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  #1  
Old 09-23-2010, 04:33 PM
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Measuring bearing clearance with Plastigauge

I thought I'd show a bit of work I've been doing recently that I hope will encourage others to do the same if they need to: Or at least be able to understand what a mechanic is doing if you decide to let someone else do this for you.

This thread is aimed at the inexperienced or the wary – I want to show that it isn't difficult and it isn't some sort of Black Magic art.

Plastigauge is cool stuff. It is used to measure very small gaps between machined parts where it is difficult if not impossible to use any other means.

The pictures below show my OM617 diesel engine in bits. I'm checking the big end shell bearings to ascertain the amount of wear before taking the block off to the machine shop for a re-bore – I hope I don't have to take the crankshaft too!

(I've already removed the engine – all of the ancillaries – the transmission – the cylinder head – oil pump - and the upper and lower oil pans. The pictures show the engine in an upside down position)

The information in this thread is nothing new – it is essentially just a copy of the instructions on the Plastigauge packet together with some photographs.
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1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
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1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #2  
Old 09-23-2010, 04:34 PM
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Step 1

Remove the big end cap and the shell that is underneath

Wipe off the oil – here you need to be as clean as clean. Dirt grit / muck / crud will kill an engine if it gets into these bearing surfaces.
Attached Thumbnails
Measuring bearing clearance with Plastigauge-plasti_remove_end_cap.jpg  
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #3  
Old 09-23-2010, 04:35 PM
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Step 2

If you remove the shell and underneath on the crankshaft you see a drilled oil hole you need to turn the crankshaft so that it is out of the way a bit. There is no point in trying to measure the clearance over a hole. Use a (27mm) socket on the bolt on end of the crankshaft and turn the engine clockwise.

Make sure the piston doesn't fall. You don't want the bolts on the end of the connecting rod scraping against your crankshaft.
Attached Thumbnails
Measuring bearing clearance with Plastigauge-plasti_turn_crankshaft.jpg   Measuring bearing clearance with Plastigauge-plasti_make_sure_piston_does_not_fall.jpg  
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #4  
Old 09-23-2010, 04:36 PM
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Step 3

Cut a bit of Plastigauge to fit the length of the shell.

Plastigauge works by being squashed between two surfaces. You then measure the deformation to ascertain the size of the gap (between these two surfaces).
Attached Thumbnails
Measuring bearing clearance with Plastigauge-plasti_cutting-length.jpg  
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #5  
Old 09-23-2010, 04:37 PM
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Step 4

In my (English genuine) Plastigauge kit there are two sachets of grease. One is a fatty oil based grease and the other is a mineral based grease. The combination of these two greases with the Plastigauge is intended to help with the removal of the deformed Plastigauge when you have finished the measurement.

Smear one type of grease onto the bearing shell and the other type of grease onto the crankshaft.
Attached Thumbnails
Measuring bearing clearance with Plastigauge-plasti_first_grease.jpg   Measuring bearing clearance with Plastigauge-plasti_second_grease.jpg  
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #6  
Old 09-23-2010, 04:38 PM
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Step 5

Lay your bit of Plastigauge onto the bearing shell or alternatively onto the crankshaft – and get it to stick as best as you can. You may want some more grease. Use enough grease to make the Plastigauge stick – so not as much as when you're greasing your front wheel bearings! LIGHTLY GREASE the two mating halves.

Try and fit the Plastigauge so that it fits in a middle position of the bearing shell.
Attached Thumbnails
Measuring bearing clearance with Plastigauge-plasti_laying_it_down.jpg  
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #7  
Old 09-23-2010, 04:39 PM
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Army:

Thanks for the post,
Please keep in mind "plastigage will not give you a true reading for roundness and taper, but it is better than nothing.

This is why a "machinist" will not use palstigage but a mic instead. Also never use any grease or such when you are measuring for precision as it can easily give you a false reading + .003-.004+ even with a thin film.

I have been a machinist for 30 years and feel qualified to help out with your post and hope you take no offense.

God Bless
John
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  #8  
Old 09-23-2010, 04:39 PM
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Step 6

Re-assemble the end cap bearing shells (both sides of the crankshaft) and torque to the specified amount. My OM617 specifies an assembly torque and then you need to rotate the nut through 90 to 100 degrees.

DO NOT TURN THE CRANKSHAFT otherwise you'll get a false reading...
Attached Thumbnails
Measuring bearing clearance with Plastigauge-plasti_re_torque.jpg   Measuring bearing clearance with Plastigauge-plasti_90deg1.jpg   Measuring bearing clearance with Plastigauge-plasti_90deg2.jpg  
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Last edited by Stretch; 09-24-2010 at 12:31 PM. Reason: Forgot to mention about not turning crankshaft
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  #9  
Old 09-23-2010, 04:41 PM
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Step 7

Once everything is torqued – undo the nuts again. Remove the big end cap and just look at the state of your Plastigauge. It should have been squashed. The THICKER widths of the deformation denote a SMALLER gap. The NARROWER widths a LARGER gap – because it has not been as squashed as far. To obtain the bearing clearance you need to measure at the narrowest point.

Compare the narrowest width with the information card supplied with the Plastigauges. You can then compare this measurement with the specifications in your FSM / Haynes manual.

For the case of a big end bearing, if the gap is too large you'll probably get knocking. If the gap is too small – too tight – then the life expectancy of the bearing will be short as not enough oil will get through.

If you get hole appearing in your deformed Plastigauge – like mine in the photograph – you probably
need to stop being so stingy with that grease.

When you've finished measuring carefully remove the deformed Plastigauge with a clean oily cloth.
Attached Thumbnails
Measuring bearing clearance with Plastigauge-plasti_measuring.jpg  
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #10  
Old 09-23-2010, 04:42 PM
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General information copied from the Plastigauge instructions

Copied from the Plastigauge instructions:-

The normal clearance in big end or main bearing should be approximately one part in 2000 of the diameter. Thus a journal of 2” (50.8mm) diameter may be expected to show a clearance of 0.0001” (0.025mm).

The oil escape from a pressure fed bearing increases by roughly the square of the clearance. Thus a clearance of 0.002” (0.050mm) can pass almost twice as much oil as with 0.0015” (0.038mm). If the pump capacity cannot meet this demand the pressure will fail and the bearing will be damaged. This illustrates the importance of accuracy in fitted bearings.

Plastigauge may be used to detect high spots in cylinder heads, pipe flanges, etc. It is useful in production, inspection and servicing.

(Like I said cool stuff)
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #11  
Old 09-23-2010, 04:47 PM
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Quote:
Originally Posted by John1 View Post
Army:

Thanks for the post, please keep in mind "plastigage will not give you a true reading for roundness and taper, but it is better than nothing. This why a "machinist" will not use palstigage but a mic instead.

I have been a machinist for 30 years and feel qualified to help out with your post and hope you take no offense.

God Bless
John
Hi John,

I completely agree - it is fiddly to get a good impression of roundness and taper with Plastigauge... they say it can be done though (well they would wouldn't they).

I think the benefit of using Plastigauge is that you can assess the "health" of a bearing once it is assembled. If for example I had measured a huge gap in my big end bearings I could then take the next step of measuring the crankshaft journal with a micrometer and more closely examining the shell bearing. To me Plastigauge is the first step measurement.

EDIT:- No offense taken John - comment is good that's the whole point of a forum isn't it?
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #12  
Old 09-23-2010, 05:16 PM
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Quote:
Originally Posted by Army View Post
I I'm checking the big end shell bearings to ascertain the amount of wear before taking the block off to the machine shop for a re-bore – I hope I don't have to take the crankshaft too!
Not trying to nitpick, but this statement deserves some comment.

Ah! a reread makes me think that you may have meant "before you take the block off to the machine shop for a "Piston Cylinder" re-bore! If so please discount the first two paragraphs!

Plasti-Gauge would as you are later seen doing, is used to ascertain the tolerance between the bearing shell and the crankshaft. This measurement will have nothing to do with the engine block's crank shaft bearing bore. Without some noticeable bearing failure the block bore would remain unaltered and would not need a "re-bore".

If there where a question regarding the block's bore the way to ascertain it's condition would be direct measurement of multiple quadrants and for square and between all the bores for alignment. Fortunately MB lower ends are exceedingly robust and absent a bearing shell being spun out and physically damaging the block bore I have never heard of a 615/6/7 block bore being tweaked!

After using Plasti-Gauge you would simply know whether the tolerance between the bearing shells and the crankshaft journals are within specs or not. If within specs, button things up, torque to spec and forget about it. If the measurement is outside of the FSM specifications the first thing would be a competent inspection by direct measurement of the crankshaft journals, once that was determined the question would be if replacement bearing shells by themselves would correct the out of tolerance issue or if the crank would require a regrind altering it's dimensions and requiring alternative repair sized bearing shells.


P. S. I almost forgot to mention it but I was wondering why you used the Plasti-Gauge in the manner you did as I’ve always used it across the bearing as described by the manufacturer and have always observed others to use it in that manner also?

http://www.plastigaugeusa.com/how.html

Last edited by Billybob; 09-23-2010 at 05:37 PM.
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  #13  
Old 09-23-2010, 05:26 PM
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I think Army is going to have the cylinders re-bored. AFIK the crankshaft will need to be removed to bore the cylinders.
I quit using plasti-gage years ago and measure the journals and bearing bores with micrometers. But the method still has merit for cases, Army's for one.
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  #14  
Old 09-24-2010, 12:35 PM
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Quote:
Originally Posted by Billybob View Post
Not trying to nitpick, but this statement deserves some comment.

Ah! a reread makes me think that you may have meant "before you take the block off to the machine shop for a "Piston Cylinder" re-bore! If so please discount the first two paragraphs!

Plasti-Gauge would as you are later seen doing, is used to ascertain the tolerance between the bearing shell and the crankshaft. This measurement will have nothing to do with the engine block's crank shaft bearing bore. Without some noticeable bearing failure the block bore would remain unaltered and would not need a "re-bore".

If there where a question regarding the block's bore the way to ascertain it's condition would be direct measurement of multiple quadrants and for square and between all the bores for alignment. Fortunately MB lower ends are exceedingly robust and absent a bearing shell being spun out and physically damaging the block bore I have never heard of a 615/6/7 block bore being tweaked!
Indeed I'm just doing a quick check to help me assess what work needs to be done before I take it to the machine shop. I'll be doing this check too when I'm rebuilding the engine to make sure the clearance is good with new shell bearings (it should be but I think it is worth a check)
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #15  
Old 09-24-2010, 12:37 PM
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Quote:
Originally Posted by Billybob View Post
P. S. I almost forgot to mention it but I was wondering why you used the Plasti-Gauge in the manner you did as I’ve always used it across the bearing as described by the manufacturer and have always observed others to use it in that manner also?

http://www.plastigaugeusa.com/how.html
Yeah strange eh? I was advised to do it this way...

'Tell you what I'll make some photographs when I'm putting the engine back together again and I'll check across and along the bearing to see if I get the same measurements.

Nice link by the way - kind of makes this thread redundant!

I could have said "Hey folks take a look at this link"...
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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