Pistons

Hi All

Well I hope you all have enjoyed this week as much as me. It has been a lot of fun going through the process of identifying the condition of the main part of my engine.

Paul is absolutely right the time has come to remove the pistons and come up with a plan.

So plan for future works is:

1. Remove pistons and rings.

2. Check the fit (My concerns with the pistons are that they are Australian. A friend had an Australian piston in a Bantam on standard fit which used to seize regularly. Obviously keen to avoid this). If the fit is supect I will replace them.

3. Modify the pistons to increase the side to side play for the small end (again to prevent seizure). Also reduce the skirt length to mimic the geniune piston skirt profile and allow improved oil access to the cylinder bore (also to get close to the weight of the original pistons)

4. Put it all back together again.

5. Move on to the rocker issues (could be another thread called 'Rockers', who knows).

I think I am going to start the strip down tomorrow. Thanks evrybody for you input on this, it has been very hellpful.

Phil

Hello,

The engine in my ABC Sopwith was also fitted with Hillman Imp pistons. No spacer plates were fitted and as in your case the pistons do not reach the top of the cylinder. I do not have the original pistons so I am not sure if these also do not reach the top of the cylinder. What I do wonder is if the compression ratio has changed from the original pistons. My engine runs very well on the Hillman pistons so I am not too worried.

Richard Johnson

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

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.

The stroke of the "Firefly" is 44mm, the barrels have a very thin wall so you cann't size the 60mm up much.

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