So, you’re ready to rebuild your Miata engine? Congratulations! You’re about to embark on an exciting journey! Rebuilding engines can be a ton of fun, extremely rewarding and add a whole new level of appreciation for your car. Or, it can be the most stressful, expensive and heartbreaking thing you’ve ever undertaken. So how do you guarantee the first outcome instead of the second? Preparation, patience, and practice. And we are here to help you succeed!
We have built a ton of engines over the years, and also helped many of our customers build their own engines. As such, we’ve seen (almost) every way something can go wrong. Most build errors fall into one of two buckets.
Pitfall #1 “I’m sure that step doesn’t matter”
Oh yes it does! There will be many steps in the process that seem like overkill or redundant. But trust us, there’s no step that can be skipped without inviting the risk of an engine failure. And most of these commonly skipped steps involve either cleaning, or measuring. As we go through the process and detail each step, remember that once an engine is complete, it’s sealed. Nothing can get in (unless it’s added intentional). But when you’re building the engine, everything is exposed. This is when dirt, metal chips and debris has the opportunity to get into the engine. Remember your bearing clearances are as small as 0.0007”, so even the tiniest piece of dirt can ruin a motor. So when a step calls to disassemble and clean a part- it’s not just a suggestion!
As for measuring, most of the measuring during a build is simply checking. But it’s important to check! We are all humans, and make mistakes. That means parts could have been made wrong, put in the wrong box, labelled incorrectly, damaged during shipping etc. Just because you have a STD sized crank, and you ordered bearings for a STD crank, and the box says STD on the side- DOUBLE CHECK!! We’ve seen it all, there’s no guarantee it’s correct- measure and record the values.
Pitfall #2 “Oh I know what happens next”
This mistake is harder to avoid. We all do it. Remember building Lego’s? Who actually reads every single step? At some point you come across a step that is so obvious, you jump ahead and do it without looking at the instructions. This is the reason for most of the engine failures we see customers bring us. They skipped a step, because they never knew the step existed and didn’t think to ask. Forgetting to clean parts, reusing a part that had a one-time use, not realizing that there was multiple types of a part, not knowing that something was or wasn’t interchangeable etc. The only way to avoid this pitfall is to read ahead on each step before starting, and assume you don’t know everything about it. Do your homework, even on the steps that you feel you already know how to do.
Specialty Tools and Equipment
The following list of tools and equipment for rebuilding an engine is by no means exhaustive, rather this is the bare minimum to do the job correctly. This is NOT the place to cut corners to save money. Having the right tools, and understanding how to use them will make or break a build.
Machine Shop
You will need to take certain parts to your local machine shop during this process. If you don’t know who your local machinist is, locating them will be step 1. Ideally you’ll find someone familiar with these old 4-cylinders, but most shops will be able to do the work. It’s ok if they are not familiar with Miata’s, as you will be providing a list with every spec and tolerance called out. You just need a quality shop that can machine a block and head to the requested dimensions.
Micrometers
You will need at least 2 sizes of micrometers. 1-2” pair, and a 3-4” pair. Get a quality brand (Starrett, Fowler, Haas, etc.) This is a precision instrument measuring down to .0001”, we all love Harbor Freight- but this is not the place. :)
Gage Block Set
A gage block stand and kit is needed to get accurate bearing clearance measurements. The brand is not super critical on this piece, as you’re looking for repeatability more than accurate numbers.
Bore Gages
You’ll probably want 2 sets of bore gauges, although it’s possible to get away with one. For the piston to wall measurement, you need a bore gage that can do 3-4” in diameter, with a .0005” resolution. For the bearings, you need a gage that can do 1.8”-2.0” diameter, with a .0001” resolution. If you find one set with the full range that’s great! We found it easier to locate two different tools.
Rod Vice
Critical tool for torquing and measuring rod bearing clearance, as well as being very handy for assembling your pistons and rings. Easy to find an most performance stores or online.
Jewelers scale
Doesn’t need to be anything fancy, but needs a resolution of .1 grams minimum.
Piston Ring Compressor
There are universal one-size fits most ring compressors out there, but we highly recommend getting a size specific tool. Makes assembly WAY easier, and reduces the risk of broken piston rings.
Piston Ring Pliers
BP Crankshaft Holding Tool
There’s lots of these on the market, and most do the same function. You’ll need to torque several items on the crank to over 100 ft-lbs, so you need a way to keep it from rotating. Our version of the tool also works on the flywheel for clutch installs, so it’s a handy tool to have! Available on our website.
Valve Lapping Tools
Gun Barrel Borescope with Right Angle Attachment
So since we are dealing with old, used crankshafts, it’s important to visually inspect the inside of the oil galleries to ensure there’s no crud or metal shavings trapped inside. The best way to do this is with a bore scope. There are several cheap borescopes available that plug into a PC and offer a good enough view of the inside of the crank to verify that it’s clean. If you’re dealing with a brand-new crank, this it’s required. But if you have an old crank, especially if it’s been turned or was in a motor that had a bearing failure- this is required.
Piston Ring Filer
Most of the time when doing an OEM build, filing the piston rings isn’t required. They tend to come with the correct gap from Mazda. But if needed, this tool is required. Don’t waste your time with the hand powered machines, they are just not accurate enough.
Most of these tools are available on our website, and for the tools we don't currently carry, a quick Google search can usually locate them. If you need help, feel free to ask us and we can help you.