CP Forged Pistons

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Peter Gidden
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Re: CP Forged Pistons

Post by Peter Gidden »

kev8611 wrote:
Peter Gidden - SBITS wrote:
kev8611 wrote:Keep in mind, when ordering, if you were to get standard 86mm (if you can) you may run the risk of them being too tight/loose in your bores due to wear. I was advised not to buy pistons and expect them to fit perfect in the standard bore due to wear.


Engine builder should be measuring the bores for wear and ovality before ordering pistons.


Yeah sorry missed that bit out. but self explanitory.

Whats the chances of an 86mm fitting as it should in the standard bores?

I was told my bores were fine but got it bored to correct spec for 86.25's using a torque plate just to be sure. Little bit extra power aswell? without pushing the wall thickness of block past safe limits


If the bores are dimensionally perfect in all respects, then the chances of STD size pistons fitting is 100%.
ashley
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Re: CP Forged Pistons

Post by ashley »

IMO you cannot get the cylinders re-bored until you have the pistons in your hands, the machine shop will generally measure the pistons for you, and then rebore to the clearance that you specify (CP supply guidance on clearances). If you haven't had the cylinders rebored to 86.25 already then I would wait...

And as for power...0.25 is not going to make any real world difference! If you can stick at 86.0 then do it to retain as much material in the cylinder walls as possible.

IMO.
Peter Gidden
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Re: CP Forged Pistons

Post by Peter Gidden »

ashley wrote:IMO you cannot get the cylinders re-bored until you have the pistons in your hands, the machine shop will generally measure the pistons for you, and then rebore to the clearance that you specify (CP supply guidance on clearances). If you haven't had the cylinders rebored to 86.25 already then I would wait...

And as for power...0.25 is not going to make any real world difference! If you can stick at 86.0 then do it to retain as much material in the cylinder walls as possible.

IMO.


Bang on.

My machine shop will not repore without the pistons.
kev8611
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Re: CP Forged Pistons

Post by kev8611 »

mines was the same. makes perfect sense to have pistons with spec sheet in hand before starting
MR2 Rich
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Re: CP Forged Pistons

Post by MR2 Rich »

Lots of good info, thank you all very much. The block I am using is on a standard bore so if I can I would obviously not want to have to rebore to 86.25 as we all know there is a chance of cracking, even on a standard bore. :thumleft:
No 2 :(
kev8611
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Re: CP Forged Pistons

Post by kev8611 »

imo 9 out of 10 blocks especially now with higher miles etc wont meet required clearances for 86mm cp's
Peter Gidden
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Re: CP Forged Pistons

Post by Peter Gidden »

kev8611 wrote:imo 9 out of 10 blocks especially now with higher miles etc wont meet required clearances for 86mm cp's


Fact or guesswork?
MR2 Rich
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Re: CP Forged Pistons

Post by MR2 Rich »

Is a higher mileage block less likely to crack due to having done more heat cycles though (is that the word?)?
No 2 :(
kev8611
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Re: CP Forged Pistons

Post by kev8611 »

purely my opinion based on my own experience with my engine.
kev8611
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Re: CP Forged Pistons

Post by kev8611 »

regarding block cracking. iirc its due to wall thickness in some areas when running big power. not sure if heat cycles or mileage play a part?
Peter Gidden
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Re: CP Forged Pistons

Post by Peter Gidden »

kev8611 wrote:purely my opinion based on my own experience with my engine.


OK.

More than half, probably nearer 3/4 of the 100K blocks we see have perfectly sized bores.
Marf
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Re: CP Forged Pistons

Post by Marf »

kev8611 wrote:purely my opinion based on my own experience with my engine.


The plural of anecdote is not data.
MR2 Rich
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Re: CP Forged Pistons

Post by MR2 Rich »

kev8611 wrote:regarding block cracking. iirc its due to wall thickness in some areas when running big power. not sure if heat cycles or mileage play a part?


My thoughts were if it has done a lot of miles if it was going to crack it would have by then, obviously running big power is tempting fate.
No 2 :(
ashley
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Re: CP Forged Pistons

Post by ashley »

kev8611 wrote:imo 9 out of 10 blocks especially now with higher miles etc wont meet required clearances for 86mm cp's


kev8611 wrote:purely my opinion based on my own experience with my engine.


Sounds like you are extrapolating a 90% failure rate based on a single data point with one engine...interesting statistical method :-k

Based on my experiences with my own engines (4 x 3SGE's) I would say 75% are still within original manufacturing dimensions (3 out of the 4).

But then I could have been lucky or unlucky with such a small sample size :eye:
kev8611
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Re: CP Forged Pistons

Post by kev8611 »

ok i get the point. il keep my opinion to myself.

mr2rich. sorry to have caused any confusion to you. after all. peter and co are the specialists on here so anyone elses points are invalid :thumleft:
GeorgeL
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Re: CP Forged Pistons

Post by GeorgeL »

MR2 Rich wrote:And JE!


JE pistons do give off some sound when the engine is cold mate yeah..
bobhatton
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Re: CP Forged Pistons

Post by bobhatton »

MR2 Rich wrote:Is a higher mileage block less likely to crack due to having done more heat cycles though (is that the word?)?


The cracking of the bores happens because the maximum cylinder pressure is taking place down in the bores and not in the cylinder head where it should be.
This is because too many engines are being built with too high a compression ratio for the fuel or boost being used. This results in the timing having to be retarded to stop detonation. This then gives the max pressure too far down the cylinder wall where there is lack of support.
Designer for turbo set ups on F1 cars, and Nitrous Oxide Systems of the USA in the 80s
ashley
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Re: CP Forged Pistons

Post by ashley »

kev8611 wrote:ok i get the point. il keep my opinion to myself.

mr2rich. sorry to have caused any confusion to you. after all. peter and co are the specialists on here so anyone elses points are invalid :thumleft:


opinions are fine, but you have to be open to challenge when throwing around statistics like that mate...all meant in good humour, apologies if I offended :thumleft:
Peter Gidden
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Re: CP Forged Pistons

Post by Peter Gidden »

bobhatton wrote:
MR2 Rich wrote:Is a higher mileage block less likely to crack due to having done more heat cycles though (is that the word?)?


The cracking of the bores happens because the maximum cylinder pressure is taking place down in the bores and not in the cylinder head where it should be.
This is because too many engines are being built with too high a compression ratio for the fuel or boost being used. This results in the timing having to be retarded to stop detonation. This then gives the max pressure too far down the cylinder wall where there is lack of support.


Except in cases like two we've had over the years where the engines were totally and utterly standard - no boost controllers to be seen. One had stock airbox.

One had hairline crack from around an inch down number 3 bore, the other cracked the deck between nos. 2 & 3 bores.
kev8611
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Re: CP Forged Pistons

Post by kev8611 »

ashley wrote:
kev8611 wrote:ok i get the point. il keep my opinion to myself.

mr2rich. sorry to have caused any confusion to you. after all. peter and co are the specialists on here so anyone elses points are invalid :thumleft:


opinions are fine, but you have to be open to challenge when throwing around statistics like that mate...all meant in good humour, apologies if I offended :thumleft:


ok so in other words. whats the chances of going and buying a set of 86mm CP's and a standard blocks measurments being bang on for required clearances on the spec sheet down to the last ten thou?
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