Piston crown heights
Turbo Mini Forums

Is the crown height on mega and omega pistons the same as standard turbo pistons. i.e do they go all the way flush to the deck like the turbo pistons?

Edited by Turbo Tel on 6th Nov, 2006.

website:- http://www.terryhunt.co.uk

the omegs do

my megas are 35 thou down with 7cc dishes i get 14.5cc total deck volume.

not ideal i know but i aint going to strip engine down for the sake of 35 thou. i will wait until my rings a goosed first.

always looking for them bigger bunches of bannanas

Ok I guess I am finding that they are all different! What's confusing me is that convention says that "standard" pistons have around 5.2 cc of deck but my measurements and theory as well say that the turbo and regular Metro engines have pistons that go up to the deck.

Perhaps there's a different block height in there somewhere as well? I'll just have to make sure I get the crown heights pinned down before I buy.

website:- http://www.terryhunt.co.uk

11 cc dished omegas

difficult to be very precise as it woud need to re mesure the used pistons which are sowhere it the worshop jungle.
but if my memory is ok, I decked the blocks for the mega's turbo +40 to be flush by 0.8 mm, and the mega's +20 by 0.5 mm, the 20831 and 832 being flush from factory , correct me if this is wrong, if really needed I"ll look for the bag and will make the mesures
cheers
fab

I contacted med and they said that Omega crown height is 1.499" I will call Avonbar tomorrow to get the height of the "standard" +40 but suspect your numbers are about right, for the 9.7 cc to work out at 9.4 CR the height must bee less than the 20831's which I also get going flush. I'll post the numbers when I get them.

btw do you know what engine the 20832's come from? I found them in my metty engine but I get 10.5 CR ( 8cc --flush to deck)but that should only be the MG..

website:- http://www.terryhunt.co.uk

yes 10.5 cr mg mety not the 9.75 ones

the metty piston had about 6 or 7cc dishes the turbo has 11cc but i think the crown hight was the same but i could be wrong

always looking for them bigger bunches of bannanas

mine are a nats cock proud of the block

I called around a bit and got the following:


Piston=height=approx deck cc (piston to deck)

Turbo original = 1.528" 38.8mm = 1cc (10THOU) (20831 12cc dish)

"Standard" = 1.497" 38.02mm = 4.1cc (40thou)

Omega = 1.499" 38.07mm = 4.1cc (40thou)

mega = 1.498" 38.05mm = 4.1cc (40 thou)

turbo+40 = no reply but must be "standard height" to work with a 9 cc dish

21253/20832 = 1.528" 38.8mm = 1cc (10thou)

forged omegas = 1.528" 38.8mm = 1cc (10thou)**CHECK THIS.. MED now saying 4cc***

Accralite forged = 38.0mm = 4.1cc (40 thou)

The piston heights are pretty accurate but the other numbers are just a guide..Thanks to Morspeed, Avonbar and MED, no thanks to Minispares who didn't even bother to answer my e-mails.

I ended up ordering the "Mega 13cc's" from Morspeed, as I felt they gave me a slight reduction in compression (8.5) without messing around with the head, and any skims the head/block will need still keeps me a bit lower than std which is where I want to be.

EDIT> And after I took out 2 sets of them (land failures) I ordered some Accralites!!

Terry

Edited by Turbo Tel on 31st Jul, 2008.

website:- http://www.terryhunt.co.uk

mega 73.5 are 0.7mm down 20831 piston

I got 10thou from the deck with my std turbo pistons so recon on 1cc deck volume

*edit* LOL i shoud have read all the topic first, its already been noted

Edited by Sprocket on 12th Dec, 2006.

Omega = 1.499" 38.07mm = 4.1cc (40thou)

So if i'm reading this right, there's a 10cc dish in the cast Omega pistons & then 4.1cc in the crown to deck height, yes ?

WWW.TURBO-MINI.COM

I would have thought that most aftermarket pistons will use the shorter deck hight to enable several engine configurations, allowing shaving of the block to what hight you want.

website:- http://www.terryhunt.co.uk

Thanks to Gavin at minispares I have been informed that their 73mm evo pistons sit approx 10thou down the block on a standard deck height.

If something is worth doing, it's worth doing half of.

acralites are a crown hight of 38mm, same as my old stock 1275 cooper S power max pistons.

I had to machine about 1.5mm off the top of the block to get the pistons flush. I have built an engine with Mega pistons and from memory, that had around 0.060" deck hight, and we took 0.040" of that off, to bring the engine back to standard compression. again from memory, building the 1399 short block, I left the deck hight standard and with the new old stock 74mm Omega's I recall a igure of 0.040".

0.040" deck hight is a safe bet with standard hight block, but could be as much as 0.060".

I feel confident enough to say that the modern 'standard' after market piston compression hights from Omega, Minispares Mega, and Acralite are all 38mm therefore with a standard hight block, will be in the region of 0.060" deck hight.

Andy, why not build the engine up without rings, maybe just one piston and rod with the crank and measure it, then shave the block? I had a conrod modified just for this purpose, its basically a floating pin little end with good used big end bearings of the apropriate size. This allows me to build up the one rod in the block and measure the deck hight before I take it back for shaving. If you want, I can bung it in the post?

Thanks for the info and the offer. I'm still at the parts hoarding bit of the engine build. I'm thinking it's pistons for my little crimbo treat.

With the accrolites no longer an option, I'm working out what to use. The plan was to buy pistons, then have the block bored and do a dry build for measurement.

Mr Austin has a set of my rods for working, including a conversion to fully floating pins. It's likely I'll ask him to do the same to the pistons (unless they come floating pin as standard). He also offers block and head decking services so that will be something else I will likely bother him for.

On my forged Omegas (18cc) I need to remove 0,75mm to get the pistons flush with the deck.

Edited by Brett on 2nd Dec, 2010.

Yes i moved to the darkside

Instagram @jdm_brett

yes, cast.

by my maths 0.8mm deck height at +0.020 ((2x pie )x 7.112) x 0.08) gives 3.575cc (approx) so is within my tolerance.

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

Cheers Carl

By +0.020 I meant the piston/bore diameter.

With a 28cc head and minimal skim to the block and head should give me around my 8:1 target.