557 SV piston

[Roger Gwynn]

The pre 1931 side valves use a much shorter piston than the later ones typically 1.56" compression height rather than 1.95" of the later ones. I don't know of an acceptable substitute. Have a look here for a list of pistons and rings https://draganfly.co.uk/ariel-technical-articles/

[RayBlack]

Thanks Roger. The piston list does not suggest one that would easily fit in the '28 SV. Which makes me wonder whether the 'mystery' piston that came with the engine had actually run in it...

[Roger Gwynn]

There is likely to be something out there to fit, I was told that my '26 has a JAP piston when I bought it, but i have not had the cylinder off it to find out.

[nevhunter]

Check what the distance from Piston pin centre to piston top (cylinder valve platform) is and the bore size. Does the earlier model have the 1'' pin? Nev

[tony.barnett]

You'll need to look in a Hepolite catalogue to find a suitable piston, and no doubt it'll need modification as will the little end bush for it to fit. From memory, the 500 Velocettes were around 85mm bore - might be a possibility there, but there will be others.

[Roger Gwynn]

Compression height = cylinder valve platform. New one on me, everyday is a learning day.

[RayBlack]

Nev: the compression height/cylinder valve platform should be 1.56in. I've no idea what the mystery piston height was as it has disappeared. The original small end bush should be 1.00in, but this has been modified in my engine to what appears to be 0.812in (although I haven't measured this accurately yet).

Looks like I still have some work to do to find a suitable piston, but there is still hope... Are Hepolite catalogues available online?

[nevhunter]

JP in Australia (Under new management) make pistons to order and better by far than they used to be. Lighter and more modern design (Better dies). In a blind cylinder it's more difficult to determine where the Piston crown should go to at TDC. These types of cylinders often get rebored too high up and the top of the cylinder can come off because it's weakened. Not ALL reconditioners can rebore them. Give a SV plenty of clearance and use 3 compression rings and plain skirt piston for more area to get the heat out..They run hotter than OHV's. Nev

[RayBlack]

I found a 1963 Hepolite catalogue online so I am going to see if there is anything in there that I can make work. If not and nobody finds a lost hoard of original pistons, I will give JP a try

[nevhunter]

The exchange rate is well in your favour. Nev