Saturday, September 12, 2015

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.

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