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Author Topic: New Engine Break in thread.  (Read 5033 times)

19Patrick83

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Re: New Engine Break in thread.
« Reply #15 on: May 25, 2009, 03:53:37 PM »
Last month I rebuilt my engine. Exhaust valve broke piston.
It was my first work and actually the engine run well, it has 600km. I think now I have finished break-in. I only had a trouble. Old pistons had 82mm of bore. Now new pistons had 82.5mm. I don't balance the weight. New and old pistons had same weight? How much is weight tolerance between that components? +-3grams? I have not good vibration at 2500 and 5000rpm... it's normal? is only after I did that work...
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Offline nhmaf

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Re: New Engine Break in thread.
« Reply #16 on: May 25, 2009, 10:51:38 PM »
Depending on brand/mfg of the piston, some of them are quite close out of the box, though they should be checked and "shaved" if out of tolerance - I don't have the factory tolerance for piston mass handy, but I'd be surprised if they were BMW pistons if they'd be more than a gram or two different, but I haven't actually rebored a BMW (all my experience is with Japanese bikes in this regard)  Are you using the same connecting rods as before?   Did you check the crank journals and bearings for play before installing new pistons?

Someone with more airhead re-bore experience than I will chime in soon..
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Offline Bob_Roller

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Re: New Engine Break in thread.
« Reply #17 on: May 26, 2009, 08:23:51 AM »
If your new pistons are .5 mm larger diameter, you have the second oversize pistons, did you get the correct piston rings ?

I just saw something in the BMW shop manual, that I didn't know before, but apparently there is a correct and an incorrect way to install the pistons.

There is a caution in the manual that states that : 'the installed direction of the piston, has the arrow and word 'vorn' face forward'.

Anybody have a piston out of their bike, to confirm this ?

It looks like the piston pin is offset from the center of the piston by 1.5 mm  (.060 in.) .

I don't know if this will help or not, or just cause more confusion .
« Last Edit: May 26, 2009, 08:43:47 AM by Bob_Roller »
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19Patrick83

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Re: New Engine Break in thread.
« Reply #18 on: May 26, 2009, 10:50:16 AM »
Thank you! Yes that's second oversize pistons. Who gave me pistons gave correct rings and pin too. I think only different weight between old piston and new +0.5mm + 5-6grams. It's possible? I think to reopen engine and cut a little portion of new piston...
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19Patrick83

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Re: New Engine Break in thread.
« Reply #19 on: May 26, 2009, 10:54:12 AM »
There's no dealer who had a pair of pistons and measure difference between that?
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