51 replies to this topic

[mini13]

the bore clearance depends on the piston material and design...

typically piston sizes are all over the shop frm manufacturer to manufacturer, I garentee there is little chance that neither the bore or piston size of any available 73.5mm pistons is 73.5mm

[Southy]

The 73.5mm or 2.8937" is the finished bore size, the piston will have to have a piston to skirt clearance of a couple of thou, this is to take into account thermal expansion of the piston whilst running. Typically on a road car you use a smaller bore to skirt clearance than you would on a full race engine.

You really need to get the block correctly measured up to find out what size it is actually at as you maybe doing all this messing about for no reason. If you were closer I would of gladly come and measured it up for yourself. If you take it to your local machine shop I'm sure they would be more than happy to size it up for you

[mini13]

NOOOOOOOOOOO!!!!!!!!!!!!!

you CANNOT!! just say the finished bore size is 73.5mm (2.8937")

my current "73.5mm" pistons are 73.42mm (2.8906") they requre 2 thou clearance, making an 2.892.6" bore or 73.472mm

if you finnished to 73.5mm they would be outside their specified max wear tollerance.

Edited by mini13, 03 February 2011 - 07:16 PM.

[1977 Loud_Mini]

Yeah i think i'm going to have to take it for measuring as i don't think i measured it accurately enough. I thought it would be around the 2 thou mark smaller than the bore as i went for 3 thou.
Thanks for the help.

[Cooperman]

Nice work, but I think they might melt

Dimensions are bore size rather than piston size.

Haha yeah i should think they would
I thought that was the case, well whoever the previous owner had bore the block must have got it a bit wrong. I made the piston around 3 thou smaller than 73.5mm which isn't right in my case. I think i will have to get the bore professionally measured rather than doing it myself with a vernier gauge and then judge the piston to bore clearance. Do you know if it's normally 3 thou clearance?

The pistons are not a constant diameter from top to bottom, so the piston to cylinder running clearance is not one single figure.

[1977 Loud_Mini]

Nice work, but I think they might melt

Dimensions are bore size rather than piston size.

Haha yeah i should think they would
I thought that was the case, well whoever the previous owner had bore the block must have got it a bit wrong. I made the piston around 3 thou smaller than 73.5mm which isn't right in my case. I think i will have to get the bore professionally measured rather than doing it myself with a vernier gauge and then judge the piston to bore clearance. Do you know if it's normally 3 thou clearance?

The pistons are not a constant diameter from top to bottom, so the piston to cylinder running clearance is not one single figure.

Thanks for the info, I realise this as it was mensioned by mini13 i think who said his powermax 73.5mm pistons were tapered 5 thou from top to bottom. I will be investigating this further so i can make amendments to my piston design.

[MRA]

the bore clearance depends on the piston material and design...

typically piston sizes are all over the shop frm manufacturer to manufacturer, I garentee there is little chance that neither the bore or piston size of any available 73.5mm pistons is 73.5mm

Agreed, we rebore on site, and always to the pistons that are going with the block..... never assuming that the pistons are a definate size because they aren't...... some are better than others, however there is always going to be a variation within a batch, and even more so between different batches