Machining debris, thread contamination, or incorrect bearing placement can invalidate an otherwise accurate setup.
Which Fastener Strategy Fits the Build?
| Build Situation | Main Consideration | Best Next Step |
|---|---|---|
| Stock rebuild | Factory clamp strategy may remain appropriate | Use new OEM fasteners where required and follow factory machining data |
| High-cylinder-pressure build | Cap control and clamp-load repeatability become more important | Let the builder specify the stud and machining combination |
| Previously machined block | A fastener change may alter the established geometry | Measure the bores before changing the joint |
Common Main-Fastener Mistakes
- Installing studs after final line honing without remeasurement
- Driving studs tightly into blind holes
- Using generic oil instead of the specified lubricant
- Reusing torque-to-yield fasteners
- Applying a universal internet torque value
- Ignoring girdle or windage-tray clearance
- Skipping final bearing-clearance verification
- Assuming stronger fasteners correct block or cap damage
- Changing the fastener system after machining is complete
Questions to Ask the Engine Builder
- Were the main bores measured with these exact fasteners installed?
- What lubricant and tightening sequence are required?
- Are the factory bolts reusable or single-use?
- Do any fastener holes enter coolant or oil passages?
- Will the girdle, windage tray, pickup, or pan clear the studs?
- Was line honing required with the final preload setup?
- What bearing clearance remains after final machining and cleaning?
Frequently Asked Questions
Do Main Studs Add Horsepower?
No. Main studs are a fastener strategy used to control clamp load and support main-cap stability under the intended engine load.
Can Main Studs Be Installed Without Machining?
Sometimes measurement confirms that the main bores remain within specification. Other blocks require corrective machining.
The answer comes from measuring the block with the final fasteners installed—not from assumption.
Are Main Studs Always Better Than Factory Bolts?
No. Factory bolts may remain appropriate for a stock or moderate rebuild. Studs should be chosen when the engine builder has a defined need and can validate the resulting geometry.
Can I Install Studs After the Short Block Has Been Machined?
Changing the fastener system after final machining can alter main-bore geometry. The block should be remeasured before assembly continues.
Key Takeaways
| Key Takeaway | What It Means for the Build |
|---|---|
| Fasteners affect bore geometry | Measure the block with the exact bolts or studs intended for final assembly. |
| Choose before machining | Changing the main-fastener system after line honing can invalidate the established geometry. |
| Torque requires context | Lubricant, thread condition, washer surface, and sequence affect preload. |
| Line honing is measurement-driven | It is neither automatically required nor safely optional without bore inspection. |
| Studs do not repair the block | Cap registers, crankshaft condition, bearing clearance, and block stiffness still matter. |
| Final verification is essential | Recheck main bores, clearances, and surrounding-component fitment before assembly. |
Related JDC Guides
Choose main fasteners before machining, follow the exact lubricant and tightening procedure, and validate the finished assembly through measurement rather than assumption. Compare application-specific hardware and supporting parts in our engine components collection after the engine builder and machine shop have defined the final assembly strategy.
