Plastigage is not that great for checking clearances. Too many variables that can mess up the readings. The correct method uses micrometers and snap gauges. It's okay for a street engine, but if you're building a performance engine the mechanical measuring tools should be used.
Fred,
I would like to know what those variables are when using plastigauge. I know the method of mechanical measurement is subject to many variables. Among them are when were the measuring tools calibrated and by who? To what standards and level of precision? Are they cheap or high quality tools? Temperature also affects expansion and contraction of both the measuring tool and pc. If you walk around with a mike in your hand it will measure different from one on a bench. What about wear and tear on the machine tools? How old is the crank grinder? How old is the boring mill and when was the last time it was squared up? How do you know the “machinist” didn’t gorilla tighten the block, machine it, measure it, and then distort it when he removed the clamps? What if some hack millwright “reconditioned” these machine tools? There are no guarantees that a line bored block is actually in a straight line parallel to the head surface. Brand new machine tools aren’t even square and wear is a constant. Every machinist has their own idea of the proper friction required between a measuring tool and pc. and I can guarantee you there is no consistency. Measuring a journal with one tool and measuring bearing thickness with another and then measuring a bore and determining a mathematical clearance is only an educated guess and introduces 3 variables. At least once a week I have to kick a machinist in the [censored] because his parts don’t meet tolerance when double checked on a coordinate measuring machine although he will swear he checked and calibrated his tools. Plastigauge is the only method that exactly confirms the amount of clearance. This is why there is a tolerance range in the first place as variation with measuring tools is assumed to be present. Sadly I believe there are many victims of shoddy machine work who don’t know any better. There are many machinists who are complete hacks and will bore blocks crooked, out of square, with improper fits, etc. If a cylinder is bored .003” bigger than the rest some won’t tell you they made a mistake and order new pistons/rings. They will give you some b.s. story about a defective piston causing that slapping sound. When an engine seizes and the machine shop swears all tolerances were met they believe it as their tired and inconsistent measuring tools told them so. However failure only happens if the tolerances were truly not met. I have been training machinists for the last 15 years and have seen every excuse related to the cutting tool, the machine tool, and the measuring tool. Experienced millwrights will confirm the reliability of plastigauge as in larger industrial applications micrometers become unpractical. Sorry to all for the long rant but let’s remember not all machinists are skilled ones. Confirming clearances with plastigauge is cheap, accurate insurance in my opinion.
Curt B.
