JCC Std Piston Size for 750

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gripper said:
So initially your machinists reported a size 2.867. now he reports that they are 2.869. Generally, digital vernier callipers are ok but a micrometer is the weapon of choice OMHO. And a proper bore gauge for the cylinder.
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Sorry for the confusion. The initial measurement of 2.867 was over the phone and from a guy in the parts shop that was selling pistons - I think he just used a caliper. Then I ordered the parts and took them to my local machinist. I watched him measure them with his micrometer and he got 2.8690.
 
wrecks said:
My machinist measured the std. sized JCC pistons this morning and they were 2.8690. He measured them with his micrometer which he checked on a test block prior to taking the measurements. My bores are 2.8750 so that will give me 0.0060 clearance.
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Sounds like you are good to go. Coulda used one fewer thousandth maybe, but better too much than too little, eh?
 
Frankie17 said:
What i find a little strange is Madnortons comment nortons cylinders / pistons have always been measured in MM since the late 40`s ??
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Check out the old manuals, even in the 30's and you will see bore and stroke in mm - it was the norm, even in the UK. Yes the piston manuals give inches, again a conversion, I have seen Hepolite criteria and working documents from which they produced the pistons and the catalogues and the documents are in mm. I recently challenged a manufacturer, and they had Hepolite designs going back to 1919. They were most surprised I was even asking if the pistons were imperial or metric, metric of course.
 
Madnorton

i have a huge collection of Hepolite information / catalogues going back to the 1930`s all the aftermarket motorcycle application quote imperial size first , after all we did not agree to adopt the metric system until sometime in the mid 70`s so imperial was our chosen method of measurement in the UK

models such as Alfa Romeo and other European models have oversizes quoted in MM not thousands ?? if all British motorcycles are measured and quoted in metric why do BSA / Triumph etc list all replacement pistons in the original parts books in imperial not millimeters

Look on Andovers web page pistons and shells are still quoted as plus or minus in imperial measurements 010/020/030/040 etc

yes early norton singles have the cases marked 82x100 etc but it would be difficult to quote imperial measurement ( 3.228 x 3.937 ) not enough physical space

i must admit the metric system is far easier to use and was probarbly quoted by manufacturers to simpify things selling motorcycles to the rest of the world using imperial measurement would be difficult

in the UK we still have wierd anomalies in our measuremt system beer in pints not litres , i will have 0.568 litres of your finest ale landlord should be how it is ordered as we officially adopted the metric system back in the mid 70`s ( only partially when it suits us )
i still order my beer in pints and order my pistons in imperial oversizes but have to buy fuel in litres and new carpet by the square meter

just remember we call the panel covering our cars engine a bonnet and over the pond its the hood
 
When I worked in manufacturing there were metric workshops and imperial workshops, the same part could and were made in both shops. The difference between the 2 types of workshops were the drawings and the measuring equipment. The metric shops had metric dimensioned drawings and metric measuring equipment and the Imperial workshop had Imperial dimensioned drawings and Imperial measuring equipment. When a workshop was converted from Imperial to Metric all the drawings were copied with Metric measurements in the weeks before the change and then over a weekend all the measuring equipment was changed, the drawing files were emptied of Imperial drawings and replaced with metric drawings. The work in progress parts were then machined and measured to the new drawings, but they were the same parts as before. The customers were from the UK, USA and Europe, they sent in their drawings and then new shop drawings were made for the planned workshop, if it was a metric shop then regardless of the dimensions on the customer drawing the shop drawing was metric.

A lot of the measuring equipment in my own workshop is Imperial cast off's. All you have to remember is there are 25.4mm in 1 inch, but better just to measure with equipment of the same type, so if its marked with imperial dimensions use imperial measuring equipment. I have a lathe with metric marked wheels so I do the conversion.
 
Frankie17 said:
What i find a little strange is Madnortons comment nortons cylinders / pistons have always been measured in MM since the late 40`s ??
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It goes back even further, as the first British motorcycles including Nortons, generally had French (or Belgian) engines!
By the early 1900s the standard of measurement for motorcycle engine bore, stroke and capacity in mm/cc had already been set.
 
Could well be, as the UK shortly after sold automotive parts to the world.

The motorcycle world would have just copied what was about at the time in the mass 4 wheel market, or they would have liaised with manufacturers to achieve a piston from a near casting for economy. This day, it can still be done, but the best result is to design a casting or forging die that is compatible with the piston design software.
 
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