11 things to know before you get a new motor

1. Transparency

Make sure that the engine builder is willing to take the time and effort to fully explain the process and why he or she chose the parts that they did.

Why is this important? This is your first line of defense from having an unskilled shop build your engine. If a builder is unwilling/unable to share basic knowledge about your motor, or the build process in general, it is usually a good sign that they are likely trying to use you as a learning project OR that they just don’t care and want to make a quick buck. Both scenarios are no good from a customer’s perspective. Make sure that you speak to the actual engine builder. Make sure they are willing to give you at least an hour of their time before starting the build to discuss plans and answer any of your questions. This will ensure that the build is done correctly and up to your standards. Make sure the engine builder and shop you’re going to isn’t just churning out dozens of motors a week. We call this the “OTS business model.” There are shops out there that build dozens of the same type of motor every week. These motors tend to fail early and often because the engines are built in production style assembly lines, where costs are cut through “Value Engineering,” and tolerances opened, cheaper internals are used, etc. Find a shop that cares about your build and doesn’t treat you like a number. These motors are priced where they are because there is supposed to be much time and labor making sure the engine is built properly. Don’t be fooled by cheap marketing ploys. Make sure your money is spent wisely with someone who knows what they’re doing and cares about your success.

2. Cleanliness

Verify the shop/engine builder has a clean and tidy machine and assembly area.

A key component in building an engine that lasts is a clean and tidy work area. Imagine walking into a dentist’s office with linens on the floors and discolored alcohol jars with utensils/tools in them. Imagine if they said “hey, its in alcohol which means its clean” or “this is what a dentist’s office is supposed to look like.” Many of us would find a new dentist, no questions asked. Far too often I see engine builders with garbage all over the floor, grime and dust on every surface, and no concept of a clean room for assembly. The number one failure of engine bearings is contaminated oil. If your new shiny expensive motor is starting its life with tiny particles of dirt and grime in the assembly lube, the first thing that will happen on initial startup is that those tiny particles will become embedded in the top layer of bearing babbit and will immediately start to wear down the bearing and journal surfaces. It is absolutely crucial that your engine is built in a clean area, using clean tools, and clean components. Tidiness is just as important in that it shows the engine builder’s ability to organize his/her thoughts and work area so that they are organized. Someone who has too much going on and has tools spread everywhere, and parts all over the place, is certainly more likely to miss a seal, torque spec, etc. when assembling your motor. Your engine builder must have a clean and tidy area.

3. Accurate machines

Using machine shops that have machines and toolings from the 1960s is fine for big block V8’s and other engines that measure clearances in the .001” but are not up to par with most modern high performance engines.

Subaru bearing clearance tolerances (at least for us) range in the .0001”. That’s 1 ten thousandth of an inch (1 tenth), 30-50 times smaller than a human hair. It’s nearly impossible to have traditional machines like a line hone be able to hit these tolerance ranges across all mains-for example. Subaru mains come in from the OEM factory with ranges of 3-4 tenths, which means they are out of spec for most high performance builds. To properly machine/blueprint the Subaru blocks, a true high quality CNC bore and CNC hone are required. A machinist who also understands what it takes to machine at this level of accuracy is also important. Understanding temperature, and heat added by the tip of the tool, and what this does to the block is all critical in properly building a high quality engine. These are all things the engine builder, especially for Subaru motors, should explain in detail when you first speak to them. So many builders just slap parts together with “extra clearance” bearings because they won’t/can’t machine at these levels. Beware of this misnomer, “extra clearance” bearings are how engine builders three decades ago dealt with a lack of highly accurate machines. Subaru blocks last longer and perform better with tighter clearances, especially in the mains.

4. Intended use and how it affects the build.

Are you building a DD street car with AC under 500whp, or going all out for 8’s in a gutted/caged 1000+whp track monster? Knowing how your car will be used is key in building you a proper motor.

Some key questions the engine builder should ask you are:

What is your boost target?

This affects the ring end gap significantly. The more boost you run, the larger the gap and thus the more blowby you will have during idle and lower RPM.

What fuel type do you plan on running?

The fuel type matters in what we can offer you. If you only want to run E85 100% of the time, well, maybe we can give you more lower end power in the motor by bumping the compression, for example. Knowing the type of fuel is crucial in building a proper motor. Ring end gaps will be dictated by both boost and fuel type.

What is the intended use of the motor?

This is a very important factor in how we build the motor, and what internals will we recommend. Ring end gaps, bearing clearance, internals, are all affected by the driving conditions. Letting us know this as accurately as possible will allow us to build you a better motor. Below are some different types of driving conditions and how it will affect your build.

Street DD car – We will typically build with tighter clearances, tighter piston to wall, tighter ring end gaps. This will give you longevity and reduce blowby and oil consumption. Our DD/Street builds are meant to last for over a hundred thousand miles and surpass the OEM life. Our stage 1 block is ideal.

500whp-600whp Street – We will recommend an upgrade to CP pistons and upgraded rod bolts at a minimum for the short block build. Ring end gaps will open up slightly but we will leave clearances tight and piston to wall at a nominal spec. A good AOS will be required to deal with the added blowby. ½” head studs are a recommended minimum.

600+whp Street – Upgraded rod bolts, pistons, and I beam rod are a minimum. Upgrading to a Manley forged dual oiling crank will increase the longevity of the motor. 14mm headstuds and a fire ring will be highly recommended as well.

Under 500whp Track Build – Upgraded arp 625 rod bolts, CP pistons, and high quality coated bearings are highly recommended. We typically open up clearances a little to allow more oil flow to remove more heat from the bearing surfaces as well as require a 12mm oil pump.

Over 500whp Track Build – Call us for more detailed information.

What is your end-goal for the car?

Are you doing a full rebuild with a new turbo and all the ancillary components at once, or are you building to future proof the car? Knowing this will help us to set ring end gaps and clearances.


Cylinder wash is exponentially worse with e85 than other types of fuel.

E85 is alcohol in its mechanical properties. When alcohol and oil combine, an unholy union is created that eats bearings in the short term. This is done by the new mixture of alcohol and oil having a reduced oil viscosity and creating a situation where the surface tension of the oil is greatly reduced, thus reducing the shear life of the oil and creating a metal to metal surface contact between the journal and bearing surface. These types of failures are easy to identify by the wiping that occurs in the bearing. Because of this, we require that you change the oil and oil filter every 1,000 miles when running e85.

Every engine also has blowby. Blowby is a mixture of fuel (burned and unburned) carbon, and oil. This blowby over time will mix with your oil in the sump and again, create a situation with lower than ideal viscosity.

6. GD&T

Understanding Datums and why they're important

This is arguably the most important factor when building an engine. GD&T (Geometric Dimensioning and Tolerancing) for the Subaru Engine builder can be broken down into; Understanding how the case halves were originally machined and why. Being able to have a basic understanding of the importance of the relationship between the cylinder centerline, main centerline, and bell housing datum will allow the engine builder to build a better motor. I can’t/won’t release too much info here on this, but if you ask an Engine builder to explain how they reference centerlines, and they give you a blank stare, you may want to find a new builder. Again, this is a differentiator between a knowledgeable builder and someone who slaps parts together en mass.

7. Supporting mods-

What it takes to make 500hp and 8500rpm 35psi boost RELIABLY.

Use this as a reference, not a true build list.

  • Short Block – true blueprinting, closed decking, forged rods, forged pistons, quality bearings
  • Heads – cams, springs, valve job with a light porting.
  • Longblock Components – ½” headstuds, upgraded head gasket, JDM 12mm oil pump, upgraded pickup, timing belt kits, etc.
  • Fuel Pump – 450LPH
  • Injectors – 1300cc Min.
  • Fuel – E85 is a cheap and easy way to net 20% power.
  • Turbo – 20g, FP Black, GTX 30r frame, etc.
  • Turbo Kits – Rotated kits are awesome, but if money is tight, go with an external wastegate at a minimum.
  • Clutch – ACT 6 pucks are cheap and do a decent job. Best value.

8. Benefits of pinning the mains

The importance of pinning the mains goes back to understanding the GD&T of the Subaru platform. The ideal situation of any engine builder is to keep the parts in their originally designed shape and form, the more a builder can do this, the longer the motor will last. But, thanks to material differences, certain materials grow faster than others, cast aluminum elastically deforms more than 4340 steel, crankshafts twist and bend, etc. The more pronounced the deformation, the more chance you have of a situation where the bearings start interfering with the journals, or timing is off under high load, etc. With this understanding, you can see the appeal of doing everything possible to keep the motor as “stiff” under load/temp changes as possible. At P2P, we build every one of our motors with pinned mains for a reason; The case halves tend to “move around” more than we would like. The main journal clearance is key to keeping high oil pressure in the rod journals. The more we can maintain a consistent main clearance, the more we can continue having longevity in rod bearings. The pinned mains achieve two things:

1. Locks the translational movement of the case halves

Think about this as taking your hands together and sliding them against each other like you’re trying to get Elmers Glue off your skin. Pinning the mains keeps the case halves from sliding against each other and thus reduces thrust bearing wear.

2. Reduces the egging of the main tunnel

This is harder to visualize, but if the pins are long enough and not stubby pins, they create a hardened core near the tunnel that creates less of a bending movement.

Machining pinned mains in a Subaru platform is extremely difficult. Most shops are not able to do this correctly and end up ruining the block. We can’t/won’t release our technique, but it is in-depth and takes an understanding of how these motors are designed to be able to achieve accurate results. If the mains are not pinned correctly, the mains will be off and will either spin or cause a rod bearing failure. I can’t stress the importance enough of choosing an engine builder who KNOWS what they’re doing in this regard.

9. Benefits of closed deck-

What is preload?

Preload is best understood as a constant load/force/stress put on an object that will not allow the object to undergo fatigue cycles unless the external force exceeds the preload. Preload has other benefits, but for this exercise, it is best understood as a “fatigue barrier.”

What is fatigue?

Fatigue is best explained with a paperclip. Taking a paper clip and bending it repeatedly, even with low force, will create microcracking which with repeated stress propagates these cracks along grain boundaries thus creating one large crack that continues to grow with repeated cycles until finally, it breaks. This is fatigue failure, and it can be measured with testing and graphical representations called S-N curves. Each material has a dedicated curve, and Engineers spend hours designing a part to be below this curve based on the number of cycles. There are Ph.D. dissertations written for decades on this subject, so to not divert too much time into this subject, we will need to understand the basics; repeated loading will cause premature failure quicker in materials like aluminum and making sure the parts are designed to last through this repeated cycling is absolutely key in a lasting part.

Now that we understand that cycles AND stress will cause a failure, we can better understand why preload is important. This “Fatigue Barrier” does not allow the part to undergo repeated cycles unless the force exerted is greater than the preload.

So, why does this pertain to a closed deck in a Subaru block?

Closed decking creates a pressure ring around the cylinder in an area that sees almost all of the cylinder pressure i.e. stress in the cylinder wall. If the shop correctly designs their closed deck inserts, they will create a preload on the cylinder wall that will prevent microcracking from occurring in the soft cast aluminum. This, in turn, allows for an exponentially greater life of the cylinder walls, especially under high loading (big HP). Over 400whp, we require all motors to be closed decked as the stock blocks were never meant for much more than stock power levels and the block WILL prematurely fail. This is in our opinion why Subaru has taken its time to increase power ratings in their motors.

10. Upgrading the head studs

Headstuds are designed at a certain size and with a certain material to be able to create a preload that is usually two times greater than the cylinder pressure.

What is cylinder pressure?

Cylinder pressure can best be explained as the pressure created by the ignition of the fuel/air combination in the combustion chamber. This explosion creates a massive pressure, which then pushed the piston downward. The greater the explosion (more fuel/air) the more power we will make. In turn, this creates more stress on every component (Rods, Crank, Pistons, Cylinder walls, Head-gaskets, Valves, etc.). So we must design these components to be able to handle this increased stress for an infinite number of cycles. Headgaskets and Headstuds are what we’re concerned about in this article.

Headstuds and their preload (estimates, each manufacturer has slightly different sizes). Use this to better understand the benefits of upgrading the headstuds:

  • Torque=KFD
  • K=.012 with Moly Lube
  • F= Force on Bolt
  • D=Diameter (Smallest)
  • 12.418in^2 is the area of the cylinder

Stock headstuds 0.433IN (11MM), 70 FT*LBS

  • 13,500 lbs of clamping force per bolt
  • 63,900 lbs of total clamping force per cylinder
  • 4300psi max cylinder pressure
  • 860psi suggested max cylinder pressure

ARP 2000 headstuds 0.433IN (11MM), 90 FT*LBS

  • 17,300 lbs of clamping force per bolt
  • 69,000 lbs of clamping force per cylinder
  • 5600psi max cylinder pressure
  • 1120psi suggested max cylinder pressure

ARP625+ headstuds 0.433IN (11MM), 100 FT*LBS

  • 19,250 lbs of clamping force per bolt
  • 77,000 lbs of clamping force per cylinder
  • 6200psi max cylinder pressure
  • 1240psi suggested max cylinder pressure

½” headstuds 0.5IN (12.7MM), 125 FT*LBS

  • 20,833 lbs of clamping load per bolt
  • 83,300lbs of clamping load per cylinder
  • 6700psi max cylinder pressure
  • 1340psi suggested max cylinder pressure

14MM headstuds .551IN (14MM), 135 FT*LBS

This is an equation for understanding peak cylinder pressures at peak torque

BMEP(pT)=(13,000*HP)/(Displacement in L*RPM)

Types of headgaskets

  • Stock
  • Cut Rings on Stock Deck (Vulcan)
  • Stainless O-Rings
  • Pressure Ring
  • Machined Pressure Ring

Decking the surface is more important than most realize. Most people think that decking of the heads/block is to; create a certain mirror surface finish for better sealing of the headgasket. This is a secondary benefit. The primary benefit of decking the heads/block is to make sure 100% (or as close as possible to 100%) of the bolt preload is put towards crushing the headgasket, not trying to bend the head surface to meet with the block. It takes a large force to bend the heads elastically to meet with the block. This force is deducted from the preload, sometimes cutting it in half or more. So now your expensive arp 625+ head studs are worse than your stock headstuds. Make sure you deck each surface every time, it’s worth the $120…

Putting it all together – People usually complain about certain types of head gaskets being “bad” or “failing early” and disregard the headstuds being used, or the tune that was too aggressive. A good tuner knows what your headstuds are and what tune to give you. Sizing these headstuds to your end-goal is key in building a proper engine.

11. Rolled thread vs cut thread

This is probably one of my favorite subjects as it is not understood in most industries, and should be used throughout high load applications in soft material. Aftermarket larger machined headstuds in soft aluminum castings such as engine blocks should have a rolled thread as it increases the strength of the female thread in the block by nearly 30-50 percent (depending on ream size and grade of thread).

For a rolled thread – the process begins with drilling to a size slightly (by 0.030”) smaller than the intended rolled thread size. Then we use a ream that is specced to an exact size (within half a thousandth of an inch). Then we use a rolled thread tap that then takes this smooth surface of the reamed hole and forges the thread into the material by sheer force (literally) and creates something similar to forging. Almost no material is removed from these surfaces. If you were to take a cross section of this process, you would see the grain boundaries actually “flow” around the thread load surfaces, reducing the chances for microcracking to occur and increasing the strength of these threads. The drawback is that it is expensive, and hard to do. Also, the threads form little “volcanoes (if viewed in a cross-section)” and make installing the bolts a pain sometimes. Cross threading can be an issue.

For a cut thread – the process begins similarly with a drilling operation to a near exact size (within a thousandth of an inch depending on speeds and feeds). A tap is then fed into the new hole and the threads are cut, creating chips or pieces of material that will need to be removed. This is a process that has been used for many years and works well for most applications. But using this process cuts into the grain boundaries and creates potential crack zones, usually in the thread roots. The benefits of using this process are that it is cheap and easy to do, most machinists can perform this operation. The drawbacks are obvious; reduced strength, reduced fatigue life, chips are created at the bottom of a blind hole that can build up and break a tap.

All of our headstud threads in our engines are rolled threads. Yes, it’s more expensive on our end, and harder, but in the end is worth it as they last. We charge the same amount for rolled threads as other shops charge for cut threads so we can be competitive in the industry