My 1040 Project

20/02/2021 - Piston Compare
I have been doing a bit of research into the Thunderace pistons and if any other machine has the same measurements. I found an early off road bike (YZ250F - 2001) had 5 valves per cylinder and a bore of 77mm (which is a Thunderace 1040 bore) so I bought a second hand one from Ebay for £10 so that I could compare. the reason being is that the one from the yz250f brand new was approx. £90, multply this by 4 would give me £360, consider then that a 1040 big bore kit for a Thunderace is £650+ this would be a great saving.

firefox You can probably see where I was coming from in this picture to the left, this is a YZ250F piston and looks remarkably like a Thunderace piston, the std bore size for the 250f is 77mm AND the bore size for a 1040 Thunderace is 77mm.
I was very excited thinking that I could save myself over £300 but this was short lived 😞

Anyway, undetered I continued to measure up the piston and compare the results with a Thunderace piston and these are the approx. results of the measurement

  Thunderace   YZ205F
Top to Bottom of Side skirt   46.20   37.80
Gudgeon pin width   58.00   49.50
Gudgeon pin diameter   19.00   16.00
Top to Gudgeon pin Center   24.00   23.00
Little end width   23.00   18.50

The main differences are around the Gudgeon pin dimensions. All the rings lined up perfect. So it cost me a tenner to find out these differences but I think it was money well spent as I now know for certain that due to the gudgeon pin diameter and width it would not have fit the Thunderace rods. If these had been the same as the Thunderace then we may have been onto something, there is also the 1mm difference in the little end pin center from the top of the piston, as the Thunderace is 1mm more this would mean that the YZ250F piston would have maybe hit the valves as it would have been 1mm closer to them.