Dishing Turbo Pistons
Turbo Mini Forums

Sorry if this has already been addressed and I just didn't find it, but can standard Metro Turbo pistons (I think mine are actually .020 oversized) be dished out a bit further? Or are we pushing the limit already?

Also, I may have to go one more oversize, and do have a set of standard +.040. Can I just mill the top down a bit to get the ring lands higher (ala turbo pistons) and then dish them some more? Or is that dangerous?

**************************************
1967 Cooper 's' Turbo build @ www.drjing.com

I'm sure the standard 'turbo' piston dish could be scooped out some more.
You want to leave the squish/quench band untouched however. You'd need to keep the dish a curved profile, as the underside of the crown has a the same curve; flat-bottoming the dish will leave it pretty thin in the area around 40/50mm diameter from the piston centreline.

The 'turbo' pistons are very difficult to remove off rods without distorting them becuase they don't have the recessed 'flat' around the pin. Worth considering before you go this route...

Don't fully understand what you're saying about the +040" pistons - same thing applies about the squish band however. You get big CC gains by machining this off, but its very bad practice and should be avoided.

Here is a section of a turbo piston. Note that if you machine a flat-bottom dish level with the centre of the original dish, it ends up pretty thin...

The shape drawn onto the cross-section (left of the picture) is more like what you should aim for...

Note the sign of a good 'forced induction' piston whailst we're here... A really thick land above the top ring, a nice thick crown, large radii, etc! No wonder these things are so bloomin strong!
Compare to the 15cc Omega behind it - these are more akin to a heavilly dished nat-asp piston than a turbo one

ours have been dished an extra 12.7cc
say the word, and i'll get some snaps.

an 'extra' 12.7???

The standard turbo piston is already somet like 12.5cc - not the 9cc quoted in Vizards bible.

i think so, they look much more angular than that pic you posted.

i'll get a pic

here ya go



:/

I had those pistons machined out to increase the dish volume. At the time I believed std turbo pistons to be 9cc as per Vizards book, and the guy that did them buretted them.

I am struggling to remember what the final volume was, I thought it was 12.7cc but after some thought and conversation I think it may well of been 14.7cc. In either case they need checking to see what the volume is. If the std items are 12.5cc then they must now be up to 14.7cc odd just from the increase in dish dimensions. The centres of the pistons were left well alone and the material taken from the edges to increase the volume whilst not making the crown too thin. I spoke to Ian at Avonbar some years ago before I carried this out and he was of the opinion that the std turbo piston was strong enough to take this mod if performed carefully. Steve at MED also has seen the pistons and commented that he thought they were well up to the job.............
Alex send me a PM if any problems?

Im sure theyll be fine mate, bit late now! altho if the dish is more than 12.7cc, we might get a bit lower comp than we need!! - the head's been fettled for 12.7 now..

As I said before, if they are no good for what you want I would refund your money no questions asked.
Jon sent me a PM asking about dish size, I think he was planning to burette them again?
What CR have you aimed for then?

Edited by Tom Fenton on 27th Apr, 2005.

cant remember.. lol, Simon @ morspeed knows 8.5:1 or something, i know feck all about comp ratios. I just told jon what i wanted out of the mini, and he told me what to ask simon to do!

its ok, just means more boost / skimmed block i guess

You should be OK down to 8:1. My engine will be about that anyway, might even be as low as 7.8:1.

Easiest way to up the cr is to skim the head if needs be anyway.

Or flash the block - good idea whilst the engie is out...

'flash' the block?

flash is skim, but only taking a nats fanny of material off the top

Team Racing

It's a sort of a very "short" skim. You only expose the block to the mill for a few hundreds of a second. In other words, you flash the block. This results in taking the ABSOLUTE MINImum of the block.

Jim. I'd say that flashing the block removes less material than a nats fanny. That equals to three, maybe 3,23 flashes.Which again is 0,76 skim.

Edited by Vegard on 28th Apr, 2005.

haha. cheers guys

never really messured a nats fanny vegard, I'll have to take your word for it! LOL

Team Racing

A nat's fanny is ever so slightly larger than an Midgie's di*k, which in itself is 3/8ths of f**k all.

Thanks for the response, guys!

Wow, I had thought that the ring lands on turbo pistons were up higher, closer to the piston top. Looks like they are actually lower, eh? Maybe I'll just use my NA +.040 pistons as they are, and just mess with the head as planned...

Thanks, again!

**************************************
1967 Cooper 's' Turbo build @ www.drjing.com

This again means that assuming you've got a thin nat's fanny, which is very similar to a midgies di*k, this means that the F'ck all is approx 8,61 flashes.

So, if youre machinist is going to remove a fuck all from the block or head. Beware. I'd say that this is near impossible with regular mills.

Nothing wrong with very low compressions.

Im somwhere between 7.25 - 7.4 to 1 and its not really that bad off boost

AlexF

Cheers Alex - I pressed 'search' and gave up after that...

12.5 is 'somet like' 12.0 however!

OK

reviving this topic from my other topic.

Dose any one know how much more cc is gained from the standard metty turbo piston, has to be more that .7cc surely.

Looking for about another 3 or 4. However, i will be machining right across the top with a slot as deep as the valve cuts i need ( I know bad for squish but Visard recons its better than valve cuts on there own).

Just need some info on whats achievable with this piston, ive got a few sets to play with.

As for them distorting, the machine shop have a saddle that the piston sat in very neatly and then pressedd the pins out. roundness checked out as round as they can be.