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Plastagage versus bore gauge

General engine tech -- Drag Racing to Circle Track

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fishman
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Plastagage versus bore gauge

Post by fishman » Sun Jan 12, 2020 3:03 am

I’m curious how much difference u have seen between the two. I’m just putting back together my 422 sbc and guess what my bore gauge went missing so I decided to try the plasti gauge . The first thing I found with it (first time using it) is not very easy to figure out exactly where it fits on the chart and jumps to far in incriminating
I realize bore gauge is the best way for accuracy
I posted as with plasti gauge says I have about .0020 clearance and I’m using exactly same standard bearings and last time I rebuild motor I had .0025 clearance. So I hoping that plasta gauge is out. I had to order a new bore gauge, if u borrowed my old one u might as well keep it. Lol 😂
Last edited by fishman on Sun Jan 12, 2020 3:30 am, edited 1 time in total.
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Re: Plasma gauge versus bore gauge

Post by raynorshine » Sun Jan 12, 2020 3:12 am

-if you're talking about plasti-gauge...i found it very accurate...

-esp if it is relatively fresh...and you torque down the rod or main without a lot of side action...

-lay a strip completely across journal...is should squish uniformly and straight...if not ...check crank journal..con rod for roundness etc.. 8)
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Re: Plastagage versus bore gauge

Post by mag2555 » Sun Jan 12, 2020 6:55 am

You will, or should always have more clearance in the Bearing parting line area which is where you need the gauge to prove it out.
Plastigauge will alway show the absolute minimum clearance that you have in any one area.

Also as mentioned above if your using plastigauge to check rod Bearings you need to place feeler gauges in between the two Rods to take up that clearance and not have the Rods move.
For your motor I could live with .002" on the Rods as a minimum, but for the mains I would want .028".

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Re: Plastagage versus bore gauge

Post by Coloradoracer » Sun Jan 12, 2020 9:15 am

Haven't used plastigauge in years, and have zero intention of using it going forward.....A quality mic and bore gauge will tell you way more than plasigauge ever will and more accurately....

It works for a quick check, but is not a good substitute for measuring clearances with the proper tools.
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Re: Plastagage versus bore gauge

Post by engineman1642 » Sun Jan 12, 2020 9:54 am

lay crank in block leaving out center bearing,plastigage and tell me how much clearance you have.

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Re: Plastagage versus bore gauge

Post by n2omike » Sun Jan 12, 2020 9:58 am

I read about all the 'evils' of Plasti-gauge... Bought quality mics, practiced and practiced using them until I was consistent and found myself proficient at using them. Still, I didn't build engines for a living, so I would check myself with Plasti-gauge. Once I got good enough with the gauges, my readings from both would be the same. I've never seen where Plasti-gauge was wrong... and with both in the hands of a person who doesn't build engines every day, my money is on the Plasti-gauge giving the more accurate measurements.

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Re: Plastagage versus bore gauge

Post by rebelrouser » Sun Jan 12, 2020 11:30 am

My issue is putting scratches on the bearings with any measurement tool I have tried, snap gauges, inside mic, even dial bore gauge, which causes the least marks. Maybe I just don't have the skill? Main Issue I see with plastigauge is people who use to much oil on the bearings when checking, as it changes the results. I have also seen wrong readings with plastigauge when people use it on engines in the vehicle and do not support the crank properly when using as well. And as mentioned my old hands are not as nimble as they once were, and plastigauge is easier for me to use accurately. I store my plastigauge in the ice box to keep it fresh, it does not like hot weather. In the shop beer ice box, the old butter tray works good.

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Re: Plastagage versus bore gauge

Post by Cris » Sun Jan 12, 2020 12:10 pm

Engineman brings up a good example of the limitations of plastigauge, but there are also some things it will tell you that your bore gauge won't.

Let's say you have measured your mains and they are perfect clearance with your bore gauge but......unknown to you, the mains are out of alignment. You throw in plastigauge as a back up and the readings are all over the map. With the information of the bore gauge AND plastigauge you pinpoint the exact cause, and even get some information on which mains are out and in what direction. This also would apply to the crankshaft itself being out of alignment.

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Re: Plastagage versus bore gauge

Post by modok » Sun Jan 12, 2020 4:13 pm

I think he means only install two bearings to check, not all of them.
This could eliminate error from alignment and/or slight bend in the crank.

Just logically it would have to read the same or slightly tighter than a bore gauge, never larger. So, basically a half thou tighter makes sense.

I use plastiguage very rarely, but I keep it around for some things.
IMO handy way to see if you need to chamfer/narrow the bearings. Lets you SEE where the edge of the bearing is on the journal.

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Re: Plastagage versus bore gauge

Post by bentvalves » Sun Jan 12, 2020 5:42 pm

cranks should be laid in protected v-blocks and indicated for straight, that is how a crank is checked quickly and easily.

main bores should be checked for straight and true with a precision round bar and a .0005 feeler gauge.

plastigauge gives you a window your in that is it, your not going to be splitting hairs and half shelling quickly and accurately down to 2-3, 5 tenths with plastiguage.

it is an in the field method of verifying you have clearance, not for dialing it in.

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Re: Plastagage versus bore gauge

Post by Mark O'Neal » Sun Jan 12, 2020 6:26 pm

Coloradoracer wrote:
Sun Jan 12, 2020 9:15 am
Haven't used plastigauge in years, and have zero intention of using it going forward.....A quality mic and bore gauge will tell you way more than plasigauge ever will and more accurately....

It works for a quick check, but is not a good substitute for measuring clearances with the proper tools.

Have you ever tried to teach someone to use a dial-bore gauge...over the internet? I have.....and plastiguage is just right for what he's doing.. :D

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Re: Plastagage versus bore gauge

Post by Coloradoracer » Sun Jan 12, 2020 9:52 pm

Mark O'Neal wrote:
Sun Jan 12, 2020 6:26 pm
Have you ever tried to teach someone to use a dial-bore gauge...over the internet? I have.....and plastiguage is just right for what he's doing.. :D

Actually, yes..... But I happen to be an instructor and I'm used to it not "clicking" with some right away....
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Re: Plastagage versus bore gauge

Post by engineguyBill » Sun Jan 12, 2020 10:36 pm

Plastigage is surprisingly accurate for what it is - an extruded wax thread manufactured to very accurate diameter. Engine bearing vertical oil clearance should be checked ONLY at a location 90 degrees from the bearing parting line. Any other location will result in wrong clearance due to eccentricity of the bearing shells. ALSO, as mentioned in previous posts, Plastigage is more accurate than inexpensive dial bore gauges that have pointed anvils. Use the Sunnen tool for best and most accurate results. But Plastigage remains a viable alternative to expensive measuring tools, just depend upon how much accuracy you are comfortable with . . . . . . . . .

Plastigage trivia: The machinery used to manufacture this product takes up as much area as a small computer desk. Very simple process.
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Re: Plastagage versus bore gauge

Post by af2 » Sun Jan 12, 2020 10:44 pm

engineguyBill wrote:
Sun Jan 12, 2020 10:36 pm
Plastigage is surprisingly accurate for what it is - an extruded wax thread manufactured to very accurate diameter. Engine bearing vertical oil clearance should be checked ONLY at a location 90 degrees from the bearing parting line. Any other location will result in wrong clearance due to eccentricity of the bearing shells. ALSO, as mentioned in previous posts, Plastigage is more accurate than inexpensive dial bore gauges that have pointed anvils. Use the Sunnen tool for best and most accurate results. But Plastigage remains a viable alternative to expensive measuring tools, just depend upon how much accuracy you are comfortable with . . . . . . . . .

Plastigage trivia: The machinery used to manufacture this product takes up as much area as a small computer desk. Very simple process.
Funny thing is that I use the plasti-gauge to check my measurements just in case.
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Re: Plastagage versus bore gauge

Post by n2omike » Mon Jan 13, 2020 10:54 am

In a pinch, or to get a quick reading without having to take several measurements... I've installed dry bearings on only two journals... such as #1 and #5... tightened the caps... then placed a standard dial gauge on the crank next to the bearing.

All you do at that point, is lift up on the crank, and see how much the gauge moves. Is a quick check, and gets surprisingly accurate results when checked against other, more accepted methods.

I do understand its limitations. I came up with this when getting suspicious results using a dial bore gauge with a pointy anvil. It was digging into the bearing slightly, making it appear larger than it actually was (compared to Plasti-gauge). This method agreed with the Plasti-gauge... which placed me in the market for a dial bore gauge without the pointy tip.

I also measured the raw iron bore without a bearing, then the bearing thickness, then the crank... and did all the math. What bothered me, was the more measurements you take, the more 'stacked up' the measured precision errors become. The fewer measurements you can make to get the results, the more accurate the final result. Sometimes, a less precise SINGLE measurement can trump a series of more precise ones.

Moral of the story... I'll check clearances with a variety of methods... and always double check them with Plasti-gauge! You never know when you'll make a math error, or something else may happen. :)

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