Page 5 - May 2016
P. 5

I came across the simple yet effective     of calipers that, while not accurately

invention of ‘plastigauge’ (trade name in calibrated, could be used to compare the

North America). This is a round-section dimensions with my type-high reference,

plastic thread of a definite diameter,      and confirm the best orientation of each

and with accurately-controlled crush       nut. For this purpose, uniformity is more

properties. In engine rebuilds it is used  important than the exact height. Then I

to measure the clearance in bearings       mounted one nut in each corner of the

etc. – you put a length of plastic thread 8x5 chase, and locked them up with a

in the bearing, assemble the bearing       quoin in the centre (see photo). I applied

shells and tighten them up, and then       a short length of plastigauge on the

when you remove them again the             upper face of each nut, and used scotch

thread has been squished according         tape (or sellotape in English!) to hold it in

to the space (clearance) left for it.      place. Having removed all packing from

By comparing the squished width of         the platen, and adjusted the screws for

the thread to a scale supplied on the      it to be pretty parallel, I put the chase

package it came in, you can read off       in place and did an ‘impression’. Worked

the clearance it experienced. The stuff    like a charm, confirming that the 4

is available at engineering and auto       corners of my chase were parallel under

repair shops, and is not very expensive. pressure to within +/- 0.001 inches or

It comes in different thicknesses          better.

according to the range of clearances       Maybe this technique (or variants of

you want to assess.                        it) could be useful to you. For example,

My next step was to visit a local if you need to figure out how much

hardware shop that sells loose or bulk shimming is needed to mount a type-

nuts and bolts. Taking along a large high block. You could even calculate

virgin character from a Stephenson how many spaces you will need to justify

Blake Founders font as a standard, I a loose line of type, but that would be

borrowed an expensive micrometer on overkill!

sale in the shop and verified the type      Best regards, Paul. (Ottawa, Canada).

height as .918 inches. I then

used the same micrometer

to select 4 large nuts that

had that same dimension

across two opposing faces.

Took a little while since

mass-produced machine

nuts are not necessarily

symmetrical, but when I

found the ones I wanted, I

labeled the reference faces,

and bought them. I left the

micrometer at the shop. The

machine nuts cost about $1

each.

Back home I had a pair                 Close up of the setup with the ‘plastigauge’ in place.

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