Episode 11: Stroker Kits - Royal Enfield 650 Twins

Hey Revheads, it's Will and Santina from Revo Recycles in beautiful Sydney, Australia. You miss us? We know it's been a hot minute, but we're back and we're ready to share that stroker kit that we promised to you some time ago. Before we get started, please remember to subscribe and like and share so you don't miss out on any of the new videos we have coming your way, including the build on this alcohol and nitrous 950 beast behind us. I know I've said this before, but as the old saying goes, there is no replacement for displacement. In this video, we're going to take a detailed look at another way to increase displacement and dramatically improve the torque, the stroker. While increasing the diameter of the piston and making the bore larger is the easiest and most common way to increase displacement, adding stroke or adding to the distance that the piston travels in the bore is another very effective way to do it. While increasing the stroke is more complicated and more expensive than adding bore, the advantage is that generally it increases torque per CC at a much greater rate than just increasing the bore. We'll get into the details of all of that in a bit. But first, we'll show you the components that are required to increase the stroke in the Royal Enfield Twin and how they work. Then we'll explain what's involved in actually doing the job and why you'd want to and what kind of results you can expect. So let's get into it. All right, let's break down what actually goes into a stroker kit and how each part works. First, this is a crankshaft. It's forged, which means it's shaped under massive pressure from solid steel. Forging improves its grain structure and enhances strength and durability, making it ideal for high performance or high RPM motorcycle engines. The crank has main journals that support the crankshaft in the crankcase and rod journals that move the rods and the pistons and the cylinders. These journals are ground super precisely so they can run on plain bearings, not rollers or balls. These plain bearings are made from soft metal alloys or composites. They sit between the crankshaft journals and the crankcase or conrod bores, and they ride on a film of pressurized oil. That oil film keeps metal parts from ever touching each other during normal operation, which means less friction and way longer life. Inside the crank, oil passages are drilled to feed oil directly through the journals. The counterweights are forged as part of the crank to balance the reciprocating mass and make the engine run smoother and reduce vibration. One more critical journal detail is here. It's the fillet radii, and that's a gentle curve at the journal's edge. That curve reduces stress points and helps prevent cracks from forming from fatigue and stress. Now let's look at connecting rods or conrods. These are the link between the piston and the crankshaft. They convert the piston's reciprocal up and down motion into the rotating motion. The big end of the rod sits around the crank like that. These use those soft metal bearings. At the other end, at the small end, the rod connects to the piston with a piston pin, or sometimes it's called a wrist pin or a gudgeon pin. In this engine, the piston pin floats in both the rod and the piston. The OEM Royal Enfield rod floats the piston pin right in the rod with no bushing, but our stroker rods, they use a very precise fit bronze bushing to float the pin. It's generally considered favorable to use dissimilar metals to prevent galling under extreme loads. Literally, the metal pieces will weld themselves to each other. Our stroker rods are forged steel I-beams, and that classic I-beam shape gives excellent strength-to-weight ratio. Compared to the stock-centric rods, which are pressed from powdered metal, forged rods can handle way more power and RPM. They're stronger under big cylinder pressures, they're more durable with knock or misfires, and they hold their shape better through heat cycles. They're especially useful in air-cooled, high-output engines. Plus, they're machined to tight tolerances, so they last longer, they balance better, and they improve bearing life. As a quick side note, while the stock-centered rods are a potential weak link in a race engine, ask me how I know, they are perfectly fine for your stock engine, and even for big bore engines that operate most of their life below the stock rev limiter. So while we're talking about the stock rods, I want you to see this really cool detail, cracked rods. These are made by intentionally fracturing the big end cap after machining. Cracking makes a perfect interlocking surface, so when you bolt it back on, it aligns exactly how it should, every single time. Centered rods are ideal for cracking because the grain structure is uniform, and it's brittle enough to split cleanly. Forged rods, not so much. That's why our forged rods are machined and precisely dowel-pinned at the big end split. Bolting it all together are high tensile strength bolts. Both these rods use stretch bolts, which clamp the cap tightly and keep it from loosening during load cycles. So, what does all this beautifully constructed metal actually do? Well, the crankshaft and the rods together convert the piston's straight-line motion into the spinning force that drives your bike forward. The plane bearings and the oil pressure keeps everything gliding smoothly with minimal vibration. If your oil pressure drops, the protective oil film will disappear, and the metal parts will grind themselves to a halt. So a well-oiled machine is critical. Using a forged crankshaft and rods provides the brute strength to handle all the forces of combustion, torsion, and inertia, allowing your performance engine to make more power, rev higher, and live longer. So let's get Will in here, and he is going to talk to you about what's involved in stroking an engine. All right, what is involved in stroking an engine? The stroker kit increases displacement, as we said, by increasing this piston's travel within the cylinder. This is done first by modifying the crankshaft to move this rod journal further away from the crankshaft's center axis. We use an original equipment Royal Enfield forged crankshaft, and we grind down those rod journals. We remove about three millimeters more from the bottom than from the rest of the journal. Then the journals are rebuilt using a submerged arc welding process that produces a clean, deep-penetrating weld that's ideally suited for forged steel crankshafts. Next, the crankshafts are reground to offset that rod center further away from the crankshaft's center line. You can see the results of that on this seven-millimeter crank. You see how much closer this rod journal is to the edge of the crank than this stock crank, quite a bit further away. After grinding, the crankshafts are inspected, magnafluxed, nitrided, hardened, and balanced. We also weld the drive gear on the end of the shaft to prevent it from slipping, which we found out the hard way happens when you hit the NOS at around 100 horsepower. Not something you're ever going to need to worry about with an 865 kit on pump fuel, but if you're building a serious race engine, it certainly matters. All right, there are several tricky bits about stroking an engine. First is deck height. When we increase the length of this crankshaft throw, we've got to either move the cylinder away from the crankshaft, or we've got to shorten the connecting rods to keep this piston from coming up out of the cylinder and making contact with the head. Deck height, all right? In a Royal Enfield engine, you can't really move the cylinder head, cylinder and cylinder head up, so your only option is to shorten the rods. Now, this is a 7mm stroke engine, so this center pin is moved 3.5mm away from the center line of this crank, and we use a rod that is 3mm shorter than the original rod, you can see here. So what that means is the deck height of this engine, if you use a standard 865 S&S pistons, this deck height is half a millimeter higher than the stock piston, which is, the stock piston is actually a half a millimeter below the deck with the thing assembled and all the gaskets in it. So this puts it basically right on the cylinder deck with the standard piston. Now, that also means, because we've shortened the rod, this thing's moving 3.5mm further up, but it has a 3mm shorter rod, so this deck is a half a millimeter higher. That deck is going to move 3.5mm further down than the stock crankshaft does, or the stock piston does, and as long as you don't go too far, that's fine. The next thing you've got to deal with is valve clearancing. Because this piston is half a millimeter further up, and a stroker, because it's traveling further, every degree of crankshaft rotation means that this piston moves further than it did before. Which means that when the valves are at maximum opening, which hasn't changed because you're still running the same head and cam and all that's still the same, what has changed is the location of this piston relative to the valve when the valve is at its maximum open point. That almost certainly means that you're going to need to make these valve pockets half a millimeter or a millimeter or so deeper than they are out of the box. Then, you've got to deal with crankcase clearancing. Because there's additional throw on this rod, the rod's moving further up and down, that also means that at 90 degrees, the rod is moving closer to the crankcase in the front, and at 90 degrees, it's closer to the crankcase in the back. Plenty of clearance at the bottom, but you've got to check the clearance on the front and the back. So, every stroker job, you have to be cognizant of deck height, valve to piston clearance, and crankcase clearance. So, it's a job that needs to be done by a competent, experienced engine builder, particularly for all the machining and clearancing. Then, once that's done, you'll also need a tuner. Because, again, the piston is traveling further, faster than it was before, so ignition timing can be critical. In fact, ignition timing in a stroker is where you get the power. So, you will need a tuner that will allow you to alter the ignition timing. I mean, you can run a stroker with a power commander and nothing to alter the ignition timing, and it's okay. But, if you're going to get the maximum power out of one, you need to be able to alter the timing to get to the maximum torque point. So, what are the options? Well, we have done strokes of 6mm, 7mm, and 8mm. And we've done bores of 81mm, 83.5mm, which is the standard S&S 750 kit, 90mm, which is the S&S 865 kit, and 92mm, which is nothing standard at all. We can do one of these cranks for you in any of those configurations. And any of these strokes can be done with any bore, including the stock 78mm bore, if you want. The 6mm conversion of your crank will just bolt in because there's no change in deck height, right? The 6mm stroke is moved 3mm away from the centerline, and the rod's 3mm shorter. So, at the top, the deck height's the same as stock, which also means that there's no crankcase clearancing needed because the rods aren't anywhere near. They're much further away from the crankcases than they are in a 7mm kit. Now, the 7mm kit is what we use in conjunction with the 865 or 90mm big bore kits. And that's what we use in our 950 engines. We strongly recommend that configuration as it, by far, provides the most cost-effective power per dollar. However, it does require fairly extensive machining of the pistons and clearancing of crankcases. The 8mm conversion makes the most torque, but requires a lot more machining. And that's really not for the faint of heart. That's the engine we're running in our 965 drag bike motor, and not sure that I'd recommend it for longevity. Then the question is, what can I expect from a stroker kit? Well, it's generally true that increasing the stroke increases the torque per cc more than increasing the bore. And that's because when you move the centerline of the crank journal away from the centerline of the crankshaft, it has a greater lever angle. So, the same force on the piston puts more force, more leverage on the crankshaft. So, it increases the torque. If you, for example, did a 6mm increase in stroke, you would get more torque per cc gained than you would if you did a 6mm overbore. 6mm overbore would give you more horsepower per cc gained, but not as much torque per cc gained. So, the magic is when you do an overbore and stroke together, you get lots more horsepower and lots more torque. For example, this 950 running on alcohol and nitrous makes 113 horsepower and 90 foot-pounds of torque at the rear wheel. That is a 164% increase in horsepower and 138% increase in torque. That is big green fun with ears. Okay, so thank you so much for watching. We hope you have a much better understanding of strokers and how they work and why you'd want one. I'd encourage you to put your questions in the comments below. We are working on the wrap-up video for this whole performance series. We've got a lot of that done. The benefit of having this two-year pause in the process is that we have built a lot of engines in the last two years. We've got a lot of engines with a lot of kilometers on them now. We can speak to the longevity of them. We've got engines running in race bikes all over the world. And we've got a lot of data around what kind of results you get from what different engine configurations and tuning and so forth. So that's going to be, I hope, a very interesting video. And we are well and truly in process with that. As I mentioned earlier, we are going to do a video on this 950 bike. We've already shot it. We just need to edit it down for you. And there are some secret squirrel projects in the works that we're capturing video of as we go. And some really pretty cool collaborations going on. So there's a lot of fun stuff coming down the pipeline. So remember, if you're having fun, you're doing it right. See you next time.