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[Webby the Bear]

As above. I had theory on Thursday and this came up. It looks like a tape measure and i quote ''is a precision clearance gauge for the measurement of clearance between fitted surfaces''.

cant see why youd need it if youve got a micrometer and feeler blades,

am i missing something? 

[Entwood]

As above. I had theory on Thursday and this came up. It looks like a tape measure and i quote ''is a precision clearance gauge for the measurement of clearance between fitted surfaces''.

cant see why youd need it if youve got a micrometer and feeler blades,

am i missing something? 

micrometer and feeler gauges are metal, and if used incorrectly can actually damage the surfaces that they are measuring the gap between

You also need full access to both surfaces so you can insert the micrometer "feelers" or the feeler gauge "tongue" ... plastigauges get around this access problem, especially when trying to measure bearing surface clearances, when access would be extremely difficult.

Imagine trying to get feeler gauges/micrometer to measure the clearance in a split bearing !! With plastigauges you can fully assemble, then disassemble and by reading the plastigauge you KNOW what the clearance was /will be when re-assembled .. even inside a closed surface.

[hotel21]

As above. I had theory on Thursday and this came up. It looks like a tape measure and i quote ''is a precision clearance gauge for the measurement of clearance between fitted surfaces''.

cant see why youd need it if youve got a micrometer and feeler blades,

am i missing something? 

micrometer and feeler gauges are metal, and if used incorrectly can actually damage the surfaces that they are measuring the gap between

You also need full access to both surfaces so you can insert the micrometer "feelers" or the feeler gauge "tongue" ... plastigauges get around this access problem, especially when trying to measure bearing surface clearances, when access would be extremely difficult.

Imagine trying to get feeler gauges/micrometer to measure the clearance in a split bearing !!

.... such as a big end bearing, once fitted and torqued to the crankshaft.  Plastiguage can measure extremely fine tolerances without damage or human error and only requires component disassembly to read what the clearance is.   

[Webby the Bear]

thanks guys.

ok so say im measuring the distance between a big end bearing shell and the cap.... i'd put a plastgauge up to measure it.... isnt it then prone to it being wrong anyway as its all by eye?

sorry if all this sounds stupid but ive never used one 

[Entwood]

thanks guys.

ok so say im measuring the distance between a big end bearing shell and the cap.... i'd put a plastgauge up to measure it.... isnt it then prone to it being wrong anyway as its all by eye?

sorry if all this sounds stupid but ive never used one 

you would put a plastigauge between the shell and the cap, the torque up the bolts to the correct torque. then undo the lot and observe the plastguage against the scale given.

The plastigauge starts off round, and is squashed/deformed by the exact amountto end up fitting in the clearance gap. so it tells you the size of the clearance gap. if it is correct, just reassemple and retorque to EXACTLY the same value you used the first time ... 

[Marks DTM Calib]

Got some here, its a controlled cross section tube of what is basically a form of plasticine. When you squash it it gets wider and by measuring the width of the squashed material you can determine the clearance

[Webby the Bear]

Aaaaaaaaaaaaaaaaha! i now understand..... i thought it was a tape measure type thing 

[Entwood]

http://www.youtube.com/watch?v=ulDX9Y4n0e0

Might help understanding ... but is fairly crap TBH

[Webby the Bear]

Right, got ya. thanks boys 

handy piece of equipment then 

[Andy B]

Aaaaaaaaaaaaaaaaha! i now understand..... i thought it was a tape measure type thing 

the clue is in the name ...... plasti(cine) gauge 

[Webby the Bear]

Aaaaaaaaaaaaaaaaha! i now understand..... i thought it was a tape measure type thing 

the clue is in the name ...... plasti(cine) gauge 

  i know. sometimes you cant see the wood for the trees 

[Andy B]

....
  i know. sometimes you cant see the wood for the trees 

you'll get there ..... 

[omega3000]

Excellent  Never heard of that and good to know 

[05omegav6]

Proper Haynes manuals actually referenced it in mechanical overhaul guides, and gave the relevant clearance specs for bearing shells and the like

None of that 'refer to dealer' rubbish that you get in the modern ones

[Marks DTM Calib]

Bearing clearances for bigs and mains are all pretty much the same across all engine applications, 3/100-5/100mm (1-2 thou for Andy B  ), the rule we used to work to is 1/100mm per 10mm of crank journal diameter (or 1 thou per inch).

When blueprinting an engine, you move bearings around until you get optimum clearance on all bearings.