Cosworth pistons major problem!!!!
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Evolving Member
Joined: Nov 2007
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From: Evoland
sounds like you made wrong assumption. oil consumption was far more likley coming from valve guides or turbo. the piston measurements are just fine. drop in pistons from cosworth should have .005 inches (.13mm ) piston to wall clearance. forget measuring top of piston, that will always be way smaller than bottom of piston because it expands far more. you only need to measure the bottom of piston. those measurements listed are perfect for drop in pistons. no way was that the cause of oil consumption. as far as pins being tight , that is normal as well. the pins move back and forth when engine is running. this puts load on the locks that distorts the groove for the locks. making it very difficult to get pins back out.
post pictures of the pistons if you want better answers. that will tell us way more than your measurements given
post pictures of the pistons if you want better answers. that will tell us way more than your measurements given
Two of the pistons have 0.17mm and 0.19mm wall clearance if the exact cylinder bore is 85.00mm
As for the photos I will post them tomorrow.
Last edited by konstantinosIX; Apr 24, 2009 at 12:20 AM.
Usually, a good machine shop will order the pistons in, measure them individually BEFORE boring/honing the cylinders, and bore/hone the cylinders individually to match the pistons and the clearance you want. The clearances are recorded on each cylinder when the block is honed and the pistons are assembled. That was apparently not done here.
Thread Starter
Evolving Member
Joined: Nov 2007
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From: Evoland
I am not talking about wall clearance difference.
I am talking about the bottom diameter of the pistons difference.
So... something is going on with the pistons before or after 5000 miles!
No real polite way to say this AL, dont know where you are getting your piston to wall specs but recommending .0029-.0035 is motor suicide for 2618 allow piston. Those clearances are good for 4032 only. 2618 allow requires .005 minimum. The cold truth is you should hear cold start piston slap when using 2618. If you dont hear them you have your clearances too tight. The loosest JE motor I built had clearance of .0065 and the piston skirts and ring seal was beautiful after 10,000 hard 35psi miles. Didnt burn oil. 4 % leakdown. no catch can needed. was quiet noisy on start up. but was quiet when warm.
so the piston to wall clearance for your pistons, given the measurements you listed are .005,005,.0065, and .0075. (inches)
if they were actually machined that far off from each other thats a pretty horrid mistake on cosworth part. more likley you are measuring by wrong method or two pistons wore from honing with too heavy grit during install. or not using torque plate or other possible mistake. cant list them all.
but even .0075 on one piston wont cause oil burning if bores were honed correctly. did you use torque plate when honing? what grit did you use? what kind of hone?
this thread is useless with out good pictures of sides of pistons. and still will likely be useless as not enough info given here
Thread Starter
Evolving Member
Joined: Nov 2007
Posts: 429
Likes: 0
From: Evoland
measuring the bottom of piston does give you wall clearance. given that you are dealing with stock bore pistons. the stock bores will be all be the same size coming from factory.
so the piston to wall clearance for your pistons, given the measurements you listed are .005,005,.0065, and .0075. (inches)
if they were actually machined that far off from each other thats a pretty horrid mistake on cosworth part. more likley you are measuring by wrong method or two pistons wore from honing with too heavy grit during install. or not using torque plate or other possible mistake. cant list them all.
but even .0075 on one piston wont cause oil burning if bores were honed correctly. did you use torque plate when honing? what grit did you use? what kind of hone?
this thread is useless with out good pictures of sides of pistons. and still will likely be useless as not enough info given here
so the piston to wall clearance for your pistons, given the measurements you listed are .005,005,.0065, and .0075. (inches)
if they were actually machined that far off from each other thats a pretty horrid mistake on cosworth part. more likley you are measuring by wrong method or two pistons wore from honing with too heavy grit during install. or not using torque plate or other possible mistake. cant list them all.
but even .0075 on one piston wont cause oil burning if bores were honed correctly. did you use torque plate when honing? what grit did you use? what kind of hone?
this thread is useless with out good pictures of sides of pistons. and still will likely be useless as not enough info given here
"Piston 1:84.40mm diameter on the top 84.81mm diameter on the end of the bottom
Piston 2:84.40mm diameter on the top 84.87mm diameter on the end of the bottom
Piston 3:84.40mm diameter on the top 84.83mm diameter on the end of the bottom
Piston 4:84.40mm diameter on the top 84.87mm diameter on the end of the bottom"
That is the reason that I am confused....
I am sure that this is Cosworth machining problem!
I do not know how many pistons from Cosworth have the same problem.
I have pistons photos at my iphone and I will try to upload them today.
You will see the piston in perferct shape like new and the piston's side green coating also is like new!!!!
I will contact again Cosworth and ask them to replace them.... but first check the diameter on new ones....
This sounds like they were intentionally machined with a slight taper to accomodate thermal expansion. It's not unheard of.
In operation, the top of the piston is exposed to higher temperatures and so expands more than the bottom.
In operation, the top of the piston is exposed to higher temperatures and so expands more than the bottom.
I am a bit confused on how even a suspected out of spec piston-wall clearance could cause a significant amount of oil consumption. If the ring gaps are correct ( my guess would be some where between 19-22 thousandths) they will seal properly when they expand. A "loose" piston within the cylinder might make noise if it has too much play, but I cannot see it causing massive oil consumption. There must be some other problem. Can you physically see evidence of oil consumption in the combustion chamber? If the engines are using that much oil then there should clearly be evidence of burning oil in the chambers. I could see valve seals causing that. How about crank case pressure? Are the engines properly vented? crank case pressure can cause alot of oil consumption.
