Piston & Ring Choice
Turbo Mini Forums

As some of you know I'm in the process of rebuilding my 1293 turbo and I'm faced with a dilemma as to the choice of pistons.

When I stripped down the engine the bores were in A1 condition, so I'd rather not re bore the block in less I have no other option, especially as the block has been lined.

Here's a bit more info on my engine for those who are interested:

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=118307

I was going to run 10cc cast omega pistons but I've heard that there rings are not up to the job. From reading Richard's post it appears that Mega pistons use the same rings as Omegas so I'm guessing that there's nothing to be gained but going to Mega (also I can't find a +20 thou Mega piston)

Both Accralite pistons supplied by Avonbar and JE pistons supplied by Mini speed are unknown quantities and cost a fair bit more.

The final option is to reuse the AE pistons that came out of my motor as they are are well with in tolerance, however I'd like something a bit more robust as I don't want it to die on me.

I suppose I could use a set of Omega pistons with my AE rings as they are showing no signs of wear, but is that just getting silly?

Finally here's an AE piston I blew up earlier:

Accralite have a good rep with other engines so I can't imagine that they will be making rubbish for the A series, also Avonbar don't normally supply crap in my experienc with them .... never cheap but never crap! Problem for you is that the smallest dish they do is 14cc.

JE pistons, well my Scirocco turbo conversion used JE's and they were beautiful things .... almost wanted to make a clear block so I could use them and still look at them. Quailty was fantastic the ring pack was top notch too, all gaps exactly what I wanted straight from the packet (something I have never found on any other piston than Powermax). As far as I can see they only come in 18cc dish though ......

From what I have read here I don't think that there is a problem with the Megapison rings in terms of quality, just the possibility that they might not be the size printed on them on a limited batch.

As you know I've never been a fan of Omagas due to the many, many issues I have had with them over the years. Now it's been a while since I bought any A series pistons but what can you get from Powermax in terms of dish these days???

What CR are you looking for and how much have you got in the head??

Every day is a school day ...........

How fast and how expensive ...... the same question...

I seem to remember the JE pistons also do not have the standard compression height and so end up down the bore, and also do not take a standard gudgeon pin either.

Interesting ... Is the pin included?? If metal off the small end then that gives you an opertunity ... if metal on then that gives you a problem!!

Every day is a school day ...........

How fast and how expensive ...... the same question...

the pins are smaller than standard, and the compression height is 0.225" lower than standard pistons.

Well dunno if this helps but TD is running the mega pistons IIRC and rates them,

as for them using the omega ring pack, if you look in the thread on dishing turbo pistons I recall there is somthing about the top ring being further down on the mega's, perhaps the omega ring pack issues ar related to them not being far enough down.

JF is also running mega pistons. These are VERY sturdy units, capable of a LOT of abuse. And they're cheap. And they're available seperately. And.....etc.

+40 mega here too since Dave recommanded them there is some time now ,they had to handle some bad treatment and never had an issu . some friends here are also using them without trouble (one at 1.3 bar (20 psi))

I've got a set og Longman S Miglia rods used with JE pistons. They'v got a bronze bush in them. Easy.

I ideally want to got for 10cc pistons to keep the compression relatively high, my head has 26cc chambers and by my maths that gives a static CR of 8.9 (assuming 3cc for the head gasket & 0.75cc for the ring lan).

I'm toying over the Accralite but being 14cc it drops the CR a bit lower than I want (8.2 by my maths) so I was thinking about getting 1mm turned off the crown and then decking the block so the pistons come within a couple of thou of the top of the block, by my maths this should give a piston dish of 11cc and a CR of 8.8 :)

Can anyone see any problems with this?

Sorry to bump up the thread, but I need to know if my idea of machining 1mm off the crowns is a sensible one or not, as my build is now getting delayed by my lack of pistons.

I'd personally rather skim the head to achieve the reduction in volume rather than risk distorting a piston when you are trying to hold it in the chuck of a lathe to machine it down.

I picked the 13cc Mega pistons from Morspeed, Not much more expensive than the stock 6cc but morspeed turn them out to 13cc for you. I worked out that this gets you to compression ratio of 8.5 without any head mods (the crown height is slightly lower than Hepolite's)

Of course can't comment on the strength yet, they are still in the post!!

Tom, dont you mean open out the chambers to Increase the volume? Skimming the head will decrease the volume and lowering the crown will increase it. Wait a bit, lowering the crown will also reduce the dish, I'm confused!!

---WOW a triple post.. Beat that!

Terry

Edited by Turbo Tel on 17th Dec, 2006.

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

No I do not mean open out the chambers to increase volume.

He is proposing to skim pistons to reduce the bowl size from 14cc to increase the compression with the head he has. I am suggesting to leave the pistons alone and skim the head to achieve the same increase in CR.

Thanks Tom, I don't know why I didn't think of skimming the head.

Yup

I agree, just skim the head. Any machining of the pistons would increase the volume, and thats the oppositeof what he is trying to achieve..

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

but then he was also going to deck the block flush with the pistons, so thee wouldnt be any gain.

Personaly, as others have said, skim the head, its only one machining process instead of five

WRONG ...

he would be lowering the piston height thus increasing the volume above the piston, OK the dish cc would reduce but as the dish is smaller than the whole piston it would reduce LESS than the volume above the piston

ding, next round..... lol

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

Please carefully read what I wrote.

The piston height from the pin to the crown would decrease, yes, but the deck height would stay the same, as he was going to deck the block in order to make the shorter pistons still flush to the deck. Therefore the unswept volume BELOW the deck surface will have DECREASED.

Ding ding round one to FENTON I think you will find.

Nahh thats just a good left hook I'm still up!

Yes Ok, granted put both together and the effect in the end is the same except the squish is possibly better?

But taken seperately you would have to make up for the extra volume caused by machining the pistons by skimming even more off the block.

and we both said leave the pistons alone, just skim it

Its a draw!!!

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

Thank you Tom, I'm glad someone understands what I'm on about.

:EDIT: Have I had too much coffee or is Charlotte dancing faster than she used to?

Edited by Rob H on 17th Dec, 2006.

Presuming the Accralites are standard height, they are 40 thou down, machine that 40 thou off the block and you reduce the volume above the piston by around 4cc

squish aside why mess with the piston? for every 10 thou you take off the piston you would have to take (at a guess) 8 thou off the block. EDIT-- or is it 2 thou off the block, did I get that the wrong way round?

Of course if the Accralites are already flush to the deck then you are absolutely right ----but you could skim the head instead

Edited by Turbo Tel on 17th Dec, 2006.

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

OK I think I see what we are both on about now..

To raise the compression..

Initially you would skim the deck which is fine until it reaches the piston crown. Then If you machined the piston down you would have to skim even more off the deck just to keep the compression the same (my point) but it also gives you the possibility of going even further (Toms point)

Right Tom??

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