This is not what a piston should look like
Turbo Mini Forums

I thought It might be due to the omega's having a "round" skirt, seeing as all the above pistons are blowing in a similar area above the gudgeon pin. Then again the 20773s seem to have a similar design and don't have a "round" skirt.. interesting

Edit**

I might be wrong, struggling to find a picture of what a standard 998 20773 piston looks like.

Those are definitely not metro turbo pistons, most likely just standard overbore 1275 pistons. AE?

Edited by CR# on 8th Nov, 2009.

Edited by Brett on 8th Nov, 2009.

Yes i moved to the darkside

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my Fekked ones are AE ... Dontknow eho makes them am guessing AE lol .. but am sure the omega's will be much better

The 'slipper' piston were used on late A+ 1275 engines and the AE 21251/ 21253 are almost the same piston. These have a slot behind the oil ring. The MG metro and the MG Metro Turbo engines used a full skirt piston with holes behind the oil ring and a thicker crown. The NA piston has a dish of around 7cc while the turbo piston has a 12cc dish. The turbo piston uses a 1.5/1.5/4mm ring pack, cannot comment on the NA MG piston piston.

Slipper pistons reduce internal losses due to reduced friction. They are not necissaraly a good thing on a high reving A series engine as the piston is more unstable with the slightly larger piston clearancies, which is why most high performance pistons have a full skirt or like the Omegas, a combination of the two designs. Does not mean you cannot use slipper pistons in such applications.

That does not stand for other designs of engine though. Its all to do with the crown/ wrist pin design with modern engines having a very compact pin to crown high compared to the MASSIVE pin to crown hight of an A series piston. By nature the A series piston is more unstable.

Also, looking at most of the piston failures on the A series engine the pistons fail either side of the pins, not always, but most of the time. Perhaps something to do with poor cooling between the cylinders?

Ring lands break due to excessive cylinder pressures and the biggest source of excessive cylinder pressures is detonation.

Thanks Sprocket. That gives me a little more to think about. Currently have what I think are standard low cr 1275 na pistons(think i calculated the cr to be around 8.5:1), which I understand I will be heavily advised not to add boost to.

On the slot/hole debate, how are some of the oz lads able to run boost on the standard overbore hypatec pistons? Iirc, its only the hugely oversized sizes ones that sport the holes over the slots.

Greg

Edit** Should add the pistons I used when rebuilding are probably one's you wont be familiar with. Think they an oz brand we get here in S.A. They were Duralite +0.060.
Should of said "how are the oz lads able to get away with running boost ... "

Edited by CR# on 8th Nov, 2009.

from what you say sprock, it seems all the more reason for someone to produce a longer rod for the a series motor, then we could egt away wih more modern piston design and/or less skimming wih short stroke cranks

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"

yes, but its expensive as we then will need custom pistons AND rods, the idea is VERY good though

Actually I think you are wrong Vegard ! Lol. The Turbo pistons are indeed 12cc.

WWW.TURBO-MINI.COM

I'm with Phil on that one. They have a thicker crown briging them to the top of the bore, so need a bigger dish to get the engine back down to 9.4:1 CR.

I'll measure a set when I get home. When that is said, a 21cc head and 12cc pistons will NOT make 9.4:1 CR in a standard engine.

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=711

Use this if you want Vegard... Turbo piston is circa 12cc. At the very least, I'll say 11.5 to 12.5cc to avoid being finicky!

The way I work it for a turbo engine is

Bore 70.61mm
Stroke 81.23mm
Gasket Volume 3.80cc
Ring land Volume 0.5cc
Piston Volume 12.00cc
Head Chamber 21.4cc
Deck Volume 0cc

Giving a CR of 9.44:1

On a standard 1275 N/A Engine I work it as 6.5cc piston dish and 5.5cc deck height.

This also gives a CR of 9.44:1

Is this right or wrong?

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

0.5cc added for ring land volume

Spooners pistons were deffo N/A AE items, I've seen them.

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

David.