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Engine Building Fundamentals: Measuring Piston To Bore Clearance

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Measuring Piston To Bore Clearance


00:00 - Measuring the piston to bore clearance, requires the use of a micrometre, to measure the piston.
00:05 And a bore gauge to confirm the clearance in the bore.
00:09 We start by taking the piston, and using the micrometre, measure the skirt diameter at a point perpendicular to the wrist pin.
00:18 If you're dealing with an aftermarket piston the manufacturer will specify the point on the skirt that you need to measure.
00:25 For example, the diameter may need to be measured half an inch, or 12.5 millimetres from the base of the skirt.
00:33 This is important, as the piston skirt is normally machined with a slight barrel shape, rather that the skirt being parallel, so we need to measure the correct dimension in order to make sure the piston to bore clearance is accurate.
00:48 Since aluminium also expands with heat, the manufacturer will normally specify a temperature to make your measurement at.
00:56 This may, for example, be 20 degrees celsius.
00:59 If your climate is significantly hotter, or colder than this, it can affect the measurement and hence, the accuracy of the final piston to bore clearance.
01:09 If you are using factory pistons, then the required information will be specified in the workshop manual.
01:16 Once the piston skirt has been measured, the micrometre will be locked at this measurement, and then be used to zero the dial gauge on the bore gauge.
01:25 Finally, the bore gauge is then used to measure the clearance in the bore.
01:30 What we want to do with the bore gauge is insert it in to the bore, and then rock it back and forth, through horizontal, while watching the dial gauge.
01:40 If the dial gauge reads a minimum value of zero, then this would mean that we had zero clearance, and the bore diameter is identical to the piston skirt diameter.
01:51 As the reading on the dial gauge becomes greater so does the clearance.
01:56 When we're checking the piston to bore clearance it's advisable to take multiple measurements in the bore, to confirm the bore is both round and taper free.
02:06 While we aren't going to have the ability to fix this ourselves, if we find the bore dimensions aren't correct, we can save a lot of wasted time and money, by abandoning the build at this point, and having the machinist correct the problems.
02:20 The lesson here is never assume that a part will be correct, or to your required standards.
02:27 I've also found that if you're dealing with the same machinist for all of your work, and they know that you'll be double checking their workmanship, you’ll often get a better of product from them, as they won't like to be shown up.
02:40 If you've used a torque plate for the boring and honing process, when the block was initially machined, you'll also want to check and confirm the piston to bore clearance with the torque plate installed.
02:52 As this can result in distortion of the bores.