So I picked up a set of bnib 85mm CP pistons for a hell of a deal on here for my GSR motor that I am currently building. A few people have told me that 85mm is a bad idea when going boost? Heres some of the points they have attempted to make:

It will work, but it does not make sense. If you are getting your motor sleeved, why would you start at 85mm? It gives you no room for error because you won't be able to overbore if something happens to the motor. I would not run a ton of power through the motor either because there will be limited surface area for the headgasket to seal. 83mm is the best option. That is what I start all my customers sleeved engines at. There is little to no difference in power from 83 to 85mm on a Turbo car from what I have seen.

My reply: The only reason I can see for starting out at 83mm is incase you have to overbore it for some reason down the road. But my feelings is if you do it right the first time you wont have a reason to do it the 2nd time.


Another person said: 85mm is doable, barely. You have ZERO room for ****ups. You **** something up with the physical assembly of the motor - new block. You **** something up with the tune - new block. You get a bad tank of gas - new block. You have a fuel pump or regulator **** on you - new block.

83mm gives you room for several ****ups - depending on how bad they are, you can go 83.5, 84, 84.5 and finally 85.

And you totally glossed over the headgasket issue! This is a big deal! headgaskets need contact between the head and the sleeves to seal. The quality of seal is related to how flat the surfaces are, the mating surface area and the pressure exerted by the headstuds. You can only tighten headstuds so far before you start to rip threads out of the block. With the limited area presented by a 85mm bore motor, headgasket sealing is a real issue.

Again another person that is seemingly building a motor that they feel is going to fail


My thinking: Ok that same theory applies to any motor no matter what the bore. So your saying its ok to **** up a smaller bore motor because you can just rebore it? Well that seems fine and dandy but you do realize that if the block is destroyed that bad then the internals will prob be toast as well, thus leaving you to have to build another motor. So really bore size has nothing to do with it, but rather the assembly and tune on the motor.



Whats your opinions on built/sleeved and boosted 85mm B series.

 

no reason for it when 84mm is perfectly fine and 84mm leaves you enough room for hg to seal whereas the 85mm is very close and not much room for headgasket to seal is the main reason

 

Reason for it being the price I paid for the 85mm CP's lol.

 

Trade them for some 83, you won't be kicking yourself in the *** later. Also the sleeve itself gets very thin in between cylinders, more chance of cracking@ 85.

 

But will an 85mm bore handle the boost if its your second time rebuilding a sleeved block that used to be 84mm. Or can you go 84.5?

 

I would start at 83mm if it were me but I dont see a problem with 85mm and lots of boost but not much room after that.

 

Kinda between a rock and a hard place right now lol. Might have jumped the gun on these pistons due to the price I got em for. Might be selling them and going either 83 or 84mm afterall

 

84mm is what most people do right from the get-go. As for 85mm you should be fine, how much are you planing to make with your setup? Garcia was right though the thickness between the walls is where you will loose strength with the bigger bore.

 

I am wanting 600whp with either a 35r or 37r

 

85 is fine honestly, no reason to go 83 like zomg i might mess up stuff, 84 is a nice starting point but 85 is ok too, you can always go to 85.5 if you have to or even 86, sure your going to lose some strength and could have hg sealing issues on high boost but it could also just work out fine and i doubt your going to crack a sleeve.

Worse case scenario you can sell it to an NA guy, they do 86mm+ all the time.

 

So starting with 84 is okay?

 

My engine is 85mm......

 

yup, its probably the most common.

 

85 is fine. I have always started with 84, but I have never got an awesome deal on pistons....

 

I paid 300shipped for BNIB CP's thats why I went 85mm. It was kind of hard to pass up a deal like that. I was looking on Laskey's site and they list their built motors as 84mm bore from the get go.

 

It's not so much people starting at 83mm plan for the engine to fail, they just like to have extra room for error if something does indeed happen.

With that being said, 85mm is on the high side, but I see nothing wrong with it. I can't see it having any real issue under 600 whp if built properly. I'd personally do 83mm or 84mm though. But I'm conservative.

 

I wonder how much you could boost with 85

 

I am sure there are plenty at 800+hp with 85mm, boost is irrelevant. As long as your surfaces are straight and you have a good headgasket/studs your fine.

My friend ran around 10k-15k miles at 85mm running 550-600whp daily, the motor is still togther on the same bore...........

 

so do you have direct experience with 85.5mm and 86mm under boost?

 

not under boost but with 15:1 compression NA they seem to hold just fine. My response was a hypotheses based on what i have seen on various setups, some freak **** works, some uber built **** fails is what i was getting at and in the end if he felt uncomfortable with it then it would be an easy sale to an NA guy and he could start over with cash in his pocket, do you directly disagree with anything i said?

 

I do, they crack.

 

I am sure they do, I was looking at my friends 85mil sleeve when we changed the headgasket and I was laughing to myself thinking it looks like a stock sleeve.

 

at what power level? what size turbo and pis?

well even if they were the same thickness of oem sleeves they are made of a far stronger material... but honestly 85mm does not look like oem thickness, granted its not as thick as a stock bore sleeved motor...

 

Kinda like B20s dont like boost either right? My B20 seemed to do just fine with Boost and the abuse I put it under, hell that motor is still going strong 10k miles later.


I might downsize to 84mm just because I can sell my 85s and profit and pick up another set of bnib cps for another 300 bucks that are 84mm lol

 

im starting with 84.50 but that's only because i let my 84mm bore get rusty on accident storing the block in a sneaky spot while i went out of town for a couple weeks. This way ill atleast be able to go 20 over down the road without getting a new block.