This is part 7 of many in Limitless’ engine build. – be sure to check out the rest of the posts there. With the pistons assembled, we can install them into the block…and finally have our engine start to look like an engine.
Pistons Part 2 – The Easy Part
Honestly, 90% of the effort of installing a piston is getting it ready, which we did in the previous post. At this point you just have to drop it in the cylinder without breaking anything, and bolt it to the crank. That doesn’t mean it’s a cake walk, but it’s certainly a lot quicker and easier than part 1. The first step to installing the assembled piston / rod assembly (which I’m just going to call “piston” from now on) is to compress the rings so that they’ll fit in the cylinder. The rings are meant to press against the cylinder wall to seal things, so you have to compress all of them into the piston groves or they won’t fit into the cylinder. Having done this once on my Skidoo 670 HO engine “by hand”, it was hard enough with just two rings…and this engine technically has 6, so there was no way I was going to manage without the right tools. Thankfully, there’s a very easy to do this, and that’s to use a ring compressor. Before using the ring compressor, it’s best to ensure that the “ends” of the rings are firmly inside their groove. This way, as the ring is compressed, it will be forced into it’s groove for the whole circumference. If one of the ends isn’t in, there’s nothing to guide it in as you compress it, and you can damage stuff pretty quick…so just be careful with this.
There are a few types of ring compressor. I went with an adjustable tapered compressor from Summit Racing – I only need it for this engine, but I might end up with slightly larger pistons years down the road when I do a rebuild. It’s effectively a “4.250+++” ring compressor, that will handle a standard bore and all the common overbore sizes, which is perfect for what I need.
Before using the ring compressor, lube everything in break in oil. This means the entire outside of the piston and rings, the entire inside of the cylinder, and the entire inside of the ring compressor. Assembly lube is too thick and you won’t be able to turn your engine over if you use it, so this is one of the few times you want regular old break in oil…which should only ever be mineral, not synthetic, and should be formulated specifically for your application. In my case, that meant Lucas 20w-50 break in oil. Yes, that’s some thick oil, but this engine has larger than normal clearances everywhere, and I’d rather rob a bit of power than be running on too little oil film thickness. After break in, I may switch to a 15w-40 depending on how the oil pressure and everything else feel.
Next, carefully insert the piston into the ring compressor…in my case because it’s adjustable I found it easiest to completely loosen the compressor, lower the piston until the skirts were poking through the bottom, fully tighten the compressor, then back off a quarter turn. Having the skirts poking out the bottom a bit makes it very easy to align the piston into the cylinder, and then a very light tap with a rubber mallet or similar can square everything up (the piston should be square in the bore, and the compressor should be lying flush on the block deck).
Just Taaaaaap it in…Tap Tap Taparoo
Now that the piston is “locked and loaded”, the next step is to get it fully into the cylinder. Ultimately, this just means giving it a few mild taps with the (non metal!!!) bottom of a hammer handle. But as always, the devil is in the details. First and foremost, you do not want to nick the machined surface of the crank, no matter what. You also do not want to nick the rod on anything on the way down. And if there’s something wrong – a ring isn’t quite inside the bore, or something is scratching your cylinder – you don’t want to keep going. So the key here is to go very slowly and steadily, take your time, and re adjust as needed. Two other very important points to note – this job is only really possible if the crank journal for the specific piston you’re installing is at bottom dead center. And, you’ll want the upper rod bearing half that sits in the rod pre-installed and lubed, ready to mate with the crank journal (which should also be spotlessly clean and lubed).
Here’s a list of the various things that might go wrong, and how to avoid them so nothing bad happens:
- A ring isn’t fully compressed, and butts up against the block deck. This one is pretty easy…if light taps with the hammer stop the piston from moving, something is wrong. DON’T KEEP TAPPING. Stop, figure out what’s wrong, and start again.
- Use *light* taps from the handle of a hammer that *is not metal*. If you smack your piston with something metal, you’ll damage it. If you use heavy taps, you’ll turn a minor “oops let’s try that again” into a major “!@#$%^ now I need new parts”.
- Watch the bottom of the rod as it travels down. There is not much clearance between it and the crank counterweights, which it can hang up on. If you keep trying to tap your cylinder in as it’s resting on a counterweight, you can gouge the rod bottom or the crank counterweight as they hit each other. To avoid this, ensure that cylinder isn’t rotated in the cylinder at all, and tap on the same side of the piston as the counter weight (which will naturally angle the rod bottom away from the counterweight).
- Similarly, as you get close to the rod hitting the crank journal itself, you’ll want to use your fingers to very carefully guide it around the journal so that it mates without scratching anything. If your rod has the bolts installed through the top, cover them in rubber tubing so that they don’t hit the crank. This wasn’t the case with me, but it’s common with a lot of OEM style rods.
- Once the top of the piston is below the head deck (i.e. fully inside the cylinder), watch your cylinder walls carefully for any signs of gouging or scraping as the piston travels further down. If you see anything, stop, carefully remove the piston, and find out what’s wrong – and pray you don’t need to re-hone the block.
Check your Torque while Torquing
With the rod snugly resting against the crank journal, the last step is to bolt on the rod cap using the correct torque, and then check the torque it takes to spin your crank. As with everything else, make sure your rod cap and bearing are spotlessly clean, then liberally coat the outside of the bearing with assembly lube. Check the torque specs for your bolts, coat them with the right lubricant – in this case ARP moly lube for my ARP rod bolts – and torque down in equal steps. My spec was 63 ft lb, but I went to 66 just to be safe.
At this point, your piston should be completely installed. Now is a very good time to check everything and make sure you didn’t mess up, before proceeding to the next piston. You really don’t want to install all 8, do a check, and find that something is wrong, because then you don’t know which one caused the issue and might have to pull them all to solve it. In my case, I only messed up once. I hung one piston backwards, and meant that I had to pull the piston, pull the rings, pull the spirlock (see tweezers / flathead above…), and redo everything. It also made me somewhat paranoid, so I re-checked all of the other pistons I’d already installed.
After a solid visual inspection, the final check you should do for every piston or two that you install is a torque reading on your crank. With no pistons installed at all, the crank spins like butter…it takes a shockingly low amount of effort to rotate it given it’s being held in place by no less than 20 bolts, all torqued to 110 ft lb. But as you install more and more pistons, the force of the rings pushing against the cylinder starts to make the crank harder and harder to spin. The great thing is, it’s a pretty predictable amount – 3-5 ft lb of torque per cylinder – and so if all of a sudden it jumps up by 10 ft lb, you know something is wrong. So pull out your torque wrench and try to spin over your crank every time you install a piston. The reading should increase by ~4 ft lb every time.
A Major Milestone – The First “Section” is Now Done!
Well, that’s pretty much it. At this point, a huge chunk of the work has been done. Sure, there’s still a bit of work left to finish the short block, but most of the major work on this half of the engine is done. As I write this post I haven’t completed the engine, but I’m quite a bit further ahead than this post would suggest – so I already have a pretty good idea of what’s all involved, and where the major work load is. The first “half” of the engine is the short block – and it’s got one big critical important job, which is installing the rotating assembly, which we just finished. Then you have to do relatively minor stuff like install the cam / timing set, fuel pump, oil pan, etc. Not trivial to be sure – but not nearly as involved as this. The rest of the engine is similar…the valve train is a lot of work, but then the intake and carb and the rest are pretty easy.
This also represents the first complete “standalone” section of the engine. By that I mean that the rotating assembly and the block are all sort of locked together in terms of what components you have. You can install tons of different heads, cams, intakes, etc going forward. But minor variations in quality / vendor aside, these pistons had to be installed with these specific rods and this specific crank in this specific block, or they wouldn’t have worked. In other words I could have used Mahle cast pistons instead of Icon forged pistons…but they still would have had to have the exactly right bore size, pin height, ring packs, etc to work with the crank / rods / block (and vice versa).
A Shortblock of Any Other Stroke…Would Not Be as Sweet.
I figured now would be a good time to revisit why I chose this specific rotating kit to go with the rest of my build. There are actually a number of factors that led to this exact setup, starting with the heads. As most of you know by now, I had to somewhat restart this build with a whole new short block after a bad experience with a previous machine shop. I’d already purchased the heads based on the old short block, so the new one had to work with the heads…they were way past the point of returning, hideously expensive, and incredibly awesome…so I’m sort of stuck with them (which is not a bad thing – they are *perfect* for this build). This meant that I needed to have a certain piston volume to match my head volume, or my compression ratio would be all out of whack. I did a lot of playing around with the 10 day trial of engine analyzer pro and, based on that, decided on the specific pistons I now have, which are flat tops with a 3cc valve relief. These are a relatively common piston as far as pistons go, which gave me lots of options everywhere else. The fact that they’re flat top means they’re about as perfect as things can get from a quench and flame front propagation standpoint, but the 3cc relief means that I’m a little low on compression – which is bad for power, albeit good for reliability. Unfortunately the next step up was to go with a +18 cc dome, which would have bumped me up into race gas territory unless I really reduced my timing (which is also bad). So even though I’m probably leaving a little bit on the table from an absolute performance perspective, I’ll already have that in spades, and now I can use shitty gas and not worry. The original engine would have needed 91 octane, and would have actually made slightly less power since it would have been smaller (see below)…so this is a much better combination.
Speaking of playing around with Engine Analyzer Pro, I highly recommend doing this. It gave me a huge amount of valuable insight into my component choices, and I could not have planned even close to as successful a build without it. Is it perfect? Definitely not. Is it overly optimistic? Probably. But does it still provide an incredible tool to see how various component choices will affect things like octane requirement, power output, etc? Yes. I cannot recommend it enough, and it’s free – for 10 days. So make sure you’re completely armed with a wealth of knowledge before you start using it, because you don’t want your simulations to result in any unanswerable questions. All of that being said, there’s nothing like coming up with the “perfect” combination, running it through a more detailed simulation, and seeing the resulting graph:
Needless to say, I’m pretty ecstatic with those results! So I didn’t mind going with the 3cc flat top piston and losing that bit of compression…because I have *plenty* of power, and it gains me some reliability.
I also new I needed a new rotating assembly, and if you’re going to buy new anyways, going for a “stroker” just makes sense. It’s almost exactly the same cost, but you get a bigger engine. In my case, I went from 454 CI to 492 CI – an 8% increase in size, torque, and power…for free. This ended up more than offsetting the reduced compression vs my original build, so I’m getting more power and more reliability than before. The crank and pistons together mostly decided the rods as well since the length is dictated by the stroke of the crank and the deck height of the block. The only real choice was I beam vs H beam…which is sort of a “Chevy vs Ford” thing. My kit just happened to come with H beam rods, which happen to be better for lower rpm / higher torque applications, which is exactly what I need!
In addition to my existing heads, I’d also found a new block, which had already been bored .030″ over. This meant that if I cleaned it up, I would have to jump to either .040″ or .060″ over, which are the next two sizes. Going .040″ is pretty rare and severely limits your choices, but it leaves room to go .060″ over down the road if you want to rebuild at a later date. And in my case, the piston volume I’d chosen happened to be available in 0.040″ from a quality manufacture, in the metallurgy I wanted, with the file fit ring pack I wanted. Ultimately, I did only have one choice of piston and ring, but I don’t mind that…because it’s what I wanted anyways.
Speaking of metallurgy, that also dictates a lot of things. In my case I wanted forged everything, not so much because I was pushing the very limits of performance, but because I want this engine to work hard for years (decades?) and not break a sweat. So forged steel crank, forged steel rods (aluminium are available, but trade reliability for performance), and forged aluminium pistons. The pistons are available in two alloys – one for extreme performance applications like NOS or super charging, and one that’s slightly less over the top, which I got. The forged pistons do expand a bit more than normal, so I’ll have to ensure my engine is properly warmed up before I really hammer it. But I plan on taking good care of it, so that won’t be a problem.
Anyways, hopefully that gives a bit more insight on why I chose what I did for this initial part of the engine. I wanted to hit a certain level of performance, but otherwise reliability was the only priority. And with my heads and block already selected, the decisions were somewhat made for me. With any luck, the engine will perform as smoothly as my cylinder bores – not a nick or scratch to be found, which is a great sign for the rest of the build going well!
