Measuring bearing clearance with Plastigauge - PeachParts Mercedes-Benz Forum

Stretch · #1 09-23-2010, 04:33 PM

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.

Stretch · #2 09-23-2010, 04:34 PM

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.

Stretch · #3 09-23-2010, 04:35 PM

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.

Stretch · #4 09-23-2010, 04:36 PM

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).

Stretch · #5 09-23-2010, 04:37 PM

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.

Stretch · #6 09-23-2010, 04:38 PM

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.

John1 · #7 09-23-2010, 04:39 PM

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

Stretch · #8 09-23-2010, 04:47 PM

Quote:

Originally Posted by John1

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?

Billybob · #9 09-23-2010, 05:16 PM

Quote:

Originally Posted by Army

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

Stretch · #10 09-24-2010, 12:35 PM

Quote:

Originally Posted by Billybob

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)

Stretch · #11 09-24-2010, 12:37 PM

Quote:

Originally Posted by Billybob

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"...

strelnik · #12 10-08-2010, 06:16 AM

Quote:

Originally Posted by Army

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"...

I have seen the Plastigage used in multiple spots on the same bearing surface on plant machinery. To each his own, use te tool for what it does to help, right?

rs899 · #13 10-12-2010, 12:43 PM

Leave it to the English to provide a Plastigauge kit with 2 types of grease.

I most recently used it on my 91 Jetta 1.6 IDI diesel where I did a quick and dirty in situ re-ring ( no chassis crossmember to get in the way on VW). I pulled the pan and checked several of big end and main bearings. It's handy stuff and will give you at least a ballpark idea on bearing condition.

Stretch · #14 10-07-2010, 02:09 PM

Quote:

Originally Posted by Billybob

...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

I've been back to the machine shop again today to check with the chap who advised me to do it that way - I got it a bit wrong - he said it doesn't really matter but that way round is really to check out of roundness if you don't have a micrometer. I misunderstood him and followed his instructions incorrectly.

I'm still planning to do the comparison I mentioned in post # 15 above on the (new) bits when I put them all back together.