Plastigage accurate?
 

Just kinda wondering what everyone thought of it, and how accurate it really is.

How are you thinking it'd be inaccurate? I mean, you must have something that's given you a bit of doubt to bring up the question, right?

Here's what I think(See sig about my thoughts): That unless the machined surfaces were improperly machined (as in, not perfectly round), and you've managed to test it in that imperfection, it's going to be accurate.

Any machinist would likely say it is not accurate enough, but for the general run-of-the-mill hobbyist and self-builder, it should be good enough. Many an engine has been measured with them and even the factory manual shows it used.

Curious: Do you by chance know how inaccurate they think it is? And what type of engine they're building that they don't think it's accurate enough? Like a street 383 with forged planning on 475hp, enough or no? Now a full on race engine, that I could understand! What do they do instead?

Technology is great but it is definetly not a must in some situations, NOW they say for accurate readings on torquing the bolts you need a bolt stretch guage but they have forgot back in the ole daze when there were no digital torque wrenches, no click type just the one with a long pointer that you tried to hold steady while shaking and reading the number at the same time, but it worked. We use plastiguage many many times back in the day on 7500-8000 RPM small blocks....No problems, I still use it today for a reference reading, is is as accurate as a mic, of course not, but it does work...

LOL Ugh, I've had to use one of those once or twice. Tell you what, I don't envy anyone for having to deal with those more than a handful and my sympathies to anyone who had to use it on a regular basis :D

I reallly would love one of those torque impacts!! Saw them on Discovery Channel during a segment on assembly lines (John Deere lawn tractors). So cool!

I've never had to use one of those thank God but I've actually heard from a few old timers that for there crudeness they are pretty accurate. Can't attest to that but I wonder if it's true...

My guess is they mic the crank and get it turned first. Then they torque the mains without the crank in and use something similar to a bore gauge. Obviously they do the math and figure their clearances needed and from there can machine the block to accept the correct oversized bearing if needed.

haha I suck at math, so I'll stick with the plastigauge :D

Pretty darn good for what it is.

One of the mags ran a long article on comparing Plasti-gauge with mics. I can't recall which mag it was, but one of the biggies, like GMHIGHTECHPERFORMANCE, VETTE, CORVETTE FEVER, CAR CRAFT, HOTROD, etc.

As you might expect, the Plasti-gauge was less accurate, but the difference wasn't all that critical.

Plasti-guage will show you if your bearing clearances are WAY out of whack. So if you're shooting for .0025 and only get .0015 with Plasti-Guage you'll know something's amiss.

I use it, especially since bore gauges are so expensive and, in my case, so seldom used.

Jake

West Point ROCKS!

It's good IF it's used right!

It's also good for showing taper on journals. :)

Like others have said Plastigauge is close enough to tell if clearance is close or way off in general.

If you want specific clearance numbers then you would measure the crankshaft journals with a micrometer and record the numbers for mains 1-5 and rods 1-8 on a piece of paper. You would then install the bearings in the block and the main caps and lube bolts, install, and torque the main caps as you will when doing final assembly on the engine.

Next set the bore gauge to the measurement of main journal number 1 on the crankshaft. Measure vertically in the number 1 main bearing journal in the block and the bore gauge will tell you your exact bearing clearance. You would then repeat for the clearance of the other 4 mains.

For the rods, you do the same thing. Put the bearings in the rods and rod caps, lube bolts, torque down in a connecting rod vise. Set bore gauge to rod journal measurements from the crankshaft and then measure the rod journal vertically from top to bottom to get your clearance.

There are different ways you can do it with setting up the bore gauge and calculating with subtraction versus setting the bore gauge to have it read the exact clearance. The method I just explained was method #2 which can be more time consuming but I take my time when building engines. If you have good parts and good machine work your parts should all measure the same or very closely anyway.

You should also always wipe your new bearings with a lint free cloth and acetone on both the front and the back. They are not clean at all.

http://www.carcraft.com/techarticles...ance_tips.html

http://www.carcraft.com/techfaq/116_...ter/index.html

I was doing some searching and this page popped up along with related search items that I have been hot after all day. I realize it is an older thread but I thought I could add to it for future references :)

I had a difference of .0026 on the mic vs. .0015 on the plastigauge. The top link is a way to adjust your clearances should you need too. The bottom is a test of accuracy plastigauge vs. dial bore and mic.

when you got to machinest school they have you use it alot in the begining to get use to it. I have never had an problem with it. just make sure your reading it correctly

Plastigage works much better and is a replacement for micrometers.....when your replacing main bearing with the motor in the car. Thats the only place I would use it and trust it for what it is.

If the motor is out of the car, its micrometers and telescoping gages at a minimum. If money permits, get a nice bore gage.

In my freshmen engine class we were required to measure for out of round and taper with snap gauges and mics. Then in my sophmore engine rebuilding class we were required to measure using dial bore gauges. We were told to check everything with the plastigauges before final assembly in both classes. I would say definitely use a dial bore gauge if you have access to one although they do take some getting used to. I found that using the snap gauges and mics was much simpler for me and i became proficient as i mic'd the whole motor by myself. I would only use plastigauge to make sure that your measurements arent far off but this can be corrected by double checking all measurements.

I had mic'd the entire assembly and motor and documented all clearances and tolerances etc. I decided to drop some gauge on there during final assembly and thats where I saw the difference. I was quite alarmed. I read it right and proceeded to check it 2 more time with the same results. I then rechecked it with mic and dial bore gauge only to find it different and the same as I had found when I originally mic'd it.Same as the second link I posted.