Pistons

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.

hi phil yes I will post the information later today.
don't worry about the diff. in piston diameter they never seem to be the same size and there is a large diff. in most of them.
one think I have noticed is that he has not put clearance in the gap around the little end it should be cut back to give side clearance for the pistons because they are offset they will rub the side of the cylinder and seize. the number on the conrod looks ok from memory but I will check

Hi All

The latest news is that the other job that I did last night was to calculate the compression ratio of the right hand piston and cylinder.

I did this this by calculating the volume of the cylinder and head with the piston at TDC and then comparing it with the Volume of the cylinder and head with the piston at BDC.

I used a mixture of measurement and calculation for the cylinder volumes in both conditions. For the head I simply measured the volume of water required to fill the cylinder head with the valves and spark plug in place. Not the most accurate method, but it would produce a result in the region of.

The expected target was to get a compression ratio in the region of 6 to 8:1.

What I actually measured was a compression ratio in the region of just under 5:1!

Granted it was not the most accurate of methods, but it does suggest that the current combination of piston and conrod will produce a very low compression engine. If anything the piston is too low in relation to the cylinder top when at TDC.

Calculation tells me that in order to achieve a compression ratio of 7:1 the piston would need to be inline with the cylinder head when at TDC.

As Paul has suggested more thinking is required.

Just for reference purposes, I've measured up a standard conrod. It is clearly a forged item, but with no implanted ref. no., as yours has. (This is not necessarily significant: I think there were alternative suppliers to the Sopwith factory). Paul.

Hi All

Ok so a quick visit to the garage to check the connecting rod dimensions against Paul's marvellous drawing has confirmed the following.

1. Gudgeon pin diameter is 11mm.

2. Connecting rod length between centres is approximately 92mm.

3. Connecting rod web thickness is 5.3mm at the small end and increasing to 6.5mm at the big end.

Conclusion is that I am happy that the fitted connecting rods are as Mr Bradshaw intended. I have to say I am very pleased about this.

So moving on, the fitted pistons could be low a compression items. A quick search on Google as found that there are two types of pistons fitted to the Hillman Imp. The cars were fitted with 'standard' pistons had a compression ratio of 10:1 whereas the Imp van had a 'low compression' engine at 8:1. So I wonder which type I have?