94 Miata Engine Rebuild - Part 4

Time to finish up the story of my engine rebuild in the 1994 Miata. The first part of this is here, here is part two and part three. The images that follow are roughly in chronological order, but not exactly. I'll just briefly explain each photo so you'll know sort of what's going on. My purpose here is just to let you know what you're in for if you're doing this, not explain exactly how to do it.

Once the pistons were all assembled and installed, and the rod bearings installed and torqued down, it was time to build the motor up. Here you can see the tops of the '01+ pistons. If you were to compare to the '94 pistons you'd see these are a bit more domed on the top, which is what provides the additional compression. You can also see where they are marked as oversize with the 0.50 stamp (upside-down in this image).

The next step is to plop the cylinder head back on the short block. I didn't get a photo of the head gasket installed but there's nothing tricky about it other than to make sure you don't block the oil feed to the head. There's only one correct orientation for the head gasket. I didn't disassemble the head for this rebuild since it was just refreshed about 25k miles ago.

Next the intake manifold goes back on.

This photo shows the VICS butterflies in the '99 intake manifold. You can see by default they are open. I don't have a way to control these yet so for now I opted to wire them shut. Even though I don't get the (admittedly miniscule) benefit of VICS, the '99 head still flows tons better than the '94 head and also has the solid valve lifters that are much less problematic than the HLAs of the '94.

Next I assembled the crank nose with the timing belt cog and a new key. You can also see the new oil pump I installed and some of the sealant spooging out from the front of the oil pan. I had to trim away this excess sealant as it interfered with the pulley boss a bit later.

This photo shows the orientation of the fuel pressure regulator and the new fuel hose connector to mate the '94 fuel system with the '99 fuel rail. This is discussed in great detail in an earlier blog post but since I got this better photo I wanted to include it.

Keep building. Here the timing belt and water pump is installed, also the throttle body, thermostat, some coolant hoses, and the A/C belt idler (my car has no power steering pump).

Close-up of the belt idler for the A/C belt. Cars with power steering will have a power steering pump here.

The front of the engine is pretty much fully dressed. The alternator is in place, the water pump pulley, and crank pulley.

Motor mounts are next. I went back to the standard mount after having Mazda Competition motor mounts for a few years. I didn't like the extra harshness I felt while driving with the competition mounts. If this was a track car it would be different.

Next I bolted on the flywheel, clutch, and pressure plate. I didn't get any photos of it! I used my trusty Flyin' Miata crank nose tool to lock everything in place while torquing things down. You can see I substituted some grade 10.9 cap head bolts for the original bolts here. No reason to do that if your old bolts are in fine shape.

Next is to mate the engine and transmission. This is much easier to do on the floor of the shop than under the car. I used the crane and the transmission jack to maneuver them in place, then just let the bolts do the work. Now it's time to fly the engine into the car.

This part was a lot harder than it looks. A helper would be very..uh...helpful right here. I was wishing for a bigger crane because this one couldn't quite reach far enough. Note I have a leveler. This is crucial for getting the whole assembly oriented and dropped in place. You have to work in fractions of inches while constantly making adjustments to the attitude of the assembly. Once the motor mounts engage with the holes in the subframe you are home free. Don't forget to reengage the driveshaft with the tail of the transmission as you move it back or you will have to do like I did and unbolt the driveshaft from the differential so you can plug the nose back into the transmission. No big deal but an unnecessary step if you think ahead.

Finally when the engine is bolted in you can start hooking everything back up. The exhaust manifold, hoses, wiring harness, radiator, A/C compressor. Everything you had to unhook before. Refill the transmission with gear lube. You'll probably forget something and wonder why the car won't start. Just be methodical and it will work out. Don't take shortcuts and don't skimp on parts or tools.

Now that this is done I'm very happy with the engine. It definitely has a lot more power than before, and runs smooth as butter. Now I have to bring the rest of the car up to the standards of this motor. It never ends. Hope you enjoyed this series of posts. Sorry they were so far between.

Assembling the Miata Engine Short Block - part 3 of a series

This the third post in a series. Part One, and Part Two came before this one. Here's part 4 which came after this one.

 

In this post I'll briefly go over the assembly of the engine short block. I'm not going to quote torque values here. Look those up for yourself. It's important you verify that yourself. I took the block, crank, rods and pistons to 3D Service in Tampa, Florida. I had them overbore the cylinders 0.5mm, resurface the block, clean it, install new freeze plugs, check and polish the crank, and check the rods. This all came to $500. I had bought a set of DNJ brand 2001-2005 pistons in 83.5mm (that's 0.5 bigger than stock to account for the overbore) from Rockauto because of the higher compression ratio (10:1) compared to my 1994's original 9:1 ratio. I supplied the pistons to the machine shop so they could properly clearance them when they did the overbore. Even at 10:1 compression this is still a non-interference engine.

 

So now it was time to assemble everything. I started with the rods and pistons. These are fairly easy to assemble. I was wishing for a better set of snap-ring pliers but I managed to get them together. Assembly is made easier by either getting the wrist pin cold, or making the piston hot. I think the hot method is better because the cold method resulted in condensation forming on the pin, which made it slightly wet. I used plenty of assembly lube and gently tapped the pin into place, and then snapped-in the snap rings. MAKE SURE YOU INSTALL BOTH SNAP RINGS! I almost left one out which would have cost me the engine.

 

Once all the pistons were assembled to the rods, I moved on to the rings.

The rings are clearly labeled as to which rings they are, and also to which way is up.

You have to space out the end-gaps so they don't line up. My shop manual spelled out exactly how to space them. This photo shows only the oil control rings assembled. Note that the spacer ring has an up-down direction, but the oil control rings don't. The first and second compression rings are not identical and also have an up and a down. Don't mess this up! I used a pair of ring pliers to get the compression rings in place. They're quite stiff so it would be hard to get them on with your fingers. The oil control rings are easy and you won't need the pliers for them.

Do not buy this tool to compress your rings, or if you did, deposit it directly in the trash. I deposited two of them in the trash before I learned my lesson.

Meanwhile, I installed the main bearing top halves and gently laid the crank in place, then torqued down the bearing caps (with the bearing halves installed, of course, and plenty of assembly lube). Don't forget the thrust washers on the number 4 journal. The crank spins freely in the block at this point.

This is the hardest part of the build - getting the pistons in the bores. The cheap ring compressors flat didn't work so I bought a better tool, which worked but was still really hard to use. This photo shows three pistons in place. Took me about an hour to get to this point. The first oil control ring likes to sneak out from under the compressor and hang up on the rim of the cylinder just as you are tapping the piston in. Like, 90 percent of the time. Note that there is a front and back to the pistons. These DNJ pistons have a round mark on the top indicating the front of the engine. There is a cut-out area of the skirt to clear the oil squirter in the block. If you install the piston backwards it will collide with the squirter.

This is the tool that actually works. Mind you, the oil control ring still hangs up 9 times out of 10, but it eventually works and you don't get your hands all sliced up and your block scratched. I got this one on Amazon and it's a GearWrench brand tool. It comes in a nice case and has a variety of rings for many different bore sizes. I'll probably never use it again, but I have it.

I found that if it kept hanging up on the rim of the cylinder if I reoriented the tool on the piston sometimes it would go in. You have to be careful not to mess up the spacing of your ring gaps while wrestling with this. I probably did. I gently tapped each piston into place with a wooden handle. Once the oil control rings get past the rim, the whole thing goes in easier. If you feel increased resistance, STOP. If you force it you will bend a ring and score the cylinder wall. Keep everything coated and lubricated with oil while doing this.

When you're tapping the pistons in, make sure to put some tape over the end of the rod bolts, so they won't scratch the crank journals. I fount it best to rotate the crank so each rod journal was at its lowest point before I installed the rod.

Finally I installed the rod bearing caps, again with plenty of assembly lube. With the pistons connected to the crank it doesn't spin quite as easily but you should still be able to move it by hand. If you can't, something is wrong.

94 Miata - Engine Rebuild, Part 2

Since my last post I've removed the engine from the car and disassembled it. I was hoping to be able to simply hone the cylinders and put the engine back together with new rings and bearings, but I found there was too much wear for that. I didn't even make any measurements. There's obviously a lot of scuffing on the piston skirts and there are enough scratches on the crank journals that I figured I'd better have it polished and maybe even ground down one size if necessary. So off to the machine shop it will go this week. I'm taking them the block, rods, new pistons, and the crank from a spare 2002 engine I previously tore down. Here are a few pics of the pieces.

In this photo you can see quite a bit of scuffing on the piston skirt. The oil control rings on all these pistons did not appear to be stuck, so I believe my oil consumption issue was simply due to wear in the bores.  

This is one of the main crank bearings. It's probably the worst one but they all looked similar. It's not the worst I've seen, but you can tell this is an engine with 175,000 miles on it. It's worse than I expected it to be. This engine has always had synthetic oil in it, and although I've pushed the change intervals pretty long a few times, it's had regular oil changes. I chalk up the increased wear to several factors, though. 1) I had a very crappy cold air induction system for several years that didn't filter the intake air very well. It was an oiled foam filter and several times I found actual grit in the airbox. Most of the bearings showed deep scratches indicating large particles in the oil. This is usually due to ingested silicon grains. 2) I had a supercharger on this engine for about 10000 miles, and 3) this engine saw some track mileage where it was at very high rpm for extended periods of time.  

On the left is a rod from the '94 engine and on the right is a rod from the 2002 engine I previously tore down. I didn't weigh them but to me the 94 rod looks like it might weigh a little less. The webbing on the I-beam portion of the rod is definitely thicker on the 02 rod. Maybe they beefed them up in later years to compensate for the higher compression the NB engines run. At any rate, I decided to stick with the '94 rods for my rebuild. I'm sure they're still strong enough and I just thought the construction looked a little better. If I was going with forced induction I'd probably use the '02 rods.  

I bought some new pistons in 0.5mm oversize (83.5mm) because I figure it's safer to oversize rather than just hone this block, based on the amount of wear I'm seeing. The bores in the block look pretty good, but the amount of scuffing on the old pistons tells me there's gotta be some wear. So just to be safe we're going to overbore.  

This pic just shows the crank still assembled to the block. The main bearing caps are stamped with numbers 1 through 5 and there are cast arrows indicating the direction to the front. The numbers are hard to see but they are there. I elected not to use this crank and will instead use the one from the 2002 engine I took apart because that one looks brand new and this one shows some wear on the bearing journals and seal surfaces.  

Here's the bare block in all its glory. Not much to it when it really comes down to it. Part three of this series is here.

94 Miata - Engine Rebuild, Part 1

Going back about 3 years on this blog you'll find a long series of posts about how I went through my Miata and refreshed a bunch of stuff. Highlights of the project included installing a '99 cylinder head on my '94 motor and painting the entire car myself in my garage. What I didn't do back then was rebuild the bottom end of the engine. Since then it's been burning oil pretty rapidly and showing some rather poor compression when cold. So, I decided it's time to pull the engine again and do a complete rebuild on the bottom end.

I'm picking this project up sort of in the middle here, because I've already got the engine out and on a stand. When I last had the engine out in 2012, the cylinder bores looked very good, so I think I can get away with a simple hone and re-ring job, with new crank and rod bearings. I plan to use a Flex-Hone rotary brush type honing tool. These are inexpensive, can be done by hand with a drill, and provides a high-quality finish with the proper cross-hatch angle to ensure a good seal by the rings. I've torn down a spare 2002 engine block we had at the Gorilla Garage and I plan to use it to practice my honing technique. I also plan to use the pistons from that engine to take advantage of the 10:1 compression ratio enjoyed by the 2002 Miata engine. This will be a true budget-build, because if the block looks and measures ok, I'm going to get by without doing any machine work. We'll see if that pans out! The head only has maybe 35k miles on it, so I'm hoping to leave it alone at this time.

To sum up part 1 of this series, here are my goals for this project:

• Rebuild engine bottom end
• Inspect head and correct any issues (don't expect any)
• Refresh suspension
• Install soft top
• Install Flyin' Miata frame-rail braces
• Detail engine compartment
• Repaint hardtop

This list is subject to change and I probably won't get to all of it before I put the car back into service as a daily driver, but it's all the stuff I'd like to get done, so I can drive this car for another three years. Stay tuned for part 2 of the series as I get into the engine over the next few weeks.