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Old 04-16-2012, 02:42 PM   #1
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Default piston to bore clerance

what piston-bore clearance should be run on old style FM 1mm over turbo wiseco forged pistons?

i do not have any spec sheet for them but from what i have found .0035" sounds about rite but cant hurt to double check.
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Old 04-16-2012, 02:49 PM   #2
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call fm
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Old 04-16-2012, 02:57 PM   #3
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spoke to Keith at FM and they no longer have the specs for these pistons but he also thinks .0035" is correct. just want to be 100% sure before i go ahead and have the machining done.
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Old 04-16-2012, 03:20 PM   #4
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Sounds about right. They are Weiscos if I'm not mistaken. Talk to your machinist, tell him what you're using the car for.. how much boost etc. and that they are Weiscos. He will know what to do. If for some reason he dosen't, find another machinist.
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Old 04-16-2012, 03:26 PM   #5
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its a full time race car running 1.3bar of boost 355bhp before oil pump failure.. i am not going to know if he is rite in what he is saying if i dont know the answer myself. my machinist will work to any specs i give him.
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Old 04-16-2012, 03:35 PM   #6
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My guy at Wiseco told me .0030-.0035 on my 85mm 2618 forgings for street/slight race turbo use. He said the big boost honda or roadrace guys run .0045".

My machinist f'ed up and my worst cylinder is .0047. Clankadee-clank! My engine isn't running yet but I'm sure it will be llloouudddddd. The skirts are coated and the pins are offset .030", hopefully that helps some. I plan to rebuild it next winter anyway.
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Old 04-16-2012, 03:37 PM   #7
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I searched high and low and ended up going 0.0040"
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Old 04-16-2012, 05:13 PM   #8
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What the piston manufacture wants for skirt clearance is machined in the skirt.

Take your Std bore and add the over bore

Working with a 1.8l BP-ZE block 94-98
Std Bore is 3.268 or 83mm

3.268 + .040 (1mm)= 3.308

Now measure the skirt 90* from the wrist pin

You should be 3.3045-3.305

That will give you .003-.0035 piston to wall clearance.
Now some "fine" adjustment can be made from this spec, to better suit your needs in your build.

Going an extra .0005-.001 will not kill your engine, nor should you hear piston slap

Get the P2W a bit too much and your knock sensor may pick up some untill the engine warms and expantion of the piston starts.
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Old 04-16-2012, 05:43 PM   #9
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great thanks for that. makes perfect sense actually. will measure up and see where i am but .0035-.0040 is looking good at the min.

is there much benefit of using a torque plate during boreing and honing?
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Old 04-16-2012, 06:12 PM   #10
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If one is available, use it.

It will distort the cylinders such as if the head was torqued on
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Old 04-16-2012, 06:17 PM   #11
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Quote:
Originally Posted by TurboTim View Post
My guy at Wiseco told me .0030-.0035 on my 85mm 2618 forgings for street/slight race turbo use. He said the big boost honda or roadrace guys run .0045".

My machinist f'ed up and my worst cylinder is .0047. Clankadee-clank! My engine isn't running yet but I'm sure it will be llloouudddddd. The skirts are coated and the pins are offset .030", hopefully that helps some. I plan to rebuild it next winter anyway.


Doubt .0002 will give piston slap.

Keep in mind a sheet of note book paper is .003 thick, so .0002 is 1/15 the thickness of a sheet of nootebook paper.

Most rod or cramk tolerance from max to min is appox .0007 or about 1/4 the thickness of a sheet of notebook paper.

I have seen engines with .008-.010 wear in them and not slap.
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Old 04-17-2012, 09:01 AM   #12
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You are the only person making me feel better about this.
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Old 04-17-2012, 11:34 AM   #13
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Last engine I heard run that had piston slap, was built by a first time guy.

When he brought it to me, and I tore it apart, I found a .030 over block with Std bore pistons and .030 rings. A new box of .030 pistons and a light hone and he was good to go.

There is no black magic to building an engine, some clearances are rather sloppy.

Now the ECU... thats all balck magic voodoo on the way it works.....
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