Attention! Forged Mahle Pistons

[T-u-R-b-O]

Hi guys,

I recently purchased race pistons from MAHLE designed for hard abuse on the RB25DET... i took standard bore pistons and slightly honed the block...

at first when I received the pistons they seemed very good and the pistons rings are amazlingly strong... they also have some special coating on them...

Now the bad points:

1) Due to them being race pistons, they are not fully round... dont know the right english term for it, but their surface on the side only has around 50% contact with the cylinder wall, which means that when pistons and oil are cold, that they have an extreme knock since they "tilt" from left to right...

2) thanks god that I installed the first piston as a test and spun the crankshaft to check all clearances... what happened? the MAHLE pistons do not have a housing for our stock RB25 oil squirters and mine broke off...

this seemed kind of strange to me, since the pistons itself is designed to support the oilsquirters... so i had to replace the broken one and really customize the other squirters and the pistons themselves in order to make both fit in the block...

very time consuming and unnecessary work for pistons which where build to go into the RB25...

everything fitted, checked the clearances and started the engine (hopefully leaving the oil squirters alive)... very loud "tack tack tack" noise from the pistons... lets the engine run for 30min to get the right temperature... once oil temp passed 80degrees, the knock from the piston reduced quite a bit...

after 500 KM of hard driving, the compression is straight 10 across the board so the pistons seem to be doing their job...

well, hope this could be of some help, and i suggest that you go with wisecos or something like that, cause otherwise you will have to do quite a bit of fusing around to make the Mahle pistons work...

[Amaru]

Thats exactly what you get for buying a high grade forged piston. The strength is determined by the silica content which expands with heat. The pistons are ovalled so when they expand they become a perfect circle. If they were this shape when cold they would not work at high temperatures once expanded (they would jam in the bore because they would physically become to big). Its not a problem, its something you have to live with when using high quality forged pistons. Arias are exactly the same.

As for the oil squirter, had the exact same problem with Arias pistons. Machined a little off the oil squirters and they cleared it fine.

[Mik]

The skirts on Mahle pistons are too short for street use in my opinion

[R31Nismoid]

I think Amaru has hit the nail on the head.

I find it funny too, saying "use wiseco instead" when your probably going to see the same thing.

[T04GTR]

ok, what is the recomended piston to bore clearance for these pistons.

bearing in mind that a "good" un damaged stock bore will have around 3-4thou clearance.

honing gives you another 1-2 thou on top.

so you could have up to 6thou clearance or more.

in regard to the oil squirter issue. thats more a suplier fault. as the recess clearly wasnt milled into them. i know some american and european parts arnt a true fit. i tend to stick to the sulpiers who "specalise" in theses engines. if i was going to build up a chev i wouldnt go to tomei and ask them to make me some pistons lol, id go je or a american product. etc etc

the reason they rattle is because of the high silicon content. BUT this often isnt the best way to go. that is why american pistons are rattely,and these obviously.

jap pistons are superior in there design and metalergy and often have the same silicon content but requir only half the clearance. and dont rattle

typical clearances for say. a JE piston is around 4 thou. for a trust/grex/os piston are 1.9thou wich is a factory clearance.

and why they are twice as expencive.

mahle pistons afre very good. they were good enough for the 1450hp turbo 1.5 liter bmw f1 engines

all pistons are barel shaped when cold.

just make sure yor clearances are ok. otherwise it will breathe to much. mesured at 90degrees fron the gudion pin

hope that is of some help

[T-u-R-b-O]

i am aware of the clearance fact and that this is why the pistons can knock while cold...

the stupid thing is that I didnt even get one of those data sheets with the pistons which states the clearance etc...

we recently put together a SR20det in our workshop with Arias pistons which require 0.1 clearance which is alot...

on my engine i have stock bore, and the clearance after honing was max 0.03 at the last cylinder... the others had 0.01 - 0.02...

i also said to get wisecos because i have seem them fit on the little brother from the RB25 (the CA) and I had no issues with clearning or high knock... i just bolted them in, and worked perfectly... CA18 also has identical oil squirters as the RB25...

a question... could somebody tell me how i can tune my engine now, because i am hitting amazingly high knock readings (255 and up) with these pistons?

[rob82]

i am aware of the clearance fact and that this is why the pistons can knock while cold...

the stupid thing is that I didnt even get one of those data sheets with the pistons which states the clearance etc...

we recently put together a SR20det in our workshop with Arias pistons which require 0.1 clearance which is alot...

on my engine i have stock bore, and the clearance after honing was max 0.03 at the last cylinder... the others had 0.01 - 0.02...

i also said to get wisecos because i have seem them fit on the little brother from the RB25 (the CA) and I had no issues with clearning or high knock... i just bolted them in, and worked perfectly... CA18 also has identical oil squirters as the RB25...

a question... could somebody tell me how i can tune my engine now, because i am hitting amazingly high knock readings (255 and up) with these pistons?

I dont know the old fashion way of just listening to the motor to detect knock!!

[Risking]

I would not let you anywhere near my engines after reading the above posts!

500Km's of hard driving on a fresh engine??

Also when your assembling such an engine you should have been aware of the side effects to using such products.

Id also have to agree with TO4GTR in saying with a jap engine generally the Japanese have them alot more wired than the americas.

Pistons are not going to effect your Knock readings unless you have changed the CR??

If the piston to bores are that bad it could be the knock sensor picking up piston slap!

[R31Nismoid]

nothing wrong with driving it hard on a fresh motor.

what are the reasons it cant be done?

Once the rings a bed, there isnt a whole lot more too it

[Risking]

Dont get me wrong, I agree with you, once the rings are bedded and its been checked over it "should" be sweet.

However when you have an engine that rattles and has been built like this Id personally tread carefully for a bit longer even if you are 500% sure its piston slap.

[T-u-R-b-O]

I would not let you anywhere near my engines after reading the above posts!

500Km's of hard driving on a fresh engine??

Also when your assembling such an engine you should have been aware of the side effects to using such products.

Id also have to agree with TO4GTR in saying with a jap engine generally the Japanese have them alot more wired than the americas.

Pistons are not going to effect your Knock readings unless you have changed the CR??

If the piston to bores are that bad it could be the knock sensor picking up piston slap!

i honestly couldnt care less whether you will let me near one of your engines...

be aware of side effects of using such products?? for example that pistons made for my engine breaking off factory oil squirters??

also, i could care less for such unnesseccary comments like yours, since i was trying to prevent people from building them it and breaking of all oil squirters... and not to discuss whether mister superman will let me near of his precious engines...

Nico

ps: there is absolutley nothing wrong with driving a fresh engine hard from the start... i drove ~100km normal, and then build my way up to the serious HP... all down to ~500km...

[r33_racer]

yeh alot of race teams...drags, circuit racing and even f1 i think dont bed an engine in for 1000ks before using the motor. They do like a half hour test of running it through the rev range and away they go. Saw something on fox about a f1 team bedding their motor in after assembly and they just had it on an engine dyno and for about 15 mins it just went through different parts of the rev range screaming its head off and that was it...they said it was good to go into the motor for any further tuning and then off to race.

Its not like its a big deal to do a set of rings in a competition/worked motor for reliable and consistent power. We didnt do a proper bed in in the rb30det...about an hour of free rev range running, change oil and did it again and then bam to the track for thrashing

Piston slap will show up on the knock sensors....the piston goes through the same motion as it would with pre ignition, its knocking itself against the bore, except its doing it on its own cause of sloppy clearances.

[Swiper the Fox]

for greater oil squirter clearance....place the correct sized copper washer under it (between squirter and block) for extra clearance (Aries pistons require this mod)

[Muz]

Sorry everyone but I just have to ask a silly question. Tomei are recognised as a leader in RB engine hardware, was the only reason for not using them cost?

cheers

Muz

[r33_racer]

Sorry everyone but I just have to ask a silly question. Tomei are recognised as a leader in RB engine hardware, was the only reason for not using them cost?

cheers

Muz

how are they a recognised leader in engine hardware? How are their pistons so much greater then say ACL, CP, Arias, JE, Mahle(with acl) and everyone else?

If you can make 1000hp using CP pistons that might cost you $1500 or 1000hp using tomei pistons which might cost you twice as much(honestly dont know but im guessing they are not cheap) what does it really matter?

[Mik]

how are they a recognised leader in engine hardware? How are their pistons so much greater then say ACL, CP, Arias, JE, Mahle(with acl) and everyone else?

If you can make 1000hp using CP pistons that might cost you $1500 or 1000hp using tomei pistons which might cost you twice as much(honestly dont know but im guessing they are not cheap) what does it really matter?

Have a look at the price of Tomei pistons...not that much more than say CP's or JE's. And the advantage (for a street car anyway) is the ability to run 0.002" clearance...so they don't rattle

[r33_racer]

I ran .002" clearance on my cp's in the rb30det. And I know cp's are in the country and a days freight away, and quite a few companies all stock them now. Tomei you could have to wait for a shipment from Japan, and that can always vary as has been my experience with parts from Japan, unless ofcourse someone is stocking them.

Edited August 24, 2006 by r33_racer

[T-u-R-b-O]

for greater oil squirter clearance....place the correct sized copper washer under it (between squirter and block) for extra clearance (Aries pistons require this mod)

i also thought about doing this, but then the squirters would hit the crank...

would be great if the exact copper washers would get supplied with the pistons...

Nico

[B-Man]

Its not a problem, its something you have to live with when using high quality forged pistons. Arias are exactly the same.

I never had any slap with my Arias Pistons - I think the clearances have to be 100% spec'd as per Arias recomendation to avoid this - also use the rings that come with the pistons - not gapless rings or anything for these.

[Mik]

i also thought about doing this, but then the squirters would hit the crank...

would be great if the exact copper washers would get supplied with the pistons...

Nico

You can space them about 2mm and still have plenty of crank clearance...