Assembling Cylinders & Pistons


Installation of an engine kit is well within the capabilities of most do-it-yourselfers with reasonable mechanical abilities and a few tools. There are some important things to know, though, and like anything else, there are little things here and there that make the job easier. Recently I took a few pictures as I put a top end on a motor and I thought I'd share a few things about the job that may be helpful to folks.


First, though, let me warn you that there are LOTS of ways to do this, and reasonable people sometimes differ on the best procedures and methods. I've tried most everything and what I'm going to show you here are the methods I've found easiest and that I've had the most success with. I have no doubt that you can find many people who disagree with some or all of this. But this is the result of doing this many times in many ways and it's been the most successful for me.


First and foremost in assembling the top end is CLEANLINESS. I can't emphasize this enough. The pistons, cylinders, and rings, as they come out of their boxes, are FILTHY, and you do NOT want to assemble them that way.


I always start by cleaning the bore with a strong solvent such as brake parts cleaner or lacquer thinner. Then I take the cylinder to the sink and using hot soapy water, I literally scrub the cylinder wall, and then thoroughly rinse the soap off. It's not unlike doing the dishes. You want it clean enough to eat off of. Likewise I do the same with the piston. I also put one wrist pin circlip into the piston now, while the piston is easy to handle.


Next I set them out on clean towels and proceed with the ring gapping. Ideally you want the cylinders in torque plates while gapping the rings, as a .001" change in diameter of the bore is literally over .003" of gap change. What happens when you stick a ring into a distorted bore is that the tight spots will touch the ring and the loose spots will not. So if the bore is distorted, it generally makes the gap measure smaller than it really is. So you may end up with a gap that's ever so slightly larger than what measured. But in reality, that's a good thing, I'd rather see people err on the too-big side than on the too-tight side when it comes to ring gaps..


Pay attention right here to piston orientation. With most of the pistons we ship, the valve pockets are the same size and the skirts are the same length, so you can put the piston in either way. However, with some custom pistons we offer, as well as some big bore pistons, orientation does matter. If one valve pocket is bigger than the other, make sure you put that pocket on the intake side. Likewise if the piston has a clearance cut on the skirt, or has one skirt shorter than the other, that side goes toward the intake (I'm assuming we're not talking about an XR type setup here).


Big bore pistons will sometimes get a clearance cut on the intake side skirt, be sure to orient this type of piston correctly in the bore.


I always start with the oil rings and work my way up. First I clean the expander with some brake cleaner and compressed air, and then I put it on the piston, taking care to put the expander gap over the wrist pin on the pushrod side of the motor as directed by the instructions.


The oil rails rarely need their gap adjusted as anything from about .015 and up is acceptable. But you should spot check it anyway. I just insert it in the top of the bore and nine times out of ten I can tell by looking that it's fine. If it looks close at all, I'll push it an inch or so into the bore, square it up carefully, and make the measurement. Oil rails are a hard spring steel and difficult to file, if I have to adjust an oil rail gap (rarely) then I'll touch it on a grinder equipped with a fine stone, and then deburr with a file.


Once you know the oil rail gap is okay, the next step is to CLEAN it thoroughly. I know, I sound like a broken record, but it's AMAZING how dirty rings can be when you pull them from the package:



This is a second ring, not an oil rail, but you get the idea. I put a little brake cleaner on it and wiped it with a rag and you can see how dirty it was. Oil rings come out of the package the same way.


Oil rails are normally put on by hand, not with a ring expander. Simply feed one end into the ring groove, just above or below the expander, and work around, sliding it on. The main thing is to pull outward as you do this enough to keep the ring from scratching the piston on it's way. But an oil rail is flexible and this is easy to do. Repeat for the other oil rail, make sure you've got the rail gaps and the expander ends in position per the instructions, and the oil rings are ready. The ends of the expander will often try to jump out and overlap each other rather than butt up to each other as you do this. Just be careful and keep an eye on it, it needs to be butted.


Now set the gap on the top and second rings. The basic idea is to place each ring in the bore, measure the gap with a feeler gauge, pull it out and file on it, and repeat until the desired gap (per the NRHS General Instruction Sheet) is achieved. But of course there's a little more to it than that.


Squaring the rings properly in the bore is absolutely essential to getting an accurate measurement, I can't emphasize this enough. It take very little error in the squaring to affect the ring gap measurement drastically, you can EASILY get fooled in your measurements if you don't square the ring properly.


The temptation is to place the ring right at the very top of the bore, because it's so easy to square it when you have the cylinder deck as a reference. BAD IDEA! The very top of a cylinder is notorious for having distortion and/or being slightly different in size. You MUST push the ring down the bore, at least a half an inch, and then you have to get it square.


I've seen people use various methods for doing this. You can go out and buy a ring squaring tool, but unless you do a lot of these, it's probably not a good investment.  Some people use a bolt as a depth gauge, pushing the ring down at various points around the bore and using the head of the bolt as a stop against the cylinder deck. Not a bad method, but a little tedious. Here's what I recommend for a simple, no cost, easy and accurate method:


A piston with only it's oil rings installed and inverted in the bore makes an excellent ring squaring tool. Note the clearance cut on the skirt on the left; at final assembly, this side of the piston needs to be oriented toward the middle of the motor.


Now you know why I recommended installing the oil rings on the piston first ;) This is a simple and effective method for pushing the ring down the bore and squaring it up for a measurement.


Never, ever, ever, use the bottom side of the cylinder for your ring gap measurements. The spigot of a Harley cylinder is unsupported by aluminum and is almost always distorted and/or not the correct size. Remember, it takes very little error in bore diameter or roundness to have a BIG effect on that gap measurement. You want to do it a half inch to an inch down from the top of the cylinder, that's a good place.


Adjusting the ring gap is best done with a gapping tool:



There are a couple different styles of gapping tools. The above is an inexpensive hand crank type tool that does a good job. Electric units are also available, although it's hard for most do-it-yourselfers to justify the expense.


Truth be told, a number of people have been very successful doing this job with a fine tooth file as well. Simply clamp the file in the vise, and then carefully work the rings back and forth squeezing the gap around the file.


Regardless of which tool you choose, there are two things to be careful of. First, KEEP THE GAP PARALLEL. Many many times filing on a ring will tend to make a "V" shape out of the gap. Keep an eye on it and work to keep the two sides of the gap parallel to each other.


Second, DON'T OVERSHOOT THE GAP. I know it sounds basic but this is a very common mistake. Go slowly as you approach the proper gap and check yourself often. It takes a little patience but it's worth it. Also be aware that second rings often file MUCH faster than top rings. If you gap your top ring and get a feel for how much filing it took to move the gap a certain amount, and then try to apply that information to the second ring filing, you're guaranteed to overshoot the gap. Patience is a virtue (and virtue won't hurt you).


Okay, once you've got the ring gap correct, next you need to deburr it carefully:



Anytime you file on metal, you create burrs. These burrs will hang up on, and even damage the ring lands, which hurts ring sealing. It's critical that you remove these burrs, on all four sides of each end of the ring. A small fine tooth file used lightly works well for this. You don't want to chamfer the end of the ring, but you need to get rid of that burr!


One last step before you put the rings on the piston: CLEAN THE RINGS! Did I say that already? They're filthy. They're even more filthy now that you've been handling and filing on them.


One area where I most certainly do not recommend cutting corners on the tools you use is the use of a ring expander:


Installing a ring with a ring expander. Note how this piston has one valve pocket larger than the other. The larger pocket always goes toward the intake valve.


A ring expander is not expensive at all, I paid less than $10 for this one at my local Sears store. Just do it, don't try to do this without it, as it's almost impossible to do without damaging the piston and/or the ring. Oh, and always CLEAN the tips of the ring expander before using it. If it's been sitting in your toolbox, chances are it's dirty, and it'll get the rings dirty.


Another EXTREMELY important point about ring installation: put the ring on right side up! Most rings have a dot imprinted onto one side; this dot always goes UP. Even if there's no dot, rings usually need to be installed a certain way. The instructions with the rings will show this. Double check this. Triple check this. Put a ring on upside down and the motor will burn oil like you can't believe.


So now you've got the rings all gapped (and cleaned), and installed on the piston. Your ring gaps are in the right place per the instructions. You're ready to put the piston into the cylinder. This is where things are going to get a little more controversial. Again, this is how I recommend doing it, but you'll have no trouble finding others who have different methods.


The service manual will talk about putting the piston onto the connecting rod, putting some support plates underneath it to stabilize it against the case deck, putting a ring compressor around it, and then putting the cylinder over the piston. And this method certainly works and I know many techs who prefer to do it this way. However, the method I'm about to describe is what I've found to be easier, and it also needs less special tools, which makes it especially easier for the do-it-yourselfer.


First and foremost, I want you to liberally lube the piston and rings, preferably with a high quality engine assembly lube. Now you will have no trouble finding all kinds of dissenting opinions on initial lubrication of pistons & rings. Many people will tell you that assembling it dry promotes better ring seating. Others will say minimal lube should be used. Total Seal sells a product called "Quick Seat" that you put on the cylinder walls to help the rings with the seating process.


I've done all those things multiple times and the conclusion I've reached is that you need lube. A new, fresh motor is at MUCH greater risk of ring microwelding than it is of failure to seat rings. If the bores are round and the rings are good and the cylinder wall finish is right, the rings are going to seat just fine. What you need to do is keep it from damaging the pistons during break-in due to ring microwelding. It's a whole 'nother subject, but the short story is that rings have very little surface area of contact with the cylinder wall during break-in, and as a result, those spots get very hot, and if they get too hot, they damage the piston's ring land, which is a sealing surface. When you do that, you've forever condemned your motor to be a mediocre performer. Proper lubrication and gentle break-in are absolutely critical to getting maximum performance from a motor.


A piston and ring assembly lubed generously with Red Line synthetic engine assembly lube (available from NRHS) and positioned to be inserted into the cylinder.


Once the piston & ring assembly are thoroughly lubed, position them as shown above, taking note of the piston orientation. It might be a good time to triple-check yourself on the ring orientation first, by the way, both that they're right side up and that the gaps are where they should be, since here in a second you won't be able to see them anymore. Now gently work the top ring into the spigot, rocking the piston slightly as needed. If you got your cylinders from us, or if we bored them for you, there's a chamfer right there at the entrance to the spigot that makes this job easier. It's really not hard at all. Now do the same for the second ring and oil rings. If you need to push on the ring, avoid the temptation to push with a screwdriver or anything made of metal for that matter, the risk of damaging the ring is just too high. Use a hard piece of plastic, for example the cap of a Bic pen works well. Don't get crazy doing this, be careful and it'll go in and there will be no damage.



Once you've got the rings into the bore, push the piston all the way down the bore and lube the cylinder wall under the piston liberally as shown. If it's hard to push down, you may have a ring cocked, and that's not good of course! But there's something else that can make it hard to push down: air. If you have the cylinder positioned like this and there are no dowels in the top side of it (bottom side as shown), air will be trapped above the piston and make it difficult to push down. Cover that possibility before you go pulling the piston out to see if you damaged a ring. If you were careful putting the rings in, the possibility you got it cocked in there is unlikely, though.


One more note before I go on. All of the prep shown so far can be done by NRHS if you desire, for a nominal charge. We do it to this point for people all the time and we charge an hour's labor. We then seal the assembly into a clean plastic bag for shipment, so it arrives ready to go on as follows.




What I'm doing here is using Three-Bond 1104 instead of a base gasket. Why? Well, it has to do with getting the squish right. This motor would've had too much squish clearance had I used a gasket. Read my piece on setting your squish.




Now that you've got the cylinder wall thoroughly lubed below the piston, push the piston back out the spigot, enough to allow the wrist pin to slide in or out, but not allow the rings to escape. Note that it is not necessary or even desirable to lube above the piston. In fact, after you push the piston back to this position, wipe the excess lube from the cylinder wall above the piston. Slide the wrist pin part way out as shown. Now take the assembly over to the waiting motor, which has already had it's gasket surface cleaned up and is positioned with the connecting rod halfway up ...



Place the assembly over the connecting rod like this, sliding the cylinder down over the studs. It helps to have three hands. Shown is my shop foreman Jon.



Wiggle, raise, lower, and twist the cylinder GENTLY while pushing in the wrist pin. When it's positioned just right it'll slide right through. Once it's in, place a rag under the piston and carefully insert the circlip, and double triple check that you've got it all the way seated in it's groove. Not a mistake you want to make! Through this whole process, do things gently enough that you don't accidentally pull the piston down and let the rings pop out of the spigot.



Next gently squeeze the piston up into the cylinder as shown. Doing it this way keeps you from banging it around and possibly letting the rings pop out.



Push the cylinder down and carefully guide the spigot into the case. Don't slam it onto the case and don't force it or wedge it, spigots are thin and somewhat fragile.



And finally push the cylinder all the way down. Piece of cake.




If you turn the motor over, it'll likely push the cylinder back up off the case deck. Here's an easy and effective way to keep the cylinder in place while you work on the other one. Use your shorter (spark plug side) head bolts with some cheap 1/2" PVC unions as spacers to hold the cylinder down. The PVC will not damage the cylinder deck.


Like I said at the beginning, this whole process is well within the capabilities of the average do-it-yourselfer with reasonable mechanical aptitude. Hopefully this article clarifies some of the finer points and gives some handy tips for doing it well. Be sure to read the NRHS General Instruction Sheet as well, and don't hesitate to call us if you have any questions. It's in our interest to make sure you're successful and we're happy to answer questions or give advice.



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