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sorry, i meant that with an original height block, the shorter stroke of the 970, coupled with the longer 970 rod meant that the factory didnt have to hack a load off the top the block or invest in new pistons.

sandman wrote:No, you need shorter rods. (i.e 1071/1275 type)

Or you could have custom pistons made up from billets.... gonna cost though.

Thanks for that info on the rods, was aware of that, its maybe the way to go...

just entering unknown territory unless somebody has done it before on this forum????!!!!!!

0.030" of the top of a piston is nothing , even less problem on a 970 as they are flat tops .

You may have to have more machined off to get the required compression ratio if you head has been faced .

What make / type of pistons do you have ?

You will probably have problems with the waterpump hitting the cylinder head too .

At the moment I will be running an 18cc taurus tuned head if that is any usefull info

I know that the 970s originally used flatop pistons, however..
the ones supplied were dished "nural"..not a make that I am familiar with either, I dont think it will matter regarding the dish, as most of it will be removed with the skim.

Incidentally, I was out of the country for a number of Months while this engine work was being "organised", but still contactable.

Hmmm....

The NURAL pistons are what used to be AE Hepolite ( there in yellow boxes ) . There are ok for a general fast road engine in 21253/ mg metro spec , good for 10.5 / 1 maximum if you want the engine to last .

If there not the MG ones , I wouldn,t use them in any performance engine .

Watch for the gudgeon pin bores being machined on the piss , and some of the ring gaps they give are " variable "

Omegas are more expensive , but they are a fantastic piston . But as you are probably running a std bore block this is not an option .

Brilliant information.

Really appreciate it, thanks.

Will know were to bring my next engine!!

As "251 eng" says:
Watch for the gudgeon pin bores being machined on the piss , and some of the ring gaps they give are " variable "

I have also seen this. The gudgeon pin bores were way off on the pistons I was shown! -I would have sent them back immediately!
-Having a piston break is not a good

The newest stock of Nural pistons are marked on the top of the piston with the mfr's part number and an arrow towards the front of the block. This tells me the offset of the wrist pin was done intentionally. In this way they are unlike the original AE/Hepolite 21253 pistons, which could be fitted either way. It is not clear if all Nural production has had the offset pins, or this is a more recent development.

I'll get a photo of the newest ones and post a link shortly.

The issue with the gudgeon pins in the Nural pistons was not a deliberate action on their part - it was an error during machining - that only occurred when Federal Mougal moved the production of their pistons to the Nural factory. The specification of the piston was not meant to alter at all with the change of production facility; rather it was a fairly simple machining error that wasn’t noticed as part of their QC.

The worse affected piston was the 21253-20.

In use the pistons didn’t break up; rather the oil control ring would rub on the bore and wear very quickly

They have now stated that the pistons are made to 'an acceptable industry standard' and to be fair to them the latest batch (ARG date code) look ok, bar the optical illusion of the casting marks inside making it look worse than it is.

This is one of the worse affected (pre ARG date code) ones...




this has been an 18 month saga that ive had the 'pleasure' of sorting out - ive unboxed hundreds of pistons to assess the quality of the machining and what not, and the complaints that we made went right to the top of both Federal Mougul and Nural - even to the extent that a mini engine went over to the Nural factory so they could grasp the concept of offset conrods........

Rich,
Thanks for shedding light on the history of this problem. Now the pistons are marked with the orientation arrow, if one installs as per manufacturer's marks, does that overcome any operational or ring-wear problems associated with the pin being accidentally offset? If the piston tops are marked, then one has to assume the wrist pin is still inadvertantly offset. If the batch was correctly machined, the pin would be in the center and there would be no need to mark the tops. Thanks.
Michael

guru_1071 wrote:The issue with the gudgeon pins in the Nural pistons was not a deliberate action on their part - it was an error during machining - that only occurred when Federal Mougal moved the production of their pistons to the Nural factory. The specification of the piston was not meant to alter at all with the change of production facility; rather it was a fairly simple machining error that wasn’t noticed as part of their QC.

The worse affected piston was the 21253-20.

In use the pistons didn’t break up; rather the oil control ring would rub on the bore and wear very quickly

They have now stated that the pistons are made to 'an acceptable industry standard' and to be fair to them the latest batch (ARG date code) look ok, bar the optical illusion of the casting marks inside making it look worse than it is


This is one of the worse affected (pre ARG date code) ones...




this has been an 18 month saga that ive had the 'pleasure' of sorting out - ive unboxed hundreds of pistons to assess the quality of the machining and what not, and the complaints that we made went right to the top of both Federal Mougul and Nural - even to the extent that a mini engine went over to the Nural factory so they could grasp the concept of offset conrods........

WHAT THE F...!!!
And I have a set of pistons made by these guys in my 970!!!!??

In all fairness, if the dish is round, does that really matter?? Please advise.

Vegard wrote:In all fairness, if the dish is round, does that really matter?? Please advise.

Hi Vegard.

Fairly subjective.
Anyway, Im off to buy some re-molds for my 911!

What...? They're round arent they?

he he..

Yes, Guru_1071 those are them! -Horrible!

-My fear would be that if they can have the pin so much on the cock, how accurate is the rest of the machining?

They would probably be OK in a standard spec road engine, but I wouldn't want to push them...

We concluded that the machining error was that as the piston was gripped in a set of internal soft jaws to drill the pin hole the operator either was rushing the speed of the the drill or the jaws where too slack, the piston would then 'jump' in the jaws - you can see a second set of witness marks inside that where not apparent (or at least not in the same position) in the ae boxed versions that didnt have any faults.

the machining error forces (in some cases) the boss of the small end against the inside of the piston, which (again, in some cases) then forces the piston into the side of the bore, its this tightness that wreaks the oil control rings - in the mighty minis (which use the 21253+20) this became apparent after as little as 20 minutes of running!

I dont think the arrow signifies anything, I think its just a standard production feature they put on their entire range of pistons to help with assembly, I dont think there is any corrolation between the arrow and the angle of the pins - the people at the Nural factory where not even aware the the conrods ran both ways in a mini engine, with the resultant 'unique' set of problems that this was causing, plus they admitted that it was an error(eventually....... )

I'd have no hesitation in using these pistons in a fast road engine, I think any decent engine builder would notice the pins, and either reject the pistons if they where really bad (and all our bad ones where rejected and returned as soon as the problem was noticed - very few where sold) , or releive some material out from the bosses to ensure that the conrods had a clear run on the entirity of their stroke.

the problem that the mighty minis faced was that their rules state that the pistons could not be modified, so the engine builders where pulling their hair out!!

Well done Rich , nice to see it,s sorted .

Any chance of some better rings on them ,

When my 970 went bang i had to go through what you are doing now but that was 20 odd yrs ago now BUT when it came to the pistons i had to have a set made in 1985 thay cost me £800 then i dont want to think about now

THE REVEREND wrote:When my 970 went bang i had to go through what you are doing now but that was 20 odd yrs ago now BUT when it came to the pistons i had to have a set made in 1985 thay cost me £800 then i dont want to think about now

I understand now why you found religion.

God help all of us mini nutters.

rosepetal wrote:

THE REVEREND wrote:When my 970 went bang i had to go through what you are doing now but that was 20 odd yrs ago now BUT when it came to the pistons i had to have a set made in 1985 thay cost me £800 then i dont want to think about now

I understand now why you found religion.

God help all of us mini nutters.

I'll second this thought

This forum surely makes me feel I'm not the only.

£800 for 4 pistons in 1985

Were they solid gold with gudgen pins made of diamonds