Pistons

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5 years 3 weeks ago #552 by Peter Gillespie
Replied by Peter Gillespie on topic Pistons
Hi Paul
I have measured my connecting rod and my measurements are the same as yours except for the Big End. My journal on the crank is 20.8mm If the rollers are 5/16inch (7.9375mm) then , without clearance the total size would be 20.8mm plus 7.9375mm plus 7.9375mm total 36.67mm say 36.7mm with clearance. Your thoughts?
I have made new connecting rods EN36B and hardened the Big end to 60Rc I will post a photo later. I have also fitted Honda GX200 pistons third oversize 68.75mm and they fit great. i will run the motor shortly and let you know.
Regards
Peter Gillespie

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5 years 3 weeks ago #554 by Paul
Replied by Paul on topic Pistons
Much has appeared in recent times on our 'site about pistons, but little about the bores they have to fit. Working back from the oft-quoted capacity of 398 cc., and stroke of 54 mm. (taken over from the earlier Firefly engine), we arrive at a bore of 68.5 mm. But working forward from the official(?) bore of 68.6 mm., we get a swept volume of 399 cc.
Bores of cylinders I have actually measured, in their unworn areas, have varied between 68.5 and 68.75 mm.
Should one conclude that Sopwith were a little free with their bore tolerances?
68.6 mm., + or - 5 thou. would cover it.
It occurs to me that if Sopwith piston tolerances were as wide, that selective assembly must have been called for! (It is definitely necessary with second-hand, used parts! I wonder what the range is, of Imp pistons...)
Lots of luck, Paul.

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5 years 3 weeks ago - 5 years 3 weeks ago #555 by degraaff
Replied by degraaff on topic Pistons
The stroke of the "Firefly" is 44mm, the barrels have a very thin wall so you cann't size the 60mm up much.
Last edit: 5 years 3 weeks ago by degraaff.

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5 years 3 weeks ago #556 by Peter Gillespie
Replied by Peter Gillespie on topic Pistons
Hi paul
I agree with what you have said about the bores, 68.6 seems correct.
I have attached a photo of the new connecting rods
Regards
Peter
Attachments:

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5 years 3 weeks ago #557 by Paul
Replied by Paul on topic Pistons
Hallo again...
In reaction to Peter's query about big-end eyes, I can only say that the conrod I measured was a spare, never used by self, and of unknown provenance. Perhaps it had been ground out to true it up, thence to be used with oversize rollers.
To Mr deGraaf, de groeten en beste wensen uit Minchinhampton, and the Firefly engine's stroke of 54 mm. I quoted appears for instance on Page 104 of Barry Jones' biography of G. Bradshaw. A stroke of 44 mm. would need pistons at 60 mm. to achieve the Firefly's 250 cc, wow, now that's what I call oversquare. (Otherwise, a square engine at 54 mm. yields the proscribed 250 cc.) P.

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5 years 3 weeks ago #558 by Bradders
Replied by Bradders on topic Pistons

Peter Gillespie wrote: Hi paul
I agree with what you have said about the bores, 68.6 seems correct.
I have attached a photo of the new connecting rods
Regards
Peter


Who made these out of interest?

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