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Engine Building Fundamentals: Using Plastigauge

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Using Plastigauge

06.48

00:00 - While the measuring technique we just discussed is the most accurate way of checking bearing clearances.
00:06 Another method that's still common is to use a product called Plastigauge.
00:11 Plastiguage is a popular alternative in particular because it's cheap since you doesn't require any expensive specialist tools.
00:20 Plastiguage is a small diameter wax strip that you could liken to a piece of string.
00:26 It's available in three different colours.
00:28 Each designed to measure a different range of clearances.
00:32 To use Plastiguage, you simply break off a short length and lay it across the journal to be measured before torquing down the bearing cap.
00:41 The Plastiguage will be squashed flat.
00:44 You can then remove the cap, and compare the width of the Plastiguage to the scale on the side of the Plastiguage pack to confirm the clearance.
00:54 On the face of it, Plastiguage sounds like an ideal solution however, you'll find that most professional engine builders frown heavily on the use of Plastiguage.
01:04 If you're just starting out and you're going to be assembling a number of engines, or you're going to be assembling any maximum effort high power engines, then I strongly suggest that you invest in the proper equipment right from the start a quality micrometer set and a bore gauge.
01:21 These tools are undoubtedly the most accurate way of measuring and confirming your oil clearance.
01:28 It's also probably quite telling that you won't find Plastiguage used by any reputable professional race engine builders.
01:37 I don't personally dismiss Plastiguage and like all tools, I feel it has its uses.
01:42 But you need to understand its limitations, as well as how to use it properly.
01:48 In particular, it's easier for a complete novice to use Plastiguage and get reasonable results than a micrometer and bore gauge.
01:57 Since Plastiguage is still depended on by many, this section of the course, we'll detail how to correctly use it.
02:05 While it can't offer the accuracy of a micrometer and bore gauge.
02:09 When used correctly Plastiguage does still give you a reasonable idea of the oil clearance at the point being measured.
02:16 When I use Plastiguage, I also insure that my piece of Plastiguage is the entire width of the journal being checked.
02:24 This gives you a very clear visual indication if there's any taper on the journal being checked.
02:30 Plastiguage is also often used as a sanity check prior to final assembly.
02:36 Even when the bearing clearances have been measured and confirmed with a micrometer and a bore gauge.
02:42 The down side of Plastiguage is that if you determine your clearance is either too tight or too loose there's no way the Plastiguage can tell you what's causing this.
02:53 Is the journal diameter out of spec, or is the bearing bore out of spec? Either can contribute to incorrect clearances.
03:02 In order to use Plastiguage, we need to assemble the crank shaft into the engine block dry which means we're not going to lubricate any of the bearing surfaces.
03:12 It's imperative here that both the crank shaft journals and bearings are clean and dry before fitting the crank shaft.
03:21 Next, we'll lay a length of Plastiguage across each journal we want to measure.
03:26 You can check all of the main journals at the same time or just an individual journal.
03:32 Now we need to carefully instal and torque the bearing caps into place.
03:37 During this process, it's imperative that the crank shaft isn't rotated.
03:42 Once the caps are torqued to spec, they are then loosened and removed.
03:47 In particular here, if you've got an engine that uses individual caps for each journal these may be quite tight once the bolts or studs are removed.
03:58 There maybe the temptation to rock the cap back and forth in order to loosen it and remove it from the register inside the engine block.
04:06 Doing so will effect the Plastiguage and prevent any chance of an accurate measurement.
04:13 Instead it's essential to carefully work the caps vertically up and out of the block.
04:19 Once the bearing caps are removed, we'll be able to inspect the Plastiguage.
04:24 Initially, what we want to look for is a consistent width to the Plastiguage across the width of the journal.
04:31 If the width fluctuates across the journal this can indicate a taper issue.
04:37 Next, we can use the reference scale on the Plastiguage pack to check the actual clearance.
04:43 By holding the pack across the crushed Plastiguage, we can confirm which measurement the Plastiguage most closely fits.
04:52 This presents one of other limits to Plastiguage.
04:56 As you're limited in accuracy to about half a thousandth an inch or about 12 hundredths of a millimetre.
05:04 Using Plastiguage to check conrod clearances is essentially a repeat of the process used for the main bearings.
05:12 These need to be checked with the crank shaft, pistons, and rods fitted to the engine block as it's impossible on the bench to prevent the conrod from moving on the crank shaft journal, and any movement can smear the Plastiguage and effect the accuracy of the reading.
05:29 In most instances, we can place the Plastiguage across the top of the big journal as we would with the main bearing journals.
05:38 With some after market conrods however this is a little more tricky, as often, we'll need to tap the rod bolts gently to separate the cap from the body of the rod.
05:49 If we place the Plastiguage strip on the top of the crank shaft journal, this tapping will tend to compress the Plastiguage and give a false reading.
05:58 In this instance, I find the best results are achieved by placing the Plastiguage on the under side of the big journal on the actual bearing and the conrod.
06:09 This is a lot more tricky to achieve though.
06:11 As we're now trying to work on the underside of the journal.
06:15 Once you've used Plastiguage to confirm your clearances, it's essential to also clean off all of the residual wax left behind by the Plastiguage.
06:25 Leaving any residue behind can quickly result in bearing damage when the engine is run for for the first time.
06:32 Removing the Plastiguage residue can be achieved with a clean rag and brake clean.