Piston Q!
Turbo Mini Forums

I got some of the hypatec 73.5mm pistons, Ive just measured the piston itself and it comes out at 72.7mm, is this right or do I have a set of 73.0mm pistons????

Box says +0.114" (73.5mm) its just I expected the piston to measure that..............

Where are you measuring it ???

I have the 73mm Hypatechs and they are 73mm where they should be, near the bottom of the skirt, front to back. (EDIT - actually 72.96 at the skirt in my case, for running clearance but a proper machine shop will measure the actual skirts and bore accordingly for whatever running clearance you specify)

If you measure at the top, above the top ring..... well don't... it will be smaller there.

Edited by Rod S on 12th Aug, 2008.

Schrödinger's cat - so which one am I ???

yep,

pistons are a funny shape to allow for expansion.

measure them on the skirt below the gudgeon pin.

oh balls, I measured just below the crown!! Ill remeasure tonight and let you know what it is.

Front to back is across the pin right???

90 degrees to the pin - on the Hypatechs you can only measure 90 degrees anyway (below the rings) as they are recessed at the ends of the pin.

EDIT - BTW, you still need to know the piston diameter above the top ring (along with the height, top ring to crown) if you are doing your CR calculations accurately - it's the smaller diameter of the piston in this place that gives the "ring land volume" for the CR calculations.

Edited by Rod S on 12th Aug, 2008.

Schrödinger's cat - so which one am I ???

Yep, Im officially a dumbass!!

Measured the skirt and it comes out as 73.45mm derrrrrrrrrrrrrrrrr

Good....

Make sure you give the machine shop at least one of the pistons - so they can measure with their own micrometers - and tell them what skirt to bore clearance you want.

As per the other threads, I would strongly recommend offset boring for this size..... Your head gasket will appreciate it !!!

Rod.

Schrödinger's cat - so which one am I ???

Rod, what sort of clearance should I be after?? This isnt going to be a turbo or supercharged engine if that makes a difference.

And is that a Megasquirt in your avatar??? Have you mastered the mapping of it yet???

5 thou us OK. That means if you have it bored 73.50 you'll be OK. I've measured 20 MEGA pistons recent years, and they are all the same size, New modern pistons are so equal that it's almost not necessary to measure.

5 thou sounds a bit large Vegard...

Difference between 73.45 and 73.50 is 0.05mm (not thou) is approx 2 thou.

Haynes quotes bottom of skirt clearance as 0.0012" - 0.0022" (1275GT) and 0.0019" - 0.0025" (Cooper S)

So all around the 2thou (or 0.05mm) area, ie a spot on 73.5mm bore should be fine.

After asking the same question on here a while ago, I personally chose to instruct my machine shop to give me 0.06mm (or 2 1/2 thou) because I'm going turbo.

Do a search on here and you'll find quite a lot on people's opinions and problems on this subject.

And yes, it is Megasquirt - but I haven't finished building the injection hardware yet so no mapping....

Schrödinger's cat - so which one am I ???

50thou as in millimetres. You have got your measurement in mm and your bore in mm. Why the fuck confuse matters with the clearance in imperial? :)

Schrödinger's cat - so which one am I ???

as a rule of thumb most cast pistons need a 2thou speil clearance and forge pistons 3 thou

always looking for them bigger bunches of bannanas

if you get the piston to bore clearance a little small by mistake you can always do what people do at all the over night mini shows. that is, get up really early, start your car and before your hand has released the key, plant your right foot to the floor and rev the absolute tits of your engine until it worms up.

PMSL @ Shed


Where should you measure clearance though? With the piston at the top/bottom/middle of the bore?

Warning:Stig in Training