Pistons

images of 3 pistons the one on the left is a bhb which came out of an abc which was part resored it was a good fit and only removed because i did not want to alter the inlet manifold i dont know what it is from but notice it is concaved so a better option than the imp.the centre one is the imp and the righhand side the original abc.does anyone know what the bhb is it has h72 stamped on the crown.

Hi All

So last night I withdrew the cylinder from the crackcase, but not completely off the piston. I don't want to remove the pistons unless I really need to. So below are some further pictures.

First picture is the piston in the end of the cylinder and the conrod small end.


Next picture is another view of the same thing.


Next is a view of the big end and crank through the crankcase cylinder port.



Things that I have noticed other than the position of the piston when at TDC are:

1. The gudgeon pin appears to be sleeved, which would follow with what Keith as said above.
2. The connecting rods do not appear to have been shortened and so I am going to assume that they are as they should be.
3. There is no play in the big end. Very happy about that.

Only area currently for concern is the tightness of the piston fit. I note that Keith is running an engine with +030" pistons and the ones I have are +040"! So the tightness could be the size difference or the fact that the engine is currently without any oil in it. Irrespective of manufacturer the size should be constant.

Last picture is a BHB Imp piston marked 'D66'. apparently these were in the engine and removed by the previous owner because there were too loose!


I am going to have a go at working out the compression ratio as Paul has suggested, next. I'll post what the result is.

Keith, can you measure the distance from the centre line of the gudgeon pin to the centre of the piston crown? I'd like to compare it with both the old Imp piston and those currenlty fitted.

I have had another look at the con rods again this evening and there are definitely no signs that they have been shortened.

Did ABC or anybody else make a shorter con rod?

Mine are marked 'S258'and I haven't been able to cross refer that to any parts information.

The 125 and 250 have a 44mm stroke and 60 mm bore so the conrod willbe shorter but i see they are differend to yours.

Exciting stuff! The Mystery Pistons! Barrels and conrods appear to be unadulterated (=not modded), and even the gudgeon pins look normal. In principal, it would be possible to fit very eccentric gudgeon bushes to the pistons, with original (diameter 7/16", I think) 'pins, to reduce the compression height. But also this, seems not to be the case...Hmmm... back to my think-tank for a bit! Paul.

The compression rate is looking good to me.