Using Plastigauge to check clearances

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK

Using Plastigauge to check clearances

Post by Guy Croft » May 29th, 2007, 4:02 pm

Plastigauge is great little product from this firm:

it's a soft, pliable material used for checking actual radial running clearances between shaft and housing. The material is non-invasive, ie: if you get any in the engine it doesn't do any damage at all. I rarely use it unless I've had a crank that has run a bearing badly, relying more often than not with just micrometer measurements of crank journals and my experienced judgement of the feel of a crank in situ when it's torqued up and turned over.

To make sense of what the gauge tells you, you need to know the OE spec for clearance between the rod and crankpin or main journal and housings. For example:
In the case of the Fiat 16v Coupe block featured the main journal housing OE spec is: 56.717-56.730mm. The bore gauge measured typically 56.74mm vert (in line with bore axis) and 56.77-78mm on diagonal.
I loosened and shifted the cap from one side to another (they have a minute clearance when in good condition) and the diagonal measurement moved accordingly. So one reading in the diagonal plane should not be taken to mean the thing has gone way_out_of_round. On older blocks you get a fair bit of shift sideways between the main bearing cap and its locating slot in the block, and building up needs care. I like to tighten the bolts just over finger-tight and rotate the crank to seat them on centreline before fully (and progressively) tightening them up.

I have NEVER resorted to line boring a TC block!

See sequence below:

GCRE 001.jpg
Here we'll check main journals to housing in a site member's Fiat Coupe block. Unit needs to be - as this one is - fully clean and ready to build.
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GCRE 002.jpg
The caps need to be torqued up to about half the OE spec. The Coupe caps have the grooves to show the order F-R, as for orientation fit as shown - the bearing lugs sit same-side.
GCRE 002.jpg (63.19 KiB) Viewed 17280 times
GCRE 004.jpg
First it's vital to measure & record crank diameters, & bores of rods and crankcase. Here a bore gauge in use. Main housings DO go slightly elliptical in service, not usually enough to cause concern, rods, well they can get severely out of round.
GCRE 004.jpg (54.75 KiB) Viewed 17274 times
GCRE 010.jpg
Wash bearings with brake cleaner (solvent) and fit, making sure to remove any debris from the brg edge that gets scored off during fitting.
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GCRE 012.jpg
Arrow shows where the debris forms. No, you really don't need that in your oilways. No ho!
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GCRE 013.jpg
Use a nice straight edge to press the bearings into place firmly in their housings.
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GCRE 014.jpg
Yes, box of Plastigauge, enough to last a lifetime really and still made in Bognor Regis West Sussex UK. Tel 01243 849125
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GCRE 015.jpg
The crank must be spotlessly clean, or as I say 'RTB' - ready to build. I give them a final wipe with WD40 or 'Duck Oil' and a lint-free cloth before fitting. Stops them corroding during build-up.
GCRE 015.jpg (75.51 KiB) Viewed 17245 times
GCRE 017.jpg
Don't lace the crank with oil, do that later. A light coating of oil on upper brg shells is all we need. Offer it up (we're not going to turn it) and stick a strip of the round gauge to the crank journal with a TINY dab of grease. I use tweezers to do thi
GCRE 017.jpg (71.16 KiB) Viewed 17241 times
GCRE 018.jpg
Get hold of some silicon spray and smear some on the lower brg shell to stop the plasigauge sticking to it.
The data at left is supplied in the Plastigauge packet and tells you how the thickness of the compressed 'wire' relates to clearance.
GCRE 018.jpg (78.18 KiB) Viewed 17244 times
GCRE 020.jpg
And this is what you get when you bolt down the housing. Don't be fooled (I always fall for it!) - the thicker the 'wire' - the smaller the clearance. This good used Coupe block & new crank has 2-2.5 thou" on diameter, which is VERY typical of bo
GCRE 020.jpg (114.66 KiB) Viewed 17229 times
GCRE 029.jpg
Just to prove the point - here an old 131 2 liter block with good used (bottom-limit) crank showing more or less the same reading as the Coupe.
GCRE 029.jpg (113.36 KiB) Viewed 17210 times


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