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i have recieved my 86.25mm je pistons and in the box theres a instruction leaflet saying a gap of 0.09mm should be left between piston and cylinder, however in my past experience with stock and hks pistons they only state a gap of between 0.02mm -.0.04mm. is the extra gap allowing for expansion in race engines that run red hot as im only using them for a road car with the odd weekend 1/4 mile?

any help much appreciated.
 

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Maybe the difference is because of material (different expansion rates).
Just guessing though.
Somebody will chime in with a proper answer...
 

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Will, a "looser" engine will require less break in. It will also be safer for your engine if you ever started to overheat, that it wont lock up quite as quickly. I know these are dumb ideas but those are reasons I can think of. JE builds alot of pistons for race cars and many of these race cars rebuild their engine and then go race without any break in. Of course, they are only expecting to get a season and maybe 1,000 miles out of the engine too.
 

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Materials. Stock pistons will not expand as much as the beefy forged pistons JE produces. You run the forged pistons looser so they can expand, which is why on some built motors in other cars you hear "piston slap" upon startup, until it warms up.
 

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My engine builder said the forged pistons expand more than the cast ones. The bigger cold clearance is to make sure the fit is right when they heat up.
 

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270whp on a CT26...
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lucky for you we just covered pistons in school. i'll give you the quick version. :)

forged pistons run cooler than cast pistons but they actually expand more during operation because they are more dense. sometimes you might get some piston slap on cold starts, but this will not damage anything, and it should only last a minute or two at the most while your pistons heat up.

for what it's worth: the spec sheet sent to me with my CP forged pistons calls for .0035" clearance, compared to my engine build sheet specs, the stock clearance range is from .0015" to .0035", so i will still be in spec according to toyota.

-Mike
 

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Yup.... they definetly expand at start up. You could hear mine when you turn the car on. Sounds weird at first... but then you get used to it.... and it definetly sounds cool. Plus it's a good investment in case later you want to upgrade blah blah blah... on whatever you pick. Like a turbo w/ higher boosts.
 

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NOTE!!!
Your piston bore clearence is most likely already built into the piston!

If JE have quoted the piston as being 86.25mm Measure it at the skirt, and you may find it is already 0.09 smaller. EG 86.25 is the exact bore diameter you should get machined - don't add the clearence again.

The Data sheet JE supplied with the piston should point this out.

Paul
 

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270whp on a CT26...
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yes, very good point. ALWAYS ALWAYS ALWAYS measure your pistons before you bore & hone. then be certain you are making the correct bore size for the clearance you want.

also, make sure to measure on the skirt, not on the ringlands, the ringlands are smaller than the skirt because the top of the piston runs hotter and expands more than the bottom. you machinist/shop should know all of this already, but just in case... :)

-Mike
 

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The best practice is to provide both the pistons, AND the boring/honing instructions they come packaged with, to the shop that will be boring/honing the block at the same time you take the block in for it's tickle job.
 
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