Building the #ProjectBRZ FA20: Understanding Tolerances

RevvProjectBRZ Building the FA20 4U-GSE: Tolerances Cover Image

Article by Brian Hannon. Photography by Ryan Randels and Nathan Leach-Proffer. Select illustrations from Subaru BRZ Service Manual.

With a bit of a lull as we wait for a few parts to show up for Project BRZ, we wanted to give a little extra insight into some of the processes employed by our partners during the build of the car. In this case, we got some great details on what SCR Performance does during the teardown and rebuild of an engine, specifically in regard to their experience with the Subaru EJ-series engines, which heavily applies to the FA20 / 4U-GSE.

RevvProjectBRZ Building the FA20 4U-GSE: Tolerances, Pieces of the puzzle

Note, this is not a step-by-step guide for a rebuild; we aim to provide some insight into the processes to help educate the enthusiast to ensure their build has the chance to provide miles and miles of increased performance, reliability, and longevity. Over the next couple of weeks, we will be rolling out several more articles that highlight our specific FA20 / 4U-GSE rebuild and the processes that SCR regularly uses. In this installment, we’re going to be specifically targeting an illusive black magic: properly capturing and documenting the build tolerances.

RevvProjectBRZ Building the FA20 4U-GSE: Tolerances, FA20 Expanded View Subaru Service Manual

Step one for any rebuild is pretty obvious: take everything apart. No surprise there, but there is a lot your engine can tell you during the teardown. An in-depth visual inspection will help shed light on excessive wear patterns in key areas like bearings, sides of the pistons and rings, cylinder bores, and crank journals, letting you identify focus areas where tolerances may need to be adjusted, materials changed, or where reinforcing is required during your rebuild. Fortunately for us (and for other FRS / BRZ owners), our 18,000-mile donor engine didn’t show any signs of excessive wear so we didn’t have any special issues to address. Be sure to check out our full assessment of the engine here.

RevvProjectBRZ Building the FA20 4U-GSE: Tolerances, Measuring tolerances Subaru Service Manual

Often times, in precise builds, you hear of the term blueprinting. This is just the measuring and recording of dimensions, weights, and clearances of the rotating assembly. For our build, since this is a new engine, we have blueprinted the stock motor so we have a solid reference point to work off of as we assemble the new engine.

RevvProjectBRZ Building the FA20 4U-GSE: Tolerances, Piston bearings

RevvProjectBRZ Building the FA20 4U-GSE: Tolerances, Piston Rings

Another key (read: mandatory) step during the disassembly is to measure all tolerances to see how closely they line up with the factory specs. Some important measurements are gap on the piston rings and bearing tolerances on the rod and main bearing journals. Once again, with our BRZ donor engine, all measurements were dead-on with the factory service manual (meaning Subaru did a good job assembling the motor, there wasn’t a wide manufacturing variance across parts, good QC, and we want to duplicate this going back together). Typically when you see near-perfect tolerances on an 18k-mile engine, it means the engineers did a great job designing the engine and setup tolerances that work well for the application. A best practice would also include recording these measurements to compare against future rebuilds.

RevvProjectBRZ Building the FA20 4U-GSE: Tolerances, Subaru BRZ Service Manual Expanded View

This is also a good time to make sure you have a copy of the factory service manual on hand. That should be a no-brainer, but it will be the best source of tolerances to reference during your build process. We love the Internet, but all the information you find should always be treated as "trust, but verify.”

RevvProjectBRZ Building the FA20 4U-GSE: Tolerances, Subaru BRZ Service Manual Expanded View Tolerances Example

When determining tolerances for your rebuild, the best starting point to reference is the factory spec, and you can adjust them based on the expected use of the engine (higher revs, added boost, or both). For example, pistons will expand as they heat up, so it is critical to ensure correct piston to wall clearances as well as proper ring fitment. If spec is too tight there may be excess wear and stress on the rings and cylinder walls. If too loose then there may be excess oil consumption or increased blow by. For our build, we will work with Crawford to adjust some tolerances to better suit our turbocharged application.

RevvProjectBRZ Building the FA20 4U-GSE: Tolerances, Rebuild

Lastly, on assembly you need to verify OEM tolerances or what you calculated to change as the engine goes back together. Accurate measurements are achieved from using tools like a dial bore gauge, micrometer, ball mic, and feeler gauges. SCR recommends putting everything together, measure tolerances, take it back apart, and rebuild it a final time with all recommended lubrications.

While there are no guarantees in life, hopefully this information leads to a more successful engine build. Arming yourself with the right tools and information can never be a detriment when undertaking a task as complex as building an engine for more power.

The Subaru BRZ / Scion FR-S Platform Series Partners

Please take a moment to check out our partners, whom without, none of this would be possible:

Heuberger Motors
The world's most enthusiast-friendly Subaru dealership.

SCR Performance
The Performance Expertise Behind the BRZ / FRS Platform Series.
Visit them on the web @

SLVA Automotive Studio
Where Form meets Function.

Our Audio and Aesthetics Partner aka the Architects of Fidelity.
Visit them on the web @

AR Design
The Fabrication Masterminds Behind our Forced Induction Systems
Visit them on the web @
Our Sport-Compact Performance Parts Source. . . Never Compromising Quality or Customer Service.
Visit them on the web @
Visit them on Facebook @

Air Lift Performance
The Progressive Behind the Build
Visit them on the web @

Maintaining Integrity & Strength through CUSCU Performance
Visit them on web @
Visit them on Facebook @

Wilwood Engineering
The Stopping Power
Visit them on the web @

Advan Racing
Bringing Together Performance & Aesthetics
Visit them on the web @

Turbo By Garrett
Forced Induction at its Finest
Visit them on Facebook @

Yokohama Tire Company
Power is Nothing without adequate Traction.
Visit them on the web @


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Brian Hannon (BHRP)

Grew up around cars and racing, a passion fueled by my whole family. Participated in numerous track days with cars and motorcycles as well as covered the NASA East Coast Honda Challenge for Grassroots Motorsports. Now that I'm in Colorado I'm enamored with the Pikes Peak International Hill Climb... borderline obsessive...
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Write a CommentCOMMENTS

What an informative article! I'm not ready to do my first rebuild yet, but with articles like this I' sure getting closer! Thank you!

Imprezions's Profile Image

It's awesome seeing a Boxer engine cracked open like this. 18K miles and the thing still looks brand new! The service manual was a nice touch haha.