Monday, July 4, 2011

Engine rebuild and swap

Used bearings and connecting rods paired to new 0.50mm oversize Mazda pistons and Rock Products rings.

This is an overdue update. Sometime last year I scored a free engine block and transmission for my Miata. The block sat outside for a winter sans cylinder head, so the bores exposed to the elements got super rusty. It needed an overbore to be clean, and new pistons to match. The rotating assembly and bearings were in good shape though; merely broken in with (allegedly) 60,000 miles. I ordered OEM pistons from Mazdaspeed, in half-millimeter oversize (twenty thousandths of an inch to you domestic types). That's also the service limit, which is legal in SCCA autocross, and nets me a whole 25cc of extra displacement. That's about a moped's worth of extra powaaaaaaaaaaaah.

I sat on this project, waiting for an opportune time to finish it. My recent career change made the decision for me however: I needed this car tip-top for a 1200-mile drive to Motorsport Marketing in Florida.

There was a broken-off motor mount bolt in the "new" block, and I didn't want to go to the trouble and time commitment to fix it myself; when I found that one, I had little time to do way too much. So I took it to a local machine shop. Sixty dollars and a few hours later, I had fresh threads on a mountable block. I've never been so glad to pay someone else to do something for me. Even the guys at the shop described that bolt as "a real bear to get out."

This explains why it kept getting tougher and tougher to slip a floor jack under the car.

One of my unpleasant discoveries while I had the car apart: Broken springs. This is a Flyin' Miata set, and I'm its third owner. Though they didn't give me a whole set for free, Flyin' Miata was kind enough to knock $75 off the sticker price on a replacement set of springs. Which covered just a bit more than the expedited shipping for the set. The woman I spoke to on the phone also personally passed the order off to the shipping department to get the order out the door that very day. Though their products are sometimes on the pricey side, I have to admit: Flyin' Miata has excellent customer service. I've always been happy to call them.

Four springs? Pfft. What kind of lame Miata are you driving? This one has twelve!

Some of the other broken parts I discovered included bad U-joints in the driveshaft and a broken pair of motor mounts. After some serious troubles with bad aftermarket motor mounts (new ones were broken or had under-rated bolts which broke upon torquing to spec), I opted to get used OEM mounts from the same guy I got my driveshaft from. Thanks, BTDT Racing, for being reasonably local to me. It was still a 3-hour round trip, but I got the parts I needed to get back on the road. (Note: I did not get discounts for any of these endorsements.)

I never realized just how long the header is in relation to the car until I saw it like this. 

I know that header looks like it's seen better days; the Raceland stainless header just doesn't handle Chicago road salt all that well. But the SpeedyRacer header I bought to replace it just didn't work, and I had no other option. Besides, though it's in ugly shape, it's still holding together and doing the job.

A bare long block is a lot smaller than a fully dressed engine.

I always thought it was funny how race cars have one tiny belt to drive its two accessories, but this shot made me realize that I've got the same thing going on. Air conditioning and power steering deleted, my car has just an alternator and water pump driven by belt.

On the right is the fresh long block with used bearings and new pistons and rings. Not having a usable cylinder head, the head work had to wait until the old engine was out. Here's a surprise: head work takes a goddamned long time. My dad and I spent at least six hours replacing sixteen valve stem seals and lapping sixteen valves from the old engine.

Another noteworthy discovery: you can't compress the valve springs with the head in the car. Don't be tempted by those valve spring compressors on the shelves at NAPA. Unless it's like a giant C-clamp, it won't work -- and yes, you do have to take the cylinder head off to do this job. My dad and mine's solution involved exactly that: a big C-clamp. He cut a window into the side of a big spark plug socket. That socket went against the valve spring, and I'd remove the spring keepers by reaching through that window with a telescoping magnet. Reinstalling the keepers involved tweezers, and grease to keep them in place. It worked very well.

In the same vein as my recent discovery of its underhood cupholders, a properly equipped Miata also has its own desk on which you can keep your workshop manual. Mine is the digital version, so I kept the laptop there. The hood sits perfectly on the luggage rack with the leading edge resting on the seat headrests. With the notches for the pop-up headlights, it even fits between the roll bar supports. Eat that, NB owners!

The used bearings in my free engine block Plasti-gauged within spec, so I was happy to keep them.

I should note: the overbore wasn't quite enough to take care of the pitting on the cylinder walls from the rusted bores. So there's still a little there on one or two cylinders. This might explain why the car still burns a little oil now. Or maybe the 500-mile break-in wasn't long enough. But despite the really cheap Eristic gasket set I bought on eBay, I have no oil leaks whatsoever. All the gaskets in that set appeared of good quality, and the set was complete -- though it did include several extra pieces, presumably for different variants of the Mazda B6 engine.

This was my first shot at an engine rebuild, and though it cost more than a used engine, I'm happy to say the experience was priceless. I had the skills to replace an engine before I took on this project, so I wouldn't have learned much if I just swapped in one that was already done. It was an adventure, a learning opportunity, and a chance to bond with my dad for a week. I wouldn't give all that up no matter how much easier the job could've been. I've always wanted to take an engine completely apart, and am pleasantly surprised at how simple it is. It's still running great a couple thousand miles later. And I got to cross an item off my list. I'm satisfied.


  1. Damn, I missed all the fun that week. You guys did a metric crap-ton of work. Well-done, chap!

  2. Metric crap-ton is pretty accurate. It was goddamned exhausting.

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