NITROUS OXIDE ( nos / n2o ) advice forum

Last year when one of my pistons failed, we just bought another off the shelf from BMW, honed the bore, stuck it in and hey presto 210psi instant cylinder pressure and still going strong after numerous NOS runs and abuse.

The supplier of my forged pistons just threw me a curved ball and put doubts in my mind, so I need some advise.

I've gone ahead with the purchase, but he questioned what I was going to do with the Alusil bores, to which my answer of "nothing as they were perfect", was met with an uncomfortable silence. He asked if I expected to just put them in and hoped for the rings to seal, well yes.
Am i missing something?

Other terms that sprung up during conversation were mops and compounds to be used because of the Alusil!

HELP

OKI engin build is not my thing but on my bike they have a coating on bores which is why i cant rebore fit new pistons ect according to Yamaha when they are gone they replaced as a complete set , now from what iv picked up if you have to fit new pistons and rings a very light hone/clean up will just work any more than that and you need a special dimoned hone or a rebore and replate amd matched to pistons

your engin builder should be able to advise , but i would guess the replacement piston would all need to be spot on , but the plated liners do need different treatment than just slapping in a new set of pistons and rings

Plated liners???

Whats wrong with good old "solid metal all the way through" type of liners???

That way you can rebore time after time

You should do a search on alusil-or ask the piston manufacturer to go into more detail.

Alusil sounds different than the nicasil type ceramic coatings.

All of the ceramic type coatings are surface finished with diamond hones.
Once they are damaged or oversize they are very expensively removed recoated and honed to size.

If the bore is undamaged i understand they can be cleaned up with scotchbright to ready them for re use.

What I'm sure your piston guy is concerned about is ...without honing of any sort your bores will be smooth,this is not ideal for oil retention - thats what the cross hatch hone finish is for-This will then cause the rings to rub preventing a proper bedding in.
The other problem with MOST forged pistons is that they need larger clearance than cast,what alloy are you hoping to use?

If you only need one piston now,you might be better to use standard again and accept you have found the weak point and it's limit.

Or get the motor out have liners fitted and do a proper job(please read this in a thick west country accent )

Hi Richard
The reason why we have cast pistons is to allow them to expand at the same rate (or close to) as the alusil cylinder walls.This creates a smoother,quieter running engine(especially when cold). Forged pistons are tough and durable but tend to slap against the cylinder walls until they expand (they are machined smaller than cast pistons because they are denser and overall expansion is greater, so measures must be taken to insure proper piston/cylinder wall clearance at operating temperature).

sleeving is the only way to go as the alusil process won't allow for use of forged pistons, but can get very expensive.

Which brings me to the point of why I am staying at the 150 shot level. Can our pistons handle more than that? Well I've seen LS1 engines run 200 shots on factory pistons like ours (hypereutectic) for a long time with no failures with the right tune. Personally I wouldn't risk it
Cheers
Hadi

Oh Crap. I'll see what the supplier has to say about this tomorrow

Thank you everyone, (especially Hadi) I have just saved a fortune.

A BMW technician has just confirmed that forged pistons MUST NOT be used in the M62 E31.
The piston rings are iron coated instead of tin coating. BMW states the pistons have a thicker fire land, and states that forged pistons should not be used in this engine.

Looks like restoration will be standard and then I will set about another project.

Totally F**king pissed off now

Time for an 840 shell with a Rover 4.6 in there Rich
Get a set of forgies and stick 300 down its neck mate

Today I have received yet more conflicting information, so my head is completely scrambled.
The piston supplier has stated that they have suppled forged pistons to Porsches with Alusil blocks and have never had a complaint, in fact quite the contrary. They also advised that JE will take all the expansion factors into the build and the piston has a special skirt because of the Alusil. Order on hold while JE are contacted with my concerns.

Richard,

your Stock pistons have a thin stainless steel coat/skirt (same on V12).
If JE does it the same or a compareable way you should be fine.

Hadi,

we have 12 pistons. Each is loaded with 25hp stock and about 38hp on a 150 shot.
300hp on a V8 is 38hp per piston. plus 100 shot it´s 50hp per piston.

I´d like to go to 50hp per piston!

Many thanks, that has just been confirmed.
This is a whole new science.
The trouble is there are no true experts, although I have confirmed this morning that both Porsche and Audi have Alusil blocks and forged pistons. JE have confirmed they supply for this combination and are well aware of the requirements.

I have also contacted a company called Boost Performance, here is what they say:

During two Porsche engine builds we discovered a viable way to recondition Alusil cylinder bores as used in engines like the Porsche 944 Turbo. Alusil bores can't be bored in the conventional manner due to the presence of silicon in the aluminium. The bores are etched in readiness for new piston rings. This has to be done to allow the rings to bed in properly, thus sealing the cylinders and restoring compression. This work can be carried out on other Alusil engines such as Lexus, BMW and Mercedes.

Just go for it Rich , etch the bores to seat the rings , use an oil additive to help , as long as your piston manufacturer is ok with the application, what is there to lose. I think the more you look into this, the info will be more conflicting ,you could also coat the pistons if required


Austin.

Have you seen Diamonds new coated nitrous pistons?? Wonder if they'd work in your aplication?? The can custom build pistons for a reasonable price. Give them a call. Lots of big nitrous guys i know use and recomend them. I'm getting a set put in my new 421" sbc
http://www.diamondracing.net/products.asp

Looks like its gonna be JE Pistons as they have confirmed:

1. Forged Pistons in an Alusil Block has been done successfully many times.
2. The honing/etching process of the Alusil bores has been explained to me and I have also spoken to a BMW mechanic who has actually done it.

My mechanic confessed to me today that when he re-fitted the standard piston to No.1 bore last year, he stone honed it. Yet against all the advise, that cylinder STILL has the highest compression at 210psi.

Feeling much better now

I thought alusil might be a high silicon etched alloy system,I've never seen one but I have read about them.I understand the etching removes aluminium leaving behind a surface of mostly silicon.

Maybe a flex hone would be a better/more gentle method of prepping the bores???

My worry is that if this swap to forged goes wrong you are screwed big time.

What's the damage that you've done to pistons?there may be other options to prevent damage.

I have blown the land between 1st, 2nd and oil seal ring.
I've already been screwed big time by this engine more than once, so this is its last chance.

That's the same sort of damage I was doing to my little beemer..K100.
I had no damage on the top of the piston,rarely any damage to top ring,frequently broken 2nd ring and sometimes pinched the oil rings.

The very interesting thing to note is that I never had any trouble when I ran a dp crossfire setup.Problems only started when I went to a pro style seperate injector set up.
And retarded the timing 2degrees!

I was in the process of increasing ring gaps when i had bore coating issues put the whole project on hold.The 2nd ring ends were definitely rubbing.In my opinion it is purely a heat issue not a detonation or stress problem.There was not a mark on the pistons other than the broken rings/lands.

Good to hear that you can hone Alusil.
Going with flex hone would be the more accurate method i think.

If you have more acid here and loger time on there and you may have ruined the block or nothing happened...


Richard,

if you ran out of luck with your V8 go for V12
Next week i have two spare engines with trans for 300 euro

Thomas

Thomas
300 euros for engine and trans. That's an awsome deal.

Richard
If the forged pistons/alusil bores combo is been successfully done before then I don't see why it wouldn't work with the BMW V8. The info I gave you is a few years old and there's always new technology and approaches to doing things.

Cheers
Hadi

Hi Hadi, I feel that between all of us, we have made tremendous progress over the last 2 days.
We have gone from MUST NOT DO (which included statements from so called expert BMW experts) to has been done successfully many times. With ,most importantly, the successful honing of the bores being the most significant factor. My block had already had that done successfully to No.1 bore, but I didn't know that until yesterday

Once the re-build is completed I will be taking very gentle steps. There will be a period of stability for several weeks before I even think of adding the base 50 shot. I will be going back to basics to build my confidence in the engine once more.

Pistons have been ordered, 3-4 weeks delivery. Car provisionally booked in for 19th April.

Sometimes BMW spares can be a real surprize
The full gasket set MUST be gold plated as the head gaskets alone were over £300,
but the biggest surprise were the big end shells, they came in cheaper by far than any rebore centre,
so its definitly swings and roundabouts.
BMW have given me a VIP card as I spend so much
The points I earn can go towards a new one

Now I'm getting withdrawal symptoms.
Not gone above 2000rpm for a month now, successfully keeping the engine in one piece.
JE pistons are delayed until 1st week of May.
I'm missing the "WON Effect"

Just not the same is it Rich

Soon ...............Soon..............

Sorry I missed this thread till now Rich, as I could have saved you all that trouble and concern.

It's a load of bollox that forged pistons can't be used in ANY type of bore as the material used for the piston makes NO difference to what bore its used in.
HOWEVER, the same is NOT the case for the ring material, because not only does the ring material need to be suitable for the bore material but the bore finish MUST also be suited to the type of ring material used. That is to say that even when you have the right ring material for the material used for the bore, you also have to have not only the correct grade of finish (which is determined by the stones or whatever material the hone is made from) but also the method that grade of finish is applied in.

For example, a typical process (from new), would be to bore to a particular size, then hone to a particular size using a particular grade of stone, followed by being honed to the next size using a finer grade and a final finish with a finer grade to the actual size using a particular method, which can be a particular angle of cross hatching.

The ring supplier should first of all make sure they supply suitable material for your bore material and then they should give you FULL bore preparation details, then you'll be fine.