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.

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?

They were made by my brother in law here in Johannesburg South Africa . They are EN36B and they were rough machined then they were painted off and only the big bore was carburised and hardened to about 60Rc the rest of the Conrod is only about 35Rc. We used the program to find the weak spots and without you being able to see have corrected a few spots.
The pistons are Honda GX200 0.75mm oversize 68.75mm. these pistons more resemble the ABC originals with the centreline to crown height being 23.3mm compared to 21.55 in the standard ABC. this should bring the compression ratio up to about 5.2:1 , very respectable, and it puts another life into old cylinders.
You can see how we have made the Gudgeon Pin . we will be using the standard Honda circlips
Regards
Peter

There is a little to be cut off the skirt as well

When it comes to pistons the options are endless. I was trying to find a piston that we could all just buy off the shelf in any part of the world using a part number but this does not seem to be possible. One of the big issues is the size of the gudgeon pin at 11,1125mm (7/16"). Most pistons that are commercially available have much larger gudgeon pins and there are multiple ways to deal with this including making a stepped pin or re-bushing the small end of the con-rod.

A further problem is the small size of the compression height of the Sopwith piston at about 22mm. Most commercial pistons like the Hillman IMP Pistons have larger compression heights and this results in higher compression ratios and the associated issues that go with this. Most pistons will need at least some form of machining and at the very least some of the skirt cut off. Many of the pistons do not have the correct piston top shape.

I have looked at casting exact replicas of the Sopwith piston and this is possible but because the numbers will be so small it is largely uneconomical.

The members feel that the right way to go is to have custom made forged pistons using modern day technology with much narrower rings that would be more suited to the Bradshaw Steel Cylinder. Most piston manufacturers are not happy to use such a small gudgeon pin in a piston of that size.

I am awaiting a quote from a piston manufacturer for pistons at 68mm and at 69mm that will need no modification other than the supply of a new con-rod bush. At 68mm we will lose a few cc's but the whole arrangement is a good compromise and after a full life will still have the option for a small re-bore.

I will let you know once I have received the quote.

Regards
Peter

Hello, thanks for investigating this. We have used arias for manufacturing pistons before, they made the pistons for our ABC car. If you would like we could find out how much they would want for a small batch to be made. They are of decent quality.

George