Forged Pistons - What size and why?

Which size forged piston to fit in a rebuild?


  • Total voters
    31
G

GTI-R Kid

Guest
I've been looking at forged pistons for some time now but i'm unsure what size to go for or what reasoning is behing it.

I understand that they need to be larger than the original as the block will be slightly rebored to fit them and get rid of any scoring caused by the originals - but which to choose - 86.5 or 87mm?

And what difference could one expect (apart from the slight volume difference)?

Can anyonme shed some light on the differences please?
 
A

AZ STE

Guest
Generally you go next size up unless you bores require the extra 0.5 mm due to damage. By going to 87 straight away you are reducing your block life as you are loosing the possibility of one rebore.
 
S

Sirnixalot

Guest
The largest you can bore these cylinder bores out before block integrity becomes an issue is 87.5mm. If you get the block resleaved i have seen a reliable motor that has 90mm cylinder bores. I would go for the 87mm just for the extra cc's if displacment. But thats only if you have the money and your engine builder has the proper facilities to bore and hone the cylinders. If the cylinder bores are off axis then your block will be screwed....think of it as your pistons going up and down like this.../....instead of this ...|...
 
G

GTI-R Kid

Guest
So the only reason for going to 86.5 is as a precaution in case the engine will need rebuilding and reboring again in the future?
 
S

Sirnixalot

Guest
that and the fact that YOU MUST hone the cylinders when putting in a new set of piston rings so the seat properly. You must make sure your engine builder knows the size of the pistons so he can know how much his honing machine will have to take out of the walls.
 
G

GTI-R Kid

Guest
Thanks Sirnix.

I thought i would have gotten more replies given the amount of members that have had engine rebuilds, but your input is very much appreciated. :)
 
G

GTI-R Kid

Guest
Eh - if Sirnix has it right (in recommending 87mm) then why are the votes in the poll 4:2 in favour of 86.5mm :?: :?
 
S

Sirnixalot

Guest
you really wont notive it but every little bit helps right! :oops:

86.5 is plenty good for a rebuild but if you want to go that extra inch.....one of those "while im in here" kinda things
 
G

GTI-R Kid

Guest
Poor Edd really is beginning to scrape the bottom of the barrel at this stage, think it might be time for some new material. :roll: :wink:

So what have the majority actually used, or plan to use then? (Piston size people - not sources for new insults for Edd :lol: )
 
S

Sirnixalot

Guest
ive seen more 87mm setups than 86.5mm to tell you the truth.
 
G

GTI-R Kid

Guest
Sirnixalot said:
ive seen more 87mm setups than 86.5mm to tell you the truth.
Thats what i believed to be the case too Sirnix, but strangely enough th voting on the poll doesnt seem to reflect this. :?
 
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