Numbers on pistons what do they mean

Post by **ssnjimb** » Sat Apr 11, 2009 8:38 pm

Hello.

when changing the cylinder head on my minor did a spot of cleaning.

I removed all the carbon from the piston tops so they now look like new.

Each piston has a 3 on it what does this mean and how can i tell if it has been rebored etc.

James

Post by **PSL184** » Sat Apr 11, 2009 8:43 pm

When engines were built at the factory all internal componants were graded depending on their size in relation to nominal (tolerances these days) The pistons were stamped either 1, 2 or 3 depending on this criteria. If it's been rebored the pistons wil be stamped +20, +30 etc...

Post by **ssnjimb** » Sat Apr 11, 2009 8:45 pm

hello psl

What are the differences between 1 2 and 3.

The pistons are not stamped with anything else except there mowog part numbers. The engine seems to be standard bore with 91k on the clock and the cross hatch markings are very visible.

James

Post by **bmcecosse** » Sat Apr 11, 2009 10:41 pm

The '3' was probably inside a diamond shape ? If so - it was a size tolerance at the factory - not significant now. The original bores were not 'cross-hatched' - so if you can see that now, someone has at least fitted new rings and 'glaze-busted' the bores! Ideally you should NOT have cleaned the carbon off the pistons - best to leave well alone!

Post by **mickessex** » Sun Apr 12, 2009 8:36 pm

bmcecosse wrote:The '3' was probably inside a diamond shape ? If so - it was a size tolerance at the factory - not significant now. The original bores were not 'cross-hatched' - so if you can see that now, someone has at least fitted new rings and 'glaze-busted' the bores! Ideally you should NOT have cleaned the carbon off the pistons - best to leave well alone!

I was always told to clean the carbon from the middle section of the piston, but not around the edge. There is also a number preceded by 'J' what is this number? I have 3 pistons with 'J31' and 1 with 'J25'.<br>

<br>

Mick

Post by **bmcecosse** » Sun Apr 12, 2009 10:07 pm

Not so bad if you leave an untouched ring around the edge - but really there is no point cleaning the piston! I have no idea about the Jxx number!!

Post by **PSL184** » Sun Apr 12, 2009 11:57 pm

mickessex wrote:
bmcecosse wrote:The '3' was probably inside a diamond shape ? If so - it was a size tolerance at the factory - not significant now. The original bores were not 'cross-hatched' - so if you can see that now, someone has at least fitted new rings and 'glaze-busted' the bores! Ideally you should NOT have cleaned the carbon off the pistons - best to leave well alone!
I was always told to clean the carbon from the middle section of the piston, but not around the edge. There is also a number preceded by 'J' what is this number? I have 3 pistons with 'J31' and 1 with 'J25'.<br><br>

Mick

Note your "3" on the block which matches the "3" on the piston.