Engine Disassembly
It’s important when disassembling your engine to document everything. Error on the side of over-documentation. Bag and tag each piece as it comes off, and take pictures as you do. We recommend using a parts sorter for all the hardware, and label it as the motor is disassembled.
Pay particular attention to the location of the pistons and rods. In most cases, it will not matter in the end as you will be replacing the pistons, but it never hurts to have the information.
If you’re rebuilding an engine that had a failure, this becomes even more important to figuring out why it failed.
Initial Measurements, and selecting your Parts
Once the block is stripped, it’s time to take some initial measurements. To decide what pistons you need, take measurements of all 4 cylinders. Measure the top, middle, and bottom of each cylinder, in both the X and Y axis. This will allow you to determine both the diameter, and taper of the cylinder.
The OEM spec for piston to wall clearance is 0.0015” – 0.0020”. If you have more than this amount, you will want to go up a piston size, and bore and hone the block.
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Cylinder #1 |
Cylinder #2 |
Cylinder #3 |
Cylinder #4 |
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X-axis Top |
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Y-axis Top |
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X-axis Middle |
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Y-axis Middle |
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X-axis Bottom |
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Y-axis Bottom |
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Selecting Your Pistons
There's a couple of options for piston selection, depending on what type of build you are looking at. Let's look first at piston types. There are three main types of pistons, cast, hypereutectic, and forged. Cast and hypereutectic are most common in OEM applications, while forged pistons are more common in aftermarket and race builds. There's pros and cons to each. OEM cast pistons are usually cheaper, and they expand less with heat. This means you can run a smaller piston to wall clearance, which results in a quieter engine. Forged pistons are stronger, and can take more heat and force. However they require more room to grow, which can lead to piston-slap, and shorter piston life.
If you're doing an OEM build, or a build with mild performance gains (anything naturally aspirated or under 6-8 psi of boost) the OEM piston is usually the right answer. If the motor is going into a track car, or anything where strength is more important than long life, the forged piston will be the better choice.
For OEM builds, we recommend Mazda OEM pistons. High quality, very good tolerance control and consistency from part to part.
For race engines, our forged piston of choice is Wiseco. We have had nothing but success with them, and the precision of their machining can't be beat. They also have a coating on the skirts that aids in reducing engine noise.
Piston Size
Now that you have measured your block, you can select the piston size you need. There's a couple of factors here:
- Cylinder piston to wall clearance. For an OEM piston, you want a P-W clearance of .0015"-.0020". If the block is old and high mileage, you might be forced to bore it out and go up a piston size
- If the bore is no longer straight (aka, the correct P-W at the top, but not at the middle) you will want to bore it out and go up one size. generally, if you have runout more than .001" top to bottom, you will want to rebore it and get it straight.
Once you have your new pistons in hand, you are ready to take the block to the machine shop! If your machinist isn't familiar with the BP engine, but sure to bring a spec sheet with your requested clearances.
Piston Rings
Piston rings need to be gapped to allow for expansion when heated. If using OEM rings, they come pre-gapped. (It's ALWAYS worth checking, but 99% of the time they are correct out of the box). If using Wiseco rings, they will need to be gapped to the desired dimension. See the spec sheet included with the rings to calculate your needed gap.
For OEM rings, the factory spec is .006-.0011" for the top and bottom rings. We like to run .002" more on the bottom than the top, so depending on the measurements, you might need to open up the bottom rings slightly.
Connecting Rods
Do you need aftermarket con-rods? Again, it comes down to application. for an OEM build, no, the stock rod is just fine to be reused. If you're putting any boost on the car, in our experience, it's not worth gambling- just buy the H-beam rods. Below is a piston that came from a 2004 MazdaSpeed, that had been turned up from the stock 7psi to 11 psi via an aftermarket boost controller. As you can see, it didn't last long. the OEM rods are cast, so the strength from one rod to another varies greatly. You could get lucky, and there's plenty of documented builds running higher HP figures on stock rods. but in our experience, that's not the norm.
Balancing the Rotating assembly
Once you have your pistons and rods selected, you want to balance the rotating assembly. on a cross-plane 4-cyl, this is fairly easy to do. The crankshaft should be balanced to itself. Most OEM cranks we've measured were very close- however it's a cheap bit of insurance to have your machine shop check it anyways.
Next, you want to match your piston and rods to make them as equal as possible. using a jewelry scale, weigh each piston, wrist pin and rod, and mix and match until they are as equal as possible. the OEM spec was +/- 2 grams. We usually shoot to get all 4 within 1 gram of each other. if you have a set of rods that are massively out of spec, the easy option is to get some more rods and find a better matched set.
Engine Bearings
Like most parts in an engine, there's different options for different applications. bigger isn't always better... it's just bigger. when it comes to bearings, there's two leading choices for the BP engines. "OEM Style" 2 layer bearings, and "Race" bearings with a tri-layer construction.
The OEM bearings are a 2-layer with a steel backing, and an aluminum bearing surface. This bearing design is engineered to give a modest level of protection, while lasting for a long time (200K+ miles).
Tri-layer "Race" bearings have a steel backing, with a bronze second layer, followed by a plastic/babbitt/lead-tin top layer (depending on bearing brand). This top most layer is very soft, designed to give extra protection in the event of oil pressure loss, or excess power pushing the crank through the oil boundary layer. This extra protection however, is more or less a 1 time use safety feature. These bearings were engineered for racing applications, where the goal is to survive the race, then rebuild the motor. Not last 20 years and 200K miles on the street. That's not to say they can't last that long, but if they do, and OEM type bearing probably would have lasted just as long under the same condition.
So, how do you decide? What's your application? Do you need the more expensive race bearings? Or is the goal to build a motor that's under lower street and can survive with a 2-layer? Choice is up to you!
Engine Bearing Clearances
For OEM engines, the target bearing clearances and journal sizes are as follows:
| Item | Min (INCHES) | Max (INCHES) |
| Crankshaft Main Journal Diameter | 1.9661 | 1.9667 |
| Crankshaft Rod Journal Diameter | 1.7693 | 1.7699 |
| Main Bearing Clearance (Mazda FSM) | .0008 | .0014 |
| Rod Bearing Clearance (Mazda FSM) | .0008 | .0017 |
| Main Bearing Clearance (Leroy Spec) | .0010 | .0020 |
| Rod Bearing Clearance (Leroy Spec) | .0010 | .0025 |
Note we run slightly larger bearing clearances than OEM. This is because we also increase the oil pressure on our OE builds, and we are running different oil than was available back in 1990.
For Forced induction and higher HP builds, using the race bearings, we recommend the following clearances. These number assume a higher oil volume and pressure.
| Item | Min (INCHES) | Max (INCHES) |
| Crankshaft Main Journal Diameter | 1.9661 | 1.9667 |
| Crankshaft Rod Journal Diameter | 1.7693 | 1.7699 |
| Main Bearing Clearance | .0015 | .0025 |
| Rod Bearing Clearance | .0015 | .0030 |
Crankshaft End Play
Once you have the crankshaft installed, measure the endplay (runout) of the crank. This is done with a dial indicator, and a magnetic stand as shown. With a screwdriver or prybar, move the crank to both extremes front to back, and measure the total travel in the system. OEM spec for endplay is .006-.011"
Oil Pump
Do you need to upgrade your OEM oil pump to a billet option? Great question! Short answer is, it depends how much power you are going to be making. The issue lies with the flat-plane crankshaft. All flat plane cranks are inherently weaker in one axis than the other. This can cause flexing in the crank, which causes the nose of the crank to oscillate. This movement of the crank snout can cause the oil pump gears to crash into each other, at which time the cast OEM gears will break.
Since there's no easy fix for the crank flexing, the solution is to use stronger gears that can take the pounding. If you are running significantly higher than stock power or RPM, then a billet oil pump is recommended.
Regardless of if you're using OEM or a billet pump, you can choose between standard (1990-2000) and high volume (2001-2005), and can shim the pump pressure to be whatever you need.
High volume pumps are needed if you are feeding external items with oil pressure (VVT system, turbo, enlarged pistons squirters etc) the increased volume of oil help prevent bearing starvation at lower RPMs. Changing the pressure cut-off by shimming the pump changes what the peak oil pressure is. If you're increase the RPM, or again, feeding extra components, then increasing the pressure will help keep everything happy. Just keep in mind, it takes power to run the pump! So calculating the correct oil pressure needed is important to prevent wasting precious HP move fluid around.
OEM BP Engine Torque Specs
| Item | Torque | Unit |
| Main Cap Bolts | 43 | ft-lbs |
| Rods Bolts | 39 | ft-lbs |
| Oil Squirters | 160 | in-lbs |
| Oil Pump to Block | 195 | in-lbs |
| Water Pump to Block | 195 | in-lbs |
| Rear Main Housing | 80 | in-lbs |
| Oil Pan | 100 | in-lbs |