Factory short stroke R7 head and pop up pistons

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jseng1

jseng1

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Here's a drawing of a Shortstroke R7 big valve head with full hemisphere. They claimed 10.5:1 C.R. but my calculations show lower than that.

Factory short stroke R7 head and pop up pistons



The Omega pistons and original steel rods were too heavy and need a complete re-design. Idea for new piston layout shown above. Crown shape is the same as original Omega.
 
I don't doubt it Jim, I still have the Omega pistons and when I built an 850 using one of those heads it was about 10.25:!

When I dropped a valve I went to Mick Hemmings for repairs. He skimmed 40 thou off the head and 60 thou off the barrel to get back to the same compression with Hepolites!

When I assembled it that time I had to have a small chamfer cut on the pistons to clear the head.

Remember that in those days you had to push start, it was an advantage not to go wild on compression, you could gain more from a clean start than a second per lap from a little more compression.
 
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SteveA said:
I don't doubt it Jim, I still have the Omega pistons and when I built an 850 using one of those heads it was about 10.25:!

When I dropped a valve I went to Mick Hemmings for repairs. He skimmed 40 thou off the head and 60 thou off the barrel to get back to the same compression with Hepolites!

When I assembled it that time I had to have a small chamfer cut on the pistons to clear the head.

Remember that in those days you had to push start, it was an advantage not to go wild on compression, you could gain more from a clean start than a second per lap from a little more compression.
Click to expand...
I agree with you. I calculated an 850 with this head to have approx 10.2 to 1 C.R. I figure a 750 short stroke would only have approx 9.3 to 1 C.R. The deep valve pockets loose about 5cm.

Original photo of Omega pistons courtesy of Ken C.
Factory short stroke R7 head and pop up pistons


rod comparison - factory original on right.
Factory short stroke R7 head and pop up pistons
 
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WP_20210201_14_43_55_Pro.jpg


Pictures courtesy of me! This one has the standard size Norton piston or gudgeon pin. So could be used with standard rod and crank. This enable a build similar to the 850s used by Thruxton when they were not running in F750. A lot of UK racing included a 501 to 1000 class. And of course Barcelona Endurance was more open. I even tried some of those long straight pipes with a slash cut at one point, similar to what Croxford used for Barcelona.

My crank (and cases were MkIII. The crank had been used at Norton Experimental to run tests on crank triggered ignition.
 
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