View Full Version : Measuring gap between curved surfaces

12-06-2005, 12:49 AM
Hi all, I am fairly new to machining and have a question regarding the proper way to take measurements. I have two curved surfaces that are parrallel to one another and need to measure the clearance between them. I cannot use any of the guages I have due to the curved surfaces. I would imagine there must be something like a gauge made from round stock or wire, but have been able to find anything. The clearance is going to be very tight, like .005". Any help is appreciated and sorry if this is a newbie question.


Forrest Addy
12-06-2005, 12:55 AM
Plastigage. You get it at auto supply houses. Important: read the directions on the package.

Your instincts are sharp. Old timey machinists used to use soft lead wire. The wire would smoosh to the clearance and when you disassemble the components the total clearance clould be miked as the flattened thickness of the wire. Another way if there is motion possible. Set up an indicator and determine the clearance by the available relative motion.

Herb Helbig
12-06-2005, 01:37 AM
Can you slip a feeler gauge between the surfaces?


12-06-2005, 09:31 AM
If you were measuring the clearance for a journal (called automotive bearing) then you can go to the auto parts store and buy plastic wire (called plastiguage). You put this in the unassembled bearing and assemble it together and torque to specs then take apart and hold it against the package that it came in and you align it to the "squished" size chart. This will tell you the clearance in thousands of an inch..

12-06-2005, 09:44 AM
Musical instrument strings (like guitar) come in small diameters down to .009" or so which can be useful sometimes as teeny plug gages.

12-06-2005, 11:22 AM
We sometimes use soft solder at work when we are trying to measure clearances. It only works well when you can apply pressure to ti though, in our case a press, or something like a bearing cap.


12-06-2005, 11:24 AM
Thank you for the information. This is actually the clearance between a Vespa crank journal and the housing/contact patch where the air/fuel enters the engine cases. The clearance on these cases increases due to many factors (debris entering the carb is probably #1) and the current mode of fixing is JB Weld or throwing away the cases. I have a 0 mile motor from the factory, so a friend is going to aluminum weld the contact patch and machine it to size if I can tell him the gap between components. Thanks again for the information, it is very much appreciated.

12-06-2005, 01:06 PM
Are you trying to measure the space between the case and the rotary valve? Probably not real hard to do, just a little tricky.

I would think they might show that in a book.

There is a huge following for these scooters, and there should be some kind of club or ten in your area.

And there should be some kind of shop that works on them.

And more than likely a Yahoo group.


Paul Alciatore
12-06-2005, 02:32 PM
Re: the wire gauge idea

They do make and sell them. I have a couple of sets of wire gauges that are used in VTR maintenance. They were sold by Sony.

Paul A.