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Engine Building Fundamentals: Installing Pistons on Conrods

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Installing Pistons on Conrods


00:00 - Before we can install the pistons and con rods into the engine block, we need to fit the pistons onto the rods.
00:07 In a lot of factory engines, the wrist pin is an interference fit in the small end of the conrod and floats in the pin bosses of the piston.
00:16 This type of installation requires the small end of the connecting rod to be heated.
00:21 The idea here is that the heat expands the wrist pin bore in the conrod, allowing the wrist pin to be installed by hand.
00:28 Once the conrod cools, it shrinks, and the interference fit returns.
00:33 An important aspect with an intereference fit wrist pin is to ensure that the wrist pin is centralised in the conrod during the installation.
00:43 Fortunately for us in the aftermarket, fully floating wrist pins, where the wrist pin floats in both the conrod and the piston bosses, are far more common.
00:52 They're also quicker and easier to install.
00:56 Since the wrist pin is floating, we need some way of securing it in the piston and preventing it from sliding out and contacting the cylinder wall.
01:05 The four common techniques employed to achieve this are circlips, or snap rings, wire locks, spiral locks, and Teflon buttons.
01:15 Teflon buttons are useful in applications that will see very frequent tear downs, such as drag engines, but realistically aren't common in use in the performance aftermarket.
01:26 The idea behind a Teflon button is that they are fitted into each end of the pin boss in the piston, and actually are allowed to contact the cylinder wall.
01:36 In this way they retain the wrist pin, as it can't move out of the piston.
01:41 Wire locks are probably one of the most popular current techniques used by aftermarket piston manufacturers.
01:49 These are a small, spring steel clip with a circular cross section, that looks a little bit like a letter C.
01:56 They locate into a grove machined in the pin boss, and are relatively easy to install and remove.
02:03 Spiral locks are a spiral of flat steel that locates into a machine grove in the pin boss.
02:10 Spiral locks do a great job of positively retaining the wrist pin, however they can be a little difficult to install if you've never used them before.
02:19 The trick is that they need to be stretched out slightly prior to installing them, so that they resemble a small coil spring.
02:28 Once you've done this, one end can be fitted into the retaining grove, and the spiral lock can simply be twisted, or spiralled, into place.
02:36 Circlips, or snap rings, are a simple pressed steel ring with either holes through which you can insert your circlip pliers, or, alternatively, some snap rings have the ends folded up so that you can grab them and install or remove them with long needle-nosed pliers.
02:53 There is a trick with these circlips, as if you look carefully at them, you will notice that one side is sharply formed, and the other is slightly rounded from where the press tool cut them out.
03:04 It's important to orient them so that the square edges face the outside of the piston.
03:11 Regardless of the technique used to locate your wrist pin, if you're using a floating wrist pin, it's important to lubricate both the pin boss and the small end bush in the conrod prior to installing the wrist pin.
03:24 You have the option here of using a moly based assembly loop or normal engine oil.
03:30 I start by installing the retaining clip in one side of the piston, then sliding the wrist pin through the other side, and through the conrod.
03:39 Finally, the other retaining clip can be installed.

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