Morris Minor Owners Club

a piston will be on its way shortly courtesy of peterdjon,the purpose of this message is...when you mucked about with pistons have you realised that there is a right and wrong way to position this on the gudgeon pin, explain further pin is nearer to one side of piston thant uther. re,john newton.

Are you saying the pin is offset?

that is right les you got the word in one,i have stripped engines in past and discovered that other people do not know this.ihave not seen evidence of damage. i should however like to know more about this. anybody?? re john newton.

You are correct for some pistons but generally they are not offset.

Where a piston is fitted with an offset gudgen pin it is done to reduce the amount of piston slap (rock) at around TDC.
If the gudgen pin is offset - it is towards the side of maximum piston thrust.
With the offset gudgen pin, a proportion of the piston thrust is transferred from one side of the cylinder to the other in an attempt to equalise the piston thrust therefore reducing the piston slap and the noise created by piston slap.
Offset of the gudgen pin is usually about 0.0125d...............d = diameter of the piston.
I am not aware that the original pistons fitted to the Minor 'A' Series had offset gudgen pins; and there appears to be no reference in the BMC workshop manual to the fitment of pistons with offset gudgen pins.
Most current aftermarket pistons have no gudgen pin offset.
With the Minor split skirt pistons the piston has to be fitted with the split skirt opposite to the thrust side. That is why the piston is marked 'FRONT'.
The shorter the piston the more prone the piston is to slap. The original pistons fitted to the 'A' Series were quite tall with quite a lot of skirt below the gudgen pin which did help reduce piston slap or rock.
If the pistons split skirt is fitted to the thrust side - the pressure on the split skirt could cause the piston to fail.
Split skirt pistons have the piston ring(s) around the split skirt secured against turning by means of a peg. If the piston ring was allowed to turn the end could jam in the split which could result in a broken ring and damaged bore.
Whilst bores are round pistons usually have a small amount of ovality built into them during production with the max diameter at 90 degrees to the gudgen pin.
As regards original Minor pistons - they were graded to the bore and that is why the top of the block has a number (adjacent to the bore) which corresponds with the number in a diamond on the top of the piston. It is no good putting a grade three (3) piston in a grade one (1) bore otherwise you could end up with excessive piston slap or a piston tight to the bore. Not all new ex factory bores were spot on.
When re-boring an engine today it is important that the bore is done to suit the piston and not just done to suit the nominal bore size.
There is a lot more to the piston and bore relationship than first imagined.
I hope that the above is of help.
Phil

thank you phillthehill that is something to add to my list.as i sit here with tape measure this piston is offset, more grist to the mill. i shall of course inspect rest of them.. re john newton